Seismic Response Spectrum, Fa & Fv values and their effect

[RockDoktor]

Hi,

I'm producing a seismic hazard spectrum for a water tank and came across something that I'm having trouble figuring out. I originally ran a spectrum analysis with the USGS Earthquake Ground Motions Parameters program using a Site Class D and was asked to change the soils definition to Site Class E. The Site Class D spectrum has a higher seismic acceleration (Sa) than the Site Class E. After re-running the site using all of the definitions (A-E) I observed that the Sa values were highest to lowest in this order D, B/C (approx. the same), E, and finally A.

I thought that the Sa values would drop from A to E, but this wasn't the case. So, my question is... why? The Fa factor appears to be the dominant variable, but the effect of the site class seems slightly contradictory. I've tried to find literature on why this is, but to no avail.

Can anyone point me in the direction of a reasonably simplified description of how and why the Fa and Fv values were chosen for their site classes?

Thanks for any help!

 

[moe333]

Site Class E will have lower SA values at high acceleration levels because they are soft (weak). These soils cannot transmit high accelerations to the ground surface. Their low stiffness reduces the accelerations they can transmit. They will however have relatively large deformations.

Google NEHRP commentary to obtain a complete discussion.

 

[moe333]

You will also notice Site Class E will have large Fa and Fv at low accelerations. This accounts for the amplification that has been observed to occur mainly due to resonance between the period of the ground motion and the soft soil column.

 

[msucog]

also, the lower site class materials will transmit seismic energies farther due to the attenuation characteristics. this helps to explain why the WUS and CEUS must consider different radial distances when performing site specific seismic assessments.

here's some educational materials. see fema 451b

http://www.nibs.org/index.php/bssc/publications/2003/