flexible diaphragm

[casasf]

I need some interpretation of section 12.3.1.1 Flexible Diaphragm Conditions per ASCE 7-05 page 124, quoting:
"...in which the vertical elements are steel or composite steel and concrete braced frames, or concrete, masonry, steel, or composite shear walls."
When the section refers to vertical elements are steel,.... do they mean only braced frames or they refer to ordinary and special moment frames?
I have a 2 story special moment frame with plywood floor over TJI. The plan checker is asking me to provide calculations to check whether the diaphragm is rigid or flexible. Am I missing something in the code?
Please help.

 

[slickdeals]

You can prove your diaphragm is flexible or rigid by calculating the deflection of the diaphragm in comparison with the deflection of the frame itself. See 12.3.1.3.

OMF/SMF are also frames. Basically it just refers to the lateral force resisting system.

 

[casasf]

Instead of calcs, can't we just say we have a flexible diaprhragm per section 12.3.1.1?

 

[RacingAZ]

Yes, I believe that is the case. If he doesn't buy it, I'll go one level above him.

I hate plan checker like this. They think we have an endless amount of time budgeted to keep working on one project.

 

[ajh1]

I think that phrase is intended to mean a steel braced frame or a composite steel and concrete braced frame. The emphasis here is on "braced" as you could then expect your deflection at the wall to be minimal, at least compared with any sort of a moment frame. While you can make the calculation for any condition, the suggestion with this statement is that it would be a trivial proof to show that the diaphragm would allow at least twice the deflection of the vertical bracing.
Similarly in the second phrase, you are not suggesting that ANY construction of concrete or masonry or steel would qualify without the qualifier "shear wall", are you?

 

[engineertipsman]

Could you ever get plywood to be rigid?

 

[Gumpmaster]

I agree with ajh1.

It should be read "...in which the vertical elements are steel (or composite steel and concrete) braced frames, or concrete, masonry, steel, or composite shear walls."

It's meant to be a no brainer clause to help us avoid calculation. For moment frames, it's not necessarily a no brainer and a calc is necessary.

 

[frv]

engineertipsman-

Plywood is semi-rigid; otherwise you couldn't get a cantilevered diaphragm to work.

 

[tbone73]

All wood design books I've seen assume plywood diaphragms are flexible.

 

[frv]

tbone73-

Yes.. you may assume they are flexible; but form an equilibrium standpoint, you MUST assume them rigid in order to design a cantilevered diaphragm. Hence, semi-rigid. (Same applies to metal deck roof as well)

 

[a2mfk]

Rigid vs flexible diaphragms can get confusing, but the main thing to remember is the relative stiffness of your horizontal diaphragm and your lateral systems must be compatible. Your analysis has to match up with your assumptions, you can call it whatever you want but the systems will behave how according to their relative rigidities. A CMU wall box ranch house with a wood roof is going to be a flexible diaphragm (99% of the time), as the walls are so much more rigid than the diaphragm. But you have to run the numbers. A residential porch addition that is later enclosed would be a cantilever diaphragm...

NDS has a whole booklet on diaphragms in their wood design package BTW that does a good job spelling it out.