Distinction between ridge beam and ridge board 2

[Buleeek]

Hello everyone,

I have the following problem. I'm designing a Tiki Bar roof. The architect sent me a plan (see attached) where there is a ridge "board", rafters (slope +-4/12) and hip beams. The hip beams are supported by 4x4 posts. Technically, a ridge board will be attached to the 4x4 posts as well.

Now, since the slope of the rafters is more than 3/12 (according to IBC) the ridge beam can be considered as a "board" (not beam) and rafters to only have a vertical force at the bottom- no vertical (very minimal) force on the ridge. That way, the vertical force transferred by the ridge on the posts will (should be) minimal.

My question is how do I distinct where the ridge will be considered a beam and when as a board in this situation? Would it help if didn't attach the ridge board to the posts? If someone with engineering experience and practice could explain that to me I would really appreciate it. I tried to find some information/articles about it, but no luck so far.

Rafters will most likely be nailed on top (no hangers).

Thank you for your help.

 

[EngineeringEric]

A board requires the the rafters to support the uniform load as well as the opposing rafter. basically they are in bending and axial. this axial component results in thrust-out at the base. the thrust needs to be resisted by a collar tie (typically at the bottom of the rafter and can be the ceiling joist). There is no gravity load in the board, so it isn't called a beam, therefore no end reaction. Still give it posts, but there is no load. boards also doesn't need to be continuous where a beam does.

If there is no way to resist the thrust, or the rafters aren't symmetrical, or don't have the capacity, then a ridge beam is required. This beam is supporting the ridge side of the rafter; and therefore has an end reaction.

i wrote this with numerous simplifications, but it hopefully helps make the difference clearer?

 

[WARose]

Basically, once the rafters get to a slope that is very low....it is no longer a situation where you are just transferring shear or axial at the apex where the rafters meet.....you are then developing (significant) bending forces that have to be transferred (as well as shear) through the ridge beam. Technically, you can still have a ridge "board' even at a shallow rafter angle (if you have no posts supporting the ridge "board").....but trying to transfer those (bending) forces can be tough.

At a high slope, the rafters (tied together at the apex via ridge beam/board) have very arch like behavior......and the force transfer isn't that bad. Furthermore any post support for a ridge "board" will likely not see much load anyway.

 

[oldinspector]

You do not mention ceiling joists. How are the outside walls prevented from "kicking out?" If no ceiling joists, you definitely need a structural ridge.

 

[JoelTXCive]

Attached is a good article from the Journal of Light Construction. It talks about ridge boards versus beams, and ceiling joists + rafter ties.

 

[Buleeek]

Can a collar tie resist a thrust completely? I'm assuming no ceiling joists, only the ties (at 1/3H from the top).

 

[jayrod12]

The collar ties cannot generally resist the thrust at all. They are typically there to keep the top of the rafters tight together. The moments induced in the rafters by having them that high up is quite large.

 

[oldinspector]

A nice post, JOELTX. I also did one for JLC on the same topic in 2018. If I can dig it out, I'll add it.

 

[XR250]

Can a collar tie resist a thrust completely? I'm assuming no ceiling joists, only the ties (at 1/3H from the top).

There was a point when the IRC allowed collar ties ALL the way at the top of the rafter with a factor of 2.0 for the span multiplier. What a disaster that was. Who makes these decisions?
I believe now you are limited to 1/3 up from the bottom.
I generally check it in a 2D frame program in lieu of the code.

 

[phamENG]

For the sake of clarity, nomenclature is important. Looks like a few people are mixing things up.

Rafter Tie: resists thrust at or near the base of the rafter. Must be located in the lower third of the roof height.
Collar Tie: resists wind uplift at the ridge. Must be located in the highest third of the roof height.

See IRC R802.3.1 if you don't believe me.