Rafter design

[m1208]

For roof rafter design (sloped roof):
1. LL is perpendicular to horizontal surface and we use the actual value from the code and design is based on the horizontal length of the beam (not the inclined length).
2. DL is perpendicular to the horizontal surface. We increase the load (DL / cosine) to compensate the sloped length. Then we use the horizontal length of the beam for the design.
3. Wind load (for the components) is perpendicular to the sloped surface of the rafter. We use the actual value and design is based on the horizontal length of the beam. The same concept as the DL.

We were questioned about this method. Is there any thing wrong with this method?

 

[AELLC]

Nothing wrong with your approach. If anyone has trouble visualizing all this, tell them to imagine what goes on as you theoretically increase the rafter slope up to the point where it is vertical as a column.

 

[AELLC]

Sorry, let me suggest using true length of rafter as its span for calculation with bending and deflection due to wind. I wasn't thinking about wind initially because we don't include wind component for design of rafters in our area here.

 

[JAE]

Your item 3 is incorrect (as AELLC corrected their own post) You should use the sloped span length for the wind load and combine shear/moment results from items 1 and 2 as you have them.

 

[msquared48]

I agree with JAE here.

Mike McCann
MMC Engineering