Dead Load Reduction for Uplift Resistance 1

[dlamoure]

I am checking the design of a temporary roof structure and am having trouble getting the design to perform against uplift. I have problems in both the capacity of my connections as well as simply not having enough dead load to resist the applied uplift forces.

I believe I will be able to make the numbers work by going to a shorter MRI and possibly a lower importance factor based on the fact that this is a short term installation (covered deck for a construction office trailer), however I get completely blown out of the water when I consider the load combination of 0.6D + W.

My question is this: Is it necessary to apply the 0.6 factor to my dead load when my only DL is the self weight of the structure? I can understand taking a reduction in DL if I had a higher degree of uncertainty in my loads (such as superimposed dead loads) but in this case I have a high level of confidence in my dead load as I am only considering the self weight of the actual structure. Is there any case when it would not be necessary to consider the 0.6 reduction factor?

 

[JAE]

No - the 0.6 factor is not ONLY to account for uncertainty in the amount of dead load, but also to provide an inherent factor of safety against uplift and overturning.

 

[civilperson]

No need for a Factor of Safety when using 0.6DL. 1.0 is sufficient, resistance/applied load.

 

[WillisV]

See my first post: thread507-202887

 

[dlamoure]

Thanks all, that is kind of what I was thinking as well, and I appreciate the lesson in the legacy codes, it's a little easier to swallow knowing the how the load combinations were developed.

I agree with your comment WillisV on the unintended consequences of this load combination, seems that it is appropriate to use 0.6D + W when considering overall stability of the structure but for individual members/connections this would introduce an over-conservative design by effectively doubling up on the factor of safety as the various material specifications already include a F.S.

To further illustrate this, consider section 2.4.4 of ASCE 7-95 that WillisV refers to - this section required a 1.5 F.S. (2/3 DL) for checking structures for sliding or overturning unless the structure was anchored to resist excess movement. Presumably this exception considered that the anchor would be designed with a factor of safety and that it was unnecessary to double up.

 

[JAE]

I think ASCE 7-95 and its associated 1.5 overturning safety factor was out before the 0.6D+W combination occurred in the codes. The 1.5 SF was replaced by the 0.6D+W combination.