Roof Deck and Cross Brace Diaphragm

[neoncookies]

Hello,

Is it possible to combine a roof deck and cross bracing of joists as a complete diaphragm? Or does it need to be one or the other? I think there may be some difficulties calculating what may fail first.

To clarify, can a specific type of roof deck be used to withstand X amount of shear load and have cross-bracing in the joists withstand the balance of the load?

Thanks,
Brian

 

[KootK]

It sounds to me as though you already understand this well. I think that the key will be this:

I think there may be some difficulties calculating what may fail first.

- It's hard to imagine a situation where the bracing wouldn't be vastly stiffer than the deck. You'd wind up loading the bracing first, possibly to failure, and then having only the deck left to work with.

- Like everything, you could distribute the load to the two systems based on relative stiffness. The trouble would be knowing the stiffness of the deck with any accuracy. I'd think that you'd end up having to bracket the design with high and low deck stiffness's. And by the time that you do that, you'll probably be stuck with a design that doesn't do a great job of exploiting the benefits of either system.

 

[KootK]

I suppose that one could make the cross bracing far less deep than the overall diaphragm dimensions such that the cross bracing system was convincingly softer than the deck. But then, what does that mean for the deck when it gets displaced beyond initial failure? Probably fasteners plowing through sheet steel. Kinda ductile but not entirely reversible.

 

[EDub24]

In my opinion anything is possible but similar to what you and KootK say, it'll be difficult to see which will fail first. It's easy to say the braces will take 'X' shear and the diaphragm will take the balance but in reality the braces will take everything until they fail and then transfer to the diaphragm as they are far more stiffer. How much shear the braces take until failure depend on a lot of variables including the material and section properties, the connection types, slenderness value etc etc. I would say go the easy route and choose one type of diaphragm and go with that.