Calculating Av Coefficient from Av

[luke648]

This is probably a pretty fundamental question, but it is not one I have had to address before.

I'm a geotechnical engineer working on a project for which the structural engineer has requested an effective peak velocity-related coefficient (Av). The project is located near Birmingham, Alabama and is being designed per the 1999 Standard Building Code. The code contains contour maps of the coefficient; however, the seismic hazard information has been recently updated, and the only information I seem to be able to find is the probablistic ground motion (%g). Can I use this information to determine the coefficient Av?

Thanks.

 

[JAE]

You should never mix coefficients between codes. With data updated, the new data will be used within a "system" in a newer code.

Use the 1999 SBC coefficient becuase the structural engineer will be (should be) using that same code as required by the governing authority.

 

[luke648]

JAE,

I appreciate your timely response, sorry mine is not as timely! You are absolutely right, the structural should be using the same code (one code) as required by the governing body. The structural did not pursue using new factors with the old code; however, we found that the following opinion from a credible source was helpful in understanding the relationship between the new and the old:

"The old Peak Velocity-Related Acceleration coefficient is very similar to the current S1 coefficient except that the return period is different. The old Av represented the 500-year mean return period, spectral response acceleration on a soft rock site for a structure having a 1 second period. The S1 coefficient is the same thing, but for a 2,500-year mean return period. For most sites, S1 will be significantly larger, because it represents a less likely earthquake.

In the newer versions of the building code (IBC-2000, IBC-2003, NFPA-5000-03), S1 is modified by the old Cv coefficient to obtain SM1. Then 2/3 of this value (SD1) is taken. This is used as the old Av value is used in the determining base shear forces.

If you want to stay with the 500-year ground motion, you can obtain this for a site by visiting the USGS web site. At that site, go to earthquake hazards. click on the map of the US and this will take you to a page where you can enter a zip code or a latitude and longitude and get the value of S1 for a 500 year earthquake. You can then use this just like Av."

Thanks again.