Changes in IBC from 2009 to 2012: Seismic Design

The transition from one building code to the next always begs the question, “how is the newer code different?” There are several changes between the 2009 IBC and 2012 IBC that will change the way designers approach seismic design. This blog post is a broad overview of some of the changes. Since it’s not practical to cover all the changes between the previous and new codes in detail in one post, the discussion will be mainly on 2012 IBC and the corresponding ASCE7-10 reference standard.

Seismic ground motion map
Seismic ground motion map

The seismic ground motion maps have been updated to match ASCE7-10. The titles of the maps in IBC were revised from “Maximum Considered Earthquake Ground Motion” to “Risk-Targeted Maximum Considered Earthquake (MCER) Ground Motion Response Accelerations” in order to reflect the titles in 2009 NEHRP and ASCE 7-10. As in previous editions, some areas will prove difficult to read due to the contour lines, so the USGS site and GPS coordinates are recommended (http://earthquake.usgs.gov). Additional information about changes made for 2009 NEHRP is available at www.nibs.org or www.bssconline.org.

The term “occupancy category” was replaced with “risk category” in the 2012 IBC for consistency with the term used in ASCE 7-10. This change was made because it was decided that the use of the word “occupancy” implied the category was directly tied to occupancy classifications in the code, while the word “risk” more accurately communicates that the category is based on acceptable risk of failure.

Seis-pic 2ASCE7-10 revised the way designers use the corresponding Drift amplification, Cd, and Overstrength factor, Ωo, of the Response modification factor, R.  In ASCE7-05, when there is a vertical combination of different R-values, the Cd, and Ωo cannot decrease as you go down each level of a building. In ASCE7-10 (12.2.3.1), the Cd and Ωo always correspond to the R-Value as you go down. The adjacent figure illustrates the new provision to use the corresponding Cd, and Ωo with the R-value at each level.

ASCE7-10 (12.3.4.1) added a clarification for out-of-plane anchorage forces where the redundancy factor, p = 1.0.  The intent of the redundancy factor was to ensure the vertical seismic-resisting system with insufficient redundancy had adequate strength. The design forces for out-of-plane wall loading are not redundancy requirements. ASCE7-10 (12.11.12) revised the out-of-plane wall anchorage force equation where the anchorage forces are reduced for shorter diaphragm spans.

Seis-pic 3Light-frame construction structures are no longer exempt from amplification of accidental torsion in ASCE7-10 (12.8.4.3).  There are many structures vulnerable to torsional effects including some “tuck under” parking buildings that are often light-frame structures. See posts  titled Soft-Story Retrofits and City of San Francisco Implements Soft-Story Retrofit Ordinance for more discussion of soft-story, light-frame buildings.

This is just a brief summary of changes related to seismic design found in the 2012 IBC.  What are other changes that will modify your approach to seismic design?