Foundations : One-third Increase

[slickdeals]

For a building situated in Seismic Design Category D, the geotech is permitting a one-third stress increase for seismic loads. ASCE 7 allows for foundation moments to be reduced by 25% when using ELF procedure.

Can the one-third increase and 25% reduction be used concurrently when using the Basic Load combinations? It appears that Section 1605.3.1.1 prohibits stress increases when using the Basic Load combinations.

The alternate load combinations allows for stress increase but doesn't allow for reduction in overturning moments.

 

[delagina]

The soil report I often get for soil bearing uses F.S. of 3.0 (1350psf) for sustained, 2.0 (2000psf) for total, 1.5 (2700psf) for transient. I just make engineering judgement which to use.
And yes, it also is confusing with 1/3 increase or no increase from ibc or asce.

 

[slickdeals]

ASCE 7 seems to allow for stress increases when it is due to rate or duration of load. This would seem to apply for seismic loads. Also, the 25% reduction in overturning moments with ELF accounts for the inherent conservatism in the calculation of these moments.

I would think that these could be used concurrently.

IBC really muddies the discussion with the alternative basic load combinations and including a statement that allowable stress increase is not permitted with the basic load combinations.

 

[TDIengineer]

I use both the 1/3 increase and the reduction in overturning.

I have found the 0.6D + 0.7E combo to be very challenging for high seismic load areas.

The CBC has an alternate load combination that I really like to use that has a 0.9D + E/1.4. You don't get the reduction in overturning moment with this combo, but when you're increasing your footing size just for stability, the 0.9D gives you more than 0.6D but a reduced overturning... for most cases in my experience.

Not really your question but somewhat part of your discussion.