Foundation Overturning Reduction in ASCE 7

[Hokie93]

For those who regularly use ASCE 7 in their practice, do you typically take advantage of the 25% reduction in foundation overturning permitted by ASCE 7-10 section 12.13.4? The same provision was included in earlier editions of ASCE 7 as well. My suspicion is that this provision is not widely employed in practice. As I read it, the provision applies strictly to overturning (not sliding) and only applies to foundation members. Of course, you have to meet the limitations set forth in ASCE 7 as well (use of the Equivalent Lateral Force procedure and the structure in question cannot be an inverted pendulum or cantilevered column structure). Since the vertical distribution of seismic forces in the Equivalent Lateral Force procedure is based on the first mode of vibration and that distribution overestimates the overturning force, it makes sense that a reduction in overturning is permitted.

 

[msquared48]

NO. Not comfortable with that provision.

Mike McCann, PE, SE (WA)

 

[JoshPlumSE]

An explanation from the 2009 NEHRP provisions:
C12.13.4 Reduction of Foundation Overturning. Since the vertical distribution of forces prescribed for use with the
equivalent lateral force procedure is intended to envelope story shears, overturning moments are exaggerated. (See
Section C12.13.3.) Such moments will be lower where multiple modes respond, so a 25 percent reduction is permitted for
design (strength and stability) of the foundation using this procedure. This reduction is not permitted for inverted pendulum
or cantilevered column type structures, which typically have a single mode of response.
Since the modal response spectrum analysis procedure more accurately reflects the actual distribution of shears and
overturning moments, the permitted reduction is only 10 percent.

 

[jittles]

I keep it in my back pocket and save it for special circumstances.

I have utilized it occasionally, but I can't think of a time where I have invoked it up-front in design.

 

[KootK]

I use it all the time. Firstly, it saves the owner a bit of money which is one of my mandates as a dutiful consultant. Secondly, I see no reason to arbitrarily increase the global safety for one particular element.

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.