huangyhg

seismic percentage
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.
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.
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.

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