Performance Based Seismic Design is the seismic design methodology of the future. It allows the design team to work together to determine the appropriate levels of ground motion and Performance Objectives for the building and the nonstructural components in order to meet the owner's expectations.
The primary focus of most model building codes, including the recently released 2003 International Building Code, is to ensure life safety. Continued operation of a facility and reduction of economic losses associated with earthquake damage to the facility are secondary considerations, if they're considered at all during the design process. Economic losses sustained in previous earthquakes have highlighted the need for a design methodology that allows the design team, which includes the building owner, architect, and engineer, to understand and choose a desired level of seismic performance for buildings and nonstructural components when they are subjected to a specified level of ground motion. Performance Based Seismic Design allows the design team to work together to determine the appropriate levels of ground motion and Performance Objectives for the building and the nonstructural components in order to meet the owner's expectations.
Current Building Code Design
Building code based seismic design has the following characteristics.
- The basic objective is life safety for the design level ground shaking.
- There are methods within the current model building codes and the new International Building Code (IBC) to increase the detailing and/or seismic design force requirements, but the resulting level of enhanced performance is not explicitly defined.
- The design of the seismic load resisting system is based on a pseudo seismic load, which is reduced from the actual load by a ductility factor (the "R" value).
- The "R" value is used to reduce the design lateral load on the entire building. There is only one "R" value for the entire building. Thus, the same reduction is applied to beams, columns, braces, foundations, bolts, welds, diaphragms, etc., independent of the element's ductility.
Performance Based Design
Over the past several years, federal guidelines were published which help to facilitate the implementation of Performance Based Design with respect to existing structures. FEMA 273, Guidelines for the Seismic Rehabilitation of Buildings, which has subsequently been updated as FEMA 356, provides specific performance objectives for both the building under consideration and the nonstructural components associated with the building. While written for use with existing structures, the Guidelines may also be used as the basis for the design of the seismic force-resisting system for new structures.
Performance Based Seismic Design has the following distinguishing characteristics.
- Performance Based Seismic Design allows the owner, architect, and structural engineer to choose both the appropriate level of ground shaking and the chosen level of protection for that ground motion.
- Multiple levels of ground shaking can be evaluated, with a different level of performance specified for each level of ground shaking.
- Target building performance levels range from Continued Operation, in which the building and nonstructural components are expected to sustain almost no damage in response to the design earthquake, to Collapse Prevention, in which the structure should remain standing, but is extensively damaged.
- Specific ductility factors ("m" values) are specified for each component of the seismic force-resisting system. The ductility factor varies depending on the target building performance level, material type, and the relative ductility of the component.
Below is a graphical representation of a performance objective matrix that matches chosen earthquake hazard levels (y axis) with selected target building performance levels (x axis). The three diagonal lines represent the performance objectives for different groups of buildings. Group I is representative of a basic commercial structure, while Groups II and III represent structures that require a higher level of protection such as hospitals, fire stations, data centers, key manufacturing facilities, etc.
The following design parameters are recommended by FEMA for ordinary buildings.
- Life Safety Performance Objective for an event that has a 10 percent probability of occurring in the next 50 years.
- Collapse Prevention Performance Objective for an event that has a 2 percent probability of occurring in the next 50 years.
The following design parameters are recommended by FEMA for essential facilities (buildings which are required to be operational after the design level seismic event).
- Continued Operations Performance Objective for an event that has a 10 percent probability of occurring in the next 50 years.
- Life Safety Performance Objective for an event which has a 2 percent probability of occurring in the next 50 years.
The estimated cost increments to raise the seismic design standard from the typical building code to the FEMA Performance Based Seismic Design Guidelines, expressed as a percentage of the total building construction cost, may be within the following ranges.
Structural Lateral Load-Resisting System
Life Safety: 0%-0.50%
Immediate Occupancy: 0.25%-1.0%
Life Safety: 0%-0.25%
Immediate Occupancy: 0.25%-0.50%
The estimated cost increment to incorporate Performance Based Seismic Design, given that no seismic design was included originally, may be in the range of 2 to 5 percent of the total building cost.
Performance Based Seismic Design is the seismic design methodology of the future. In addition to meeting the basic safety of objective of preventing loss of life, Performance Based Seismic Design can provide a cost-effective means to:
- Reduce earthquake financial loss due to structural and nonstructural damage.
- Design a structure for a range of performance levels when subjected to different levels of ground motion.
- Obtain minimum structural and nonstructural damage in a moderate seismic event.
- Allow for continued operations in a structure following a design level seismic event.