1/3 soil bearing increase vs 25% overturning reduction for seismic loads

[AaronMcD]

The CBC (mostly IBC) allowable soil pressures may be increased by 1/3 for seismic loads that use the alternative load combinations, and geotech engineers typically allow a similar increase in their reports.

ASCE 7 12.13.4 allows a 25% reduction to overturning loads, and CBC does not allow this when using alternative basic load combinations.

Thus ASD would be 0.6D + 0.75(0.7E) and alternative would be 0.9D + E/1.4 with 1/3 increase in allowable soil pressure. Would you assume that the 25% reduction in overturning loads should not be used with the 1/3 increase from virtually every geotech report? Or is it acceptable to use the increased soil capacity with ASD load combinations and the 25% reduction in overturning forces? Maybe a question for geotech engineers but maybe more of a code-intent question.

http://www.hellodwell.design

 

[HTURKAK]

Pls look ASCE 7-16 C12.13.6 Allowable Stress Design for Foundation Geotechnical Capacity.

[quote... In traditional geotechnical engineering practice, foundation design is based on allowable stresses, with allowable foundation load capacities, Qas, for dead plus live loads based on limiting static settlements, which provides a large factor of safety against exceeding ultimate capacities. In this practice, allowable soil stresses for dead plus live loads often are increased arbitrarily by one-third for load combinations that include wind or seismic forces. That approach may be both more conservative....][/quote]

 

[r13]

Correct. First - don't mix codes and mixing factors; second, if you follow IBC, apply the 25% reduction on overturning stability check only, and use 33% overstress allowance on bearing check as called out by your geotechnical report.

 

[Deker]

We use the overturning decrease and bearing increase simultaneously where applicable. The two are really trying to capture different things. Usually the 1/3 increase for soil bearing is permitted when the allowable soil bearing pressure is controlled by long term settlement. Since seismic / wind loads are transient, there's no risk of long term settlement when the bearing pressure is exceeded (ignoring liquefaction concerns).

The 25% reduction on overturning applies to structures designed to ELF because you are designing as if 100% of the building mass responds in the first mode of vibration, and the code recognizes that this can be overly conservative for overturning. Note that the reduction doesn't apply to cantilever columns or inverted pendulums, and the reduction for buildings designed to MRSA is capped at 10%. Although not explicitly prohibited by code, a logical extension of the reasoning above should preclude one story structures from using the reduction.

So for your situation, your options would be to use the basic load combinations with the reduction (if applicable) OR the alternative basic load combinations without the reduction. The 1/3 increase in allowable bearing pressure is permitted in either case.

 

[r13]

Traditionally, the safety factor for over turning is 1.5, but 1.0 for seismic loading condition. The 25% reduction results in a sfaety factor of 1.5*0.75 = 1.125 > 1.0.

 

[sandman21]

CBC 1605.3.2 ".... Where using these alternative load combinations to evaluate sliding, over-turning and soil bearing at the soil-structure interface, the reduction of foundation overturning from section 12.13.4 in ASCE 7 shall not be used." Bold and underline added by me.