NGA-West2 research spurs updates to Ground Motion Site Amplification Factors in Building Code

The site amplification factors used to develop design-basis ground motions will be changing in the next revisions of several building code documents due to research conducted as a part of PEER’s NGA-West2 project.  These factors, commonly called Fa, Fv, and FPGA are found in Tables 11.4-1, 11.4-2, and 11.8-1 of the ASCE-7 code and have not been updated in several decades. 
The new site factors have been approved by the Provisions Update Committee of the Building Seismic Safety Council and are expected to appear in the 2015 version of the NEHRP Provisions, followed shortly thereafter by ASCE-7 and various building code documents. 
“After several years of comprehensive and extensive research, PEER is proud to have spurred this long awaited update,” said Yousef Bozorgnia, PEER’s Executive Director and Project Manager for the NGA-West2 project. 
Engineers use site factors to modify ground motions from a reference rock site condition to reflect the influence of geologic conditions at the site of interest, i.e. the location where a new building is being constructed. 
The current NEHRP site factors are based on a very small database of recordings in the San Francisco Bay Area from the 1989 Loma Prieta earthquake, which established relatively weak motion amplification empirically, in combination with nonlinear effects established from numerical simulations performed in the early 1990s.  Over the years misfits have been identified between NEHRP site factors and more modern ground motion prediction tools, such as the 2008 ground motion prediction equations (GMPEs) developed in PEER’s NGA-West1 project.
Not surprisingly, experience in practice and from research indicates that the site factor discrepancies are consequential for ground motion hazard analyses, loss estimation and other applications.  To resolve these issues, the new NGA-West2 ground motion data set was used by PEER researchers to develop a new site amplification model, which was then used to update the NEHRP site factors.   Because the updated site factors operate seamlessly with the NGA-West2 GMPEs (which will soon be applied for the next-generation of seismic hazard maps used in the code), a long-standing problem in earthquake engineering will be resolved.
PEER researcher Jonathan Stewart from UCLA led this effort and is the first author of PEER Report 2013/13 that proposed the new factors.  This report titled “Semi-Empirical Nonlinear Site Amplification and its Application in NEHRP Site Factors” was just published by PEER on November 7th and is available for free download at the PEER website.
“This long-awaited change will allow engineers and earthquake financial loss modelers to have more accurate ground motion hazard estimates when using generic building code procedures or site-specific analyses,” said Professor Stewart.  “This will likely lead to more confidence in the levels of seismic risk considered in design and loss estimation practices. In some areas ground motions estimates will rise as a result of the change, in other cases they will fall. In either case, the ground motions that buildings are designed for will be more scientifically sound.” 
NGA-West2 has been financially supported by the California Earthquake Authority (CEA), California Department of Transportation (Caltrans) and Pacific Gas & Electric Company (PG&E).  Other products from this project, including final project reports describing the newly updated GMPEs and the database flatfiles, can be found at the NGA-West2 website.