Cantilever Diaphragm 1

[haynewp]

I have a metal deck diaph. w/ conc. shear wall bldg.

As we all know the chord force in a simple span diaphragm is (wl^2/8)/(bldg depth). For a cantilever diaphragm, I have shear walls on 3 sides. At first I thought the max chord force would be (wl^2/2)/(bldg. depth), just like in a true fixed cantilever beam. The force is assumed to be transferred all the way through the roof, the roof starts to deflect, and then it gets into the 3 sidewalls.

But then I thought about the concrete shear walls acting to take the T/C couple all the way down the bldg length, and the moment wouldn't reach wl^2/2 at the end. The rotation is taken out immediately along the length. Or maybe I am being stupid and thinking too much again.

 

[gauger]

Think of the shear walls as vector components of the required reaction. They always exist as reactions and always in balance with applied loads.
All loads parallel to the single wall form a moment with their reaction, which has to be resisted by a force couple in the double walls.

I have kept this simple, trying to avoid confusion.
Good Luck,
Fred

 

[haynewp]

I am thinking of it like an open channel with torsion. Is that what you are also saying?

 

[DaveAtkins]

haynewp,
There are no chord forces in a three sided diaphragm. As you stated, the moment is resisted by the two shear walls perpendicular to the direction of the applied force. DaveAtkins