L/60
L/120
L/175
L/180
No deflection limit?
Stress only?
Center of panel ?
Gross or net?
What about the stiffener itself?
Why the inconsistency in metal panel deflection criteria? Is it a lack of understanding on the part of specifier’s? Is there no cohesion or clarity in the panel industry? Not a clear standard? All of the above? I have my viewpoint, but I’ll be silent on that for the moment.
I’ve performed deflection and stress calculations on thousands of panels and hundreds of jobs. Allowable deflection criteria changes on a job by job basis depending on the specifier, consultant, or architect. There’s no need to have a subjective performance standard.
A panel is a flexible membrane element. It’s not stiff like glass; it’s not brittle like stone. It takes multiple forms. It’s an infill element in a curtain wall across a pressure gradient. It’s a pressure equalized panel in a spandrel cavity. It’s a rain-screen panel over a stud wall. It’s formed, bent, part of a sunshade or louver. It’s a soffit panel or fascia. It’s composite ACM, sheet, plate, perforated, aluminum, stainless, copper. Panels take many forms.
Whatever form it takes as a facade element in the enclosure, AAMA (the American Aluminum Manufacturers Association) calls for a deflection limit of L/60 for panels under pressure (glazed-in panels or across pressure gradient.) This is the correct standard and most balanced with stress considerations for panels under load. However, some manufacturers, installers, and facade engineers don’t even want to recognize deflection as a criteria. They only want to consider stress. This is a worthy debate.
For sheet and plate that is 4mm, 6mm, 1/8″ or thicker , or gauged stainless steel, we should establish and support a consistent standard of L/60 for panel deflection. I’ve never seen a metal panel fail at a Mock-Up or on a Project due to deflection issues if stress is within allowable limits and L/60 is used as a criteria. Just be mindful of the context (like all design considerations.)
Certainly there may be contexts where a panel should have a smaller deflection limitation, but this should only matter if it’s at a seal line or adjacent element could be jeopardized due to a typical deflection. Design professionals making subjective decisions based on lack of knowledge or fear, creates conflict and difficulty for panel manufacturers, engineers, and others that support the facade industry.
I’d like to hear from others in the industry.
JOHN L WHEATON BLOG 