Dead Load Reduction for Uplift Resistance
Dead Load Reduction for Uplift Resistance
(OP)
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?
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?





RE: Dead Load Reduction for Uplift Resistance
RE: Dead Load Reduction for Uplift Resistance
RE: Dead Load Reduction for Uplift Resistance
RE: Dead Load Reduction for Uplift Resistance
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.
RE: Dead Load Reduction for Uplift Resistance