Compression Flange Buckling

[jdm86]

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.

 

[ToadJones]

Doubt it was built with a gap...if the sheathing happened to splice there, then it would be "bouncy". A good builder would not do this IMO.
How much shrinkage are glulams subject to compared to dimensional lumber?
Surely glulams are much more dimensionally stable.

 

[BAretired]

Toad,

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://www.anthonyforest.com/pdfs/APA/Glulam_Connection_Details.pdf

BA

 

[jdm86]

There are no splices at the W33, the detail calls for a minimum 6" lap onto the glulam on the other side of the W33 so that loads will be continuous over the beam.
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.

 

[BAretired]

SEIT,

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

 

[Lion06]

All,

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'.

 

[hokie66]

Maybe not, but if braces are required at 12', and you provide them at 4', there is no way you have made the situation worse, which I think is what you are saying.

 

[BAretired]

Article 9.2.7 of S16-01 states that when bracing of a compression flange is by a slab or deck, the bracing force required is 0.05 times the maximum force in the flange and is distributed uniformly along the compression flange.

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

 

[dik]

You might want to reduce the number of bolts and have a single bolt near the bottom... Glulam suffers from dimensional shrinkage in the depth.

Dik

 

[hokie66]

That might be an issue, dik, but if you reduce the number of bolts, it reduces the bracing for the steel beam. One way of dealing with the splitting issue is to use a through vertical bolt rather than the lag screw I suggested. Provides a clamp to prevent splits from opening.