Flexible Diaphragms in RISA 3D 1

[IngeIvan]

Hello,

I am trying to model a flexible diaphragm (steel metal deck) as I need the lateral stiffness that this would provide to some elements that are not part of the lateral system.

To my understanding, there are two methods: Plate elements (semi-rigid) diaphragm and to model "X" braces to provide this stiffness. Here are my questions:

1) If I model the plates, do you guys know any tips to get a good approximation of the actual properties of the deck? (How to model G'?, how will my flexural stiffness of the plates affect my in-plane stiffness of the diaphragm, etc).
I was thinking that the ratio of the diaphragm will play a big role, as the shear deformations may control over flexural deformations in some cases, so I think I need to model both flexural and shear deformations accurately. Any thoughts?

2) If I do X braces, how can I estimate the stiffness I should give to them? I was thinking to use round, weightless rods. Any thoughts?

Any tips will help a lot.

Thanks!

 

[JAE]

IngeIvan,

What we've done is to create a model of the diaphragm only (separate from your base model) with the diaphragm proportions/dimensions with plane stress plates (i.e. only in-plane DOF).

Then apply a 1 kip/ft load along each edge of the diaphragm (two force directions) with fixity on either end of the diaphragm width.

Separately, we calculate the G' of a proposed deck system using Vulcraft or other deck manufacturer tables. Vulcraft has a complete formula for G' based on deck gage, support fasteners and sidelap fasteners.

With that G', we calculate the diaphragm deflection:
1. Deck width L (inches)
2. Deck depth (parallel to force), B, inches
3. Lateral force, w, kips/inch (here we would use 1 kip/ft)
4. Deflection, Delta (max) at "midspan" of diaphragm = wL^2 / (8G'B) (inches)

We then see what our RISA diaphragm deflection is with an initial thickness and adjust the thickness of the plates to match the deflection we got from the calculations above.
This then provides you with a plate thickness that mimics, to some degree, the deflection of the diaphragm consistent with a theoretical deflection based on G'.





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[JoshPlumSE]

I concur with JAE's procedure for using plates. I believe there are some published references on how to do this. SDI Diaphragm Design Manual.

For the X brace option, this is really more of a "ball park" estimate. It comes in handy when you are getting some P-Delta failures because the diaphragm doesn't have any stiffness and isn't bracing your columns in the weak direction. What do you set the properties of the X braces to? Well, that's a matter of engineering judgment. I usually use weightless gen_steel rods with an cross section equal to the thickness of the deck time some representative width for the area of deck it is replacing. Though I don't like to see the area get too large.... My tendency is to err on the side of more flexible. My goal is usually just to provide stability to the structure, but to get behavior that is close to what you get from a flexible diaphragm.