5.1 Program Objectives and Scope
For new construction, it is possible to anchor, brace, or restrain all of the critical nonstructural items at the same time according to a chosen set of performance objectives and in conformance with current building code requirements. It is generally more efficient and less costly to install anchorage details during construction and at the time of initial occupancy than to upgrade them after the fact.
The planning stage for new construction is the ideal time to consider the desired seismic performance of a facility. It is an opportunity to coordinate the structural and nonstructural aspects of the design, for instance by selecting a structural system that provides a greater level of seismic safety and that will provide for a higher level of both structural and nonstructural performance. It is also critical to communicate concepts of seismic performance, risk, and related options to a building owner, in order to establish project specific design and construction strategies.
The following questions might help define the project objectives, including the nonstructural risk reduction objectives:
- What type of organization or business will occupy the facility?
- What type of functionality is needed during and after a minor, major, or severe earthquake?
- How much structural and nonstructural damage can be tolerated after a minor, major, or severe earthquake?
- Do the design professionals have experience with bracing and anchorage of the types of nonstructural components proposed for the facility, particularly if the facility will need to be operational following an earthquake?
- For an important project, is there a third party peer reviewer for the seismic design, including the design for the nonstructural components?
- What is the value of proposed architectural finishes? MEP systems? Furniture, fixtures, and equipment (FF&E) and contents? What would be the financial impact of damage to or failure of each of these items?
- How much of the potential earthquake losses will be covered by insurance?
It is worth repeating that the nonstructural components and contents typically represent the major portion of the capital investment for new construction; per Figure 2.1.3-1, this is 82% for office buildings, 87% for hotels, and 92% for hospitals (Whittaker and Soong, 2003). Incorporating seismic damage control measures into the design for new construction makes good business sense, particularly for buildings that have a high probability of experiencing damaging earthquakes several times during their life span. For new construction of essential buildings in high seismic areas, damage control measures are now required, in order to increase the likelihood that these facilities will remain functional following a major earthquake.
- What is the design life of the facility?
- What are the magnitudes and frequency of earthquakes the building is likely to experience during its life?
- Has a structural system been chosen that will provide the level of structural and nonstructural protection required? Is the structural system very stiff? Very flexible? Are the inter-story drifts large? Does the structural design include base isolation or energy dissipation devices such as structural dampers?
- What types of nonstructural components are proposed? Would damage to or failure of the proposed components be a life safety hazard or result in heavy property loss, or compromise building function? What is the cost of upgrading to more seismically resistant components and detailing?
- Does the design team have any control over future FF&E and contents? If not, who will have control? Can the design team coordinate the design and installation of these components with design representatives for the initial building occupants?
Although code provisions historically have been written with the primary intent to provide a minimum level of life safety and to avoid legislating property damage control measures, code provisions now mandate an increasing level of damage control for certain types of essential and high occupancy facilities. In Minimum Design Loads for Buildings and other Structures (ASCE/SEI 7-10), structures are assigned to a Risk Category. The Risk Category is related to the consequences of failure, from the lowest risk to human life (Risk Category I) to the highest (Risk Category IV). Most structures are placed in Risk Category II. Facilities where higher standards are currently mandated include hospitals, aviation control towers, designated emergency shelters, police and fire stations, power generating stations, water storage or pumping facilities, facilities that handle hazardous materials, and a number of others identified as Risk Category IV in ASCE/SEI 7-10. Except in areas with the lowest seismicity, the structural and nonstructural design of these facilities must meet more stringent design requirements than for standard construction.
For standard construction, a "code design" is intended to provide a minimum level of life safety, now considering both structural and nonstructural components, but it does not provide for significant damage control.
- To avoid serious injury or loss of life
- To minimize repair costs to the extent practical.
In order to achieve enhanced performance (e.g., Operational, Immediate Occupancy, or a higher level of structural and nonstructural damage control), the design objectives must be targeted higher than the life safety level implicit in the minimum code provisions. Essential structures (Risk Category IV) are designed for higher forces, and more of the nonstructural components must be designed for seismic loads.
