DHS Cold Form Purlin Capacity

[moyseh]

Hi All,

My question is regarding a cold form steel purlin capacity. I do not have much experience with the local code (AS/NZS4600), however, the local supplier provides capacity tables which are given in the form of uniform loaded bending capacities (ΦbWbx)(Snip below). For cases where we have point loads and need to determine the uniform bending capacity, DHS provide conversion tables which are approximate but work. For my specific case I have an AC unit being placed on top of the purlins which is quite heavy but would be a UDL that doesnt span the whole length of the purlin. Am I able to back-calculate to find the ΦMn based on the ΦbWbx given or is it more complicated than that? Any help much appreciated.

 

[hokie66]

I think you can do as suggested, but be careful to respect the bracing requirements.

 

[Lomarandil]

Aside from any bracing assumed inside the span length (you'll have to check for any table notes), sure, ΦMn should be at least ΦbWbx*l^2/(8*1.14) -- where that 1.14 is an adjustment assuming that Cb is applicable and used in their derivation of Wbx. Depending on the span, bracing conditions, and governing limit state ΦMn might be better than that, but that should be the lower bound.

Practically the A/C unit is more rigid than the purlin, so it probably becomes two point loads at the edges of the A/C as soon as the purlin starts deflecting. Not sure if that's included in the DHS conversion tables or not.

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just call me Lo.