5.3 Building Code Requirements
- 5.3.1 2012 Edition of the International Building Code (IBC 2012)
- 5.3.2 Enforcement of Code Requirements
- 5.3.3 Requirements for Contents
- 5.3.4 Other Standards and Protocols
- 5.3.5 Validation and Refinement of Code Requirements
Structures designed to the provisions of IBC 2012 are expected to have a low likelihood of collapse in the very rare seismic event defined as the Risk-Targeted Maximum Considered Earthquake (MCER) ground motion. Nonstructural components and systems are designed to protect occupants from life threatening damage or failures of nonstructural elements in an unusual but less rare earthquake ground motion, which is referred to as the Design Earthquake ground motion (defined as two-thirds of the MCER). There is no implicit performance goal associated with the MCER for nonstructural components in IBC 2012.
The current code requirements for nonstructural components are contained in ASCE/SEI 7-10 Section 13 which is adopted by reference in IBC 2012 (ICC, 2012). In recent years, engineers, researchers, and code committees have paid increasing attention to the issues of nonstructural performance. As a result, ASCE/SEI 7-10 now includes a 15-page chapter devoted to nonstructural components and contains design requirements for both force- and displacement-controlled nonstructural components. In contrast, the Uniform Building Code (ICBO, 1994) covered the nonstructural requirements in less than two pages, where the focus of the requirements was primarily on position retention of the components. The requirements are now more detailed and include explicit provisions for more items that apply to facilities that require postearthquake functionality.
- Minor earthquake ground motions—minimal damage; not likely to affect functionality
- Moderate earthquake ground motions—some damage that may affect functionality
- Design Earthquake ground motions—major damage but significant falling hazards are avoided; likely loss of functionality.
The code formulas used to compute the design forces on nonstructural components in buildings contain a number of simplifying assumptions regarding damping, response amplification, possible resonance between the equipment and the structure, and the distribution of forces over the height of the structure. For some facilities, more sophisticated analyses may be warranted. The Tri-Service Manual “Seismic Design Guidelines for Essential Buildings” (TM 5-810-10-1) describes an approximate floor response spectrum method that considers two earthquake levels and takes the multi-mode response of the building into account. Beyond that, an analytical model of the building can be used to generate floor response spectra at critical equipment locations, and these spectra can be used to determine the design forces for the nonstructural components and equipment.
- Seismic Design Category A: All Risk Categories in areas with minimal seismicity; these facilities are exempt from the nonstructural requirements
- Seismic Design Category B: Risk Categories I, II, and III in areas with low seismicity
- Seismic Design Category C: Risk Categories IV in areas with low seismicity and Occupancy Categories I, II and III in areas with moderate seismicity
- Seismic Design Category D: Risk Categories IV in areas with moderate seismicity and All Occupancy Categories in areas with high seismicity
- Seismic Design Category E: Risk Category I, II or III in areas of very high seismicity and near an active fault
- Seismic Design Category F: Risk Category IV in areas of very high seismicity and near an active fault
The seismic design forces are based on a variety of factors including the weight of the item, the ground acceleration and soil type, the flexibility of the component and its attachments, the location in the building, and an importance factor. In general, design forces are higher for flexible components and flexible attachments; higher for items anchored in the upper levels of the building; higher for items that contain hazardous materials, that are needed for life safety functions, or that are needed for continued operations of an essential facility; and lower for items with high deformability or high ductility.
Minimum and maximum limits on design forces are specified in the code. For a given acceleration and importance factor, the range from minimum design forces (0.3SDSIpWp) to maximum design forces (1.6SDSIpWp) is a factor slightly greater than 5. Thus, a flexible item anchored at the roof of a building might be designed for up to 5 times more force than a rigid item anchored at the base of the same building.
For items affected by differential movement and building story drift, the code requires that the design consider the relative lateral displacements both within and between structures. This will affect the design of such components as pipe risers and precast panels, which are connected to adjacent floors, and piping, cable trays, ductwork, or architectural finishes crossing seismic joints.
