4 DECREASING DEMAND ON EXISTING SYSTEMS 4.0 INTRODUCTION The design seismic forces (or demand forces) prescribed by most building codes generally are proportional to building weight and inversely proportional to the two-thirds power of the fundamental period of vibration of the building and to a response reduction factor that represents the capability of the structural system to absorb energy in the inelastic range of the building response. Within this context, the earthquake demand of a building may be reduced by reducing the weight of the building, increasing the fundamental period and the energy dissipating capacity of the structural system, or using alternate procedures. 4.1 REDUCING THE WEIGHT OF THE BUILDING In relatively low buildings (i.e., below 3 to 5 stories), reducing the weight of the building will result in a reduction of the seismic forces on the structural elements. Although a reduction in weight will decrease the fundamental period of vibration, the code-prescribed seismic force coefficient remains constant (i.e., is not affected by a change in fundamental period) for these buildings, so the reduction in the seismic forces is directly proportional to the weight reduction. For taller buildings (i.e., 6 to 10 stories), the reduction in the fundamental period resulting from a reduction in weight (i.e., the period is proportional to the square root of the weight) also will result in an increase in the seismic force coefficient. This increase will tend to offset the decrease associated with the reduction in weight. For very tall buildings (i.e., 20 stories or more), the effect of the fundamental period is minimal and the seismic forces are essentially proportional to the reduced weight. Techniques. Techniques that have been utilized to reduce weight include: 1. Removing the upper stories of a building. 2. Changing the use of the building (e.g., converting from heavy warehouse loading to office or residential use). 3. Removing a heavy roof system and replacing it with a lighter assembly. 4. Removing heavy appurtenances (i.e., parapets, balconies, water towers, or equipment). Relative Merits. Removal of the upper stories is an effective technique for decreasing the earthquake demand on a building. As indicated above, this technique may be less effective for buildings of moderate height than it is for low or very tall buildings. An additional benefit associated with this technique is the reduction in gravity loads. Use of this technique will result in reduced forces on the existing vertical-load-resisting elements in the remaining stories and foundations thereby providing additional capacity for seismic forces. The primary disadvantage of this technique is the loss of usable space and the associated loss of rental income and resale value. The American Iron and Steel Institute has written a minority opinion concerning this section; see page 193. 103 Changing the use of the building in order to eliminate heavy floor loads that contribute to the seismic force also is an effective technique to reduce the seismic demand. Since the ground floor and its tributary loads do not contribute to the building seismic forces, reducing the floor loads in the upper floors of a multi story building is most effective. This technique also reduces the forces on the vertical-load-resisting elements and, thus, increases the capacity of these elements for seismic forces. The elimination of heavy floor loads that are regarded as dead loads in seismic provisions will affect the fundamental period of the building in a manner similar to that discussed above for the removal of upper stories. Also as discussed above, the advantage of weight reduction may be partly offset for moderate height buildings by an increase in the seismic force level due to the period changes. An additional factor to be considered for this technique is whether the change in use or occupancy will trigger other building code requirements (e.g., fire protection, egress) that may be costly to meet. Removal and replacement of a heavy roof system is particularly effective in reducing the seismic demands on an existing one-story building. As the number of stories is increased, this technique becomes less effective and it is also subject to the limitations for moderate height buildings discussed above. Removal of heavy appurtenances has the same effects on seismic demand as discussed above for the removal of stories or the elimination of heavy floor loads. 4.2 INCREASING THE FUNDAMENTAL PERIOD AND THE ENERGY DISSIPATING CAPACITY OF THE STRUCTURAL SYSTEM By increasing the fundamental period of vibration of some structures, the seismic demand can be decreased. The most effective method for increasing the fundamental period of the building system without modification of the structural system of the building is by introduction of seismic isolators at the base of the building. The seismic isolators can increase the effective fundamental period of the system, thus reducing the base shear of the structural system of the building; energy dissipation also can be included within the isolator system. In addition to seismic isolation, energy dissipation devices may be added to the structural system. The energy dissipation system increases the system damping and modifies the building response and provides the equivalent of increasing the value of the response modification factor, R. Techniques. The response reduction factor (i.e., energy dissipating capacity) applicable to an existing building can be increased by: 1. Modifying the existing structural systems, 2. Supplementing the existing structural systems, or 3. Replacing the existing structural systems. Relative Merits. Modification of an existing structural system to improve its energy absorbing capacity is seldom feasible except in the case of an ordinary steel moment frame. In this case, it may be possible to upgrade the frame to a special moment frame or to the minimum frame requirements for a dual system in conjunction with existing shear walls. Similarly, removal and replacement of an existing structural system seldom will be economically feasible unless dictated by other than engineering considerations (e.g., complete architectural retrofit of the exterior of the building). A possible exception to this statement could occur in existing steel frame buildings with concentric steel bracing or unreinforced masonry infill walls. In these cases, it may be feasible to remove the bracing or the infill walls and install eccentric bracing or reinforced concrete shear walls. Supplementing the existing structural system is, by far, the most common technique for seismic strengthening and, in many cases, it is possible to reduce the seismic demand by improving the energy absorption characteristics of the combined system. 104 4.3 ALTERNATE PROCEDURES The NEHRP Recommended Provisions as well as model building codes provide for approval of alternative procedures that can be demonstrated to be equivalent to code-prescribed procedures concerning strength, durability, and seismic resistance. In recent years, several innovative alternative procedures for the reduction of seismic demand have been subjected to analytical and experimental research and have seen limited application in both new and existing buildings. These procedures include: * Seismic isolation techniques and * Supplemental damping techniques. 43.1 SEISMIC ISOLATION Techniques. Base isolation is a generic term for procedures whereby the response characteristics of a building are altered by the introduction of devices or special construction at the base of the building. The discussion here is confined to the use of base isolation to reduce seismic demand by lengthening the fundamental period of vibration of an existing building. Relative Merits. Most base isolation devices are capable of developing a fundamental period of about 2 seconds. This can effectively reduce the seismic demand for buildings founded on rock or firm soils that have a natural fundamental period of about 1 second or less ( i.e., buildings less than about 10 stories). Base isolation may be detrimental to buildings founded on very soft soils where a 2 second period base-isolated building may be in resonance with similar periods in the ground motion transmitted by the soft soils. Implementation of base isolation for existing buildings usually will require that the building be underpinned for the installation of base isolation pads. A competent diaphragm also is required above the isolation pads to distribute the lateral forces and, for existing buildings, a new concrete slab generally has been provided to serve this purpose. Finally, provision must be made to accommodate the large displacement of the isolation pad; this usually is done by providing both adequate clearance around the building to accommodate this displacement and sliding or flexible connections for all utilities and services to the building. 43.2 SUPPLEMENTAL DAMPING Techniques. Structural damping may be defined as an internal energy absorption characteristic of a structural system that acts to attenuate induced free vibration. Damping is commonly expressed as a percentage of critical damping. A zero damped elastic system, when displaced, theoretically would vibrate continuously at its natural period and at the same amplitude. A critically damped structure when displaced would return to its original position without vibration. Damping also tends to reduce the dynamic amplification of vibration particularly when the period of the building is in resonance with the ground motion. The seismic provisions in most building codes are based on 5 percent of critical damping as being representative of most building structures. The upper limit of the required seismic forces, before division by the response reduction factor, assumes dynamic amplification of the ground motion by a factor of 2 to 2.5 depending on the soil conditions. If the structure can develop 20 percent damping, the above amplification (and the displacements) would be reduced by one-half. The various concepts that have been proposed for providing supplementary damping are: * Viscous damping, * Friction damping, and * Natural yield damping. 105 Viscous damping involves taking advantage of the high flow resistance of viscous fluids. A simple shock absorber like that on an automobile is one example. Other devices such as a pair of flat plates with viscous fluid between them have been proposed. Shock absorbers have been implemented in connection with nuclear power plant piping systems but they have proved to be very high maintenance cost items. Friction between dry surfaces produces a constant force, always opposed to the direction of motion, that is proportional to the contact force between the surfaces and the coefficient of friction of the materials. A number of friction damping devices usually associated with diagonal bracing in buildings, have been proposed. Major concerns with friction dampers in connection with the long-term periods between earthquakes are ensuring that the contact forces between the sliding surfaces do not change and ensuring that the coefficient of friction does not change. Natural yield damping of structural elements in buildings (e.g., beams) has long been recognized as providing added damping to structures. Material yielding is very commonly used in earthquake engineering in conjunction with the ductility, seismic isolation, and supplemental damping concepts of design. In recent years, a variety of mechanical devices that incorporate the yielding deformation of mild steel to provide supplemental damping have been implemented in earthquake-resistant designs of buildings and other structures. Mild steel bars in torsion Similarly, lead and cantilevers in flexure have been developed, tested, and installed in buildings and bridges shear and lead extrusion devices also have been developed. Relative Merits. The application of supplemental damping in the seismic rehabilitation of existing buildings is in its infancy; hence, the benefits and problems of the various alternatives have not been thoroughly investigated. In general, devices that involve material yielding as the means for increasing energy dissipation or damping can be regarded as being very reliable. Mild steel and lead are very stable materials with predictable yield deformation characteristics. Irrespective of the type of damping involved, the installation in buildings of devices commonly proposed thus far in connection with supplemental damping involves distributing the devices throughout a structure. The seismic response of a damped building would be similar to that of a conventional building. This is in contrast to the seismic isolation concept where virtually all of the relative displacement occurs at the isolation level. Change in period of vibration and stiffness associated with material yield damping can be significant Practical supplemental depending on the ground motion demand and the elastic strength of the damper damping devices that involve material yielding generally result in a reduction of stiffness during earthquake -response and, thus, periods lengthen. Although the change in period may be of little importance, the change may result in decreased demand forces. The seismic analysis of buildings using supplemental dampers requires sophisticated nonlinear time-history analytical tools because of the yielding (i.e., inelastic) response requirements. 106 5 REHABILITATION OF NONSTRUCTURALARCIUTECTURALCOMPONENTS 5.0 INTRODUCTION Nonstructural architectural elements can be damaged in an earthquake, and some of this damage may result in life-threatening hazards. The two principal causes of architectural damage are differential motion and lack of component capacity: For example, the differential seismic displacement between stories (i.e., drift) can cause window breakage. Architectural cladding, such as a granite veneer, with insufficient anchorage capacity is an example of a component with a lack of capacity. 