Compression Flange Buckling
Compression Flange Buckling
(OP)
I'm investigating the integrity of a steel building that is already built. The ability for the building to resist the uplift forces from strong winds on the leeward side has been called into question. It is a large a-frame building that has glulams at 4' OC for framing members. The glulams are attached to the steel beam by means of a stiffener plate.
Q1: Can the stiffener plate acts as compression flange bracing in reverse loading? The beam is a W33 and the plate extends down 2'5 5/8" from the top flange. The plate is 3/8"x9 1/2"x2-5" and is welded to the web of the beam.
I'm really tight on time, we need to make a decision as to how we will move forward in the next few days and my boss wants my input on this by Monday. Thanks for your help.
Q1: Can the stiffener plate acts as compression flange bracing in reverse loading? The beam is a W33 and the plate extends down 2'5 5/8" from the top flange. The plate is 3/8"x9 1/2"x2-5" and is welded to the web of the beam.
I'm really tight on time, we need to make a decision as to how we will move forward in the next few days and my boss wants my input on this by Monday. Thanks for your help.






RE: Compression Flange Buckling
A detail would be nice, and how the compression force is handled once in the glulam will also be of interest.
An expert is a man who has made all the mistakes which can be made in a very narrow field
RE: Compression Flange Buckling
The glulams are supporting the plywood roof, they span 28'. They are spaced at 4' OC going up the steel, the steel is on a 12/12 pitch.
When the leeward side of the structure experiences uplift, it puts the bottom of the W33 in compression. I don't know how to figure out how much the 3/8" plate that stiffens the glulam connection acts as compression flange bracing to reduce my unbraced length.
RE: Compression Flange Buckling
My gut reaction is to say that it is not braced for uplift at the glulams. If that were a WF framing into a stiffener of shown height My gut would say yes. The problem with the detail as it stands is that the three bolt connection into the wood likely doesn't have the moment capacity or ratational stiffness to be considered a brace point per AISC -especially since they're so closely spaced. I know it seems counter-intuitive, but the closer the brace points are the stronger and stiffer they need to be to be considered brace points. The reason is as the get closer together it is trying to force the beam to buckle in a higher mode, which takes more force to brace it.
Can you add a bent plate from the bottom flange of the WF to the underside of the glulam at maybe a 2H to 1V? Without running any numbers that seems to feel a lot better. You'd still have to do the strengh and stifness checks, but at least it would be axial strength and stiffness and not rotational. It's tough to get moment connections in wood and the biggest reason is that the rotational stiffness is difficult to achieve so even is the moment capacity is adequate there is so much rotation that the connection never sees the moment.
RE: Compression Flange Buckling
RE: Compression Flange Buckling
@Hokie66 - I like your idea about lagging through the seat of the glulam connection. I am leary of considering the bolted connection a moment connection because I know that the bolts can shift in the oversized holes slightly before taking any moment resistance. If I lag the underside, I will not have any shifting. The alternative perspective to that is that the plywood sheathing will act as the tensile reinforcing for the moment transfer across the top flange of the beam if the beam goes into uplift. You just have the glulam compressing against the web of the beam and the tension taken in the plywood.
It still comes down to how much the stiffeners can transfer to the glulam.
Thanks for all your input
RE: Compression Flange Buckling
Calculate the compression in the bottom flange due to wind uplift and ensure that the brace can provide 2% of that force laterally.
BA
RE: Compression Flange Buckling
You might be leary of counting on this, but it is there.
RE: Compression Flange Buckling
BA
RE: Compression Flange Buckling
RE: Compression Flange Buckling
The glulam connection is identical with or without the sheathing. The glulams are free to rotate with the sheathing there as the wood be without the sheathing there.
No force will be transferred through the sheathing across the beam.
If the glulam experiences uplift, the sheathing will not see any force and will not add to the global stiffness of all members involved.
Having built some similar structures with my hands and not just on paper, I'd be very surprised if there isn't some good ole' PL 400 subfloor adhesive between the steel beam and sheathing....just a hunch.
RE: Compression Flange Buckling
RE: Compression Flange Buckling
I don't agree with SEIT on the rational that if you have more braces these must be stronger. I would think you would be able to spread the load over a few connections.
An expert is a man who has made all the mistakes which can be made in a very narrow field
RE: Compression Flange Buckling
Often your replies go only towards furthering your engineering rectitude and do nothing for the furthering of the topic and their condescending tone is a bit off-putting.
....Just a peanut-gallery observation.
RE: Compression Flange Buckling
RE: Compression Flange Buckling
BA
RE: Compression Flange Buckling
I do not believe it can provide reliable bending resistance required to stabilize the bottom flange of the steel beam.
BA
RE: Compression Flange Buckling
RE: Compression Flange Buckling
I know it's counter-intuitive, but the closer spacing of braces forces the beam to buckle in a higher mode. This requires each brace to take more force. Check out the equations in App. 6. Part of the reason is that, typically speaking, more braces means greater beam capacity which means greater brace force.
RE: Compression Flange Buckling
It may work that way in the US, but not in Australia. Sometimes I wonder about the way things are going in my home country...
RE: Compression Flange Buckling
How much shrinkage are glulams subject to compared to dimensional lumber?
Surely glulams are much more dimensionally stable.
RE: Compression Flange Buckling
Glulam shrinks much less than dimension lumber, it is true, but it is still a consideration. The following link provides some guidance on hanger connections and the expected results of incorrect details.
http://
BA
RE: Compression Flange Buckling
There is a gap between the decking and the top flange of the beam, it was to avoid interference between the bolted splice connection and the diaphragm, not for shrinkage of the glulams.
RE: Compression Flange Buckling
What about continuous bracing such as a steel deck welded to the flange of a beam? The spacing is usually about 12" between spot welds.
If the beam flange in the current thread required a spacing of 4' in order to sustain its compressive stress due to wind uplift, then I agree with you that the bracing requirement would be substantial. If the beam flange requires a spacing of, say 12' in order to safely sustain the compressive stress, the bracing requirement should not be based on a 4' wavelength (my opinion).
I don't have App. 6 but I expect it is similar to the Canadian Code.
BA
RE: Compression Flange Buckling
My apologies, I downloaded the spec and the distance between braces does not enter into the strength equations for the braces, but it is in the required stiffness for the braces. If you cut the brace points in half, then the required brace stiffness doubles.
So even though it's not for the brace strength, the distance between brace points is an important consideration as it will affect the required brace stiffness significantly.
BA-
I agree that you should only brace it as needed. If you only need it braced at 12' o.c. there is no reason to brace it at 4'.
RE: Compression Flange Buckling
RE: Compression Flange Buckling
On that basis, the bracing force needed in this case is 0.05*C*sp/L where C is the compressive force, sp is the spacing and L is the span.
BA
RE: Compression Flange Buckling
Dik
RE: Compression Flange Buckling