Effective length of sign post

[handex]

We have been asked to check if two 460UB67s will work for supporting a 15m tall x 3.55m wide sign. We usually do big monopole signs which have circular posts, so the member capacity is equal to the section capacity.

On this one however Im not quite sure as to what to take as my effective length? There will be girts fixed off to each side of the posts continuously. Is there any guidance as to whether these girts can be used as restraints, so each seperate post is effectively restraining the other?

Bit of a grey area for me, so any guidance / references is appreciated

 

[csd72]

Handex,

How do the girts restrain the beam? What stops both beams buckling in the same direction?

 

[hokie66]

The girts would provide some restraint, but it is difficult to quantify. Sheeting on the girts could be considered as a diaphragm, but I would prefer to provide diagonal bracing.

 

[handex]

csd72,
The girts will be fixed off to the beams with purlin cleats and a single bolt. As for "What stops the beams buckling in both directions?", thats exactly what I am thinking too. It seems like in reality it will be better off than a unsupported 15m cantilever, but quite worse than being restrained at the girts.

Hokie,
Thanks for the response, but even if there is a diaphragm on each side of the beams, what stops them both theoretically buckling in the same direction?

 

[csd72]

The trick that I have used in the past is to put some diagonal bracing behind the sign panel thus forcing the beam buckling into a double curvature.

If you dont do something like this then I would say you have an unrestrained cantilever.

By the way, the purlins do actually help if they have two bolts as they provide torsional restraint but this is hard to quantify by some codes.

 

[hokie66]

Yes, I would say with two bolts to the purlins, the double diaphragm (one each side) would be able to take the weak axis load to the ground. With single bolts, no. Hard to justify, though, so I would put bracing on the centreline of the 460UB's.