In roof framing, how does a ridge beam prevent lateral pushout of home walls? 10

[CivilSigma]

I have been researching site-built roof structures with ridge beams such as the following:

Typically, you see collar ties or ceiling joists to help tie the truss and prevent lateral movement. How would a ridge beam prevent lateral movement in this case?

Thank you.

 

[dik]

jayrod... They were U straps to act as joist hangers. I should have made it more clear...

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?
-Dik

 

[jayrod12]

Thanks for the clarification dik,

However, could you confirm whether that's with a ridge beam, or a ridge board? With just a ridge board, that is inadequately sized to span the entire roof ridge, I fail to see how that limits the thrust.

With a ridge beam, there would be no thrust regardless as long as the connection between rafter and ridge beam loads the beam vertically. Your straps would work for that, but wouldn't it be easier to just provide joist hangers?

 

[Ron]

MIStructE IRE said it nicely. Mitigate deflection at the top of the rafters and you mitigate lateral thrust. That's what a ridge beam does

 

[jimstructures]

George,

In the picture you seem to have one wooden beam at the top of the structural ridge, one wooden beam in the middle of the structural ridge and one steel beam at the bottom of the structural ridge. Are those beams attached to each other so there is no slip between the individual beams as they deflect with load from the roofs applied by the rafters? Fastening the respective beams together will increase the total stiffness of the structural ridge beam (i.e. preventing any slip). The entire ridge beam looks like overkill to beam, but I don't know the respective stiffnesses.

Jim

 

[GeorgeTheCivilEngineer]

HI Jim,

The beams are all tied together. It's a long story but in essence, the steel beam was supposed to be set lower than the rafters and it ended up being between 50-100mm too low. So the timber is essentially packing and the steel is the ridge beam, but is all nailed and bolted to the steel so it's additional stiffness. There are plenty of things in the house where I go 'ooph, wish I hadn't done it that way', but fortunately the vaulted ceiling and slightly-lower-than-necessary beam is not one of them!

Edit - plus the end user/ultimate client/the wife is happy with it.

 

[CivilSigma]

@Retired13

Exactly, the ties would be laoded in tension and prevent the rfaters from "falling" down. Also, in terms of modelling, two rafters without a tie is an unstable truss system

 

[r13]

Yes. I believe you remember that triangle is the most stable shape. For pitched roof, the triangle is formed by the rafters and a tie. Without the tie, the apex needs to be held in position by something, and the ridge beam is just that something. Don't forget that the stiffness of the ridge beam has paramount importance, on top of adequate strength to support the rafters, it needs to be check against deflection criteria to avoid sagging, and displacement associate problems. Glad you got this matter cleared up.

 

[CivilSigma]

Thank you all very much !

 

[sdz]

As the ridge beam deflects, which it will, there will be some push out at the walls. So long as it is well designed this should be within acceptable limits.

 

[BAretired]

If the ridge beam sags Δmax at midspan, each wall moves out a maximum of (h/l)*Δmax, assuming that Δmax is small compared to h and l.

For a maximum ridge beam deflection of 1", and roof slope of 4/12, each wall moves out approximately 0.333".

BA

 

[r13]

In the old house, the gaps are easily covered by decorative trims. How do you accommodate the uneven gaps in ceiling of a modern straight edge house? For an event occurs only every time after a big snow storm. The gaps might, or might not, be closing up after the event. I guess you have to rely on the stiffness of the edge beam then.

 

[Ron247]

Place your elbows on a table and touch your fingertips of each hand together. There are 3 ways your fingertips can lower. 1. Elbows move apart thus allowing your fingertips to drop (no ceiling joist). 2. Fingertips drop while unconnected elbows separate (no ridge beam and no ceiling joists) 3. Elbows lower due to poor vertical supports. This is not relevant to this thread.

A ridge beam (not a ridge board) keeps scenarios 1 and 2 from occurring.

Reality is that a ridge beam installed rather than a ridge board still has some downward movement. Scenario #2, Ridge beam keeps finger tips from dropping other than the unavoidable beam sag. Due to beam sag, there is still some separation of your elbows. A stiffer ridge beam means less elbow separation.