ASCE 7-05: Dead Load Reduction 0.6 + 0.7E or W

[gvot]

What is the basis of chapter 2.4.1 reduction of dead load to 0.6? Some theories are to provide SF against theoretical DL to actual as-built...also to insure overturning SF of 1.5 is met. However, would this combination be needed in a case where DL is well defined and overturning is taken by tenion piling? The piling will have its own SF on failure so is the 1.5 from this load case in addition, therefore, the SF on the piling with 0.6D reduction could be reduced?

 

[JAE]

No - you have to use the load combination.

If you have a tension element that resists overturning, then well and good. However, this load combination may control the tension in that element.

Yes the 0.6 is a measure of safety against instability that would occur when the stability is based upon some dead load helping.... i.e. 1/1.5 is approximately 0.6.

 

[WillisV]

gvot - I have had the same issue:

A real world example where the length of soil anchors required to resist uplift forces on a spread footing was to be determined by a geotechnical engineer. These soil anchors were to be extended below the footing to form a "breakout" cone of soil whose dead weight was adequate to resist the applied uplift. The service dead load on the footing was 7,000kips. The service wind uplift on this same footing was 6,500 kips, so in reality there was no net uplift. But, using 0.6D+W we provided the goetechnical engineer with a net uplift load of 2,300 kips. The geotechnical engineer then insisted on using a safety factor of 3.0 for the soil uplift requirements, and therefore provided anchors long enough to mobilize 6,900 kips of soil. The 6,900 kip resistance was now greater than the original wind uplift not including dead load at all - on a footing that had zero nominal uplift!?!?!?

I think that the above is ridiculous, and I think that the S.F. on the soil anchors in this case and the piling in your case could be reduced somewhat based on engineering judgement when using the 0.6D load combination to recognize the safety factor built into that equation, but that is just my opinion...

 

[JAE]

WillisV - agree that with the 0.6D AND a SF of 3 by the geotech is overkill.

 

[wannabeSE]

I don't have an opinion on the safety factor used by geotechs. However, this situation is similar to anchor bolt (except the safety factor for typical bolts is 2). The commentary in ASCE 7-05 states: "This eliminates an inconsistency in the treatment of counteracting loads in allowable stress design and strength design. . ." The strength design combinations with 0.9 D seem reasonable. So, the corresponding ASD combinations also seems reasonable to me.

 

[DST148]

@WillisV - Your post ..... "We provided geo with a net uplift of 2300 kips. Geo provided soil anchors to mobilize 6900 kips of soil not including dead load at all..."
ASCE 7-05, ASD Combo 0.6D + 1.0W addresses stability. For stability checks against sliding, overturning, and uplift a FOS of about 1.67 has always been there. Although geo is counting on the weight of soil to counteract uplift, he chose to use 0.33 (1/3) factor based on his design philosophy. Thus we have 0.6D (structural) + 0.33D (geo) + 1.0W.
Given division of design scope, do you think the outcome would have been different had you provided the info in the following format?
Dead load (including overburden on the footing?) = 7000 kips (service)
Wind uplift = 6500 kips (service)
A minimum FOS of 1.67 shall be provided against uplift.