5.1 EXTERIOR CURTAIN WALLS Rigid nonductile curtain wall panels, (e.g., those constructed of precast concrete) attached to the exterior of a flexible structure (e.g., a steel moment frame) may have insufficient flexibility in their connections to the frame and insufficient spacing between panels to prevent damage due to racking. The connection details therefore may have to be modified to allow flexibility, and Figure 5.1a shows a typical connection detail that precast panel provides ductility and rotational : capacity. The panel is rigidly attached at the base and held ; beam with a flexible rod at the top usually is desirable to provide a Insert rigid support at one end of each panel and to allow the other end to translate to accommodate the interstory deflection of the frame without racking of the panels. Another common deficiency gap weld (t p) is that the existing connections may not provide adequate freedom for accommodating the calculated horizontal and vertical story distortions. A feasible remedy may be to remove the existing connections at one end .r of the panels and replace them with flexible rods (as indicated in Figure 5.1b) or with other connecting devices provided with adequate oversized and slotted FIGURE 5.1a Flexible connection for precast concrete cladding holes. In implementing these techniques, the capacity of the modified connection for gravity loads and for out-of-plane seismic loads must be checked and strengthened if necessary. 107 Inserts or attachments secured to panel reinforcing, FIGURE 5.1b Detail for flexible connection for precast concrete cladding. 5.2 APPENDAGES Cornices, parapets, spandrels, and other architectural appendages that have insufficient anchorage capacity must Cornice anchorages can be strengthened be retrofitted to prevent damage and, most important, falling debris by removing the cornice material, adding anchorages, and reinstalling the material. A technique that has been used in rehabilitating heavy and ornate cornice work is to remove the cornice and reconstruct it with adequate anchorage and new lighter material such as lightweight concrete or plaster. Parapets can be reduced in height so that the parapet dead load will resist uplift from out-of-plane seismic forces or they can be strengthened with shotcrete (Figure 5.2a) or braced back to roof framing (Figure 5.2b). All elements must be checked for their ability to sustain new forces imposed by the corrective measures. 108 (E) masonry (N) shotcrete (E) concrete floor FIGURE 5.2a Strengthening a masonry parapet with a new concrete overlay. (N) drilled and grouted bolt (E) masonry parapet (N) channel (N) brace (E) roof FIGURE 5.2b Strengthening a masonry parapet with steel braces. 109 5.3 VENEERS Stone and masonry veneers with inadequate anchorage should be strengthened by adding new anchors. Veneers Typical details for approved anchorage of masonry typically must be removed and replaced for this process veneers are published by the Brick Institute of America. 5A PARTITIONS Heavy partitions such as those of concrete block may fail from excessive flexural stresses or excessive in-plane shear stress caused by interstory drifts. Such partitions should be retrofitted with connections like those shown in Figure 5.4a that restrain out-of-plane displacement and allow in-plane displacement. Alternatively, unreinforced masonry partitions can be removed and replaced with drywall partitions. Partitions that cross seismic joints should be reconstructed to allow for longitudinal and transverse movement at joints. Plaster or drywall partitions in office buildings generally need lateral support from ceilings or from the floor or roof framing above the partition. Steel channels are sometimes provided at the top of the partitions. The channels are attached to the ceiling or floor framing, they provide lateral support to the partition but allow vertical and Partitions that do longitudinal displacement of the floor or ceiling without imposing any loads to the partition. not extend to the floor or roof framing and are not laterally supported by a braced ceiling should be braced to the framing above (as indicated in Figure 5.4b) at a maximum of 12 foot spacing between braces. Hollow clay tile partitions occur in many existing buildings as corridor walls or as nonstructural enclosures for elevator shafts or stairwells. Hollow clay tile is a very strong but brittle material and it is very susceptible to shattering into fragments that could be hazardous to building occupants. In many cases it is not possible to isolate these partitions from the lateral displacements of the structural framing and, in those cases, it is advisable to consider either removal of these partitions and replacement with drywall construction or "basketing" of the potential clay tile fragments with wire mesh. 110 FIGURE 5.4a Bracing an Interior masonry partition. 111 Bracing an interior masonry partition. 112 5.5 CEILINGS Unbraced suspended ceilings can swing independently of the supporting floor and cause damage to the ceilings, particularly at the perimeters. Providing four-way (12-gage wire) diagonals and a compression strut between the ceiling grid and the supporting floor at no more than 12 feet on center and within 6 feet of partition walls will significantly improve the seismic performance of the suspended ceiling. Figure 5.5 shows a typical detail of the four-way diagonals and the compression strut. In addition to the braces, the connections between the main runners and cross runners should be capable of transferring tension loads. Lay-in ceilings are particularly vulnerable to the relative displacement of the supporting grid members. Splices and connections of the T-bar sections that comprise the grid may have to be stiffened or strengthened with new metal clips and self-threading screws. (N) 12 gage wires (N) adjustable compression struts to prevent vertical movement (E) main runner FIGURE 5.5 Lateral bracing of a suspended ceiling. 5.6 LIGHTING FIXTURES Suspended fluorescent fixtures are susceptible to several types of seismic damage. Fixtures that are supported by suspended ceiling grids can lose their vertical support when the suspended ceiling sways and distorts under seismic shaking. Independent wire ties connected directly from each of the fixture corners (or at least diagonally opposite corners) to the structural floor above can be added to prevent the fixture from falling (Figure 5.6). Pendant-mounted fixtures often are supported by electrical wires. Wire splices can pull apart and allow the fixtures to fall. The fixtures also may swing and impact adjacent objects resulting in breakage and fallen fixtures. 113 Safety wires can be installed to prevent the fixtures from falling and diagonal wires can prevent them from swaying. Some fixture manufacturers also provide threaded metal conduit to protect the wiring and to support the fixture as well as wire straps or cages that can be added to prevent the fluorescent tubes from falling away from the fixture if they become dislodged. 5.7 GLASS DOORS AND WINDOWS Inadequate Seismic rehabilitation of glass windows and doors to prevent breakage may be a significant effort. edge clearances around the glass to allow the building and, hence, the window frame to rack in an earthquake without bearing on the glass is the principal cause of breakage. Redesign (along with close installation inspection) of the frame and/or glazing to provide sufficient clearance is necessary to prevent seismic breakage. A technique suggested by Reiterman (1985) to reduce life-safety hazards from falling glass is to apply adhesive solar film to the windows. The film will hold together the glass fragments while also reducing heat and glare. The application of solar film to insulating glass may cause heat build-up inside the glass and the possible adverse effects of this build-up need to be considered since damage can result. 114 5.8 RAISED COMPUTER ACCESS FLOORS Access floors typically are constructed of 2-foot by 2-foot wood, aluminum, or steel panels supported on adjustable column pedestals. The column pedestals frequently are fastened to the subfloors with mastic. Some assemblies have stringers that connect the top of the pedestals (Figure 5.8a) and others have lateral braces. When subjected to lateral loads, access floors typically are very flexible unless they are specifically designed to be rigid. This flexibility may amplify the ground motions such that equipment supported on the floor may experience significantly high displacements and forces. The high displacements also may cause connection failures that could precipitate a significant collapse of the floor. Existing floors can be rehabilitated by securing the pedestals to the subfloor with expansion anchors or by adding diagonal bracing to pedestals in a regular pattern (Figure 5.8b). Rehabilitated floors should be designed and tested to meet both a stiffness and a strength criterion. 115 FIGURE 5.8b Strengthening of access floor 116 6 REHABILITATION OF NONSTRUCUURAL MECHANICAL AND ELECTRICAL COMPONENTS 6.0 INTRODUCTION Nonstructural mechanical and electrical components are often vulnerable to seismic damage in moderate to large earthquakes. Damage to mechanical and electrical components can impair building functions that may be essential to life safety. This chapter presents common techniques for mitigating seismic damage of the following typical mechanical and electrical components: * Mechanical and electrical equipment * Ductwork and piping * Elevators * Emergency power systems * Hazardous material storage systems * Communication systems * Computer equipment 6.1 MECHANICAL AND ELECTRICAL EQUIPMENT Large equipment that is unanchored or inadequately anchored can slide during an earthquake and damage utility connections. Tall, narrow units may also be vulnerable to overturning. Positive mechanical anchorages (Figure 6.1a) will prevent seismic damage. Electrical equipment frequently is tall and narrow and may overturn and slide, causing damage to internal instruments and utility connections. This type of equipment can be secured against sliding or rocking in many ways depending on the location of the units relative to adjacent walls, ceilings, and floors (Figure 6.1b). In all cases, the capacity of the wall to resist the seismic loads imposed by the connected equipment must be verified. Mechanical or electrical equipment located on vibration isolators may be particularly vulnerable to being shaken off the isolator supports. Rehabilitation to mitigate the potential for damage involves either replacing the vibration isolation units or installing rigid stops. Vibration isolation units that also provide lateral seismic resistance are available from isolator manufacturers and these units (Figure 6.1c) can be installed in place of the existing isolators. Alternatively, rigid stops designed to prevent excessive lateral movement of the equipment can be installed on the existing foundation (Figure 6.1d and e). A sufficient gap needs to be provided between the stop and the equipment to prevent the transmission of vibrations through the stops. Where equipment is tall relative to its width, stops in the vertical direction are required to prevent overturning. The equipment itself, its attachments to the isolators or support rails, and the rails themselves can be points of weakness that need to be assessed and strengthened where required. 117 typical angle clips (N) weld (E) transformer FIGURE 6.1a Typical detail of equipment anchorage. 118 Q FIGURE 6.Ib Alternate details for anchoring equipment. 119 FIGURE 6.1b continued. FIGURE 6.1c Prefabricated vibration isolation assembly with lateral seismic stops. 121 (N) provide gap as required (E) vibration isolation assembly (N) angles with resilient pads (N) anchor bolt FIGURE 6.1d Seismic restraints added to existing equipment with vibration Isolation. 122 (E) vibration isolation assembly (N) bar stock (N) channel with resilient pads f a (N) weld <1 FIGURE 6.1e Multidirectional seismic restraint. 123 Suspended mechanical or electrical equipment may sway during an earthquake, damaging utility connections and the vertical support components. This equipment should be braced to prevent swaying (Figure 6.10. (N) diagonal bracing (E) vertical support FIGURE 6.1f Typical bracing for suspended equipment. 124 Water heaters are tall, heavy, narrow components that, if unanchored, are vulnerable to damage in an earthquake. Sliding or overturning of water heaters may result in broken water and gas lines. Water heaters should be anchored as shown in Figure 6.1g, and flexible gas lines should be installed with a sufficient loop to allow the heater some movement without stressing the gas lines of domestic water heater. 125 6.2 DUCT WORK AND PIPING Seismic retrofit of ductwork and piping primarily consists of providing lateral sway braces. The Sheet Metal and Air-Conditioning Contractors National Association (SMACNA) has published guidelines for the design of seismic restraints of new mechanical systems and plumbing piping systems (September 1982) that can also be used for rehabilitation of existing systems. These guidelines were developed for use in areas of relatively high seismicity and engineering judgment should be utilized in their application elsewhere. The SMACNA guidelines for seismic bracing of ductwork recommend that: 1. All rectangular ducts 6 square feet in area and greater and round ducts 28 inches in diameter and larger should be seismically braced. 2. Transverse braces should be installed at a maximum of 30 feet on center, at each duct turn, and at each end of a duct run. 3. Longitudinal braces should be installed at a maximum of 60 feet on center. 4. No bracing is required if the top of a duct is suspended 12 inches or less from the supporting structural member and the suspension straps are attached to the top of the duct. The SMACNA guidelines for seismic bracing of piping recommend that: 1. Braces for all pipes 2-1/2 inches in diameter and larger (and also for smaller piping used for fuel gas, oil, medical gas, and compressed air and smaller piping located in boiler rooms, mechanical equipment rooms, and refrigeration machinery rooms). 2. Transverse braces should be installed at a maximum of 40 feet on center. 