Reduction in roof live loads 1

[PostFrameSE]

A co-worker and I were having a discussion on reductions in roof live loads and the appropriate tributary areas to use. If I have 8' o.c. trusses on a 60' wide building, the tributary area for the truss is 480sf. For live load, using 480sf, I calculate a reduction factor R1 = .72. That is very simple. However, when I look at the reaction that the truss places on an isolated column, I tend to think that I must use a live load reduction of .96 since my tributary area is now 240sf. Is that correct?

Assuming that is correct, how then do I look at the reactions from the truss on a girder that is 30' long? Is the tributary area that I use to determine my R1 for the concentrated loads at 8' o.c. on the girder equal to 1/2 the building width multiplied by:

1. Girder length of 30'?
2. Actual tributary width of roof carried by the girder?
3. Truss spacing of 8'?
4. Other??

It makes a huge difference how you look at it, and I'd appreciate some perspectives on this.

Thanks.

 

[WillisV]

You should use the tributary area of the member being designed to calculate live load reductions for the design of that particular member, not the tributary area of each member delivering the load to that member (so every beam, truss, column, etc has its on live load reduction factor for design, you don't reduce the live load on a beam and then carry that reduced live load to the girder and then the column and so forth - you run the full unreduced live load through to the foundation, then reduce the forces for each member based on that members tributary or effective area).

So the answer to your question is 2.

 

[msquared48]

I agree with Willis...

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[BAretired]

In the NBC, neither snow load nor minimum roof live load can be reduced based on tributary area. For floor members, I agree with WillisV.

BA

 

[ToadJones]

Willis is correct IMO.

 

[PostFrameSE]

Thank you all for your thoughts. I appreciate the advice!!