Bowstring stability?

[MIStructE_IRE]

Hi,

I guess I’m thinking about this incorrectly. But looking at the tension chord in the roof support member below, why does it not want to ‘guitar string’ itself and flip - if that makes sense?

To justify this I’m assuming the verticals need to be cantilevered down from the horizontal tubular compression member. This induces torsion into the tubular horizontal member but the end connections are not torsionally stiff by the looks of it.

 

[human909]

It seems that the CHS member is lateral and twist restrained by he roof it is supporting. Also I'd expect the ends to have sufficient torsional restraint. This is just a LTB scenario with a twist.

 

[MIStructE_IRE]

I think you’re right, must be restrained from twisting by the roof members above.

I’m guessing wind uplift is resisted by the CHS member alone.

 

[KootK]

I don't dispute that, with the right detailing, the skylight mullions can brace the truss against LTB and probably do. That said, I agree with human909 that the CHS end connections (shaped w/pin) should also be able to provide torsional restraint. And I'd expect that to be the designers original intent rather than relying on the skylight mullions. That, becaue:

1) The mullions are somewhat less permanent feeling.

2) The mullions are often a delegated engineering item and, thus, less under the EOR's control.

3) The aluminum mullions may have slip joint detailing. The aluminum will be exposed to the elements and has a relatively high coefficient of thermal expansion.