Does lateral restraint attached to only web prevent Lateral Torsional Buckling 1

[abhussain]

Hi guys,

We make our roof structure for our circular tanks out of variously sized purlins. We have one end of 2 main purlins attached to a centre bracket and the other ends bolted to the tank wall. Now we have secondary smaller purlins regularly spaced attached to the main and the tank wall. Then we have bridging purlins regularly spaced and attached to the second. and the tank wall.

Now my question is the bridging purlins are bolted to the secondary (inside web) via plates bolted to the web (for both). Given that the spacing is within limits, would that prevent lateral torsional buckling, even though the top (and bottom) flange is unrestrained?

Thanx.

A.

 

[KootK]

I'd still like to know what kind of roof diaphragm we're dealing with here.

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

 

[abhussain]

Hi

Could you elaborate a bit more what you mean by diaphragm.

I am a mech. eng. so am not familiar with all the structural terms.

A.

 

[Ingenuity]

In reply they are C25024 purlins cold formed.

abhussain,

1. Are these C25024 secondary purlins part of a proprietary roofing system like LYSAGHT purlins incorporating HOOK-LOK II bridging, or STRATCO purlins with RAPIDFIX bridging etc?

2. Are the C25024 secondary purlins framed into the primary/main purlins in approximately the same plane (web to web) or seated on the top flange of the main purlins?

3. Where is the roof cladding (aka diaphragm) connected to - direct screw attachment to top flange of C25024 secondary purlins?

 

[hokie66]

Don't call roof cladding, as opposed to roof deck, a diaphragm. It is not considered to be a diaphragm, at least in the global sense. It does brace the top of the purlins.

Purlin systems like the Lysaght and Stratco ones which Ingenuity referenced are proprietary systems, including the bridging, and the capacities are well recognized and based more on testing than on calculation. These purlin systems are basically commodities, and the manufacturer's details need to be followed.

 

[Ingenuity]

Don't call roof cladding, as opposed to roof deck, a diaphragm. It is not considered to be a diaphragm, at least in the global sense. It does brace the top of the purlins.

I meant to place inverted commas/quotes around "diaphragm". I was trying to give the OP a loose terminology cross-link that KookK referred.

But totally, agree - roof cladding is NOT a diaphragm, in that it provides lateral restraint to the top flange of the purlin to which it is fastened.

 

[abhussain]

1. Are these C25024 secondary purlins part of a proprietary roofing system like LYSAGHT purlins incorporating HOOK-LOK II bridging, or STRATCO purlins with RAPIDFIX bridging etc?

Similar, but not the same. Our bridging is similar to that shown in their manual, except we donot use tie-rods

2. Are the C25024 secondary purlins framed into the primary/main purlins in approximately the same plane (web to web) or seated on the top flange of the main purlins?

All Purlins are attached web to web (90 deg.)> The 2 main are attached to a centre brkt. and far end of secondary and main are attached to the tank wall.

3. Where is the roof cladding (aka diaphragm) connected to - direct screw attachment to top flange of C25024 secondary purlins?

Yes roof sheeting (polyvinyl) is directed screwed onto the top flanges. I cant imagine it of being reliable enough to be considered as a lateral support.

Anyway thanx guys for your input, much appreciated. We had the designs checked by a structural engineer (certified) and he Oked it.

Regards,

A.