Structural design of aluminium panels 2

[canadiancastor]

I recently got a job for an exterior siding installer. They want to get into bending of aluminum panels for use as exterior cladding. The architect requires engineering calculations for these panels. I stumbled upon this thread and am hoping to get the attention of berkshire who said he had done something similar in the past:

https://www.eng-tips.com/viewthread.cfm?qid=317129

I'm wondering if this can be done 100% with regular structural engineering calculations or if I'm going to have to do some lab testing. Some things that come to mind:
[ol 1]
[li]Fatigue of aluminium: I think I can find the fatigue curves of the aluminum we will be using. I'm not sure where to look for number of cycles under wind loading for 50 years[/li]
[li]Residual stress after bending. Our panels will have 90 degree bends on all sides, 90 degrees is way past Euler beam theory, not sure if this is problem[/li]
[li]FEA modeling: I'm thinking of using shell elements to check bending stress. I'm thinking of modeling screws as pined supports[/li]
[li]Stiffness: Competitors seem to glue stiffeners in the back of large panels, probably to stop them from fluttering in the wind. I think the stiffeners are simple aluminum angle extrusions and they are glued with epoxy, but I have not confirmed this yet. As with all aluminium, I suspect deflexion might control design. [/li]
[/ol]

Any advice or tips as to where to start would be appreciated!

 

[phamENG]

canadiancastor - I'd heed CWB1's comments a little more. As a structural engineer I can appreciate your approach to the wind loading, but remember where you are now. You're not designing an entire building or even a tried-and-true cladding system, you're looking at the performance of an individual and somewhat specialized component. Maybe I'm misunderstanding your background, but for most of us these are just a matter of checking what the manufacturer has certified they'll do. Well...you're the one doing the certification. To get them to perform properly with an efficient design (important for the fabricator to maximize profits - if they are going to mass produce these finding an engineer who can cut 1/10th of the thickness for twice the fee will be worth it) it will require more sophisticated analysis than we typically use in general building design.

If wind can't get behind it then that'll certainly help, but remember that even though it's strong enough to resist the static wind pressures doesn't mean that the incredibly random and varied application of actual wind loads could cause some interesting vibrations in a panel that is thin enough and lacks sufficient damping.

 

[canadiancastor]

cause some interesting vibrations in a panel that is thin enough and lacks sufficient damping.

I agree with everything you said. I think vibration and resistance to fatigue is something I need to look into, as I'm not familiar with it. I'm just not going to wind tunnel test every type of panel I have to see how the wind blows off it.

 

[CWB1]

I can't speak to common practice in the CE/SE world, however the SEs I know commonly use modal analysis and have their own large and small-scale test methods to correlate, my favorite being the good ol kabong. Comparing notes, its interesting how experience crosses over between industries, forces scale, and various vibratory failure modes common in relatively small structures are also common in bridges and buildings as are their solutions.

In any case, my concern isn't reaching a natural frequency but rather a lack of damping across the panel. Both residential and commercial siding are notorious for fastener (ultimate) failures caused by wind and vibration. Wind is like any force, neither perpendicular nor constant velocity so you will have varying levels of vibration & feedback (flutter) continuously working against the fasteners. Unlike modal, static analysis doesn't really tell you anything about stress concentration or how a structure flexes, much less fails so is only a first step toward a complete analysis. You need to understand where and how the panel moves to dampen it efficiently. I'd use a shaker to correlate.

 

[canadiancastor]

@CWB1
Thanks for the input, I will look into this as well.

 

[IRstuff]

I would be concerned about the "paint" and how UV resistant it is. I have an aluminum roof with colored Kynar, which appears to extremely robust and I think that all similar roofs easily withstand 20+ years in Southern California sun.

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[Dan D]

Hey canadiancastor,
Check sites related to the ACM Panel Industry (Aluminum Composite Material). Check United Laboratories Testing data on ACM panel systems. Wealth of panel design on both. What you are designing is already a wheel. You shouldn't have to go through the intricacies of design. It's already out there. I worked in the engineering department of an ACM panel fabricator. Sheet aluminum or a composite will have similar designs.

Your perimeter extrusions are going to be the integral part of most importance. Thermal expansion is your enemy! Water is your second enemy. Never doubt that it won't find the least point of resistance and ruin your whole day.

Tips:
ACM is .020 aluminum skinned on both sides of a plastic composite. I wouldn't worry about bend stresses with the thickness you are using. ACM is vee cut leaving only one side of the .020 aluminum and bent to 90 degree for returns with no problems. But, you only get "1" bend. Try to rework it and it breaks.

Two types of systems are well proven. Waterproofed substrate with uncaulked joints, and, caulked joints. Joints are typically 3/4" and up The systems are called wet & dry. And just the opposite of what you would call wet or call dry. Dry is uncaulked wet is caulked. It's referring to the joint condition not water penetration.

Design stackable mounting extrusions at the panels perimeter with enough play to combat the thermal coefficient of your panel material. There are standard extrusion profiles already available on the market. They are pop rivit or screwed to the panels at a return. Typically 8" o.c. There are also panel ZEE substrate members designed for wall mounting use and readily available.

Pinning your lapped panels to the structure with slotted holes or neoprene washed screws is a low life cycle method. Both expansion deformation and dissimilar-metal corrosion are inevitable. With a shop painted system, corrosion blistering at the holes will be a problem. A true weathertight warranty is unachievable with pinned systems. Trust me, taking down panels, repainting them, and then putting them back is no fun. Review purchasing kynar pre-painted sheets.

