Basic Concrete Diaphragm Question

[zrck99]

I am designing a middle school gymnasium storm shelter using 6" total thickness non-composite concrete slab over 1.5" form deck. I've read through the diaphragm sections of ACI 318 and saw the 3:1 aspect ratio requirement for considering the diaphragm rigid. For simplicity sake, I would like to analyze the full shelter as rigid in order to avoid putting together a computer model. With this being a storm shelter in a school, the Calculation Review process is pretty involved so I want to make sure that I have code backing for my analysis.

In the attached pdf, I've shown with highlighter what I'm considering my separate diaphragms for loading in each direction. In order to meet the 3:1 aspect ratio, I've broken the southern 9'-5" x 53'-9" bumpout as two separate diaphragms with each meeting the 3:1 aspect ratio requirement. So the question is, is this an acceptable way of analyzing my scenario? Is there any reason that I can't break the lower section into two separate rigid diaphragms as shown?

I am still getting familiar with the full code provisions for rigid vs semi-rigid so please feel free to point out anything that you think would be of value.

Thanks

 

[Agent666]

What is the lateral load system. Where are you trying to get the load to? I think that's a more pertinent question regarding whether or not a 3D model is required accounting for diaphragm flexibility rather than simply an aspect ratio argument driving that important decision. Is it irregular by code definition?

As far as I'm concerned it's one large diaphragm with a small extension on the bottom. Arguing some split like done is simply twisting the numbers. Fundamentally if it's all monolithic then it's not going to behave as if the extension is split into two as you're reinforcement is arranged. If you're physically splitting it then again it seems like you're trying to satisfy some code provision at the expense of a better detail by actually connecting everything.

 

[KootK]

I get what you're trying to do and I agree, that would satisfy code intent and be a compliant solution. I do think that there's an easier, more natural story to tell though:

1) Bumpout is connected to the main diaphragm so often that it's really just ride along mass rather than a separate diaphragm.

2) Only the main diaphragm needs to be considered with respect to aspect ratio concerns.

Your system is conceptually similar to what is often used in flexible diaphragm situations.

 

[KootK]

I had not initially realized that this was form deck instead of just CIP slab. Will you have rebar, WWF, or something in the slab? If so, I stand by my previous comments. Given the orientation of the deck flutes, you might be able to tie the bump out to the main diaphragm even without the WWF.

 

[zrck99]

KootK,

Right, this is similar to my previous wood diaphragm question that you helped me out on but with rigid diaphragms. I totally get the logic that the bumpout will just be along for the ride with the main diaphragm. I just know with the line by line review that my calculations will get on this one that I need to clearly present how I'm meeting the rigid diaphragm requirements and have detailed load paths for chords/collectors/etc.

 

[zrck99]

It's 1.5" form deck with 6" total concrete thickness. We will reinforce with #4's at 18" o.c. each direction.

 

[KootK]

I get where you're coming from with the review but I'd encourage you not to be too fearful of that. For the most part, they're reasonable design professionals just like us. If not, either add your collectors back in or make the case that every piece of rebar entering the bump out is a collector separating a thousand tiny diaphragms. That's pretty much the truth and makes a mockery of any concern.

 

[zrck99]

Kootk,

Agreed. With bars at 18" o.c., you could effectively argue that you have 35 or so 1.5' wide rigid diaphragms in that direction.