Deflections in collar/rafter tie assemblies 3

[ClearCalcs]

Hello! We've had a question come up in our team and it's got us curious and looking to find out a bit more. What deflections would you typically consider in a roof assembly with rafter and/or collar ties? Some ideas here:
[ol a]
[li]Absolute deflection at ridge (for your L/ value, would you use L = total roof span?)[/li]
[li]Rafter deflection (would you use L/ based on total rafter span or based on span between wall/ridge & tie?)[/li]
[li]Deflection of roof eave (checked against 2x the L/ value since it's a cantilever)[/li]
[/ol]
I've added an image below for reference. Thanks so much!!
-Laurent

http://www.ClearCalcs.com

 

[phamENG]

Laurent -

a) This one is primarily important for ridge beams. In that case, it's compared to an L = span of the ridge beam. While it could be important for a traditional rafter framed roof, it's not practical to check it and won't be a problem in most cases. To calculate it with any degree of accuracy, you'd need to determine the horizontal stiffness of the wall at the top plate. Worst case scenario would be that it has zero stiffness. If this were the case, I'd probably go with the the same L/ridge span as I'd want to limit any apparent sag along that ridge line.

b) Rafter deflection is wall to ridge. That's the span of the rafter. For rafters at 16" on center, 2/3 of the rafters won't have a collar tie attached, so it's not a good idea to consider it as a support point.

c) Most roofs this is a non issue as the rafter span is so much longer than the eave (if the rafter deflection is okay, the eave will be, too). If you have an eave longer than 2', sure, check it, but there's not much of a serviceability concern there that I'm aware of.

 

[jayrod12]

I'll add to Pham's accurate response to a) by including, if there is in fact a rafter tie at the bottom, the deflection at the ridge would be fairly negligible and be based highly on the elongation of the rafter tie and its connections.

I agree with his answers fully to b) and c).

 

[phamENG]

Thanks, jayrod. The only potential serviceability concern for an eave I could think of was maybe ice damming? But if you don't think so, I'll go along with it. (It's 50[sup]o[/sup]F here today, as opposed to 22[sup]o[/sup]F in Winnipeg...)

 

[XR250]

If the there is a rafter tie at the bottom of the rafter, I don't check it as long as the rafter span meets code. (as Jayrod said, the ridge deflection would be negligible)

If the rafter tie is higher up, I typically check the deflection at the peak and use the plate to plate distance as the "L". (same as the truss guys do for scissors trusses).

The collar ties tend to minimize the gravity load deflection along the rafters by going into compression but this adds to the tension in the lower rafter tie (no free lunch).

 

[ClearCalcs]

phamEng, jayrod, XR250 - thank you all so much for the great responses. This clears up a lot! Really appreciate it

http://www.ClearCalcs.com