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Reducing Seicmic Over-turning moment by 25% for deign of foundation

Reducing Seicmic Over-turning moment by 25% for deign of foundation

Reducing Seicmic Over-turning moment by 25% for deign of foundation

(OP)
Hi,

Has anyone used Section 12.13.4 of ASCE for designing foundation with reduction for foundation overturning? I am designing foundation for overturning with combination 0.6D+0.7E with a factor of safety equal to 1.0. I would like to make use of provision 12.13.4 for the design. Any suggestions? I am using ELF Method for seismic analysis.

Can also the shear be reduced by 25%?

Thanks

RE: Reducing Seicmic Over-turning moment by 25% for deign of foundation

"Basic allowable strength design load combinations (IBC 2012 §1605.3.1) are adopted from ASCE 7, with the high overturning load combination defined as 0.6D + 0.7E. The alternative basic allowable strength design load combinations (IBC 2012 §1605.3.2) are legacy load combinations from the Uniform Building Code, with the high overturning load combination defined as 0.9D+E/1.4. A major difference between these load combinations for the design of a mat foundation is the reduction of seismic overturning effects at the soil-foundation interface as permitted by ASCE 7-10 §12.13.4. This reduction of seismic overturning (10 % or 25 % reduction depending on the analysis type) can be applied only if the basic allowable strength design load combinations is used, not when the alternative basic allowable strength design load combinations."

Taken from another source, but essentially yes you can use the reduction as long as the appropriate load combination is utilized is my understanding.

RE: Reducing Seicmic Over-turning moment by 25% for deign of foundation

Here is some commentary concerning reducing the overturning with equivalent lateral force

Quote (Commentary to FEMA 450)

5.2.5
In the design of the foundation, the overturning moment calculated at the foundation-soil interface may be reduced to 75 percent of the calculated value using Eq. 5.2-14. This is appropriate because a slight uplifting of one edge of the foundation during vibration leads to reduction in the overturning moment and because such behavior does not normally cause structural distress.

Quote (FEMA P-750)

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.

RE: Reducing Seicmic Over-turning moment by 25% for deign of foundation

Provided you satisfy the limitations, I would not hesitate to use the provision. I have applied it to both overturning moment and shear. The NEHRP Commentary quoted by wannabeSE provides the rationale behind the provision in case you think it is sketchy or questionable. The ASCE 7-10 Commentary provides no insight as I recall.

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