Purlin Outwards Capacity vs "No Cladding" capacity?

[Tomfh]

Purlin tables typically list two capacities. 1. inwards capacity (where cladding restrains the critical flange) and 2. outwards capacity, where there the cladding is on the non-critical flange.

Does outwards capacity depend upon cladding on the non-critical flange? Presumably the cladding provides *some* rotational and lateral stiffness to the purlin, I'm just not sure whether the tables are including it or not.

 

[Toby43]

Tomfh,
I believe some utilize FE analysis to validate tabulated values.
In a more simplistic sense, I would consider the cladding to not provide torsional resistance, only translational, but it acts as a "stabilizing" load (loads away from shear centre in outwards mode, assuming it is the cladding applying the load). Together with bridging, the cladding will provide a couple and thus torsional restraint (bit of a lean on effect I suspect, if no direct load path provided), hence increased capacity for increased number of bridging.

Toby

 

[Tomfh]

Thanks for the comment. I tend to agree it will help stabilise the Purlin. What I'm really wondering is have the purlin msnifacturers included that effect in their "outward capacity" tables?

Put more simply - is "outwards capacity" ok to use if there's no cladding at all?


For simplicity let's assume 0 bridging

 

[Toby43]

How is the purlin been loaded if not via the cladding? Compression?

 

[Tomfh]

Heavy Beams stacked perpendicular on top of the purlins.

 

[Ron]

Since claddings can take a variety of forms, from sheets, to sheathing, to brick, to stucco,.....and the stiffness of such claddings will also vary significantly, unless there is a table available for each cladding or group of claddings I would consider that no cladding influence is included.

 

[Tomfh]

Ron, so in your opinion it would be safe to load unclad purlins based on the "outward loading" tables?

 

[Once20036]

"What I'm really wondering is have the purlin msnifacturers included that effect in their "outward capacity" tables"

Have you called the manufacturer?

 

[Ron]

Tomfh..check both and be conservative, but "yes".

 

[j19]

The AISI Specs have procedures on how to design a zee or cee for uplift loads when there are through-fastened cladding panels attached to the outside (tension) flange. For example, for an 8" deep simple span zee there is a reduction factor of 0.65. More than likely the purlin manufacturer used these reduction coefficients to come up with the values in their table.

 

[Tomfh]

J19

That AISI factor does sound similar to the tables in the stramit and lysaght purlin tables I'm familiar with. The outwards load is often around 60% of the inward.

So in the case of AISI specs, my question is - would it be appropriate to use that factor if there is no cladding, or should a lower factor be used?

 

[j19]

No, you shouldn't use those factors if you don't have cladding. The Spec also has a list of criteria that the cladding has to meet in order to be able to use the factors. If you don't have cladding for your situation I assume you will have some type of bridging between the purlins?

 

[Tomfh]

Thank you all for the pointers.