Live Load Reduction per area

[AELLC]

Case 1: Suppose we have a beam with tributary area of 300 sq. ft. and roof live load can be reduced to 0.9 x L (10% reduction).

Case 2: Now suppose we add a second (continuous), equal span so that we have 2 spans of 300 sq. ft. tributary area each for a total of 600 sq. ft.

Now, it is clear that the beams in both cases should be designed with roof live load reduced 10%, but what about the design loads for the center supporting post?

Should the live load acting on the post be reduced for:

A) Trib area = 600 sq. ft.

B) Trib area = 300 x 1.13 = 338 sq. ft. (based on the actual reaction at the center support, i.e., the wl x 1.125 reaction at the center support of a 2-span continuous beam equally loaded both spans)

 

[msquared48]

Yes, the post can be reduced based on the sum of the areas it sees, but remember that only half the area of each beam is supported by the column.

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com

 

[BAretired]

Well, I don't know which code you are using, but roof live load is usually snow load. In the NBC, snow load cannot be reduced on the basis of area supported.

BA

 

[AELLC]

BA, I use IBC...it says any construction (1.25) or snow (1.15) between 12 and 20 psf can be reduced for area - I am in Arizona it is 20 psf construction here.

 

[AELLC]

ooops -

Snow load S is not allowed to be reduced for area in the IBC. That makes sense. I never did reduce snow load...it has been a few years since I had a snow load project.

Actually, my OP asks the same question regarding reducible floor loads.

 

[msquared48]

It's true that snow cannot be reduced. I missed the "roof" part of your post. Sorry 'bout dat.

In lower floors, though, it can be reduced.

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com

 

[asixth]

That area reduction is for roofs which only support the nominal roof loading. If it is acting like a floor than the more area needs to be accumulated before a reduction can occur. Normally I would not consider area reduction for the spanning member (not 300sq.ft anyway). Only really consider area reduction for sizing members that support lots of area such as columns and footings in multistory buildings.