Design Spectral Acceleration Parameter 2

[movasra]

Hi everybody,
I am trying to find an accurate value for SDs, Design spectral acceleration, for the zip code 90745, I got 1.443 using Staad-Foundation and by assuming R=2 and I=1.25, I get such a big value for Cs=0.9 which doesn't make sence to me. Could anybody give me an acuurate value for this zip? where am I wrong? I use Cs=Sds/(R/I) and Sds=2/3Sms and I have Site calss D and Fa=1. I am using ASCE 7-05.
Thanks Guys

 

[moe333]

You can download free the USGS calculator to calculate these parameters.

I get SDs=1.038 for that zip code with a site class D using the 2006 IBC

 

[movasra]

Thank you so much Moe333 for the link. But the problem is that I dont know which values are more reliable, I got 2 significant values from 2 different design softwares and the other one from USGS. and Its too hard for me to find an acurate contour from the ground motion figure, at least for this latitude and longitude. But thanks again anyway.

 

[moe333]

I would obtain a relatively accurate lat.-long. from Google Earth and then use the USGS software for the parameters. I would generally consider the USGS values as more reliable than commercial software.

 

[msucog]

http://earthquake.usgs.gov/research/hazmaps/design/index.php

based on the value, i guess you're in a high seismic area. after looking up the zip code, it looks like you're in california which is likely considered a high seismic area. i'm not familiar with staad but the difference could be reference to different maps.

example
2006IBC SDs=1.038 SD1=0.615 for site class D
2003IBC SDs=1.262 SD1=0.742 site class D

here is my estimate of what happened but you should confirm it if necessary. the link above takes you to the calculator for the building code seismic parameters (or others depending on what you need). the ibc maps are based on the usgs maps. these maps are updated every few years and the values have tended to go down the past few revisions where i live(the next revision looks like it might go up for some high seismic areas). the biggest difference is that the building code puts an upper bound in some high seismic areas through a different sort of analysis to remove huge "spikes" over short distances. the usgs maps are not created with these upper limitations however the ibc maps are adjusted to include these upper limits. so i think your staad program may be set up to reference the actual usgs maps and not the building code maps (i did a quick check and came up with 1.42 so that looks about right). you should obtain an accurate lat/lon (from google, terraserver.com, etc)or you could see dramatic differences especially for where you live and use the calculator for ibc2006 or asce7(if that's what you need).

see also this thread where the maps are discussed a little

http://www.eng-tips.com/viewthread.cfm?qid=198387&page=1

hope this helps. if someone else happens to see another explanation (or sees where i may be wrong), please post.