3. Longitudinal braces should be installed at a maximum of 80 feet on center. 4. Thermal expansion and contraction forces, where present, must be considered in the layout of transverse and longitudinal braces. 5. Flexibility should be provided where pipes pass through seismic or expansion joints. Figures 6.2a through 6.2c show typical seismic brace details for ducting. Duct diffusers also should be positively attached with mechanical anchors to rigid ducts or secured with wires to the floor above when connected to flexible ducts. Figures 6.2d through 6.2g show typical details for installing seismic braces for piping. 126 I I (N) angle brace .0(N) strap around duct FIGURE 6.2a Lateral and longitudinal braces for large-diameter ducting. 127 FIGURE 62b Lateral and longitudinal braces for small-diameter ducting. 128 (N) angle brace (E) duct FIGURE 6.2c Lateral and longitudinal braces for rectangular ducting. 129 (N) pipe clamp (N) channel (N) strap around pipe FIGURE 6.2d Lateral braces for piping. 130 (N) angle brace (E) pipe (N) pipe clamp (N) angle brace (E) pipe FIGURE 6.2e Longitudinal pipe brace. 131 (N) angle brace (E) pipe (N) strap around pipe FIGURE 6.2f Lateral brace for multiple pipes. (N) angle brace (E) pipe (N) strap around pipe FIGURE 6.2g Longitudinal brace for multiple pipes. 1132 6.3 ELEVATORS Elevator machinery and controller units should be anchored like other mechanical and electrical equipment to prevent the units from sliding or toppling. Rope retainer guards should be provided on sheaves to inhibit displacement of wire ropes. Snag points created by rail brackets should be provided with guards so that compensating ropes or chains, governor ropes, suspension ropes, and traveling cables will not snag. Retainer plates should be added to the top and bottom of the cars and counterweights to prevent them from becoming dislodged from the rails. Seismic switches should be installed to provide an electrical alert or command for the safe automatic emergency operation of the elevator system and to detect lateral motion of the counterweight. For more information on the requirements for elevator seismic safety refer to ANSI 17.1, Safety Codes for Elevators and Escalators. 6.4 EMERGENCY POWER SYSTEMS Although emergency power systems typically containing batteries, motor generators, fuel tanks, transformers, switchgear, and control panels are designed to be activated in the event of an emergency, many are inadequately protected from earthquake forces. Batteries are frequently stored in racks as shown in Figure 6.4a, and structural members should be installed to restrain the batteries to the racks, the racks should be (N) foam ( foam spacers e A d G braced, and adequate anchorages should be provided to carry (N) restraining the lateral loads. Foam spacers members also should be fitted snugly between the batteries to prevent them from impacting each other. Motor generators typically are mounted on vibration isolators, and these units should have seismic stops installed as shown in Figures 6.1d or 6.1e. Fuel tanks frequently are mounted on legs to facilitate gravity feed of the fuel, and these tanks should be braced as shown in Figure 6.4b and provided with adequate anchorage. Flexible fuel piping with adequate loops also should be installed both at the fuel tank and at the motor generator (transformers, switchgear, and control panels should be anchored as shown in Figure 6.1b. FIGURE 6Aa Bracing of existing battery racks. 133 (N) brace (E) leg (N) anchor bolt FIGURE 6Ab Bracing of horizontal tank. 134 6.5 HAZARDOUS MATERIAL STORAGE SYSTEMS Seismic-activated shutoff valves should be installed on hazardous materials supply lines. These lines also should be adequately braced as shown in Figures 6.2e and 6.2f and should be provided with flexible connections at storage tanks. Bottles of laboratory chemicals should be prevented from falling by using elastic straps or shelf lips as shown in Figure 6.5a. Liquid oxygen and similar pressurized tanks also should be restrained as indicated in Figure 6.5b. FIGURE 6.5a Protective measures for hazardous materials. 135 (E) gas cylinder (N) chain restraint FIGURE 6.5b Anchorage detail for pressurized tanks. 136 6.6 COMMUNICATIONS SYSTEMS The operation of communication systems following an earthquake is of vital importance to individuals, communities, federal agencies, and private businesses that depend on them to aid in assessing damage and responding to problems. Telephone communications equipment consists of input and output data processing units, disk drives, central computers, and remote regional and central switching units, much of which is located on raised access floors; this computer type equipment is discussed in Sec. 6.7. Remote switching units not located on raised floors should be secured like other mechanical and electrical equipment as discussed in Sec. 6.1. Essential facilities such as hospitals and fire and police stations that must have communication capabilities in the event of an earthquake should have backup external and internal communication systems. Radio equipment should be secured to prevent sliding or toppling. Desk top equipment also should be secured or tethered to prevent falling. 6.7 COMPUTER EQUIPMENT Computer equipment vulnerable to seismic damage includes electronic data processing equipment such as mainframes, peripherals, telecommunications cabinets, and tape and disk storage units. Seismic rehabilitation to protect computer equipment is different from that required for other mechanical and electrical equipment for several reasons: (1) computer equipment typically is located on raised access floors that complicate traditional anchorage techniques and may amplify seismic loads, (2) computer equipment design is rapidly evolving and advancing, units frequently are replaced or rearranged, and (3) some computer equipment may be sensitive to high-frequency vibrations such as those that may be caused by ground shaking. The remainder of this section briefly identifies rehabilitation techniques for data processing equipment and tape and disk storage racks. For more information on the subject, refer to Data Processing Facilities: Guidelines for Earthquake Hazard Mitigation (Olson, 1987), which provides detailed seismic design recommendations for new computer facilities and the rehabilitation of existing facilities. Electronic data processing (EDP) equipment typically is located on raised access floors; hence, the traditional techniques of anchoring electrical equipment to the floor are complicated by the fact that the anchorage needs to pass through the access floor to the subfloor. This reduces the access to the space beneath the raised floor and greatly reduces the flexibility to rearrange and replace equipment. Some dynamic tests of EDP equipment also have shown that certain vibration-sensitive equipment may be more prone to seismic damage if it is rigidly anchored to the building and is subjected to high-frequency seismic ground motions than if the equipment is free to slide on the access floor. However, if EDP units are unrestrained, they may slide into structural walls or adjacent equipment or their support feet may slide into an access floor penetration, and the unit will topple. Two general solutions may be used to reduce the potential for seismic damage of EDP equipment: rigidly restraining the equipment or allowing the equipment to slide. Rigid restraints (Figure 6.7a) may be appropriate for equipment that is not vibration-sensitive, is not likely to be relocated, or is tall and narrow (and, hence, susceptible to toppling). Air-handling units, modem cabinets, and power distribution units fall into this category. Tall, flexible equipment such as modem cabinets may require stiffening or bracing near the top. If anchored only at the base, the seismic motions at the top of the units may be significantly amplified and may result in equipment damage. Figure 6.7b shows a detail that will prevent toppling but does not transmit high-frequency ground shaking to the unit. Equipment that is vibration sensitive or is likely to require frequent relocation can be isolated to reduce the potential for seismic damage. Some of the considerations necessary for isolating equipment include protecting the equipment from sliding to prevent a supporting foot or caster from falling into an opening in the access floor (provided for cable penetrations). This can be prevented by tethering the equipment to the subfloor (Figure 6.7c) so that the equipment cannot slide far enough to impact other equipment or walls or to fall into a penetration. Precautions should be considered for tall equipment restrained with a tether to prevent the equipment from reaching the end of the tether, which may cause the equipment to overturn. Floor penetrations also can be provided with guards (Figure 6.7c) that will prevent the equipment feet from entering. Adjacent 137 equipment either should be separated by about 1 foot to prevent potential pounding or should be strapped together (Figure 6.7d) so that the separate pieces move as a unit. FIGURE 6.7a Rigid anchorage of computer equipment. 138 (E) computer equipment (E) leveling pads (N) tension rods (N) turnbuckle (N) expansion anchor FIGURE 6.7b Flexible anchorage of computer equipment. 139 FIGURE 6.7c Tether and equipment. 140 FIGURE 6.7d Strapping of electronic data processing units. 141 BIBLIOGRAPHY ABK. 1981. Methodology for Mitigation of Seismic Hazards in Existing Unreinforced Masonry Buildings: Categorization of Buildings. Washington, D.C.: National Science Foundation. American National Standards Institute. 1987. Safety Code for Elevators, "Appendix F -Recommended Elevator Safety Requirements for Seismic Risk Zone 3 or Greater," ANSI 17.1. Applied Technology Council. 1986. Base Isolation and Passive Energy Dissipation: Proceedings of a Seminar and Workshop, ATC-17. Redwood City, California: Applied Technology Council. Applied Technology Council. 1987. Evaluating the Seismic Resistance of Existing Buildings, ATC-14. Redwood City, California: ATC. 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In Proceedings of U.S. -People's Republic of China Bilateral Workshop on Earthquake Engineering, Vol. 1. Zarnic, Roko, Miha Tomazevic, and Tomaz Velechovsky. 1986. "Experimental Study of Methods for Repair and Strengthening of Masonry Infilled Reinforced Concrete Frames." In Proceedings of the 8th European Conference on Earthquake Engineering. Zarnic, Roko, and Miha Tomazevic. 1986. "Aseismic Strengthening of an Historical XVII-Century Urban Building--A Case Study." In Proceedings of the 8th European Conference on Earthquake Engineering. 148 APPENDIX A SEISMIC-FORCE-RESISTING ELEMENTS IN BUILDINGS This handbook discusses techniques for rehabilitating the seismic resistance of the following 15 common building types: 1. Wood Light Frame 2. Wood, Commercial and Industrial 3. Steel Moment Frame 4. Steel Braced Frame 5. Steel Light Frame 6. Steel Frame with Concrete Shear Walls 7. Steel with Infill Masonry Shear Walls 8. Concrete Moment Frame 9. Concrete Shear Walls 10. Concrete Frame with Infill Walls 11. Precast/Tilt-Up Concrete Walls with Lightweight Flexible Diaphragm 12. Precast Concrete Frames with Concrete Shear Walls 13. Reinforced Masonry Bearing Walls with Wood/Metal Deck Diaphragms 14. Reinforced Masonry Bearing Walls with Precast Concrete Diaphragms 15. Unreinforced Masonry Bearing Wall Buildings The lateral-force-resisting elements of buildings can be categorized into the following subsystems: vertical elements resisting lateral forces, diaphragms, foundations, and the connections between the subsystems. The 15 common building types considered in this report can be composed of various subsystem types. The construction of each subsystem can vary. For example, diaphragms can be constructed of timber, steel or concrete. The technique to rehabilitate a deficient subsystem and hence a deficient building depends upon the type of construction of that subsystem. The following tables present common construction of the lateral-force-resisting subsystems for the 15 common building types. These tables are provided to aid the reader in determining the types of subsystems likely to be present in a building of a given type. With an understanding of the subsystem construction and the subsystem deficiencies, the techniques presented in Chapter 3 can be investigated to determine effective ways to rehabilitate the seismic resistance of a given existing building. 149 TABLE Al STRUCTURAL ELEMENTS OF COMMON BUILDING TYPE 1--WOOD, LIGHT FRAME Floor Roof Vertical-Resisting Diaphragm Elements Foundations Connections Timber framing with Wood stud walls with Spread footings, piles, Diagonal wire or strut plywood, straight-laid, let-in or cut-in timber or drilled piers. bracing of ceilings from or diagonal sheathing. bracing or plywood, floor roof diaphragms. straight-laid, or diagonal sheathing. Bracing or lateral support of walls and partitions from ceilings or diaphragms. Nailing and blocking for direct shear transfer from horizontal diaphragms to shear walls or vertical bracing. Drag struts to collect shear from horizontal diaphragms for transfer to shear walls or vertical bracing. Bolting of shear walls and vertical bracing to concrete slabs or foundation walls. Tension ties or hold-downs for shear walls and vertical bracing. Nailing or bolting of vertical bracing. 150 TABLE A2 STRUCTURAL ELEMENTS OF COMMON BUILDING TYPE 2--WOOD, COMMERCIAL AND INDUSTRIAL Floor Roof Vertical-Resisting Diaphragm Elements Foundations Connections Timber framing with Wood stud walls with Spread footings, piles, Diagonal wire or strut, straight-laid, let-in or cut-in timber or drilled piers. of ceilings from floor diagonal sheathing. bracing or plywood, roof diaphragms. (Floor roof deck- straight-laid, or may be 2-inch sheathing. Bracing or lateral support of material). walls and partitions from Knee bracing or ceilings or diaphragms. bracing of timber columns. Nailing and blocking for direct shear transfer from horizontal diaphragms to shear walls or vertical bracing. Drag struts to collect shear from horizontal diaphragms for transfer to shear walls or vertical bracing. Bolting of shear walls and vertical bracing to concrete slabs or foundation walls. Tension ties or hold-downs for shear walls and vertical bracing. Nailing or bolting of vertical bracing. 