Glulam bracing
Glulam bracing
(OP)
Would you consider 2x12@16" roof joists flush framed with "U" style hangers into both faces of a 5.125x28.5 cantilevered roof glulam as providing bracing against compression face buckling?
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS Come Join Us!Are you an
Engineering professional? Join Eng-Tips Forums!
*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail. Posting GuidelinesJobs |
|
RE: Glulam bracing
RE: Glulam bracing
RE: Glulam bracing
RE: Glulam bracing
RE: Glulam bracing
I have a problem with that last statement "A cantilever is nearly always restrained by the support it crosses over."
I realize that you said "nearly", but with the width of the glulam at 5.125" and the depth at 28.5", that's over a 5"1 depth to width ratio. As this ratio increases, the lateral contribution from the column for restraint of the bottom flange decreases. Indirectly, the AITC recognizes this fact by limiting the compression stress in areas of higher D/t ratios. Here, I would install a kicker on each side of the column, at least, regardless of the stress seen.
Mike McCann
MMC Engineering
RE: Glulam bracing
Mike McCann
MMC Engineering
RE: Glulam bracing
Agreed, the d/b is a concern, and in fact would violate the Canadian code for using straight L as Le... But in this case we also have purlins that are abutting the face (unless I misunderstand, always a possibility!) and as such I do not believe the compression face is going to be able to move latterally, IF given a good load path.
Sorry for the run-on sentence; Hopefully my meaning is clear.
RE: Glulam bracing
I would agree with you, but you are talking about the tension face of a cantilever with the joist serving as support, not the compression face which is at the top of the supporting column.
Mike McCann
MMC Engineering
RE: Glulam bracing
The top of 2x12s is flush with the top of glulam for its full length. The column (colium or columb per some other post) cap at the cantilever is 14" high and there is no additional column bracing. I suppose that some kind of lateral restraint is provided by the column/cap against the buckling of the the bottom/compression face of the glulam at the column, but I would think it to be minimal, almost impossible to calculate, and even if I concocted some kind of calculation I probably wouldn't feel comfortable with it. I think it is a hinge. I am specifying an added brace for the top of the column.
Regarding the contribution of the 2x12s at 16" o.c. to the restraint against buckling of the bottom face of the glulam where it is in compression, whereas a full depth 28.5" deep joist at 16" o.c. would plainly provide adequate bracing, and a 3.5" deep joist would plainly not provide adequate bracing, and there must be some intermediate depth somewhat less than 28.5" that we might consider as providing adequate bracing, I do not think 11.25" is that depth, and quite likely not even close to that depth. How's that for a running on sentence!! I'm going to specify additional compression (bottom) face bracing. But I would like to know how to determine that "intermediate depth" that would provide bracing.
Thanks!
RE: Glulam bracing
Mike is right; It is not a great plan to extend this to a cantilever. The risk far outweighs the benefit, however in the case of the joists framing into your compression flange and "only" being over half the depth of your beam, the beam is very effectively braced and your effective length should be computed based on that bracing being present.
RE: Glulam bracing
RE: Glulam bracing
The greatest trick that bond stress ever pulled was convincing the world it didn't exist.