The code includes provisions for architectural, mechanical, and electrical components, supports, and attachments. Tables of design coefficients ap and Rp are provided for dozens of architectural, mechanical, and electrical components. Where design of nonstructural components or their supports and attachments is required by code, such design must be shown in construction documents prepared by a registered design professional. It is not sufficient to provide a note saying "All ceilings to be braced"; the bracing details must be included on the plans and covered in the project specifications.
Earlier provisions related to nonstructural components in the IBC 2000 and IBC 2003 were concerned primarily with position retention, i.e., preventing components from becoming dislodged or overturned during an earthquake. ASCE/SEI 7-10 contains additional provisions related to postearthquake functionality that are applicable to components with hazardous contents and to equipment that is required to remain operational following an earthquake. For such designated seismic systems, where Ipis 1.5, certification based on approved shake table testing or experience data must be submitted to the authority having jurisdiction.
The code contains a number of significant exemptions (see sidebar below). Exemptions are granted for:
- Structures that are subject to low-level earthquake demands (accelerations and relative displacements).
- Components that possess inherent strength and stability
- Items that are not part of the building architecture. This includes items such as temporary or movable items, equipment that is not permanently attached to the structure such as desktop items (computers, copiers, lamps, etc.), and most furniture, except permanent floor-supported storage cabinets, shelving or book stacks over 6 feet tall.
- Lighter nonstructural components that meet certain prescriptive requirements provided they are positively attached to the structure.
Although the building code does not contain requirements for many components, such as furniture, and movable fixtures, equipment and contents that are supplied by tenants and building occupants,thesemay still pose a significant risk in a strong earthquake, and consideration should be given to tethering or anchoring these items to reduce damage and disruption. For areas with moderate or high seismicity, the risks associated with many of these components can be reduced by following the suggestions contained in Chapter 6.
The following items are specifically exempt from the ASCE/SEI 7-10 seismic design requirements for nonstructural components:
- Most furniture and temporary or movable equipment.
- Most components in Seismic Design Categories B and C (i.e., normal occupancies in areas of moderate seismicity).
- Mechanical and electrical components in Seismic Design Categories D, E and F, where all of the following apply:
- Ip is equal to 1;
- The component is positively attached to the structure;
- Flexible connections are provided between the components and associated ductwork, piping and conduit are provided, and either
- the component weighs 400 lb or less and has a center of mass located 4 feet or less above the adjacent floor level; or
- the component weighs 20 pounds or less or, for distribution systems, weigh 5 pounds per foot or less.
The effectiveness of model code requirements governing seismic design of nonstructural components depends on technically sound code provisions, proper application by designers, and code enforcement. Proper enforcement requires both comprehensive plan review and thorough construction inspection.
A comprehensive plan review includes determination of which items require seismic design; and an examination of the details for compliance with code requirements. Determining which items require seismic bracing involves a review of the construction drawings and specifications for each discipline (e.g., architectural, electrical, mechanical, plumbing, and other specialties). Few jurisdictions, if any, have resources devoted to such a comprehensive review of construction documents, and few jurisdictions have reviewers qualified to comprehensively evaluate compliance with all nonstructural code requirements.
An additional challenge in plan review arises from the many items that are commonly excluded from construction drawings, but are identified in the project specifications to be procured from the contractor on a "design-build" basis, or may be “owner furnished and installed.” Unless these items are carefully tracked and submitted for review, building department plan review can be nonexistent. Few jurisdictions have mechanisms in place to track and support ongoing review of nonstructural seismic bracing designs developed during construction. The responsibility matrices included in Appendix B are intended to aid project managers in the assignment and tracking of responsibility for nonstructural seismic protection. Used in conjunction with the specification section provided in Appendix A, the responsibility matrices can be used to facilitate compliance with nonstructural performance objectives.