151 TABLE A3 STRUCTURAL ELEMENTS OF COMMON BUILDING TYPE 3--STEEL MOMENT FRAME Floor Roof Vertical-Resisting Diaphragm Elements Foundations Connections Timber framing with Moment resisting * Spread footings, piles, Diagonal wire or strut, straight-laid, structural steel frames. or drilled piers of ceilings from floor diagonal sheathing. roof diaphragms. Reinforced concrete Bracing or lateral support of slab supported on walls and partitions from structural steel floor ceilings or diaphragms. framing members. Nailing and blocking for Steel decking with or shear transfer from without concrete fill. horizontal diaphragms to shear walls or vertical bracing. Drag struts to collect shear from horizontal diaphragms for transfer to shear walls or vertical bracing. Shear studs or other connections of concrete diaphragm to steel chord members. Welding, shear studs, or other connections of steel deck diaphragms to structural steel framing. Splice detail of steel chord members. Beam/column connections. Beam/column panel joint details. Column splice details. Column base details. 152 TABLE A4 STRUCTURAL ELEMENTS OF COMMON BUILDING TYPE 4--STEEL BRACED FRAME Floor Roof Vertical-Resisting Diaphragm Elements Foundations Connections Timber framing with Concentric steel Spread footings, piles, Diagonal wire or strut, straight-laid, in diagonal, X, K, or drilled piers of ceilings from floor diagonal sheathing. or chevron diaphragms. Reinforced concrete Bracing or lateral support of slab supported on May also have moment walls and partitions from structural steel floor resisting structural ceilings or diaphragms. framing members. steel frames. Nailing and blocking for Steel decking with or shear transfer from without concrete fill. horizontal diaphragms to shear walls or vertical bracing. Drag struts to collect shear from horizontal diaphragms for transfer to shear walls or vertical bracing. Shear studs or other connections of concrete diaphragm to steel chord members. Welding, shear studs, or other connections of steel deck diaphragms to structural steel framing. Splice detail of steel chord members. Beam/column connections. Beam/column panel joint details. Column splice details. Column base details. Bolted or welded bracing connections. 153 TABLE AS STRUCTURAL ELEMENTS OF COMMON BUILDING TYPE 5--STEEL, LIGHT FRAME Floor Roof Vertical-Resisting Diaphragm Timber framing with I Elements Moment resisting Foundations Spread footings, piles, Connections Diagonal wire or strut 1 plywood, straight-laid, structural steel frames. or drilled piers of ceilings from floor diagonal sheathing. roof diaphragms. Concentric light steel Reinforced concrete bracing in diagonal or Bracing or lateral support of slab supported on X configuration. walls and partitions from structural steel floor ceilings or diaphragms. framing members. Nailing and blocking for Steel decking with or shear transfer from without concrete fill. horizontal diaphragms to shear walls or vertical bracing. Drag struts to collect shear from horizontal diaphragms for transfer to shear walls or vertical bracing. Shear studs or other connections of concrete diaphragm to steel chord members. Welding, shear studs, or other connections of steel deck diaphragms to structural steel framing. Splice detail of steel chord members. Beam/column connections. Beam/column panel joint details. Column splice details. Column base details. Bolted or welded bracing connections. TABLE A6 STRUCTURAL ELEMENTS OF COMMON BUILDING TYPE 6--STEEL FRAME WITH CONCRETE SHEAR WALLS Floor Roof Vertical-Resisting Diaphragm Elements Foundations Reinforced concrete Non-moment-resisting Spread footings, piles, slab supported on steel frames. or drilled piers. structural steel floor framing members. Reinforced concrete shear walls. Steel decking with or without concrete fill. May also have moment resisting structural steel frames. Connections Diagonal wire or strut bracing of ceilings from floor roof diaphragms. Bracing or lateral support of walls and partitions from ceilings or diaphragms. Shear studs or other connections of concrete diaphragm to steel chord members. Welding, shear studs, or other connections of steel deck diaphragms to structural steel framing. Splice detail of steel chord members. Beam/column connections. Beam/column panel joint details. Column splice details. Column base details. Connection of concrete shear walls to floor roof diaphragms. Development of boundary members for concrete shear walls. 155 TABLE A7 STRUCTURAL ELEMENTS OF COMMON BUILDING TYPE 7--STEEL WITH INFILL MASONRY SHEAR WALLS Floor Roof Vertical-Resisting 1 Diaphragm Elements Foundations Connections Timber framing with Nonmoment-resisting Spread footings, piles, Diagonal wire or strut bracing plywood, straight-laid, steel frames. or drilled piers of ceilings from floor diagonal sheathing. roof diaphragms. Unreinforced masonry Reinforced concrete walls. Bracing or lateral support of slab supported on walls and partitions from structural steel floor May also have moment ceilings or diaphragms. framing members. resisting structural steel frames. Nailing and blocking for Steel decking with or direct shear transfer from without concrete fill. horizontal diaphragms to shear walls or vertical bracing. Drag struts to collect shear from horizontal diaphragms for transfer to shear walls or vertical bracing. Shear studs or other connections of concrete diaphragm to steel chord members. Welding, shear studs, or other connections of steel deck diaphragms to structural steel framing. Splice detail of steel chord members. Beam/column connections. Beam/column panel joint details. Column splice details. Column base details. Connection of masonry walls to steel framing. TABLE A& STRUCTURAL ELEMENTS OF COMMON BUILDING TYPE 8--CONCRETE MOMENT FRAME Floor Roof Vertical-Resisting Diaphragm Elements Foundations Connections Reinforced concrete Reinforced concrete Spread footings, piles, Diagonal wire or strut bracing monolithic with frames. or drilled piers. of ceilings from floor reinforced concrete roof diaphragms. beams and girders. Bracing or lateral support of walls and partitions from ceilings or diaphragms. Beam/column panel joint details. Column shear reinforcement and confinement. 157 TABLE A9 STRUCTURAL ELEMENTS OF COMMON BUILDING TYPE 9--CONCRETE SHEAR WALLS Floor Roof Vertical-Resisting Diaphragm Elements Foundations Connections Reinforced concrete Reinforced concrete Spread footings, piles, Diagonal wire or strut bracing slab monolithic with shear walls. or drilled piers. of ceilings from floor reinforced concrete roof diaphragms. beams and girders. Bracing or lateral support of walls and partitions from ceilings or diaphragms. Connection of concrete shear walls to floor roof diaphragms. Development of boundary members for concrete shear walls. Concrete diaphragm chord details. 158 TABLE A10 STRUCTURAL ELEMENTS OF COMMON BUILDING TYPE 10--CONCRETE FRAME WITH INFILL WALLS Floor Roof Vertical-Resisting Diaphragm Elements Foundations Connections Timber framing with Reinforced concrete Spread footings, piles, Diagonal wire or strut plywood, straight-laid, frames. or drilled piers. bracing of ceilings from floor diagonal sheathing. roof diaphragms. Unreinforced masonry Reinforced concrete walls. Bracing or lateral support of slab monolithic with walls and partitions from reinforced concrete ceilings or diaphragms beams and girders. Connection of timber floor roof diaphragms to concrete frames. Connection of concrete floor roof diaphragms to concrete frames. Connection of masonry walls to concrete frames. Beam/column joint details. Column shear reinforcement and confinement. 159 TABLE All STRUCTURAL ELEMENTS OF COMMON BUILDING TYPE 11--PRECAST/TILT-UP CONCRETE WALLS WITH LIGHTWEIGHT FLEXIBLE DIAPHRAGM Floor Roof Vertical-Resisting Diaphragm Elements Foundations Connections ] Timber framing with Precast concrete walls. Spread footings, piles, Diagonal wire or strut plywood, straight-laid, or drilled piers. bracing of ceilings from floor diagonal sheathing. roof diaphragms. Reinforced concrete Bracing or lateral. support of slab monolithic with walls and partitions from reinforced concrete ceilings or diaphragms. beams and girders. Welding, shear studs, or Steel decking with or other connections of steel without concrete fill. deck diaphragms to structural steel framing. Connection of timber floor roof diaphragms and precast walls. Connection of concrete floor roof diaphragms to precast walls. Connection of steel deck floor roof diaphragms to precast walls. Vertical precast panel connections. Tension ties or hold-down connections for precast panels. Diaphragm chord details for timber, steel decking, and concrete diaphragm. Base detail for precast panels. 160 TABLE A12 STRUCTURAL ELEMENTS OF COMMON BUILDING TYPE 12--PRECAST CONCRETE FRAMES WITH CONCRETE SHEAR WALLS Floor Roof Vertical-Resisting Diaphragm Elements Foundations Connections Reinforced concrete Precast concrete Spread footings, piles, Diagonal wire or strut slab monolithic with frames. or drilled piers. bracing of ceilings from floor reinforced concrete roof diaphragms. beams and girders. Reinforced concrete shear walls. Bracing or lateral support of walls and partitions from ceilings or diaphragms. Connection of concrete floor roof diaphragms to precast frames or shear walls. Development of boundary members for concrete shear walls. Beam/column joint details. Column shear reinforcement and confinement. Concrete frame splice details. 161 TABLE A13 STRUCTURAL ELEMENTS OF COMMON BUILDING TYPE 13--REINFORCED MASONRY WALLS WITH WOOD/METAL DECK Floor Roof Vertical-Resisting Diaphragm Elements Foundations Connections Timber framing with Unreinforced masonry Spread footings, piles, Diagonal wire or strut plywood, straight-laid, bearing walls. or drilled piers. bracing of ceilings from floor or diagonal sheathing. roof diaphragms. Steel decking with or Bracing or lateral support of without concrete fill. walls and partitions from ceilings or diaphragms. Welding, shear studs, or other connections of steel deck diaphragms to structural steel framing. Connection of timber or steel decking floor roof diaphragms to masonry walls. Tension ties or hold-downs for masonry walls. 162 TABLE A14 STRUCTURAL ELEMENTS OF COMMON BUILDING TYPE 14--REINFORCED MASONRY WALLS WITH PRECAST CONCRETE DECK Floor Roof Vertical-Resisting Diaphragm Elements Foundations Connections Precast concrete units Reinforced masonry Spread footings, piles, Diagonal wire or strut bracing (planks, cored slabs, walls. or drilled piers. bracing of ceilings from floor tees, etc.) roof diaphragms. Bracing or lateral support of walls and partitions from ceilings or diaphragms. Connection of precast floor roof units to shear walls. Connections between adjacent precast floor roof units. Tension ties or hold-downs for masonry walls. 163 TABLE A15 STRUCTURAL ELEMENTS OF COMMON BUILDING TYPE 15-UNREINFORCED MASONRY BEARING WALLS Floor Roof Vertical-Resisting Diaphragm Elements Foundations Connections Timber framing with Unreinforced masonry Spread footings, piles, Diagonal wire or strut plywood, straight-laid, walls. or drilled piers. bracing of ceilings from floor or diagonal sheathing. roof diaphragms. Reinforced concrete Bracing or lateral support of slab supported on walls and partitions from structural steel floor ceilings or diaphragms. framing members. Connection of timber or Reinforced concrete floor roof slab monolithic with diaphragms to masonry walls. reinforced concrete beams and girders. Development of diaphragm chords in timber or concrete floor roof diaphragms. Tension ties or hold-downs for masonry walls. 164 APPENDIX B SUMMARY OF STRENGTHENING TECHNIQUES The deficiencies and alternative strengthening techniques discussed in Chapter 3 are summarized here as follows: Table Bi Moment Resisting Systems Steel Moment Frames Concrete Moment Frames Moment Frames with Infill Walls Precast Concrete Moment Frames Table B2 Shear Walls Reinforced Concrete or Reinforced Masonry Precast Concrete Unreinforced Masonry Shear Walls in Wood Frame Buildings Table B3 Braced Frames Table B4 Diaphragms Table B5 Foundations Table B6 Diaphragm to Vertical Element Connections Table B7 Vertical Element to Foundation Connections 165 TABLE Bi MOMENT RESISTING SYSTEMS STEEL MOMENT FRAMES Deficiency Inadequate moment/shear capacity of beams, columns, or their connections Inadequate beam/column panel zone capacity Excessive drift Strengthening Techniques 1. Increasing the moment capacity of the members and connections by adding cover plates or other steel sections to the flanges Or by boxing members. 2. Increasing the moment and shear capacity of the members and connections by providing steel gusset plates or knee braces. 3. Reducing the stresses in the existing frames by providing supplemental vertical-resisting elements (i.e., additional moment frames, braces, or shear walls) as discussed in Sec. 3.4. 4. Providing lateral bracing of unsupported flanges to increase capacity limited by tendency for lateral/torsional buckling. 5. Encasing the columns in concrete. 1. Providing welded continuity plates between the column flanges. 2. Providing stiffener plates welded to the column flanges and web. 3. Providing web doubler plates at the column web. 4. Reducing the stresses in the panel zone by providing supplemental vertical-resisting elements (i.e., additional moment frames, braces, or shear walls) as discussed in Sec. 3.4. 1. Increasing the capacity and, hence, the stiffness of the existing moment frame by cover plates or boxing. 2. Increasing the stiffness of the beams and columns at their connections by providing steel gusset plates to form haunches. 3. Reducing the drift by providing supplemental vertical-resisting elements (i.e., additional moment frames, braces, or shear walls) as discussed in Sec. 3.4. 