Seismic Reductions and Increases

[chamokinawan]

ASCE 7-10 12.13.4 allows a reduction of seismic loads (demand) at soil interactions (i.e. foundation) by 25%. Our california building code allows an increase in soil capacity by 1/3 for seismic loads. And lastly, we are allowed a 1.1 safety factor instead of 1.5 in retaining walls when we use seismic loads. Are all of these factors applicable simultaneously?

My understanding was that the 25% reduction in demand (3/4 of demand) was the same as the inverse (4/3 Capacity), hence, the 1/3 increase in capacity. But I was told that they are two different things and I can use both at the same time.

 

[wannabeSE]

The way I interpret ASCE 7-10 §12.13.4, the 25% reduction in force is for overturning (foundation uplift) not bearing pressure. I would consult with the project's geotech concerning the 1/3 increase in soil capacity. Frequently, the bearing pressure is governed by settlement and the 1/3 increase assumes the foundation won't have significant settlement from the short term seismic loads.

 

[chamokinawan]

@wannabeSE: So you wouldn't decrease your overturning force (checking down push) and increase your soil capacity by 1/3 at the same time? The 25% reduction when checking uplift makes more sense to me. I'm a bit skeptical on double dipping when checking down push/bearing. Thanks.