Enforcement of nonstructural seismic requirements is often lacking in the construction inspection process. Since details associated with seismic restraint of nonstructural components are not often fully shown on approved drawings, inspectors are left without the tools necessary to evaluate the adequacy of as-built installations. Historically, building inspectors have not been systematically trained to inspect the seismic restraint of nonstructural components, and few inspectors have sufficient experience to field review seismic restraints of nonstructural components that are not covered by a well known standard.
Many design professionals have the necessary training and experience to evaluate the adequacy of nonstructural seismic restraints; however, field observation of nonstructural component installations is often not included in their scope of work. As a result, it is not uncommon for nonstructural components to be installed without inspection.
IBC 2012 contains requirements for special inspection of designated seismic systems. For most buildings, a written statement of special inspection must be prepared by a registered design professional. In buildings assigned to Seismic Design Categories C, D, E or F, the statement of special inspection must include seismic requirements for selected HVAC components, piping systems and electrical equipment. These code requirements are expected to increase the construction oversight of nonstructural installations and ultimately, to improve the seismic performance of nonstructural components.
Building contents, such as furniture, kitchen and laundry equipment, movable partitions, and storage shelving are typically considered separate from the building and are usually the responsibility of the building occupant, not the owner or the original design team. Many such items are specifically exempted from seismic provisions in model building codes (e.g., furniture, floor-mounted equipment weighing less than 400 pounds, and suspended items weighing less than 20 pounds). Regulated by the code or not, contents can pose an additional risk to safety and continuity of operations after an earthquake. The seismic protection of contents is dependent upon an understanding of potential seismic risk, followed by action to mitigate that risk on the part of business owners, homeowners, and tenants. The content examples included in Chapter 6 provide guidance for the bracing and anchorage of many common furniture and content items and can be adapted for other similar items. Building code provisions, guidance documents, or other resources listed in the references can be effectively applied to the design and installation of seismic protection measures for building contents.
Many of the challenges related to design, plan review, and construction inspection are resolved when installation in accordance with nationally accepted standards becomes a construction standard of practice. For example, IBC 2012 accepts seismic restraint of fire protection systems designed in accordance with the National Fire Protection Association's Standard for the Installation of Sprinkler Systems (NFPA 13). As a result, verification of NPFA 13 compliance is a common occurrence in the field. Similar examples exist for other major nonstructural components: Installation of suspended ceilings in accordance with ASTM C635/C635M-07, ASTM C636/C636M-06, and the Standard Practice for Installation of Ceiling Suspension Systems for Acoustical Tile and Lay-in Panels in Areas Subject to Earthquake Ground Motions (ASTM E580/E580M-10a) is included in the IBC by reference. Selected additional industry standards are listed in Appendix B of the State-of-the-Art and Practice Report (ATC 69).
Qualification testing is an acceptable alternative to the analytical requirements of the code. IBC 2012 accepts seismic qualification based on nationally recognized testing procedures, such as the, Acceptance Criteria for Seismic Certification by Shake-table Testing of Nonstructural Components (AC156) by the International Code Council Evaluation Service. Method of Test of Seismic Restraint Devices for HVAC&R Equipment (ANSI/ASHRAE Standard 171-2008) provides additional test methods used in the HVAC industry. Selected additional testing protocols, such as Interim Testing Protocols for Determining the Seismic Performance Characteristics of Structural and Nonstructural Components (FEMA 461) are listed in Appendix B of the State-of-the-Art and Practice Report (ATC 69) and repeated here in Appendix F.
Seismic design requirements for structural systems have evolved over time as a result of documented earthquake performance and laboratory testing. Seismic design requirements for nonstructural components have also evolved over time; however, comprehensive evaluation of these requirements, either by testing or through postearthquake observations, has been limited. Future earthquakes might be able to provide the information necessary to validate or refine current design requirements, but comprehensive and systematic postearthquake documentation of nonstructural performance is needed. Obstacles to gathering such perishable data will need to be overcome before a quantitative review of nonstructural seismic design requirements can become possible.