4. Increasing the stiffness by encasing columns in reinforced concrete. 5. Reducing the drift by adding supplemental damping as discussed in Sec. 4. CONCRETE MOMENT FRAMES Inadequate ductile bending or shear capacity in the beams or columns and lack of confinement, frequently in the joints 1. Increasing the ductility and capacity by jacketing the beam and column joints or increasing the beam or column capacities. 2. Reducing the seismic stresses in the existing frames by providing supplemental vertical-resisting elements (i.e., additional moment frames, braces, or shear walls) as discussed in Sec. 3.4. 3. Changing the system to a shear wall system by infilling the reinforced concrete frames with reinforced concrete. 166 TABLE Bl--continued MOMENT FRAMES WITH INFILL WALLS Crushing of the infill at the upper 1. Eliminating the hazardous effects of the infill by providing a and lower corners due to the gap between the infill and the frame and providing out-of- diagonal compression strut type action plane support. of the infill wall 2. * Treating the frame as a shear wall and correcting the deficiencies as described in Sec. 3.2. Shear failure of the beam/column connection in the steel frames or direct shear transfer failure of the beam or column in concrete frames Tensile failure of the columns or their connections due to the uplift forces resulting from the braced frame action induced by the infill Splitting of the infill due to the orthogonal tensile stresses developed in the diagonal compressive strut Loss of infill by out-of-plane forces due to loss of anchorage or excessive slenderness of the infill wall PRECAST CONCRETE MOMENT FRAMES Inadequate capacity and/or ductility 1. Removing existing concrete in the precast elements to expose of the joints between the precast the existing reinforcing steel, providing additional reinforcing units steel welded to the existing steel (or drilled and grouted), and replacing the removed concrete with cast-in-place concrete. 2. Reducing the forces on the connections by providing supplemental vertical-resisting elements (i.e., additional moment frames, braces, or shear walls) as discussed in Sec. 3.4. 167 REINFORCED Deficiency Inadequate shear capacity Inadequate flexural capacity TABLE B2 SHEAR WALLS CONCRETE OR REINFORCED MASONRY SHEAR WALLS Strengthening Techniques 1. Increasing the effectiveness of the existing walls by filling in door or window openings with reinforced concrete or masonry. 2. Providing additional thickness to the existing walls with a poured-in-place or pneumatically applied (i.e., shotcrete) reinforced concrete overlay anchored to the inside or outside face of the existing walls. 3. Reducing the shear or flexural stresses in the existing walls by providing supplemental vertical-resisting elements (i.e., shear walls, braces, or external buttresses) as discussed in Sec. 3.4. 1. Increasing the effectiveness of the existing walls by filling in door or window openings with reinforced concrete or masonry. 2. Providing additional thickness to the existing walls with a poured-in-place or pneumatically applied (i.e., shotcrete) reinforced concrete overlay anchored to the inside or outside face of the existing walls. 3. Reducing the shear or flexural stresses in the existing walls by providing supplemental vertical-resisting elements (i.e., shear walls, braces, or external buttresses). as discussed in Sec. 3.4. Inadequate shear or flexural capacity 1. Eliminating the coupling beams by filling in openings with in the coupling beams between shear reinforced concrete. walls or piers Inadequate shear in the wall panels 2. Removing the existing beams and replacing with new stronger reinforced beams. 3. Adding reinforced concrete to one or both faces of the wall and providing an additional thickness to the existing wall. 4. Reducing the shear or flexural stresses in the connecting beams by providing additional vertical-resisting elements (i.e., shear walls, braces, or external buttresses) as discussed in Sec. 3.4. PRECAST CONCRETE SHEAR WALLS or flexural capacity 1. Increasing the shear and flexural capacity of walls with significant openings for doors or windows by infilling the existing openings with reinforced concrete. 2. Increasing the shear or flexural capacity by adding reinforced concrete (poured-in-place or shotcrete) at the inside or outside face of the existing walls. 3. Adding interior shear walls to reduce the flexural or shear stress in the existing precast panels. 168 TABLE B2--continued PRECAST CONCRETE SHEAR WALLS-continued Inadequate interpanel shear or flex-1. Making each panel act as a cantilever to resist in-plane capacity forces (by adding or strengthening tie-downs, edge reinforcement, footings). 2. Providing a continuous wall by exposing the reinforcing steel in the edges of adjacent units, adding ties, and repairing with concrete. Inadequate out-of-plane flexural 1. Providing pilasters at and/or between the interpanel joints. capacity 2. Adding horizontal beams between the columns or pilasters at mid-height of the wall. Inadequate shear or flexural capacity 1. Eliminating the coupling beams by filling in openings with in coupling beams reinforced concrete. 2. Removing the existing beams and replacing with new stronger reinforced beams. 3. Adding reinforced concrete to one or both faces of the wall and providing an additional thickness to the existing wall. 4. Reducing the shear or flexural stresses in the connecting beams by providing additional vertical-resisting elements (i.e., shear walls, braces, or external buttresses) as discussed in Sec. 3.4.2 UNREINFORCED MASONRY SHEAR WALLS Inadequate in-plane shear and out- 1. Providing additional shear capacity by placing reinforcing of-plane flexural capacity of the walls steel on the inside or outside face of the wall and applying new reinforced concrete. 2. Providing additional capacity for out-of-plane lateral forces by adding reinforcing steel to the wall utilizing the center coring technique. 3. Providing additional capacity for out-of-plane lateral forces by adding thin surface treatments (e.g., plaster with wire mesh and portland cement mortar) at the inside and outside faces of existing walls. 4. Filling in existing window or door openings with reinforced concrete. 5. Providing additional shear walls at the interior or perimeter of the building or providing external buttresses. Inadequate shear capacity of the 1. Filling in openings with reinforced concrete. coupling beam 2. Removing existing connecting beams and replacing them with properly designed new reinforced concrete beams. 3. Providing additional shear walls at interior or perimeter of building or external buttresses. 169 TABLE B2--continued SHEAR WALIS IN WOOD FRAME BUILDINGS Inadequate shear capacity of the wall 1. Increasing the shear capacity by providing additional nailing to the existing finish material. 2. Increasing the shear capacity by adding plywood sheathing to * one or both sides of the wall. 3. Reducing the loads on the wall by providing supplemental shear walls to the interior or perimeter of the building. Inadequate uplift or hold-down ca-1. Increasing the tensile capacity of the connections at the edge of the shear walls by providing metal connectors. pacity of the wall 2. Reducing the overturning moments by providing supplemental vertical-resisting elements as discussed in Sec. 3.4. 'W 170 TABLE B3 BRACED FRAMES STEEL CONCENTRICALLY BRACED FRAMES (including chevron or K-bracing) Deficiency Strengthening Techniques Inadequate lateral force capacity of 1. Increasing the capacity of the braces by adding new members the bracing system governed by buck- thus increasing the area and reducing the radius of gyration ling of the compression brace of the braces. 2. Increasing the capacity of the member by reducing the unbraced length of the existing member by providing secondary braces. 3. Providing greater capacity by removing and replacing the existing members with new members of greater capacity. 4. Reducing the loads on the braces by providing supplemental vertical-resisting elements (i.e., shear walls, braces, or eccentric bracing) as discussed in Sec. 3.4. Inadequate capacity of the brace 1. Increasing the capacity of the connections by additional bolt- connection ing or welding. 2. Increasing the capacity of the connections by removing and replacing the connection with members of greater capacity. 3. Reducing the loads on the braces and their connections by providing supplemental vertical-resisting elements (i.e., shear walls, braces, or eccentric bracing) as discussed in Sec. 3.4. Inadequate axial load capacity in the 1. Providing additional axial load capacity by adding cover columns or beams of the bracing plates to the member flanges or by boxing the flanges. system 2. Providing additional axial load capacity by jacketing the existing members with reinforced concrete. 3. Reducing the loads on the beams and columns by providing supplemental vertical-resisting elements (i.e., shear walls, braces, or eccentric bracing) as discussed in Sec. 3.4. 171 TABLE B3-continued ROD OR OTHER TENSION BRACING Inadequate tension capacity of the 1. Increasing the capacity by strengthening the existing tension rod, tensile member, or its members. connection 2. Increasing the capacity by removing the existing tension members and replacing with new members of greater capacity. 3. Increasing the capacity by removing the existing tension member and replacing it with diagonal or X-bracing capable of resisting compression as well as tension forces. 4. Reducing the forces on the existing tension members by providing supplemental vertical-resisting elements (i.e., additional tension rods) as discussed in Sec. 3.4. Inadequate axial capacity of the 1. Increasing the axial capacity by adding cover plates to the beams or columns in the bracing existing flanges or by boxing the existing flanges. system 2. Reducing the forces on the existing columns or beams by providing supplemental vertical-resisting elements (i.e., braced frames or shear walls) as discussed in Sec. 3.4. ECCENTRIC BRACING Nonconformance with current design 1. Ensuring that the system is balanced (i.e., there is a link standards beam at one end of each brace), the brace and the connections are designed to develop shear or flexural yielding in the link, the connection is a full moment connection, where the link beam has an end at a column, and lateral bracing is provided to prevent out-of-plane beam displacements that would compromise the intended action. 2. Providing supplemental vertical-resisting elements such as additional eccentric braced frames. 172 Deficiency Inadequate shear capacity of the diaphragm Inadequate chord capacity of the diaphragm Excessive shear stresses at diaphragm openings or at plan irregularities Inadequate stiffness of the diaphragm resulting in excessive diaphragm deformations Inadequate the concrete diaphragm TABLE B4 DIAPHRAGMS TIMBER DIAPHRAGMS (straight-laid or diagonal sheathing or plywood) Strengthening Techniques 1. Increasing the capacity of the existing timber diaphragm by providing additional nails or staples with due regard for wood splitting problems. 2. Increasing the capacity of the existing timber diaphragm by means of a new plywood overlay. 3. Reducing the diaphragm span through the addition of supplemental vertical-resisting elements (i.e., shear wall or braced frames) as discussed in Sec. 3.4. 1. Providing adequate nailed or bolted continuity splices along joists or fascia parallel to the chord. 2. Providing a new continuous steel chord member along the top of the diaphragm. 3. Reducing the stresses on the existing chords by reducing the diaphragms, span through the addition of new shear walls or braced frames as discussed in Sec. 3.4. 1. Reducing the local stresses by distributing the forces along the diaphragm by means of drag struts. 2. Increasing the capacity of the diaphragm by overlaying the existing diaphragm with plywood and nailing the plywood through the sheathing at the perimeter of the sheets adjacent to the opening or irregularity. 3. Reducing the diaphragm stresses by reducing the diaphragm spans through the addition of supplemental shear walls or braced frames as discussed in Sec. 3.4. 1. Increasing the stiffness of the diaphragm by the addition of a new plywood overlay. 2. Reducing the diaphragm span and hence reducing the displacements by providing new supplemental vertical-resisting elements such as shear walls or braced frames as discussed in Sec. 3.4. CONCRETE DIAPHRAGMS (monolithic concrete diaphragms--i.e., reinforced concrete or post-tensioned concrete) in-plane shear capacity of 1. Increasing the shear capacity by overlaying the existing concrete diaphragm with a new reinforced concrete topping slab. 2. Reducing the shear in the existing concrete diaphragm by providing supplemental vertical-resisting elements (i.e., shear walls or braced frames) as discussed in Sec. 3.4. 173 TABLE B4--continued Inadequate diaphragm chord capacity Excessive shear stresses at the diaphragm openings or plan irregularities 1. Increasing the flexural capacity by removing the edge of the diaphragm slab and casting a new chord member integral with the slab. 2. Adding a new chord member by providing a new reinforced concrete or steel member above or below the slab and connecting the new member to the existing slab with drilled and grouted dowels or bolts as discussed in Sec. 3.5.4.3. 3. Reducing the existing flexural stresses by providing supplemental vertical-resisting elements (i.e., shear walls or braced frames) as discussed in Sec. 3.4. 1. Reducing the local stresses by distributing the forces along the diaphragm by means of structural steel or reinforced concrete elements cast beneath the slab and made integral through the use of drilled and grouted dowels. 2. Increasing the capacity of the concrete by providing a new concrete topping slab in the vicinity of the opening and reinforcing with trim bars. 3. Removing the stress concentration by filling in the diaphragm opening with reinforced concrete. 4. Reducing the shear stresses at the location of the openings by adding supplemental vertical-resisting elements (i.e., shear walls or braced frames) as discussed in Sec. 3.4. 