LRFD Load Combinations for roof

[Busybardo]

I am confused as to which load combination to use for roof loads. I hear people say to use the max value from 1-7 of LRFD load combinations. Then I also hear that (D+L) will govern. Here's my example.

I have 20psf dead load, 20 psf live load, 10psf snow load, and a negative wind pressure of 11.2psf.

So my thought is that load combination 2 (LRFD) will govern;
=1.2(20psf dead) +1.6(20psf live)+ 0.5(10 psf snow) = 61 psf.

So this is the value i am using to calculate for my linear load on joist, etc.

I am wrong here?

 

[steellion]

The code considers "Roof live load" and "live load" to be two different things. You would not apply Roof Live Load and Snow Load simultaneously, the ASCE load combinations reflect this. The logic is that if it's snowing, you won't also have roof occupants creating live load at the same time.

 

[frv]

Depends..

LRFD are strength-level combinations.. You only use these to determine whether the member will fail, using LRFD provisions

There are also ASD combinations which are based on nominal loads. These may be used to determine strength if you are using ASD provisions, and must be used to check serviceability (deflection).

If you were told that serviceability criteria will govern, you are likely using wood, which is frequently limited by serviceability considerations.

On another note, there is a difference between Live Load (L) and Roof Live Load (Lr). The LRFD combination you show above is not appropriate (it is overly conservative).

I suggest that if this is more than an academic exercise, you consult with a Structural Engineer.. There are other issues to consider, such as Lateral Torsional Buckling and, if using wood, load duration factors.

 

[rscassar]

For a roof, I would be considering the load combinations:

1. 1.2*D+0.5*Lr+0.5*S
2. 1.2*D+1.6*Lr ... Governs
3. 1.2*D+1.6*S

 

[frv]

Neither the IBC nor ASCE 7 require consideration of simultaneous Snow and Roof Live Load.