Glulam Cable Truss

[MIStructE_IRE]

Hi,

An architect I’m working with is looking to span a sports hall roof with large double Glulam beams with a Macalloy tension member below, creating a very simple truss. There will be a single vertical element at midspan very similar to the attached photo (only I don’t have the glulam arch above).

He doesn’t want anything tying the bottom chords of the truss together and I have no uplift case here as the roof is more than heavy enough.

My question is, since I can’t tie the bottom chords together, meaning that I can’t tie the midspan struts together (much like the photo) should the strut be designed as a mini cantilever with fixity to the top glulam chords? I have a feeling it may try to whip laterally otherwise..

Any thoughts?

 

[Agent666]

Yes, the vertical strut would need some restraint, cantilevering it seems like the logical answer. You'd just need to be careful about the fixity at the top, would you have members perpendicular at top chord to take out the twist of the Glulam rafter on which the fixity relies?

 

[MIStructE_IRE]

Thanks! Yes, I will have them anyway to fully restrain this chord. I’m thinking of the strut being a tapered cruciform. The crucifix representative of what I’m doing to myself on these jobs!!

 

[Agent666]

Just be aware that cruciforms have another torsional/twisting buckling mode under axial loads that might need to be investigated. Here's some background information on this aspect.

 

[MIStructE_IRE]

Great resource! It will likely end yp as a flanged cruciform. At 8m overhead the flanges won’t even be noticeable!

 

[JAE]

MIStructE IRE:
Check out this thread - thread507-124363


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[MIStructE_IRE]

Thanks JAE. That’s very useful.