174 TABLE B4--continued POURED GYPSUM DIAPHRAGMS Inadequate in-plane shear capacity of the concrete diaphragm Inadequate diaphragm chord capacity Excessive shear stresses at the diaphragm openings or plan irregularities 1. Increasing the shear capacity by overlaying the existing concrete diaphragm with a new reinforced concrete topping slab. 2. Reducing the shear in the existing concrete diaphragm by providing supplemental vertical-resisting elements (i.e., shear walls or braced frames) as discussed in Sec. 3.4. 3. Increasing the flexural capacity by removing the edge of the diaphragm slab and casting a new chord member integral with the slab. 4. Adding a new chord member by providing a new reinforced concrete or steel member above or below the slab and connecting the new member to the existing slab with drilled and grouted dowels or bolts as discussed in Sec. 3.5.4.3. 5. Reducing the existing flexural stresses by providing supplemental vertical-resisting elements (i.e., shear walls or braced frames) as discussed in Sec. 3.4. 6. Reducing the local stresses by distributing the forces along the diaphragm by means of structural steel or reinforced concrete elements cast beneath the slab and made integral through the use of drilled and grouted dowels. 7. Increasing the capacity of the concrete by providing a new concrete topping slab in the vicinity of the opening and reinforcing with trim bars. 8. Removing the stress concentration by filling in the diaphragm opening with reinforced concrete. 9. Reducing the shear stresses at the location of the openings by adding supplemental vertical-resisting elements (i.e., shear walls or braced frames) as discussed in Sec. 3.4. 10. Adding a new horizontal bracing system may be the most effective strengthening alternative. PRECAST CONCRETE DIAPHRAGMS (precast or post-tensioned concrete planks, tees, or cored slabs) Inadequate in-plane shear capacity of the connections between the adjacent units 1. 2. Replacing and increasing the capacity of the existing connections by overlaying the existing diaphragm with a new reinforced concrete topping slab. Reducing the shear forces on the diaphragm by providing supplemental vertical-resisting elements (i.e., shear walls or braced frames) as discussed in Sec. 3.4. Inadequate diaphragm chord capacity 1. Providing a new continuous steel member above or below the steel slab and connecting the new member to the existing slab with bolts. 2. 3. Removing the edge of the diaphragm and casting a new chord member integral with the slab. Reducing the diaphragm chord forces by providing supplemental vertical-resisting elements (i.e., shear walls or braced frames) as discussed in Sec. 3.4. 175 diaphragm openings or plan irregularities Inadequate in-plane shear capacity TABLE B4--continued PRECAST CONCRETE DLAPHRAGMS--continued Excessive in-plane shear stresses at 1. Reducing the local stresses by distributing the forces along the diaphragm by means of concrete drag struts cast beneath the slab and made integral with the existing slab with drilled and grouted dowels. 2. Increasing the capacity by overlaying the existing slab with a new reinforced concrete topping slab with reinforcing trim bars in the vicinity of the opening. 3. Removing the stress concentration by filling in the diaphragm opening with reinforced concrete. 4. Reducing the shear stresses at the location of the openings by providing supplemental vertical-resisting elements (i.e., shear walls or braced frames) as discussed in Sec. 3.4. & STEEL DECK DIAPHRAGMS (steel decking on steel framing) which may be governed by the capacity of the welding to the supports or the capacity of the seam welds between the deck units Inadequate diaphragm chord capacity Excessive in-plane shear stresses at diaphragm openings or plan irregularities 1. Increasing the steel deck shear capacity by providing additional welding. 2. Increasing the deck shear capacity of unfilled steel decks by adding a reinforced concrete fill or overlaying with concrete filled steel decks a new topping slab. 3. Increasing the diaphragm shear capacity by providing a new horizontal steel bracing system under the existing diaphragm. 4. Reducing the diaphragm shear stresses by providing supplemental vertical-resisting elements to reduce the diaphragm span as discussed in Sec. 3.4. 1. Increasing the chord capacity by providing welded or bolted continuity splices in the perimeter chord steel framing members. 2. Increasing the chord capacity by providing a new continuous steel member on top or bottom of the diaphragm. 3. Reducing the diaphragm chord stresses by providing supplemental vertical-resisting elements (i.e., shear walls or braced frames) such that the diaphragm span is reduced as discussed in Sec. 3.4. 1. Reducing the local stress concentrations by distributing the forces into the diaphragm by means of steel drag struts. 2. Increasing the capacity of the diaphragm by reinforcing the edge of the opening with a steel angle frame welded to the decking. 3. Reducing the diaphragm stresses by providing supplemental vertical-resisting elements (i.e., shear walls, braced frames or new moment frames) such that the diaphragm span is reduced as discussed in Sec. 3.4. 176 TABLE B4-continued HORIZONTAL STEEL BRACING *1 Inadequate force capacity of the 1. Increasing the capacity of the existing bracing members or members (i.e., bracing and floor or removing and replacing them with new members and roof beams) and/or the connections connections of greater capacity. 2. Increasing the capacity of the existing members by reducing unbraced lengths. 3. Increasing the capacity of the bracing system by adding new horizontal bracing members to previously unbraced panels (if feasible). 4. Increasing the capacity of the bracing system by adding a steel deck diaphragm to the floor system above the steel bracing. 5. Reducing the stresses in the horizontal bracing system by providing supplemental vertical-resisting elements (i.e., shear walls or braced frames) as discussed in Sec. 3.4. 177 TABLE BS FOUNDATIONS CONTINUOUS OR STRIP WALL FOOTINGS Deficiency Excessive soil bearing pressure due to overturning forces Excessive uplift conditions due to overturning forces Strengthening Techniques 1. Increasing the bearing capacity of the footing by underpinning the footing ends and providing additional footing area. 2. Increasing the vertical capacity of the footing by adding new drilled piers adjacent and connected to the existing footing. 3. Increasing the soil bearing capacity by modifying the existing soil properties. 4. Reducing the overturning forces by providing supplemental vertical-resisting elements (i.e., shear walls or braced frames) as discussed in Sec. 3.4. 1. Increasing the uplift capacity of the existing footing by adding drilled piers or soil anchors. 2. Increasing the size of the existing footing by underpinning to mobilize additional foundation and reduce soil pressures. 3. Reducing the uplift forces by providing supplemental vertical-resisting elements (i.e., shear walls or braced frames) as discussed in Sec. 3.4. INDIVIDUAL PIER OR COLUMN FOOTINGS Excessive soil bearing pressure due to overturning forces Excessive uplift conditions due to overturning forces 1. Increasing the bearing capacity of the footing by underpinning the footing ends and providing additional footing area. 2. Increasing the vertical capacity of the footing by adding new drilled piers adjacent and connected to the existing footing. 3. Reducing the bearing pressure on the existing footings by connecting adjacent footings with deep reinforced concrete tie beams. 4. Increasing the soil bearing capacity by modifying the existing soil properties. 5. Reducing the overturning forces by providing supplemental vertical-resisting elements (i.e., shear walls or braced frames). 1. Increasing the uplift capacity of the existing footing by adding drilled piers or soil anchors. 2. Increasing the size of the existing footing by underpinning to mobilize additional foundation and soil weight. 3. Increasing the uplift capacity by providing a new deep reinforced concrete beam to mobilize the dead load on an adjacent footing. 4. Reducing the uplift forces by providing supplemental vertical-resisting elements (i.e., shear walls or braced frames). 178 TABLE BS--continued INDIVIDUAL PIER OR COLUMN FOOTINGS--continued Inadequate passive soil pressure to 1. Providing an increase in bearing area by underpinning and resist lateral loads Excessive tensile or compressive loads on the piles or piers due to the seismic forces combined with the gravity loads Inadequate lateral force capacity to transfer the seismic shears from the pile caps and the piles to the soil Inadequate moment capacity to resist combined gravity plus seismic over- turning forces Inadequate passive soil pressure to resist sliding enlarging the footing. 2. Providing an increase in bearing area by adding new tie beams between existing footings. 3. Improving the existing soil conditions adjacent to the footing to increase the allowable passive pressure. 4. Reducing the bearing pressure at overstressed locations by providing supplemental vertical-resisting elements such as shear walls or braced frames as discussed in Sec. 3.4. PILES OR DRILLED PIERS 1. Increasing the capacity of the foundation by removing the existing pile cap, driving additional piles and providing new pile caps of larger size. 2. Reducing the loads on overstressed pile caps by adding tie beams to adjacent pile caps and distributing the loads. 1. Reducing the loads on overstressed pile caps by adding tie beams to adjacent pile caps and distributing the loads. 2. Increasing the allowable passive pressure of the soil by improving the soil adjacent to the pile cap. 3. Increasing the capacity of the foundation by removing the existing pile cap, driving additional piles, and providing new pile caps of larger size. 4. Reducing loads on the piles or piers by providing supplemental vertical-resisting elements (i.e., braced frames or shear walls) and transferring forces to other foundation members with reserve capacity as discussed in Sec. 3.4. MAT 1. Increasing the mat capacity locally by providing additional reinforced concrete (i.e., an inverted column capital) doweled and bonded to the existing mat to act as a monolithic section. 2. Providing new shear walls above the mat to distribute the overturning loads and also to locally increase the section modulus of the mat. 1. Constructing properly spaced shear keys at the mat perimeter. 179 TABLE B6 DIAPHRAGM TO VERTICAL ELEMENT CONNECTIONS CONNECTIONS OF TIMBER DIAPHRAGMS Deficiency Inadequate capacity to transfer in-plane shear at the connection of the diaphragm to interior shear walls or vertical bracing Inadequate capacity to transfer in-plane shear at the connection of the diaphragm to exterior shear walls or vertical bracing Inadequate out-of-plane anchorage at the connection of the diaphragm to exterior concrete or masonry walls Inadequate tensile capacity between floors due to overturning moments Strengthening Techniques 1. Increasing the shear transfer capacity of the diaphragm local to the connection by providing additional nailing to existing or new blocking. 2. Reducing the local shear transfer stresses by distributing the forces from the diaphragm by providing a collector member to transfer the diaphragm forces to the shear wall. 3. Reducing the shear transfer stress in the existing connection by providing supplemental vertical-resisting elements as discussed in Sec. 3.4. 1. Increasing the capacity of existing connections by providing additional nailing and/or bolting. 2. Reducing the local shear transfer stresses by distributing the forces from the diaphragm by providing chords or collector members to collect and distribute shear from the diaphragm to the shear wall or bracing. 3. Reducing the shear stress in the existing connection by providing supplemental vertical-resisting elements as discussed in Sec. 3.4. 1. Increasing the capacity of the connection by providing steel straps connected to the wall (using drilled and grouted bolts or through bolts for masonry walls) and bolted or lagged to the diaphragm or roof or floor joists. 2. Increasing the capacity of the connections by providing a steel anchor to connect the roof or floor joists to the walls. 3. Increasing the redundancy of the connection by providing continuity ties into the diaphragm. 1. Increasing the tensile capacity of the connections at the edge of the shear walls by providing metal connectors. 2. Reducing the overturning moments by providing supplemental vertical-resisting elements as discussed in Sec. 3A. 180 TABLE B6--continued CONNECTIONS OF CONCRETE DIAPHRAGMS Inadequate in-plane shear transfer 1. Reducing the local stresses at the diaphragm-to-wall inter- capacity face by providing collector members or drag struts under the diaphragm and connecting them to the diaphragm and the wall. 2. Increasing the capacity of the existing diaphragm-to-wall connection by providing additional dowels grouted into drilled holes. 3. Reducing the shear stresses in the existing connection by providing supplemental vertical-resisting elements as discussed in Sec. 3.4. Inadequate anchorage capacity for 1. Increasing the capacity of the connection by providing out-of-plane forces in the connecting additional dowels grouted into drilled holes. walls 2. Increasing the capacity of the connection by providing a new member above or below the slab connected to the slab with drilled and grouted bolts similar to that for providing a new diaphragm chord. CONNECTIONS OF POURED GYPSUM DIAPHRAGMS Inadequate in-plane shear transfer 1. Providing new dowels from the diaphragm into the shear wall. 2. Removing the gypsum diaphragm and replacing it with steel decking. Inadequate anchorage capacity for 3. Adding a new horizontal bracing system designed to resist all out-of-plane forces in the connecting of the seismic forces. walls CONNECTIONS OF PRECAST CONCRETE DIAPHRAGMS Inadequate in-plane shear transfer 1. Increasing the capacity of the connection by providing capacity additional welded inserts or dowels placed in drilled or grouted holes. 