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Roof rafter design

Roof rafter design

Roof rafter design

(OP)
In designing the roof rafters, the California Building Codes requires (using allowable/working stress design) the 8 load combinations. Usually we use the DL+LL. To include the wind load (DL+0.75LL+wind), does it apply vertical to the rafter? Is there any design example showing the design criteria.
DL+LL
DL+.75LL+wind

RE: Roof rafter design

Wind is always normal to the surface. So you dead and live loads would be vertical and the wind would be perpendicular to the rafter.

RE: Roof rafter design

The reason for the question, JAE, is that nothing is normal in California. Nothing.

Mike McCann
MMC Engineering

RE: Roof rafter design

(OP)
According to ASCE 7, figure 6-2, the wind can also be in the direction of the DL and LL. See the attached figure for loading E & G. Therefore, we need to use vertical load case DL+0.75LL+wind acting on the rafter. Same what we do for the DL+LL case.

RE: Roof rafter design

m1208. Figure 6-2 is for the MWFRS wind. Not the components and cladding wind. You started your post with the question about designing a single rafter. That is a small area (for a rafter) and thus would be a component and cladding design. If you look at Figure 6-3 (which is C&C wind) you will see that the wind is normal to the rafter.

The MWFRS wind is for the overall building design (i.e. lateral force brace systems) and would not be used for a single rafter.

RE: Roof rafter design

(OP)
For design of the rafter, The DL+LL is perpendicular to the rafter (using the projected horizontal length of the rafter and increasing the DL to account for the roof slope; DL /cosign of roof slope). I think it would be conservative to add the wind load of figure 6-3 table (with coefficients) , directly to the DL+LL. For example if my roof DL=18, LL=12, Wind=12, then my
DL+0.75LL+Wind = 12+.75*12+12 = 33.

RE: Roof rafter design

(OP)
Jae:
Do you know of a design example showing the roof wood rafter design for DL+LL+WIND using the California Building Codes 2013 (using allowable/working stress design)? Does the above total load of 33 lb/sq ft load is reasonable or over killed?
Thanks

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