You can also review "sliding clip" installation methods. (review standing seam roofing for ideas) The clip is a two part. A base that is pinned to the substrate and the second half, pinned to panel, slides up to 3/8" in both directions. Pin the panel at one end and the sliding clips spaced along the panel edge take up the expansion. On long runs where the expansion exceeds the clip slide limit you pin the middle of the panel for a 50/50 expansion in both directions. Note: In wall applications be sure your pin location design will support the panel load in shear. The clip design also has a very high wind lift resistance factor and can be design spaced to achieve any wind limits.

The stiffeners on the panel are to prevent oil canning and sag. They are used at 3rd points up to 5th points (applied vertically) depending on panel area size. They do not need to extend to the perimeter. They are attached with 3M structural tape. Great stuff when you follow the application requirements. The stiffeners have no structural properties. They are there for panel exterior aesthetics only. A channel profile works best. Typically a 1/8" x 3/4" x 3/4".

Hope that helps.
Dan

PS: Look into wall panel weep hole design. If you do have a leak it can get out.

 

[canadiancastor]

Dan D,
Thanks very much for the wealth of information. I started to have a look for online design guides, but I didn't find anything specific with the keywords you suggested. I will get to looking at this more seriously in the coming weeks.

I have a couple of question to better understand some of your advice. Sorry if any of these questions seem ignorant, english is not my first language.

Design stackable mounting extrusions at the panels perimeter with enough play to combat the thermal coefficient of your panel material

What do you mean by stackable? Our #1 panel is only bent twice, so in some sense the panel edge becomes the extrusion. Our #2 panel seems to have what you refer to as the "Zee substrate member".

There are standard extrusion profiles already available on the market. They are pop rivit or screwed to the panels at a return. Typically 8" o.c.

Is this the type of extrusion you are referring to? What would be the added value of this type of extrusion over the "Zee substrate member" or the bent edge?

Pinning your lapped panels to the structure with slotted holes or neoprene washed screws is a low life cycle method.

What do you mean by pinning? Using self-tapping screws? Using any kind of screw in predrilled holes? Also what do you mean by "lapped" pannels? I was thinking of oversized holes with washers + neoprene washers to deal with thermal expansion, are you saying this is a bad idea for all aluminum pannels, or just for "lapped" pannels?

With a shop painted system, corrosion blistering at the holes will be a problem. A true weathertight warranty is unachievable with pinned systems.

Interesting, why is that? When the screw is set it damages the coating?

The stiffeners on the panel are to prevent oil canning and sag. They are used at 3rd points up to 5th points (applied vertically) depending on panel area size.

Are these prescriptive rules of thumb or are there formulas and rules to calculate when a certain panel size will sag? Or is it just past experience?

Thanks again for all of the information.

 

[Dan D]

Canadiancastor,

The Zee system you proposed leaves you with a wide variety of problems when working with aluminum. Pinning 2 sides will push all the expansion diagonally in one direction on the entire wall. With the extremes of your weather you will need to pay very close attention to: What temperature you install in; Single expansion direction, and A design with maximum floating characteristics.

If you are looking for the simplicity of a Zee substrate system use the standing seam concept. Way simpler and you can shop fab - clip attach - no stiffeners and field fold the seams. You'll have a reverse aesthetics profile though. See these for fabrication ideas:

https://www.metalsales.us.com/wp-content/uploads/2020/01/Magna-Loc_90-2D.png

ACM Product Specifications might help you find the testing data that will lead you to more product details like the one you posted.
ex. =

https://northclad.com/acm-specification-guide/

Contact the Aluminum Composite Manufacturers and they will also help you with industry standard details.

* "stackable" - see #1

* "Pinning" - It's where you attach the panel to the substrate. Expansion needs to be calculated over your entire panel span. Assume it's 1" and you want only 1/2" top and bottom because of space limitations. You would only fasten it at the mid point. Sliding attachment would be required at all other attachment points per shear/pull design requirements. Pin the top 1" goes to the bottom .... pin the bottom 1" goes to the top. (pin bottom includes gravity factor- poor choice)

Pinning a lap induces the coefficient of expansion of two independent items into the design, possibly at different temperatures based on wall locations (i.e. Corners, soffit, offsets, etc.) sharing a lapping hole and fixed by the same screw. One may want to shrink and the other expand. And every lap in a wall system should be treated like the head of an engine to the block. Two faying surfaces, mechanical attachment, and a gasket. The gasket you'll need will throw a whole new array of problems.

Extrusion mounted panels isolate each panels expansion locally with gaps in the extrusion system. Essentially, the extrusion is an expansion carrier substrate system.

* "Shop Painted" - The corrosion will come from the dissimilar metals. Unless your fasteners are aluminum? Not likely as they are poor structural fasteners. Inevitably the thread of those fasteners (under the washer) will come into electrical contact with the hole no matter how you try to avoid it. It may not be a catastrophic occurrence but any corrosion will lead to paint warranty and paint system failure problems. And adequately prepping hole edges for paint adhesion is not economical. The hole edge paint will fail over time. Not enough surface area.

* "Stiffeners" - Use the span deflection criteria of the metal you are using and decide how much visual deflection is acceptable for your product. Applying stiffeners reduces the span thus reduces deflection.

The detail you've shown is a "horizontal" joint, no caulk, with filler plate. It is designed to allow water intrusion. The extrusion joints at the panel corners are 45'd and sealed. Allows the water to travel across and down. Any water intrusion inside the panel weeps into the panel below. It requires a weather tight substrate.

#1 - The lower panel extrusion is on the wall. Notice it extends into the panel bend then to the wall and up to create a receiver for the panel above. It accomplishes three tasks. Gives water a channel to ride in, pins the lower panel to the wall, and becomes the receiver for an adjacent panel. It's a male / female system. So if you are installing panels left to right you would use a female extrusion on the right side and top of your panels. A male extrusion on the left and bottom of the panel. You can vary the male female connections to start from either corner or even the middle of a wall. That's the "stackable" term I used.