Seismic Drift Question

[Lion06]

I haven't done a lot of high seismic design, so I have a few questions for the folks who deal with it more often. This is all based on ASCE 7-05.

1. In 12.3.4.2, is the 35% reference based on the applied story force or the story shear?

2. If rho=1.33, then the seismic force is amplified by this factor. 12.12.1.1 then requires that in addition to amplifying the seismic load by rho, that the allowable seismic drift be reduced by rho. This appears to be double-dipping on the rho factor. Is that the intent?

 

[jdgengineer]

1) It's my understanding that this would be story shear not story force. Therefore, top levels of a multi-story building may not need to be included in the check, but the lower levels would.

2) When checking drift the rho factor = 1.0 (12.3.4.1). So it's not double-counting it i's just making you reduce the allowable drift by rho. Therefore, discouraging moment frame buildings with only a few lines of resistance (i.e. typical pre-northridge type building)

 

[jdgengineer]

1) It's my understanding that this would be story shear not story force. Therefore, top levels of a multi-story building may not need to be included in the check, but the lower levels would.

2) When checking drift the rho factor = 1.0 (12.3.4.1). So it's not double-counting it i's just making you reduce the allowable drift by rho. Therefore, discouraging moment frame buildings with only a few lines of resistance (i.e. typical pre-northridge type building)

 

[jdgengineer]

Here is an article by S.K. Ghosh which goes into a little more detail about 12.12.1.1.

http://www.gostructural.com/magazin...ismic_design_categories_d_through_f-7919.html

 

[Lion06]

jd - Thanks for the clarification and article.