Glulam bracing

[Triangled]

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?

 

[CELinOttawa]

As long as they have a decent load path (ie: will resist the imparted load), yes.

 

[jdgengineer]

I would consider the joists bracing the top flange. You mention the glulam is cantilevered which means there is an inflection point where the bottom of the beam will be in compression. I would not consider these joists effective in bracing the bottom flange.

 

[CELinOttawa]

The bottom flange is braced if the section is more than 50% boxed in, which is the case for this example, so long as there is an effective load path. I cannot remember the source of this, but there is an excellent paper from HERA on a more in-depth version with purlins or girts and steel beams or columns.

 

[CELinOttawa]

Oh, and remember that the MOST important part of all of this is that the compressive flange or extreme fibre can be effectively restrained against lateral movement. It does not matter how you achieve this, but rather that you do. A cantilever is nearly always restrained by the support it crosses over.

 

[msquared48]

CEL:

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

 

[msquared48]

I meant a 5:1 depth to width ratio. Sorry for the typo.

Mike McCann
MMC Engineering

 

[CELinOttawa]

Hi Mike,

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.

 

[msquared48]

The use of run-on sentences is one way to get exercise.

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

 

[Triangled]

Thank you all!

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!

 

[CELinOttawa]

Half the depth of the beam is sufficient, so long as the load path is reasonably stiff... See this paper for a detailed discussion.

http://folk.ntnu.no/bell/Projects/Stability.pdf

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.

 

[Triangled]

Thanks for the paper! On a totally different subject, the pics of the presenters remind me of a show I've been starting to watch... Lillehammer.

 

[KootK]

It`s worth remembering that, for a cantilever, it is tension flange bracing that is most effective for LTB control. Of course, you still need to provide rotational restraint at the support as well.

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.