Modeling of flexible diaphragm 1

[Luiz163]

I am modeling a steel structure using Staad.Pro v8i.Some areas of the floors are covered with bar grating instead of steel plates,so I am considering to model these areas as flexible diaphragm.A reference suggests using Floor Load with no plates for these areas.However,Floor Load doesn't work properly for irregular geometric layouts.So I would like to put plates on and change material properties.Does anybody know if this is possible for flexible diaphragm modeling in Staad?Or do you have a better way on flexible diaphragm modeling?Thanks.

 

[structuresguy]

FLexible diaphragm is a bit tough, in my experience. Staad does not have a database for deck types, so even modeling standard metal roof deck is tough. In your case, even tougher I think. Personally, I don't think I would rely on grating to give any kind of diaphragm strength.

But if you want to give it a shot, I think your best bet is to create a material with the density and Modulus of Elasticity of your grating. Then create plates of the appropriate thickness. This is all easy. The difficult part is trying to figure out what elastic modulus value to use. You want your stiffness (EI) to be approximately correct. Your I comes from the plate thickness. But E is always tricky, especially in your case since you have a "hollow" floor plate (grating).

I have used this appraoch to simulate metal roof deck in the past, where I had complicated diaphragms. I calibrated my E and my I (plate thickness) using a simple diaphragm, and known diaphragm stiffness and deflections from diaphragm design manual. Once I had an E and a thickness that gave the correct deflection and diaphragm shear on the simple model, I used this on my complex roof. The results were very reasonable, IMO.

Good luck with your unique situation.

 

[Luiz163]

The reason I would use plate elements to model bar grating is that it is easier to apply vertical loads(dead & live loads) on plates.Otherwise dead and live loads have to be converted into line loads and applied on the beams supporting these panels.That's much more work to do.

So I would like to model bar grating with plate elements,and a new material shall be created which best reflects the mechanical properties of flexible diaphragms.Like light-gage metal deck,bar grating has some out-of-plane shear stiffness but little in-plane shear stiffness,i.e.,the in-plane G(shear modulus) is zero or a very small value while the out-of-plane G is not.So this material shall be orthotropic instead of isotropic.

In 2D Orthotropic material property definition,I am not sure about the direction of "Shear Modulus".I am also confused by "Transverse Shear".Is this the shear modulus in another direction?

 

[ChipB]

Maybe I'm a little bit late on this reply but:

Bar grating should NEVER be used as a flexible diaphragm. Its sole purpose is to distribute vertical loading to the support beams. NO LATERAL LOADING. I honestly have never heard of an engineer using it as a diaphragm. Does it truly help? Probably a little, but are you going to trust they tack weld it properly and not use G-Clips? Is it in a corrosive environment? I've seen bar grating corroded to the point I wondered how it was still supporting anything.

That being said, I'd model it as a thin plate to match the weight of the grating you are using, and release everything on the plate nodes supported by the beam except the Fy direction (specification page).