When is diaphragm action a necessity in floor deck systems 2

[IJR]

This one has always been a gray area to me.

You have a steel frame with joists to form a floor structure and you top it with trapezoidal deck and concrete topping.

In a seismic area I would utilize diaphragm action for better load distribution between frames. I would do this by using shear studs. This also gets me some composite action.

When seismic or wind load is not dominant I would simply brace the floor with horizontal bracing and I would ignore studs. The deck will be in place with the simple "nail " kind of fasteners. The braces distributes the load just like a diaphragm would at a lesser cost.

When no lateral load is available and I am relaxed with the stiffnesses of my sway frames and about sure that they have almost proportional sway stiffness, I would ignore bracing.


How do my friends approach this subject?

Regards

IJR

 

[Taro]

Even without shear studs and composite members, a steel deck will act as a diaphragm if it is properly fastened (welds, screws, etc.). Most deck manufacturers publish diaphragm values in their catalogs. If you have an unusual case that's not covered in a catalog, you can calc out the capacities using the Steel Deck Institute's Diaphragm Design Manual by Larry Luttrell.

 

[JAE]

I agree with Taro - normally, with a metal deck with or without concrete, you do have a good diaphragm action if the deck is properly attached to the supporting steel and the lateral forces required for your diaphragm are checked.

The critical issue with flexible diaphragms come at the boundary where the lateral load is passed off to a supporting edge collector. This is usually a fairly brittle connection (the weak/brittle link in the chain) and its very important that you design for a higher load capacity here (in fact: the codes usually require you to do so).

Also, a deck diaphragm without concrete is considered flexible (usually) and additional requirements for attaching load-bearing masonry or concrete walls is prescribed as well.

We almost never use additional bracing in the plane of the diaphragm when using metal deck diaphragms unless we can't get a particularly high shear to work with the deck we specify.