Anticipated Changes to Los Angeles Building Code Due to IBC 2012: Impact on the Seismic Design of Building Foundations on Liquefiable Soils at the Port of Los Angeles

摘要

The 2009 International Building Code (IBC 2009) for the seismic design of building structures specifies the use of the Design Earthquake (DE) ground motion for most structural and geotechnical evaluations. However, the revised IBC 2012 specifies the use of increased ground motion requirements for liquefaction-related geotechnical evaluations based on the Maximum Considered Earthquake (MCE) rather than DE. This change will result in a 150 percent increase in the ground motion, since MCE is equivalent to 150 percent of the DE. As the IBC 2012 requirements will be adopted by the California Building Code, which in turn will be reflected in the City of Los Angeles Building Code, the revised IBC 2012 requirements may significantly impact the design of waterfront building structures at the Port of Los Angeles (POLA) built near dikes or slopes where liquefiable soils are present because of larger lateral spreading deformations. However, the new requirements will have minimal impact on building structures constructed on level ground since lateral ground deformation is not generally an issue for level-ground conditions. To illustrate the impact of the new IBC 2012 requirements, two dike configurations at the POLA, existing and new, were considered in a study commissioned by the POLA. The study concluded that lateral dike and slope deformations for the two dike configurations using site specific MCE ground motions developed per IBC 2012 requirements are expected to be about 200 percent to 400 percent of those estimated using the IBC 2009 requirements. New building construction at port and harbor facilities may require increased costs related to ground improvements to mitigate the increase in slope deformations. Similar to new construction, upgrades, repairs, and retrofits for existing structures may also require increased costs related to ground improvements and may result in structure replacements due to excessive ground movements under MCE. In this paper, background to the IBC 2009 liquefaction evaluation requirements and revisions incorporated in IBC 2012 are described together with impact case studies conducted for the POLA. Geotechnical and structural analyses for two of the case studies are then discussed to illustrate the impacts.