Although new construction must meet the minimum life safety standards, owners concerned with building functionality or future earthquake losses may choose to implement a higher standard and to incorporate damage control measures into the design for new construction on a voluntary basis.
At their discretion, owners may adopt more stringent seismic design standards than those in the prevailing code.
- Beginning in the late 1970s, some owners of high tech research and manufacturing facilities in California started to use higher standards for the seismic design of critical buildings and nonstructural components on a voluntary basis.
- In the mid-2000s, several thermoelectric power plants in Chile were designed using a special seismic performance criteria stipulated by the owners that require that any damage to the plants from a major earthquake be limited to that which could be inspected and repaired within 14 days time; further, the criteria require that these plants remain operational during moderate seismic events.
In these examples, the owners developed special seismic design criteria to meet the needs of their organizations, primarily motivated by a desire to limit costly postearthquake outages.
The use of performance-based design concepts requires a discussion between building design professionals and their clients about performance expectations and seismic risk tolerance. Performance-based design provides terminology to characterize seismic risk and seismic performance and provides a framework for making comparisons between varying levels of seismic hazard, structural and nonstructural performance, postearthquake functionality, acceptable and unacceptable damage, and total earthquake losses over the expected life of the facility. Design professionals, organizational risk managers, building owners, business owners, and tenants all need to develop an understanding of the tradeoffs between risk and reward; that is, an understanding that seismic design and investment choices have a relationship to expected future performance and potential future losses. The parties all need to understand that they make choices, both passive and active, based on their understanding of the issues and their seismic risk tolerance. One may choose to live with known seismic risks or choose to initiate programs to reduce some or all of the known hazards; either way, a choice must be made.
Performance-based design concepts have been in development for several decades; this development is ongoing. These concepts are gradually finding their way into the building codes used for new construction, such as 2012 International Building Code (ICC 2012) and ASCE/SEI 7-10. These codes now specify higher seismic design forces and more comprehensive requirements for nonstructural components while at the same time imposing more stringent drift limits on the building structural systems in certain types of facilities in an effort to reduce the earthquake damage and improve the performance of these facilities. Nevertheless, the code generally does not address damage control or postearthquake operations for standard occupancies. If an owner wants to specify higher performance standards than those embodied in the code, it is important that those performance expectations be identified early in the planning process. For example, some owners interested in enhancing the structural and nonstructural performance of their facility will choose a structural system incorporating seismic isolation and/or supplemental damping. Seismic isolation provides the most significant benefit by greatly reducing both drift and acceleration demands imposed on nonstructural systems as well as demand on the structural system itself. Seismic isolation is discussed in more detail in Section 6.1.
Borrowing some terminology used in Seismic Rehabilitation of Existing Buildings (ASCE/SEI 41-06) for the rehabilitation of existing construction and previously described in Chapter 4, target building performance levels may be described as basic or enhanced. Limited performance objectives, those that provide less than the minimum life safety standard, while permissible for existing construction, are not allowed for new construction.
- A basic level of safety is achieved by following the code requirements for standard occupancies. This type of design should not be expected to provide significant damage control for structural or nonstructural components or to provide for continued operations or immediate occupancy after an earthquake.
- An enhanced performance level is achieved by following both structural and nonstructural code requirements for essential facilities. Enhanced performance could be achieved for nonessential facilities by using some or all of these additional requirements.
- Enhanced performance might also be provided by developing project-specific seismic design criteria to meet the needs of a particular organization (see sidebar on previous page). These criteria should be developed and implemented by design professionals with specific experience with performance-based design. Engineering analysis methods, such as those using nonlinear or push-over techniques, are available that can be used to check whether or not the design meets the target performance objectives.
It is important that the performance objectives be clear from the outset, so that the owner and design professionals are in agreement on what they are trying to achieve. Design and construction contracts must all include language describing the responsibilities of the designers, contractors, subcontractors, specialty subcontractors, vendors, and inspectors to provide systems and details that will meet the project objectives; this is particularly important if these are enhanced performance objectives that are higher than for a "code design." Budgets and schedules will all have to take into account the resources and time required to achieve the project goals.