Question on Wood Diaphragm

[Lion06]

I've come to the conclusion that there is no way I have time to study anything for the PE in less than three weeks and now I'm scrambling just to get codes tabbed.

In doing some tabbing, I noticed that the wood chapter of IBC talks about cantilevering a wood diaphragm. It gives the parameters under which this is acceptable, but lists it under RIGID diaphragms. I haven't done a great deal of wood design, but I've always thought that wood diaphragms were always considered flexible? Does this cantilevered wood diaphragm really require it to be rigid and needs to be confirmed per the rigid/flexible criteria in IBC?

 

[frv]

The diaphragm must be considered rigid; otherwise you don't have rigid body rotation that would allow the two perpendicular shearwalls to contribute to lateral stability.

Plywood is really semi-rigid in that you are allowed to consider it flexible for most situations. But it really is rigid enough to to act as a cantilevered diaphragm.

 

[Lion06]

If it's a flexible diaphragm and it has two walls parallel to the load it's still stable, right?

 

[frv]

No.. that is equivalent to a pin-free beam..

It has to be rigid in order to rotate.. it's the rotation that enables the two perpendicular shearwalls to be engaged.

btw.. up until a year and a half ago, I was under the exact same impression as you.. The only reason I know otherwise is that shortly after I was laid off, I had an interview with a company and I was given an impromptu oral technical exam.. This came up.. not really a good time to learn something new..

 

[msquared48]

frv:

What SEIT says is correct - read it again.

The rotation comes in where you have three walls, two transverse to the load, and one not asligned with the center of gravity of the building, inducing an inherent torsional force.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[frv]

Yep.. you're right.. I misread it..