Luncheon – June 4, 2010

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Talk: Colorado’s Earthquake Hazard: Seismic Provisions of the International Building Code [IBC] – How this Model Code Addresses Seismic Hazards

Speakers: Vince Matthews, Colorado State Geologist and Alexander H. Abel, P.E. Plans Review Engineer Building Department, City and County of Denver

Location: Marriott City Center

Date: June 4, 2010

Abstract: Published in The Outcrop, June 2010

Colorado is noted for its induced earthquakes at the Rocky Mountain Arsenal, in Rangely oil field, and in the Paradox Valley. Lesser known is Colorado’s natural earthquake activity which includes a magnitude 6.6 earthquake in 1882 that caused damage along the eastern margin of the Front Range. Three of Colorado’s faults have been studied sufficiently to be included in the USGS National Earthquake Hazard Map which ultimately drives building codes in the country. Numerous other faults offset Quaternary deposits around the state, but have not been sufficiently studied to warrant inclusion in the hazard map. Calculations of economic loss using FEMA’s HAZUS modeling program indicate that large earthquakes on these faults could cause damage as high as $20+ billion. In order to better understand the earthquake hazard in Colorado, six new broadband stations will soon be added to the existing four permanent stations in the state.

The second portion of the talk will provide a brief overview of the current model code approach to seismic forces and their impact on structures and nonstructural components.

The seismic provisions of the IBC are, for the most part, not intended to produce structures with an elastic response to the design event. The building code seismic provisions are intended to prevent catastrophic failure and loss of life in the design event. There are exceptions to this rule for structures of a higher “occupancy category,” for example, hospitals, power generation facilities, fire stations, jails, aviation structures, designated emergency shelters and some large venues.

The current approach to determining seismic loads on structures has changed dramatically from the days of the Uniform Building Code (UBC) where the country was divided into seismic zones and the forces on a structure taken from the generalized seismic risk, the magnitude of the 50 year recurrence interval event, for all structures within the zone. Investigations of structure failures resulting from the Northridge and Loma Prieta events resulted in a dramatic change in understanding of seismic risk and loads for structures. The new approach resulting from this improved understanding of seismic hazards was firs incorporated into the 2000 IBC. Details of this new understanding of seismic risk will be presented.

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