I am having a discussion with a client about allowable deflection for MCM panels. He shared a spec requirement stating a maximum bow of 0.8% of the panel width and height. That would be L/125 deflection limit.
He shared a technical bulletin from MCA - Metal Construction Association that suggests L/60 deflection limit. That seems excessive considering it would allow 1" deflection in a 5’ span. For all the concern of oil-canning, I imagine this would not be acceptable to most architects.
Do you specify MCM deflection limits? If you do, what limit do you require?
Are you referring to deflection from wind loads? Warpage of the panel would be a whole different story. Speclink’s standard text includes:
Maximum deflection of perimeter framing member of L/175 normal to plane of the wall; maximum deflection of individual panels of L/60.
I don’t see anything directly stating this, but I would expect that to be without permanent deformation too.
I do not see why a MCM panel would require, or need, different deflection limits than any other metal wall panel, such as insulated metal wall panels or metal plate wall panels.
For wind loads, I typically specify 1/240 deflection limit, which clearly is a decision I have no memory of how it was determined.
Asking a thin MCM panel to perform the same as the structural supports may not be practical. The moment of inertia property responsible for resisting deflection for a 4 mm panel will be significantly less than a structural member designed to carry loads over a span.
The only option to control bending for MCM is to add stiffeners to the back of the panel. The more restrictive the allowable deflection, more or deeper stiffeners will be required.
When I was specifying, per my old spec, I had structural performance of the MCM Panel Systems tested in accordance with ASTM E330. Deflection limits for wind loads, panel deflection no greater than L/60 of the span.
The system you are using I think matters (system being a wet system, drained and back-ventilated, or pressure-equalized rainscreen system).
I would also specify a 4mm panel. Did these on tons of projects and we never had any issue with the panel. Our biggest thing was always making sure the wall assembly was NFPA 285 compliant.
I did have an option for stiffeners, but I don’t ever remember specifying them. We placed max 30-inch center.
This was over 4 years ago, so somethings probably changed.
For some reason Im also thinking the structural engineer would have provided calcs, but not entirely sure. DSA always asked for weird things.