Seismic percentage

[FSS]

I have been doing some wind/seismic calculations for relatively light structures. The wind forces are much higher than the seismic forces in this case due to site locations.

When using the equivalent lateral force procedure I come up with basically a percentage of the weight to use as my base shear. In the high-seismic areas, how high can this percentage get? Just wondering what would be the all-out worst case? 50%? I deal with seismic on such an infrequent basis that I don't have a good feel for the numbers.

 

[RockEngineer]

It really varies depending on your seismic response factor R which is determined by your material and type of construction. From ASCE 7-05 Table 12-14.1 you can have R values ranging from 1.5 for plain concrete to 8 for buckling restrained braced frames. Normal light framed wood construction is 6.5.

In Washington State which is moderately high seismic the percentage of horizontal load to weight for a wood framed structure usually runs below 16%. My understanding is when you get into the range of horizontal acceleration greater than about 25% the equations aren't realistic.

 

[JStephen]

Referring to ASCE 7-05-
Pg. 220 shows areas with Ss=300% = 3.0
Then Fa=1.0 maximum, Sms=3.0, SDS=2.0
In Table 12.2-1, R can be as low as 1.0.
I, importance factor, can be 1.5.
Then Cs = SDS/(R/I)= 3
And V = Cs * W = 300% of W in 12.8.

That's not to say you'd actually see this- you'd try to use a structure type with higher R, for example. And the jobs I've seen where seismic loading was really high also had site-specific response spectra, so you weren't working off of standard code provisions.