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
RE: Structural Steel Ridge Beam Deflection
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
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
RE: Structural Steel Ridge Beam Deflection
RE: Structural Steel Ridge Beam Deflection
RE: Structural Steel Ridge Beam Deflection
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
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
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
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
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
RE: Structural Steel Ridge Beam Deflection
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
=> 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
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.