The 10/50 Seismic Acceleration 1

[steellion]

I'm working on a project in California in a high-seismic region. The geotechnical engineer recommended use of Ss and S1 based on a 10% chance of exceedance in a 50-year period, instead of the normal standard of 2% for seismic and wind. The 10/50 seismic accelerations are about half of the 2/50.

When is it acceptable to use a 10% exceedance probability? This is an Occupancy Category II, two-story building.

 

[OHIOMatt]

As far as I know, never for the building that you describe. 2% in 50 years equates to a once in 2500 years event. 10% in 50 years equates to a once in 500 years event. 10% in 50 may be acceptable for a temporary structure, but I would have to do some research first.

 

[MarkAJohn]

I have heard of reduced seismic for retrofit of some existing buildings. Check International Existing Building Code (hope I got that right). But I don't think it was as much as 50% off.

So, why would the Geotech suggest doing that? I wonder if the Geotech would be willing to stamp the structural calcs.

MJ

 

[cvg]

Geotech may not know the relevant code.
10/50 is the data which was originally published by the USGS in 1991. It is used by various federal agencies for design, although not necessarily for buildings. See the link.

http://pubs.usgs.gov/of/1995/596/appxC.html

the USGS hazards map site also lists 10/50, 5/50 and 2/50 maps all updated in 2008.

http://geohazards.usgs.gov/hazards/apps/cmaps/

 

[shin25]

Make sure that performance based seismic design is not a requirement for the project. This requires certain performance of a structure to be maintained based on a given recurrence interval of a seismic event.

 

[muuddfun]

Ask the goetech what he is getting at. It may be that he is giving you something for Ss and S1 that he recommends you use instead of SDs and SD1. If that is the case then they may be higher or more conservative values than what you would get if you use 2 in 50 for Ss and S1 then reduce them by 1/3 to get Sds and SD1. The arbitrary reduction factor for Sds and Sd1 were put into the code to mimic the 10/50 values under the old way of doing things for sites like california. If we all used 2 in 50 straigt up in california the only thing you would be able to build is bunkers.

 

[steellion]

I figured it out. The geotech was giving Sds and Sd1 values based on 2%/50yr criteria; it was simply mislabeled as Ss and S1. So it looks like 10/50 should not be used unless the project specifically requires it. Thank you to everyone for your responses!