Light Gauge Metal Composite Floors

[OlafTheBlack]

I am working on a wood truss roof replacement. The fire treatment turned the wood brittle. The truss bottom chord is attached to the supported ceiling. Non bearing walls attach directly to the ceiling. Hence, snow loads deflect the truss and pass into the non bearing walls below. These walls are supported on a concrete deck which is made composite with light gauge metal C-joists spaced at 32 inches. I have never encountered a light gauge metal composite floor (as defined on the original drawings).

I have no details of the method of attachment of the concrete floor to the C-joists. I presume the concrete was placed on form deck that was puddle welded to the C-joistss. I expect the only composite traction the concrete gets with the joists is through welds between the metal deck and joists and the ribs with the concrete. I expect there are no mechanical fasteners as one has with Hot-rolled steel to concrete composite floors.

I am operating in the belief that the shear transfer is adequate without mechanical connections. The only benefit I see with mechanical connections is that they prevent the slab from lifting off the deck. I see any load that generates shear will be sufficient to push the slab down tight to the deck.

I would appreciate any comments about this floor construction and my thinking to not worry over its likely lack of mechanical connections.

 

[KootK]

Based on my experience with similar systems:

1) I would expect the concrete to be composite with the steel deck.

2) I would not expect the concrete to be composite with the floor joist.



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.

 

[XR250]

I agree with KootK but why would they even bother with composite for 32" O.C.?
Never heard of a composite light gage floor system before.
You might want to try shimming up the truss some at their bearing and add a deflection clips on the interior walls.

 

[jayrod12]

Is it at all possible they are custom made channel joists with something similar to the hambro floor joist tab at the top?

 

[OlafTheBlack]

To XR250. LIght gauge steel salesmen were promoting cold formed steel in the northeast as non combustible and lightweight and installed by carpenters. Some would push anything to make a sale. I suspect that's where the composite idea cam from as a product competitor to the Hambro system as Jayrod12 asked. The Hambro would have required 49 1/4 inch spacing to drop in a sheet of plywood between the curved top chords.

I plan to resupport the ceiling by sistering the truss bottom chords to new LSL joists and pre-deflect the joists for the dead load deflection of the existing ceiling. Can't clip the walls as we would have to rip out the ceiling triggering electrical and mechanical redesign. Additionally, I will put down a new shoe support on the joists to gain a 1 1/2 inch space to allow for new truss deflection.