timber roof - buckling of main glulam beams 1

[mats12]

Timber roof - main frames (arranged at 6,5 m) - glulam beams b/h = 200/1000 mm supported on RC walls.
On top of glulam beams are timber beams b/h = 100/240 mm arranged at 1m.
Can this beams be considered/act as lateral supports for glulam beam buckling verification?

Thank you for help.

 

[JStructsteel]

Can you calculate your buckling force and check them for the load?

 

[SandwichEngine]

It looks like your smaller beams will be at the top of the main glulam beams. They would be good at bracing the top of the glulam beam for LTB when they are in compression. However, they would not brace the bottom of the glulam beams for LTB when wind suction controls. In the PEMB industry, we provide flange braces to the bottom of the main frames to brace them for this scenario.

 

[ChorasDen]

Agree with JStruct, first step would be to figure out what the buckling force is. After that is determined, how are you resolving the load, will you take the load up through the beams into your roof diaphragm, or how will this load be resolved at beam ends?

SandwichEngine, kinda tough to tell from his sketches, but appears that it may be a compression style system using glulams, since OP also shows steel tie rods. If so, the glulams are acting more as axially loaded members than bending members, so it might not be that wind uplift is causing significant load reversals in the glulams, however, I still agree, if these members are thought of as axial loaded, OP should consider bracing both the top and bottom edges for buckling.

 

[KootK]

The graphic shows the purlins coming into the side of the glulam but I'm assuming that we can take OP at his word that they actually come in on top. That being the case:

1) The purlins will laterally brace the glulam top face if, and only if, the purlins themselves are adequately restrained against axial translation. Usually this restraint is got by attaching the purlins to a competent diaphragm or by introducing plan bracing into the roof.

2) The purlins will not provide torsional restraint to the glulams and therefore, by extension, will also not provide lateral restraint to the bottom face of the glulams. As Sandwich mentioned, the usual way to address this is by introducing fly bracing. Without fly bracing, you may struggle to deal with net uplift loads or the axial loads that will develop in the glulams as a result of their truss action.