Roof Bracing - Litigious

[kauri]

This is in terms of a legal case with respect to the earthquake susceptibility of a warehouse. The roof structure is DHS purlins w/fastbrace (lateral restraint to purlins

https://www.dimondstructural.co.nz/page/purlin/)

with corrugated iron cladding. There is no cross bracing.

What does the term "roof bracing" refer to? Is this specifically diagonal members, or would the horizontal bracing also be termed "roof bracing"?

Do you consider horizontal bracing with corrugated iron taking some diagonal loads to be a roof bracing system? I am not asking if it is adequate for the loads, merely whether it will provide some capacity worth accounting for.


I know what I think, however I have some disagreements with my supervisor.

 

[Tomfh]

The sheeting does provide some capacity, and is often relied upon in place of bracing where the lateral loads are minor - e.g. a canopy roof.

For warehouse you normally design the structural bracing system to carry all the bracing load, but you can count the sheeting/purlins and consider it as "stressed skin" design if you're so inclined:

https://www.istructe.org/journal/vo...ed-skin-design-and-construction-a-state-of-a/

 

[hokie66]

Folks use corrugated sheeting to brace minor structures like garden sheds, etc. But for a sizeable warehouse, discrete bracing should be provided, and just use the roofing to shed water.

 

[civeng80]

Definitely agree with Hokie66.

 

[Agent666]

In NZ (I think you are in NZ from previous posts) you can make the argument that for structure it requires a design life of 50 years under the NZBC, cladding require 15 years durability. There is a disconnect right there if you argue the cladding as structure.

When you say horizontal bracing, what exactly do you mean, can you share a sketch of the structural system. I'd associate 'bracing' with diagonal bracing specifically.

Designing as a stressed skin requires specific requirements, which often are not conducive with fixing sheeting through ridges of cladding with screws which effectively cantilever 30-50mm above the purlins all at larger 200-300mm spacings. Many times the skin should be welded to the supporting members with puddle welds.

No doubt it does 'something' as currently detailed, but it's hard to justify, so any benifit is typically ignored in practice. In my mind a few cycles of a decent earthquake end up slogging out all the screw holes and you lose the effectiveness of your roof diaphragm, as it was never designed or detailed to be a proper structural diaphragm. It just has some diaphragm qualities up to a point, after that it's less effective or ineffective.

 

[Agent666]

In peer reviews I've done where people have tried to argue the corregated claddings as diaphragm without any specific design. I've simply asked them to prove the load path from the sheeting to cantilevered screws work for transfering loads from cladding to perimeter purlins to wall structure, this fails horribly usually so it's a good place to start proving the flaws in the designers thinking.

 

[kauri]

Hi all,

Thanks for your responses, much appreciated.

After looking into the limitations of stressed skin design, I've convinced my supervisor that it is not worth arguing that the corrugated iron will provide any bracing.

 

[hokie66]

Good decision. Roofing is for watertightness, not strength of structure.