In the first of three previous blog posts in our Basics of Seismic Design series, we discussed earthquake forces and how a structure responds to them, elastically or inelastically. In the second blog, we explained why the building code allows structures to be designed for inelastic response to the design earthquake of the code, thereby lowering the seismic design forces. The third blog, not particularly relevant here, explained why seismic detailing is required even where design is “governed” by wind forces. This blog discusses “performance objectives” as they relate to seismic design.

In the context of seismic design, “performance objectives” express how we want our structure to perform when it is shaken by an earthquake, which then forms the basis of how we design the structure. The following are important considerations when it comes to performance objectives:

1. A structure may experience earthquakes of different intensities during its service life, and
2. The performance expected of a structure should depend on the earthquake intensity considered.

Different performance levels for different earthquake intensities

While it is really up to the owner to decide what performance level to set for what level of earthquake, our building codes have set some objectives that all structures need to meet as a minimum. When a structure is designed by the seismic requirements in our building codes and the materials standards adopted by them, it is deemed to satisfy these performance objectives. Three earthquake intensities (seismic hazard levels) and four performance levels are considered by the building codes for this purpose:

Earthquake intensities (hazard levels)

1. An earthquake of very high intensity that has a low probability of occurring during a building’s service life – This is referred to as the Maximum Considered Earthquake (MCE), and there is only a 2% probability that an earthquake of the same or greater intensity would occur at the location of a structure in 50 years (in other words, an earthquake of the same or greater intensity will happen on an average every 2475 years). Note: Starting with ASCE 7-10 and the 2012 IBC, the high-intensity earthquake is MCE_R for risk-targeted Maximum Considered Earthquake, which targets a 1% risk of structural collapse in 50 years. However, this detail is not important for the purpose of this blog and is not discussed here any further.
2. An earthquake of more moderate intensity that has a higher probability of occurring than the MCE – This is the Design Earthquake (DE), and its intensity is two-thirds that of the MCE. Its probability of exceedance changes from one location to another, varying from about 500 years in coastal California to roughly 1500 years on the Eastern seaboard.
3. An earthquake of lesser intensity that has a higher probability of occurring than the DE – This is the Frequent Earthquake (FE) that is typically defined by a 50% probability of exceedance in 50 years (FEMA 450, Commentary). In other words, an earthquake of this size is exceeded on an average every 87 years.

Performance levels

1. Collapse Prevention: The building suffers extensive damage in an earthquake, but remains standing, even if barely.
2. Life Safety: The building sustains substantial damage in an earthquake, but remains stable and with significant reserve capacity. Occupants have an opportunity to egress the structure. Nonstructural elements remain secured to the structure.
3. Immediate Occupancy: The building remains essentially elastic in an earthquake, with most or all of its strength and stiffness intact. The building can be occupied immediately after the earthquake, even though minor repairs may be necessary.
4. Operational: The building remains occupied and operational during an earthquake.

These performance levels (a – d) are combined with the three hazard levels (1 – 3) to define performance objectives (FEMA 450, Commentary), as shown below. So, a building of standard occupancy (such as an office or an apartment building), when designed in accordance with the seismic provisions of the building codes and standards, should meet the Collapse Prevention performance level, as defined above, when subjected to the MCE, the Life Safety performance level when subjected to the DE, and the Immediate Occupancy performance level when subjected to the FE. A building that is designed as an Essential Facility, on the other hand, will meet the Life Safety performance level even when subjected to the MCE, and will be fit for Immediate Occupancy even after the DE.

Meeting performance objectives – by code compliance

So, how do we make sure our buildings are meeting those performance objectives? Just complying with building code requirements is adequate for most buildings, as the seismic provisions in the building code are set up to at least roughly meet these performance objectives. For instance, the Design Earthquake (DE) is determined as 2/3 of the Maximum Considered Earthquake (MCE). This factor of 2/3 comes from the verified fact that a building properly designed by our seismic code provisions will have a margin of safety against collapse of at least 1.5 (2/3 being the reciprocal of 1.5). So, if a standard-occupancy building is designed for Life Safety under the DE, it will have 50% more reserve strength available to ensure Collapse Prevention under the MCE. Higher performance objectives for buildings with higher and more critical occupancies are met by requiring higher design seismic forces by the application of an Importance Factor, I_e (which will be discussed in a later blog).

Meeting performance objectives – by performance-based design

Explicit checks for performance objectives become necessary when an alternative design methodology is adopted, which does not strictly comply with all the prescriptive requirements of the building code. That is performance-based design; it is often done, for instance, for tall buildings where certain building code requirements such as height limitations or limitation on the use of certain structural systems may prove to be overly restrictive. It may be more economical to carry out exact analyses of the nonlinear response of every structural member and connection to different levels of seismic excitation, to make sure their performance meets or exceeds the minimum objectives implicit in the code provisions. This is permitted by the IBC (Section 104.11) as well as ASCE 7 (Section 1.3.1). Needless to say, a performance-based design approach should only be undertaken by competent structural engineers with a thorough understanding of the inelastic response of structures to high seismic excitation, who possess the appropriate tools to perform such analyses. A document published by the Pacific Earthquake Engineering Research Center Tall Building Initiative, Guidelines for Performance-Based Seismic Design of Tall Buildings, is an excellent resource for anyone who wants to undertake such a project. A similar resource is An Alternative Procedure for Seismic Analysis and Design of Tall Buildings Located in the Los Angeles Region, published by the Los Angeles Tall Buildings Structural Design Council.