Bending of Fastener

[xez]

Hello all,

I have a condition where I have a sandwich panel with metal facings let's say 4'' apart and is fastened down into steel purlins. However, there is a 3'' air gap (left out details for simplicity) between the sandwich panel and the girts. The panels are subject to a snow drag load, lets say 100 PSF. How do I account for the bending of the fastener between the purlins and the sandwich panel? I have attached a drawing for reference.

Thanks for the help.

 

[rb1957]

? "air gap" ... fill the gap with a cylindrical spacer, so the spacer takes the moment ...

another day in paradise, or is paradise one day closer ?

 

[KootK]

there is a 3'' air gap (left out details for simplicity) between the sandwich panel and the girts

I suspect that the left out details may, in fact, be necessary to generate a reasonable answer.

How do I account for the bending of the fastener between the purlins and the sandwich panel?

Mobilizing fastener bending in these situations is usually not the way to go. Even if the fasteners can take the bending, the cold form probably can't handle it locally. If there's actually something of substance in the air gap, even insulation, you may be able to use it in compression to set up a strut and tie kind of mechanism with the fasteners in tension. Some wood screws work that way.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[xez]

The sandwich panels are actually installed onto a metal deck so some of the fasteners will hit the "highs" of the deck and some fasteners will hit the "lows" of the deck. When it fastens into the "lows", there will be a 3'' space.

 

[KootK]

I expect that your "highs" will end up doing the lion's share of the work and the "lows" will be extraneous. You could fill in the lows with some blocking but that would be tough to locate and may compromise the insulation strategy.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[rb1957]

still, cylindrical spacers at each fastener.

if the metal deck is castellated, why not pick up the "highs" ?

another day in paradise, or is paradise one day closer ?

 

[hokie66]

I take it the deck is fastened through the "lows", or troughs of the deck. Then the sandwich panels which fasten through the "highs" will have to support the lateral loading. You can't do that with fastener bending. I suppose these fastener spacings are controlled by the sandwich panel design, and that is one of the problems with sandwich panels. If you can't engage enough of the "highs" with fasteners, you may have to apply a flat layer, maybe plywood, between the deck and sandwich panel.

 

[xez]

Thanks for the responses everyone.

 

[Ron]

Agree with hokie66....the fasteners can't handle the bending. They are designed for shear resistance, not bending. You would have to extraordinarily upsize the fasteners to account for bending. We see this same condition routinely in window opening gaps.

 

[xez]

Thanks. I ended up working out the calculations and checked it against some manufacturer's test data as well.

Here's another scenario:

Same drawing as the one I previously posted. Say there isn't a 3'' air gap between the sandwich panel and supports (and that it'snot attached to a metal deck.) The sandwich panels have 22 gauge facings on the top and bottom side with foam in the center. Assuming adhesion between facings and foam is adequate, would the two facings resist the same amount of shear? That is, if the shear load for the 22 gauge facing is 600 lbs, can this be treated as 1200 lbs? I would think in a homogeneous material, say a 1/2'' steel plate, the plate or fastener/bolt would shear. But can we treat it the same way in a composite material?

 

[rb1957]

in-plane shear ?

another day in paradise, or is paradise one day closer ?

 

[SWComposites]

At the fasteners, most of the shear load will transfer in the facing adjacent to the attached structure. Over some distance it will flow thru the core and both facings will carry the load. The distance depends greatly on the geometry, material properties, fastener spacing, etc.