SEISMIC: USGS Unified Hazard Tool Parameters vs. Design Code Seismic Design Parameters

[NZtoUSA]

I'm curious why the unified hazard tool PGAs I'm getting are 0.05g smaller than those from AASHTO or my state seismic tool. There really is no good explanation on the USGS, except to say that they are different so you shouldn't use the UHT... Anyone have an idea why? Are they disregarding some models for the design spec for some reason?

from the "Unified Hazard Tool" page:
"Please do not use this tool to obtain ground motion parameter values for the design code reference documents covered by the U.S. Seismic Design Maps web tools (e.g., the International Building Code and the ASCE 7 or 41 Standard). The values returned by the two applications are not identical."

from the "USGS design ground motions" page
"The USGS collaborates with organizations that develop building codes (for buildings, bridges, and other structures) to make seismic design parameter values available to engineers. The design code developers first decide how USGS earthquake hazard information should be applied in design practice. Then, the USGS calculates values of seismic design parameters based on USGS hazard values and in accordance with design code procedures."

 

[BridgeSmith]

The various specs are calibrated differently, with different load factors, and the responses of buildings and bridges, for instance, are substantially different. I suspect the vibration period of most bridges is typically much shorter than it would be for most buildings, also.

 

[BridgeSmith]

Unfortunately for those of us who use AASHTO, the USGS hasn't gotten around to adding the AASHTO spec to the UHT, and they removed the previous tool that we had been using, so now we're back to squinting at the maps in the spec, trying to figure out where the heck our bridge site is on the 3 different crappy maps, and then trying to interpolate between the contours. What used to take 30 seconds has become an annoying process that takes an hour or more.

 

[NZtoUSA]

The UHT has an option to change the period, so I wouldnt have thought the difference came from there. Regardless, being geotech I'm usually only looking for PGAs. Like you say, different load factors, or maybe general conservative(ness?) during model development might be the reason. Perhaps with other types of hazard modeling its necessary to be more conservative.