2. Increasing the capacity of the connection by providing a reinforced concrete overlay that is bonded to the precast Inadequate anchorage capacity at the units and anchored to the wall with additional dowels placed exterior walls for out-of-plane forces in drilled and grouted holes. 3. Reducing the forces at the connection by providing supplemental vertical-resisting elements as discussed in Sec. 3.4. 181 TABLE B6--continued CONNECTIONS OF STEEL DECK DIAPHRAGMS WITHOUT CONCRETE FILL Inadequate in-plane shear capacity or 1. Increasing the capacity of the connection by providing anchorage capacity for out-of-plane additional welding at the vertical element. forces in walls 2. Increasing the capacity of the connection by providing additional anchor bolts. 3. Increasing the capacity of the connection by providing concrete fill over the deck with dowels grouted into holes drilled into the wall. 4. Increasing the capacity of the connection by providing new steel members to effect a direct transfer of diaphragm shears to a shear wall. 5. Reducing the local stresses by providing additional vertical- resisting elements such as shear walls, braced frames, or moment frames as discussed in Sec. 3.4. CONNECTIONS OF STEEL DECK DIAPHRAGMS WITH CONCRETE FILL Inadequate in-plane shear capacity or 1. Increasing the shear capacity by drilling holes through the anchorage capacity for out-of-plane concrete fill, and providing additional shear studs welded to forces in walls the vertical elements through the decking. 2. Increasing the capacity of the connection by providing additional anchor bolts (drilled and grouted) connecting the steel support to the wall. 3. Increasing the capacity of the connection by placing dowels between the existing wall and diaphragm slab. 4. Reducing the local stresses by providing additional vertical- resisting elements such as shear walls, braced frames, or moment frames as discussed in Sec. 3.4. CONNECTIONS OF HORIZONTAL STEEL BRACING Inadequate in-plane shear transfer 1. Increasing the capacity by providing larger or more bolts or capacity by welding. 2. Reducing the stresses by providing supplemental vertical- resisting elements such as shear walls or braced frames as discussed in Sec. 3.4. Inadequate anchorage capacity when 1. Increasing the capacity of the connection by providing supporting concrete or masonry walls additional anchor bolts grouted in drilled holes and by providing for out-of-plane forces more bolts or welding to the bracing members. 182 TABLE B7 VERTICAL ELEMENT TO FOUNDATION CONNECTIONS CONNECTIONS OF WOOD STUD SHEAR WALLS Inadequate anchorage Inadequate stud walls Inadequate Inadequate anchorage Inadequate stud walls Inadequate Inadequate Deficiency shear capacity of the shear capacity of cripple uplift capacity Strengthening Techniques 1. Increasing the shear capacity by providing new or additional anchor bolts between the sill plate and the foundation. 2. Increasing the shear capacity by providing steel angles or plates with anchor bolts connecting them to the foundation and bolts or lag screws connecting them to the sill plate or wall. 1. Adding plywood sheathing over the cripple studs (usually on the inside) by nailing into the floor framing and the sill plate. Anchorage of the sill plate to the foundation also must be provided. 1. Increasing the capacity by providing steel hold-downs bolted to the wall and anchored to the concrete. 2. Reducing the uplift requirement by providing supplemental shear walls as discussed in Sec. 3.4. CONNECTIONS OF METAL STUD SHEAR WALLS shear capacity of the shear capacity of cripple uplift capacity 1. Provide anchor bolts, grouted in drilled holes, through sill plate of wall. 2. Provide steel angles with anchor bolts to concrete and bolts or screws to wall. 1. Provide plywood sheathing, nailing into cripple studs, sill plate, and first floor framing; anchor sill plate to foundation. 1. Provide steel hold-down with bolts or screws to wall and anchor bolts to concrete at ends of shear wall. 2. Provide additional shear walls or vertical bracing. CONNECTIONS OF PRECAST CONCRETE SHEAR WALLS capacity to resist in-plane 1. Increasing the capacity of the connection by providing a new or out-of-plane shear forces steel member connecting the wall to the foundation or the ground floor slab. 2. Increasing the capacity of the connection by adding a new thickness of concrete (either cast-in-place or shotcrete) placed against the precast wall doweling into the existing foundation or ground floor slab. 183 TABLE B7-continued CONNECTIONS OF PRECAST CONCRETE SHEAR WALLS--continued Inadequate hold-down capacity to 1. Increase the hold-down capacity by removing concrete at the resist seismic overturning forces edge of the precast unit to expose the reinforcement provide, new drilled and grouted dowels into the foundation, and pour a new concrete pilaster. 2. Reduce the uplift forces by providing supplemental vertical- resisting elements such as shear walls or braced frames as discussed in Sec. 3.4. CONNECTIONS OF BRACED FRAMES Inadequate shear capacity 1. Increasing the capacity by providing new steel members welded to the braced frame base plates and anchored to the slab or foundation with drilled and grouted anchor bolts. 2. Reducing the shear loads by providing supplemental steel braced frames as discussed in Sec. 3.4. Inadequate uplift resistance 1. Increasing the capacity by providing new steel members welded to the base plate and anchored to the existing foundation. 2. Reducing the uplift loads by providing supplemental steel braced frames as discussed in Sec. 3.4. CONNECTIONS OF STEEL MOMENT FRAMES Inadequate shear capacity 1. Increasing the shear capacity by providing steel shear lugs welded to the base plate and embedded in the foundation. 2. Increasing the shear and tensile capacity by installing Inadequate flexural capacity additional anchor bolts into the foundation. 3. Increasing the shear capacity by embedding the column in a Inadequate uplift capacity reinforced concrete pedestal that is bonded or embedded into the existing slab or foundation. 184 APPENDIX C REHABILITATION EXAMPLES Two examples are included in this appendix to demonstrate the process of selecting appropriate seismic rehabilitation techniques: a two-story steel frame building and a two story unreinforced masonry building. Both buildings were evaluated to determine their seismic deficiencies in accordance with the NEHRP Handbook for the Seismic Evaluation of Existing Buildings (which includes the evaluations as Examples D1 and D6 in Appendix D). STORY STEEL FRAME BUILDING EXAMPLE C1.1 DESCRIPTION OF BUILDING The building is 200 ft by 340 ft in plan with 20 ft by 20 ft bays. The girders in the transverse direction are connected to the column flanges with top and bottom clip angles. The beams in the longitudinal direction are connected to the column webs with beam web connections. The floor and roof diaphragms consists of steel decking with concrete fill. C12 DEFICIENCIES Inadequate moment capacity in both directions. C13 STRENGTHENING ALTERNATIVES This building could be strengthened by providing adequate moment capacity to the existing frames, by providing new diagonal bracing, and/or by providing new shear walls C13.1 Providing Adequate Moment Capacity Assuming that the first story shear of 2,970 kips can be equally distributed to all columns, it is calculated that there is excess capacity for the columns in the transverse direction (i.e., the strong axis of the columns) but grossly inadequate capacity for the columns in the longitudinal direction (i.e., the weak axis of the columns). This indicates that it is not feasible to develop adequate frame action to resist the seismic forces in the longitudinal direction, but it is feasible in the transverse direction. The structural modifications (Figure C1.3.1) required to develop moment frame action will involve: 1. Removal of the concrete fill and steel decking over the ends of the transverse girders at the columns. (It is assumed that the steel decking is supported on secondary floor beams that frame into the transverse frame girders so that there are no adverse effects associated with removal of the decking over top flanges of these girders adjacent to the columns.) The American Iron and Steel Institute has written a minority opinion concerning this appendix; see page 193. 185 2. Addition of new vertical shear connections between the girder webs and the column flanges. 3. Removal of existing clip angles at the top and bottom flanges of the girders. 4. Addition of new moment plates welded at the top and bottom flanges of the girders. (E) transverse =girder (N) web shear connection (E) clip angles, to be removed. removed (E) longitudinal beam SECTION a-a (E) transverse girder (N) moment plate (E) longitudinal Remove (E) concrete beam fill and steel decking PLAN FIGURE C1.3.1 Providing moment capacity to an existing steel frame. The design of these modifications should provide moment plates that are sized so as to yield prior to inducing yield stress in the columns. The new girder web shear connections should be sized for the gravity load shears (i.e., dead and live load) plus the shears associated with the formation of yield hinges in the moment plates. The column section of the new frame joint must be checked to determine the possible need for horizontal stiffeners opposite the girder flanges. The column web also should be checked to determine the need for doubler plates. Stiffeners probably can be fitted above or below the existing longitudinal beam-, at the column, but if doubler plates are required, this alternative may not be feasible because of interference with the existing longitudinal beam connection. C1.3.2 Providing New Diagonal Bracing Assume that diagonal bracing is to be considered for the longitudinal direction of the building. If the existing diaphragms have adequate capacity, the new bracing can be located in the exterior walls to avoid possible interference with the internal circulation within the building. If the diaphragm has inadequate capacity to transfer 186 the seismic shears to the exterior longitudinal walls, it probably would be more cost-effective to brace one or more of the interior longitudinal frames rather than to strengthen the diaphragm. In the design of the vertical bracing, X-bracing will be more effective than diagonal or chevron bracing for most braced bays because the tension diagonal will provide lateral support for the compression diagonal. Many designers assume that the effective length of the compression diagonal for X-bracing may be taken as one-half of the diagonal length for the in-plane direction and two-thirds of the diagonal length for the out-of-plane direction. Since the greater L/ will govern the capacity of the brace, this leads to the use of brace members with different radii of gyration, r, about each axis. The number of braced bays must be adequate to resist the story shears; however, in this building the story shears are not severe and can easily be resisted with only a fraction of the number of bays available in the exterior longitudinal frames. Next, the existing columns and foundations must be investigated for the overturning loads in the bracing. If it is assumed that all braces are equally loaded, it should be noted that with multiple contiguous bays of X-bracing there are no additional vertical forces in the columns and foundations except at the extreme ends of the braced bays. Therefore, if the existing columns or foundations do not have adequate capacity for the calculated overturning loads in the bracing, the engineer may be able to reduce these loads to acceptable limits by using smaller brace members and increasing the number of braced bays. The required structural modifications (Figure C1.3.2) involve: 1. Removal of the existing concrete fill and steel (E) column decking at second floor VP and roof levels to permit welding of gusset plate to (E) beam shear beam flange. Since the connection gusset is to be welded along the center of the (E) floor beam flange, only a narrow section of decking needs to be removed. Care must be taken that adequate bearing remains (N) gusset plate for the decking. 2. Welding of gusset plates to the beam/column joints and to the column/base plate joints. (N) X-bracing 3. Welding of new diagonal braces to the gusset plates. The design of the new plate A' w e n bracing system must include a e ()tension structural investigation of the to base plate ad of the existing provide I additional capacity of the existing co-' anchor bolts, as, beams, and foundations A required to resist the additional forces associated with the new FIGURE C1.3.2 Providing new diagonal bracing to an existing steel frame. It should be noted that the floor beams in the braced frames are required to function as collector members to "collect" the diaphragm shears and distribute them to the braced bays. The beam-to-column connection must therefore be capable of transferring tensile or compressive forces as well as resisting the vertical reaction of the floor beams. 