How to determine strength of small portal frame (<6m wide) including strength of cladding? 2

[FLR84]

Hi everyone,

I'm trying to determine how much wind load a cold formed portal frame can take which is, for the sake of argument, 5m wide, 4m high, with 3m bays. With small structures such as this, the steel cladding can contribute significantly to the strength of the portal frame. I know methods to find out the effect of bracing that the cladding provides ('stressed skin'), but not how to find out how much the cladding directly contributes to the strength of the columns and rafters in bending as a composite section.

I can input the composite sections into my structural analysis software but because the code only gives advise on calculating the strength of typical sections (eg. Cee, Zed etc), the software refuses to analyse it. I have worked out the Ixx of the cladding, if that is of any help.

Does anyone have any ideas on how I would do this?

 

[MIStructE_IRE]

Composite effect of cladding? The calc for that is very easy:

Composite effect = 0.00

It can of course offer restraint to member buckling depending on the details/fixings.

 

[FLR84]

Hi MIStructE_IRE,

I think the cladding does have some strength - in fact you can basically build a building of this size entirely from cladding without the portal frame. Of course as the span of the building increases the relative effect of the cladding rapidly diminishes but for smaller buildings it is a factor... maybe 20%? The Ixx is 9400mm4 (190mm wide section) and it is placed quite far from the centroid of the composite section (around 60-100 away because of the purlins/girts) so it's effect is increased even further.

 

[MIStructE_IRE]

I’m thinking corrugated cladding - That will simply crimp when loaded along its ‘weak’ axis. I could do that with my hands! If its separated by purlins then there’s zero composite action anyway.

Are there any published guides on this? I can’t say I believe it at the moment.

I certainly agree re stressed skins for stability and the restraining effects against LTB etc.. But I’m yet to see anything on cladding acting compositely with beams for bending. Always happy to learn though!

 

[rowingengineer]

https://www.google.com/url?sa=t&sou...WMAZ6BAgJEAI&usg=AOvVaw3gWCDKK3n2nKxconJlCzEr

 

[saplanti]

Cladding can only provide stiffned in its plane. There is no additional stiffness which will effect a portal frame other than its own stiffness.

 

[FLR84]

Thank you rowingengineer, that paper is very helpful!

 

[MIStructE_IRE]

The paper discusses stressed skins and diaphragm effects. I still don’t believe there’s a composite/compound effect from cladding which will increase your I or Z values whatsoever.

Also bear in mind the crappy tec screws that fix cladding to steel or cladding to rails. They’re tiny and certainly won’t cope with any great longitudinal shear required to gain composite action - even if the cladding could take it.. which it can’t.

 

[FLR84]

MIStructE_IRE:

From the paper:
"The action of these crest-fixed steel claddings with braced end gable frames has produced a reduction of 33% in the maximum moment and 72% in the maximum horizontal deflection"

 

[MIStructE_IRE]

Yes, that’s due to the diaphragm skin action. Ie, its stiffening effect reduces the applied moment, it does not increase the moment capacity of the members.

I think we’re talking about two different things here.