deflection

[I-beam]

Looking for some feedback on what to use for deflection limitations for
roof framing subject to just snow and dead loads in areas with high snow loads, ie Pg = 60psf +
My question is specifically for the design of a LVL ridge beam on a low slope 5/12 roof, and the roof
dead load is fairly low, ie: 10 psf.
Thanks

 

[dhengr]

I-beam:
The question isn’t so much some exact code dictated deflection limit for this exact situation. Keep in mind that that ridge beam will settle (creep) a bit over time, in addition to load caused deflections. Study a cross sectional view of the bldg. Allow the ridge beam to deflect 1”, then calculate how much that movement causes the rafters to tend to push out at their lower bearing points, they don’t change in length. Can you live with the push-out at the top of the walls? Alternatively, this ridge beam movement induces a thrust which must be taken by ceiling joists, or some such, to prevent this lateral movement at the top of the wall.

 

[Tomfh]

As a reference, the ridge of our 50year old house is 12m (39 feet) long. It has settled 50mm (2 inches) at the middle, which is about L/240.

It’s too much, but it still looks ok, and the average person would never notice. If it went another 25mm (1 inch) it would look bad.

If I built a house I’d want to limit long term dead load deflection of ridge to about L/360. Less if you can see down the ridge.

Also check the wall spread, as noted above. You want to keep that within limits too, which will depend a lot on the type of walls, and architectural detailing. You don’t want your roof cracking the walls every time it snows.