187 C133 Providing New Shear Walls New shear walls of reinforced concrete or reinforced masonry may be provided in lieu of bracing or frame action in either direction of the building. If shear walls are provided, they should be infilled bays on a column line and preferably in a location where window or door openings are not required. With infill walls, the columns can function as boundary members for overturning loads and the beams or girders as collector members for the shear walls. The shear walls probably will require new foundations and also will add significantly to the building mass, which will increase the seismic story shears. CIA RELATIVE MERITS OF THE ALTERNATIVE STRENGTHENING TECHNIQUES As indicated above, the frame columns have inadequate capacity to resist the seismic story shears in the longitudinal direction; therefore, new vertical bracing or shear walls are the available options. It appears that the bracing could be installed in the exterior longitudinal frames without strengthening the columns or the foundations whereas the shear walls probably would need new foundations and be more disruptive as well as requiring more time for construction. In the transverse direction, providing moment capacity to the existing frames (Figure C1.3.1) appears to be feasible. It appears that this would be required for about two-thirds of the frames in the transverse direction at the second floor level and only about one-half of the frames at the roof level. Preliminary design of the structural strengthening concepts should be performed to define the location and extent of the modifications and to size the new structural members. Relative costs for the various alternatives also should be developed and attention should be given to the other considerations described at the beginning of Chapter 3. With this information, the most appropriate seismic strengthening technique for the building can be selected. C2 UNREINFORCED MASONRY BUILDING EXAMPLE C2.1 DESCRIPFTIONOF BUILDING This building is a two-story structure, 30 ft by 100 ft in plan. The first level has an open front at the east end and a longitudinal bearing wall on the centerline of the building. There are no crosswalls in the first level, but the second level contains apartments with many crosswalls. The roof diaphragm is constructed of diagonal timber sheathing. The floor contains finished wood flooring over timber diagonal sheathing. The existing conditions are shown in Figure C2.1. C22 DEFICIENCIES The building's deficiencies involve: 1. Torsion--The east end of the building has negligible resistance to lateral loads at the first level and constitutes a severe seismic hazard. 2. Adjacent Building--The adjacent building on the south side is not separated from the south wall and would act as a buttress for the diaphragms of the subject building. This could result in damage to both buildings. 3. Wall Stability--The gabled east and west walls at the second level are too slender (i.e., 9 in.) for the calculated out-of-plane seismic response imparted by the roof diaphragm. 4. Wall Anchorage--There is a serious inadequacy in the anchorage of all walls to the floor and roof diaphragms. 188 5. In-Plane Shear Strength of Walls--In addition to the obvious deficiency in the open east wall at the first level, there also are potential deficiencies in the remaining east and west walls at both levels. 6. Parapet--The 9-in. unreinforced masonry walls in the second level terminate in an unsupported 18-in. high parapet above the roof level that may be a hazard to life safety in a severe earthquake. SECOND LEVEL 13-inch masonry exterior walls N I, 9-inch masonry : t-partition A._; FIRST LEVEL FIGURE C2.1 Existing two-story unreinforced masonry building. C23 STRENGTHENING TECHNIQUES The structural evaluation of this building was conducted using the ABK Methodology for unreinforced masonry bracing wall buildings with wood diaphragms. The recommended strengthening techniques (Figure C2.3) also follow that methodology. C23.1 Torsion The east wall of the building is deficient in both strength and stiffness. In addition, extensive wall anchors are required at both the first and second levels. Although the open front condition at the first level could be improved with either a concrete or steel moment frame, the extensive additional work required for this wall and its foundation combine to make replacement an attractive alternative. 189 Replacement of the existing east wall with a two-story reinforced concrete frame is the recommended strengthening alternative. Since the roof and second floor joists are supported on the longitudinal walls, shoring will not be required as the east wall is removed. Temporary lateral bracing in the north-south direction should be utilized during the replacement of the east wall. The new (N) 4-inchl reinforced n (N) wall anchorages second level wall would be a concrete overlay at second and roof metal stud wall with window on (E) wall (E) wall levels levels > openings similar to the existing ones and with brick veneer to F I [II fill I I1 I match the other brick walls, desired. I C2.3.2 Adjacent Building The proposed solution to the problem with the adjacent (N) reinforced concrete building is to provide a new frame with brick veneer>/ reinforced concrete shear wall on steel studs --in the first level of the subject SECOND LEVEL building. The wall would be in line with the west wall of the (N) plywood sheathed (N) reinforced adjacent building. In addition crosswalls concrete frame to the new shear wall, three new timber cross walls will be provided in the first level (Figure C2.3) to reduce the diaphragm deflection. The shear wall, the cross walls, and their connections to the floor diaphragm will be designed in accordance with the ABK Methodology. This strengthening reinforced concrete may not completely solve shear wall the adjacent building problem, but the new shear wall and the FIRST LEVEL cross walls will significantly reduce the inertia forces transmitted FIGURE C2.3 Proposed structural modifications to the adjacent building. C233 Wall Stability The height to thickness ratio, h/t, of an unreinforced masonry wall is used as an index of the stability of the wall for the out-of-plane seismic response induced by the diaphragm. The east wall is to be replaced with a reinforced concrete wall so that the west wall in the second level is the only remaining wall with an excessive h/t ratio. The deficiency can be corrected by providing anchors at the ceiling level and bracing this anchorage up to the wood diaphragm or by designing vertical wall braces that span from the floor to the roof anchorage level. 190 C23A Wall Anchorage All anchorages of masonry walls to diaphragms were found to be inadequate at both levels of the building. Supplementary anchors must be provided for the calculated anchorage. The anchors should be similar to those indicated in Figure 3.7.1.4a or b. Significantly greater allowable loads are permitted for anchors that extend through the masonry wall with a large metal washer on the outside of the wall. This type of anchor should be used in all locations where access is available to the outside face of the wall. C2.3.5 In-Plane Shear Stress The new reinforced concrete frame at the east wall, the new shear wall, and the new concrete overlay for the west wall at second level have eliminated the calculated in-plane overstress in the east and west walls. C2.3.6 Parapet The unreinforced and unbraced parapet is a life safety hazard because of its h/t ratio. It is recommended that the parapet be reduced in height by the removal of several courses of brick (i.e., 8 to 10 in.). This should be preceded by a horizontal saw cut at a mortar joint on both sides of the wall to avoid damage to the remaining brickwork. The top of the reduced parapet then should be sealed with a mortar cap to prevent intrusions of moisture into the wall. 191 MINORITY OPINIONS The comments concerning this handbook presented below are included at the request of the representative of the American Iron and Steel Institute. • Concerning Chapter 1, Sec. 1.4: It is questioned if rehabilitation techniques need to be fully consistent with the NEHRP Recommended Provisions for new buildings. The NEHRP Recommended Provisions were developed with modern buildings as the underlying basis. Force fitting detailing provisions developed for modern structures onto older structures may overlook adequate details built into many older buildings which can provide adequate toughness. * Concerning Chapter 3, Sec. 3.0.4: In the 5th line of the first paragraph the word "tested" should be changed. A test implies that a structural system has been subjected to known loading conditions which is typically not the case with existing buildings. * Concerning Chapter 3, Sec. 3.1: We strongly object to the organization of this section. Typically when several subjects are presented the most significant is placed first. Since we are dealing with techniques of rehabilitating seismically deficient structures this section should be organized with the most significant (deficient) structures first. The scope of the section should then explain the reason for the organization of the section. For whatever reason this section has been organized with steel moment frames placed first. Steel moment frames have been observed to be one of the most reliable seismic resisting systems worldwide, the majority of which were not designed to modern seismic detailing practices. * Concerning Chapter 4, Sec. 4.1: Additional techniques such as reducing the weight by eliminating hollow clay tile partitions and substituting with lightweight partitions should be included. * Concerning Appendix C: Where a limited number of examples are to be presented they should be based upon the highest risk structural systems. Certainly steel moment frames do not fall into that category. The two most common types of seismically deficient structural systems observed in past earthquakes are unreinforced masonry and poorly detailed concrete frames. The inclusion of steel moment frames as one of two examples does not serve justice to the potential risk of the various structural systems. The comments concerning this handbook presented below are included at the request of the representative of the American Institute of Steel Construction: * Concerning Chapter 3: In Sec. 3.1.1.1, modify the first sentence to read: "The principal deficiencies of ordinary steel moment frames in high seismic areas are:" 193 In Sec. 3.3.1.1, modify the first sentence to read: 'The principal deficiencies of steel concentrically braced frames in high seismic areas are:" Users of this document may not read the Introduction and/or Sec. 3.0.4 for a proper are orientation on seismic zonation. Thus, explicit reminders in the actual design chapters needed. 194 BUILDING SEISMIC SAFETY COUNCIL BOARD OF DIREMION --1992 Chairman Gerald H. Jones, Director of Codes Administration, Kansas City, Missouri Vice Chairman Allan Porush, Dames and Moore, Los Angeles, California (representing Structural Engineers Association of California) Secretary Harry W. Martin, American Iron and Steel Institute, Newcastle, California Ex-Officio Warner Howe, Gardner and Howe, Memphis, Tennessee Members John C. Canestro, PE, City of Orinda, Pleasanton, California (representing the National Institute of Building Sciences) S. K. Ghosh, Portland Cement Association, Skokie, Illinois Mark B. Hogan, National Concrete Masonry Association, Herndon, Virginia Nestor Iwankiw, American Institute of Steel Construction, Chicago, Illinois H. S. "Pete" Kellam, Graham and Kellam, San Francisco, California (representing the American Society of Civil Engineers) Les Murphy, International Association of Fire Fighters, (representing AFL/CIO Building and Construction Trades Department) F. Robert Preece, Preece/Goudie & Associates, San Francisco, California, (representing Earthquake Engineering Research Institute Blair Tulloch, Tulloch Construction, Inc., Oakland, California (representing the Associated General Contractors of America) David Tyree, National Forest Products Association, Georgetown, California Martin Walsh, City of St. Louis, Missouri (representing the Building Officials and Code Administrators International) Richard Wright, National Institute of Standards and Technology, Gaithersburg, Maryland (representing the Interagency Committee for Seismic Safety in Construction) Staff James R. Smith, Executive Director 0. Allen Israelsen, Project Manager Claret M. Heider, Technical Writer-Editor Karen E. Smith, Administrative Assistant 195 BSSC MEMBER ORGANIZATIONS AFL-CIO Building and Construction Trades Department AISC Marketing, Inc. American Concrete Institute American Consulting Engineers Council American Institute of Architects American Institute of Steel Construction American Insurance Services Group, Inc. American Iron and Steel Institute American Plywood Association American Society of Civil Engineers Applied Technology Council Associated General Contractors of America Association of Engineering Geologists Association of Major City Building Officials Bay Area Structural, Inc. Brick Institute of America Building Officials and Code Administrators International Building Owners and Managers Association International Building Technology, Incorporated California Geotechnical Engineers Association Canadian National Committee on Earthquake Engineering Concrete Masonry Association of California and Nevada Concrete Reinforcing Steel Institute Earthquake Engineering Research Institute General Reinsurance Corporation* Interagency Committee on Seismic Safety in Construction International Conference of Building Officials Masonry Institute of America Metal Building Manufacturers Association National Association of Home Builders National Concrete Masonry Association National Conference of States on Building Codes and Standards National Elevator Industry, Inc. National Fire Sprinkler Association National Forest Products Association National Institute of Building Sciences National Ready Mixed Concrete Association Permanent Commission for Structural Safety of Buildings' Portland Cement Association Precast/Prestressed Concrete Institute Rack Manufacturers Institute Seismic Safety Commission (California) Southern Building Code Congress International Steel Deck Institute, Inc. Steel Joist Institute* Steven Winter Associates, Inc. Structural Engineers Association of Arizona Structural Engineers Association of California Structural Engineers Association of Central California Structural Engineers Association of Illinois Structural Engineers Association of Northern California Structural Engineers Association of Oregon Structural Engineers Association of San Diego Structural Engineers Association of Southern California Structural Engineers Association of Utah Structural Engineers Association of Washington The Masonry Society Western States Clay Products Association Western States Council Structural Engineers Association Westinghouse Electric Corporation 'Affiliate (non-voting) members. * U.S.G.P.O. 1999 -722-072 / 94347 197