ground response spectrum usage

[muuddfun]

What usage do you generally make of the ground response spectrums that are prepared by the geotechnical engineers?

Do you use the whole spectrum or just the Sa at the fundamental period?

Would different modes of the structural response contribute more or less response back to the base of a structure?
For example does only the first mode count the most, or do higher harmonics contribute more force to the base of a structure? We geotechs go to a lot of trouble to prepare these reports on ground response that are required by code, but I am wondering what actually is useful out of these?

This question is related somewhat to my other thread on bearing capacity, but I started a new one to specifically find out about the structural inputs.

http://www.eng-tips.com/viewthread.cfm?qid=295834&page=1

Thanks

 

[larsacious]

Essentially we only need the site class from the geotech. Typically, we determine the ground accelerations (Ss & S1) from the USGS website and by having that information in conjunction with the site class we can determine Sa on our own.

We determine Sa by finding the corresponding value on the design response spectrum where the fundamental period of our structure falls. Higher level modes are typically not considered in seismic design using ASCE.

 

[SrVaro]

The most important piece of information is the soil site class. Any other information (Ss, S1, etc) would just be used a double check as I also look up and calculate on my own. Usually we are pulling the data from the place so there is not a difference but since it is a critical part of the design load, I like to calculate myself.

 

[muuddfun]

For all projects we include the site class, S1, Ss, Sds, Sd1 in our reports. For many projects like those requiring review by DSA or OSHPOD we are required to generate a site specific spectrum analysis, and provide the full ground spectrum in the report for the structural engineers. Do you use any of this? It sounds like you do not. If you are not using it then why does the code require us to develop it (at significant expense)?

For sites classified as F where the structure is tall and has a fundamental period greater than 0.5 seconds, then a ground response is required per ASCE 21.1. Of that response report how much would you actually use in the analysis?

I am checking the foundation design of a 300 foot tall amusment park ride, and was trying to determine how the tower response from the ground motions supplied by the geotechnical of record were utilized by the tower structural engineer to determine a foundation loading for bearing capacity under liquefied conditions? I was not able to follow his calculations from the ground response, input to the structural response, then back to the foundation loading. It appeared that he may be using some static equivalent value, when is this appropriate and how is an equivalent static value derived from a ground response report?