Max considered earthquake response acceleration to Design acceleratio

[dcceecy]

In codes, SDS=2/3*SMS.
SMS is from Fa*Ss, and Ss is from the map.
but where is the 2/3 from?

Does anybody know where is the 2/3 from?
I am not sure if the 2/3 is used for 500 year return period since the map is for 2,500 year return?
Thanks.

 

[larsacious]

The Sms and Sm1 values are the maximum considered earthquake motions that are possible at any given site and are an upperbound limit. Since all structures have an inherent seismic capacity which is not accounted for, it is believed that structures designed for 2/3 of the maximum accelerations will have a low likelihood of collapse and thus it is not necessary to design for the maximum possible seismic accelerations.

It essentially lets you take a 150% overstress in the applied seismic accelerations (2/3 = 100/150).

Hope this explains it!

 

[msucog]

search for "fema451b". search eng-tips site and i have posted the link many times. basically the concept is that it's doubling up the same factor of safety. others suggest that it's just a "touchy feely" jock factor.