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1/3 stress increase applicability

1/3 stress increase applicability

(OP)
A provided geotech report states that "Bearing capacities may be increased by 1/3 for transient loads, such as wind loads." (Transient defined as temporary loads of limited duration). ASCE7-05 states that "Increases in allowable stresses shall not be used with load combinations given in this standard unless it can be demonstrated that such an increase is justified by structural behavior caused by rate or duration of load." Commentary (p255) from same code says that LRFD load combinations use factors less than unity applied to VARIABLE loads. Commentary (p257) says ASD applies 0.75 factor to variable loads. Then states "Some material design standards that permit one-third increase in allowable stress for certain load combinations have justified increase by same concept. Where that is the case, simultaneous use of 1/3 increase and 25% reduction in combined loads is not permitted." "In contrast, allowable stress increases that are based upon duration of load or load rating effects, which are independent concepts, may be combined with the reduction factor for combining multiple variable loads. In such cases the increase is applied to the total stress; that is, the stress resulting from the combination of all loads."

Is there an official "material design standard" for geotechnical that permits the 1/3 increase in stress available for "duration of load or load rating effect"? Do stress levels resulting from ASD formulae need to be treated differently from those found thru LRFD with regard to the 1/3 increase in bearing capacity? Only in some cases? Wind and Seismic are variable AND of limited duration. So which rules apply? Is the added capacity only applicable to loads resulting from wind and seismic, or to the total from worst case load combo? Is this confusing?

Thanks in advance for your help.

RE: 1/3 stress increase applicability

The geotech report would be wrong if it conflicts with the underlying governing code and its referenced standards.

Under the IBC, Section 1804 requires you to use either of the two types of ASD load combinations.  One type allows the 1/3 stress increase and the other doesn't.

RE: 1/3 stress increase applicability

Remember that soils don't settle or "deflect" instantaneously.  Even sandy soils take some finite amount of time to settle.  Given that our typical transient loads last a few seconds or a few minutes, and settlement often governs your bearing capacity, it is reasonable to allow an increase for short term loading.   

Of course, it is up to you to decide when and if it should be used.  I tend not to.

RE: 1/3 stress increase applicability

If a geotech report has such a statement, I DO tend to use the increase in combination with the building code load combinations with the (maybe misguided) thinking that it falls under the statement "...allowable stress increases that are based upon duration of load or load rating effects, which are independent concepts, may be combined with the reduction factor for combining multiple variable loads. In such cases the increase is applied to the total stress; that is, the stress resulting from the combination of all loads" for the same reasons that UcfSE mentions.

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