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Structural Steel Ridge Beam Deflection

Structural Steel Ridge Beam Deflection

Structural Steel Ridge Beam Deflection

(OP)
Working on a church project - open sanctuary will have a steel ridge beam supporting wood rafters. The ridge spans nearly 75 feet. The question came up about deflection. I had detailed the beam to have a camber in it to counter the dead load deflection. My concern is at the supporting walls. I am thinking that the rafters will rotate down with the beam as it deflects. But I am concerned that they may instead push on the tops of the walls causing the walls to spread. Thoughts?

RE: Structural Steel Ridge Beam Deflection

what's the difference in actual length vs. developed length?

RE: Structural Steel Ridge Beam Deflection

It is likely that the walls will spread slightly, but it takes a lot of reflection for this to be a problem. You should try to make the walls able to take the lateral load in any case... How else do you ensure they are stable?

Please post a sketch of your planned steel layout. That will give us all a good idea of what is going on.

We need to know:

- How is the steel ridge beam supported?
- What braces the ridge laterally?
- What bracing, if any, are you providing to the roof? If diaphragm action, you need to ensure the contractor uses a great deal of quality bracing during construction.

There are more thoughts, but this will help us all start helping you figure this out.

RE: Structural Steel Ridge Beam Deflection

The walls will spread, but I have never seen this to be an issue. They will spread the most at their most flexible point - the middle.
All exterior walls spread to some degree at the bottom chord of a roof truss or a ceiling joist as they elongate under tension. It is just significantly more when using a structural ridge in a vaulted ceiling. You can figure out the spread based on the beam deflection and geometry. The roof diaphragm will prevent the roof from spreading as much as you calculate under live loads.

RE: Structural Steel Ridge Beam Deflection

even with a camber in the ridge bm, the top of the walls will experience the affects of the deflection of the bm....one would assume that using a deeper ridge bm or better yet a truss would be more effective for the ridge, that it a substantial span...what deflection are you getting ?.....what type of walls?

RE: Structural Steel Ridge Beam Deflection

(OP)
Wood walls - 2x6 at 16" oc. The beam deflects 1.8" under dead load. The ridge beam is supported on each end by tube columns. The diaphragm is a wood panel diaphragm (OSB - 15/32" Structural 1). The bottom portion of wall is rock veneer, but the top is siding. There is enough gap between the rock and wall to prevent contact between the two. Detail of rafter connection attached.

RE: Structural Steel Ridge Beam Deflection

(OP)
Also, we are actually considering a truss as well. The architect has asked me to come up with an approximate depth. Is there a pre-engineered product to handle this, or would it be best to do a custom design? I couldn't find anything that seemed to apply online. Seems like a lot of fabrication effort - which is why I didn't go that route to begin with.

RE: Structural Steel Ridge Beam Deflection

Generally speaking a ridge beam prevents any significant lateral thrust from the rafters because the rafters are hung from the ridge beam; that is what the ridge beam is there for. In this case, however, with a span of 75', then yes, I think the deflection from the ridge beam will be enough that there will be more appreciable lateral thrust from the rafters near the center of the beam. I don't think an upward camber will do much to alleviate that as you'll still have the same amount of total deflection; the difference is that you'll start from a higher position.

I think what might help is a very stiff ridge beam and whether that is practical only you would know. My $.02 worth.

RE: Structural Steel Ridge Beam Deflection

as above, it may not be a concern, but if it is,
with about 1 truck of concrete, and a tension column at midspan, and a kind of reverse jacking operation, I suppose you could pre-deflect your beam to its dead load deflection, then install your complete roof dead load and remove the column.

regarding "Is there a pre-engineered product to handle this" the DLH and SLH series steel joists should easily work if your architect can work the appearance into his overall scheme. an advantage of the truss, of course, is that it will be considerable stiffer.

i am supposing a relatively small tributary width as you mention wood roof rafters.

RE: Structural Steel Ridge Beam Deflection

(OP)
Good thoughts by all. I decided to model this situation - complete with wood structural diaphragm. My model seems to indicate that the ridge beam is not needed at all. I think this is because the diaphragm is acting like a deep beam and actually supporting the ridge. I am also getting virtually no deflection at the wall and barely any in the ridge. Not comfortable taking it out, but also having trouble justifying a beam that size.

I like the idea of the long span joists, but the bridging won't work with the architectural design.

RE: Structural Steel Ridge Beam Deflection

With the span in question I would expect the roof system to have to transfer significant lateral wind load to the endwalls/bracing @ each end of the 75' span.This would raise the following questions which the OP may have already addressed.
1. What is the integrity of the OSB when used as a diaphragm for thses magnitude of loads and span.
2. When the roof develops a leak how does this affect the durability of the OSB.
3. What type of conn between the wall and roof and also what is the pitch of the roof.
Also something does not look correct with the results of your model and I would definitely be unconfortable taking the ridge bm out.

RE: Structural Steel Ridge Beam Deflection

@bpstruct
Without the ridge, you can count on your walls spreading. What are you using for diaphragm chords?
I have seen this tried numerous times and it will not perform as intended.

RE: Structural Steel Ridge Beam Deflection

Kingnero, what do you mean?

RE: Structural Steel Ridge Beam Deflection

(OP)
Definitely not taking the ridge out. The model doesn't seem right to me either, but it is not an overly complex model. At the risk of sounding like a narcissist, I don't see where I could have made a modeling mistake.

Pitch is 10:12, the osb is good for the loads, and the chords are double 2x6 wall plates - which do work. However, the diaphragm deflection is high too. I may add a steel flitch plate between the two 2x6 wall plates - although I've never done that before.

RE: Structural Steel Ridge Beam Deflection

Difference between horizontal projection of the beam, and the actual beam length.
=> by how much would the walls spread apart when the beam deflects?

If the beam deflects , say, 2", the walls can't spread much more than 1/8 of an inch (wild guess)? This depends on the pre-camber that the beam is given.
If he is talking about a few hundreths of an inch, there is nothing to worry about.

RE: Structural Steel Ridge Beam Deflection

@BPstruct:

I think you are overly counting on the transfer of the diaphragm shear to the chords without alot of movement in the process. You will have to add blocking between the rafters to make the transfer. Lots of possibilities for long-term movement, nail slip there.
No offense, but you are really wasting your time here other than performing an academic exercise. Much easier to put in a properly sized ridge and be done with it.
Even if the walls spread 1" under full design loads, so what. It is not enough to crack finishes.

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