This past week marked the 20th anniversary of the M6.7 Northridge earthquake. In addition to 57 fatalities, over 40,000 buildings sustained damage amounting to an estimated $25 billion in losses, making it the most costly earthquake in the United States.
Good friend and extraordinary structural engineer Gene Trahern shared with us an update on changes in earthquake engineering during the intervening years. Although I had not yet met Gene in 1994, I worked in a support role on the Northridge response effort, a big project for Gene and a pivotal time in my career. Since then Gene has further developed his expertise in evaluating seismic vulnerability of existing structures and founded Cascade Crest Consulting Engineers. More information about Gene’s practice is here: http://www.cccengr.com/
IBC replaces UBC, BOCA, and SBC
Three semi-regional codes were combined into one uniform document, the International Building Code, causing engineers nationwide finally to use the same design procedures. As part of that change, design ground motions changed from the 475-yr earthquake to two-thirds of the 2500-yr earthquake. The change elevated design forces in areas with large but infrequent earthquakes like some parts of the Midwest, Salt Lake City, and the Carolinas. Design forces increased in Seattle and slightly decreased in Sacramento and Portland.
ASTM Seismic Loss Estimation Standards
Two new standards, E2026 and E2557, now define a uniform method for assessing potential earthquake damage to existing buildings. The standards define several terms that used to have various meanings, eliminating conflicts in insurance and valuation procedures. They also establish four discrete levels of analysis, improving quality and reliability of seismic loss estimation studies.
Improved Seismic Evaluation and Upgrade Methods
At the time of the Northridge Earthquake, seismic evaluations of existing buildings used basic ground motions similar to the outdated UBC seismic zone system. ASCE-31 (2003) improved evaluation methods by changing to the current USGS seismic hazard maps. ASCE-41 followed 3 years later and carried the evaluation method improvements forward into seismic upgrade of existing buildings.
Gene Trahern can be reached at (541) 549-1331 and firstname.lastname@example.org