Load Combination Question/Poll
Load Combination Question/Poll
(OP)
A question for the structural engineering community: The ASCE/IBC load combinations include a combination under the ASD Basic Load Combinations for D+(W or E)+L+(Lr or S or R). When two or more transient loads are combined in this section a 0.75 factor on the live loads may be used. Now here is the question, how many engineers use this load combination for C&C design of say a rafter or bar joist?
Here is an example that would be typical for my part of the country. Assume a roof snow load of 20 psf (unreducible) and the wind load minimum inward pressure of 10 psf.
S = 20 psf
W = 10 psf
0.75(20 psf + 10 psf) = 22.5 psf > 20 psf for snow
However, I don't know personally of any engineers who would design for 22.5 psf in this case as nobody combines the roof live load with wind.
The typical interpretation I hear is that this load combination is intended for design of elements such as columns where axial load results from the lateral loads.
Do you agree or disagree???
Here is an example that would be typical for my part of the country. Assume a roof snow load of 20 psf (unreducible) and the wind load minimum inward pressure of 10 psf.
S = 20 psf
W = 10 psf
0.75(20 psf + 10 psf) = 22.5 psf > 20 psf for snow
However, I don't know personally of any engineers who would design for 22.5 psf in this case as nobody combines the roof live load with wind.
The typical interpretation I hear is that this load combination is intended for design of elements such as columns where axial load results from the lateral loads.
Do you agree or disagree???






RE: Load Combination Question/Poll
RE: Load Combination Question/Poll
DaveAtkins
RE: Load Combination Question/Poll
RE: Load Combination Question/Poll
woodengineer
RE: Load Combination Question/Poll
RE: Load Combination Question/Poll
RE: Load Combination Question/Poll
Yes, that is what I meant. In the example I used in the orginal post, I combined the 10 psf positive wind pressure with the assumed 20 psf roof snow. Thanks for clarifying that.
RE: Load Combination Question/Poll
Here is where engineering judgment comes in--when COMBINING wind load with live load, I think you could use 3/4 of the 5.92 psf plus 3/4 of the 20 psf, which is 19.4 psf. And so 20 psf live load would still control. I think wind acting ALONE with dead load would need to be the 10 psf minimum, but then that wouldn't control either.
I suppose if you have a higher eave height, Exposure C, or Iw > 1.0, live load plus wind load would control.
BUT--isn't the 20 psf an ARBITRARY minimum roof live load? If you are in a five psf snow load area, for example, could you combine 3/4 of the snow load based on five psf with 3/4 of the 5.92 psf wind load, and THEN compare that to 20 psf?
Just some food for thought.
DaveAtkins
RE: Load Combination Question/Poll
DaveAtkins is correct in that for areas of the country with small roof snow loads this particular load combination would not control. It would also not control for large snow loads as the 25% reduction would more than account for the (net) 7.5 psf wind.
I've always said that the day I stop learning will be the day I die.
RE: Load Combination Question/Poll