Semi rigid diaphragm
Semi rigid diaphragm
(OP)
I am trying to model a semi-rigid diaphragm in a finite element program. I am looking for very accurate results and would like some help.
The diaphragm is a standard wide rib 1 1/2" deck. But the shear deflection calculation uses a G' for the shear modulas, which combines a lot of factors in it. I am currently modeling using flat plate elements with a mesh that is about 5% of the total area. My idealized model (pined on one corner with rollers at the support nodes) doesn't deflect anywhere close to where the diaphragm design guide says it should. I have reduced the shear modulas so that it matches G', but it doesn't come close (off by more than a factor of 10).
I could sure use some help.
The diaphragm is a standard wide rib 1 1/2" deck. But the shear deflection calculation uses a G' for the shear modulas, which combines a lot of factors in it. I am currently modeling using flat plate elements with a mesh that is about 5% of the total area. My idealized model (pined on one corner with rollers at the support nodes) doesn't deflect anywhere close to where the diaphragm design guide says it should. I have reduced the shear modulas so that it matches G', but it doesn't come close (off by more than a factor of 10).
I could sure use some help.






RE: Semi rigid diaphragm
Is it possible that your software develops its own G from E and Poisson's ratio? It might be worth changing the diaphragm E as well, to see what happens.
RE: Semi rigid diaphragm
For a given steel deck spanning a distance L and extending a depth B(parallel with the applied load), the maximum lateral diaphragm deflection at midspan is:
D = q x L^2 / (8 x G' x B)
where q is the lateral load in kips per inch
G' is the diaphragm stiffness.
If you try a particular deck, you can hand calculate the G' from the diaphragm charts provided by SDI or the manufacturer. Calculate D from the above equation using a distance L and depth B which is similar to your model. When you create your model, add you finite elements with E = 29000 ksi and G = 11,200 (for basic steel).
What we do is vary the finite element thickness. Compare your hand calculation under a unit load with the computer model under the same unit load, varying the thickness until the deflections are close. You now have a diaphragm that should behave pretty close to what the deck will provide.
Your calcs won't be perfect...there is a level of approximation and variation in deck behavior (second order buckling between the supports).
RE: Semi rigid diaphragm
Thanks for the tips. I have used the varied thickness of deck and E & G for steel, and I have also used the 1 inch deck varying the shear modulas. The G = 20 ksi seems to give me the most consistant answer (at least one that I can more easily justify mathematically).
What I think that I am running into is that the finite element plates are derived using beam theory, with varying levels of shear deformation accuracy (often ignored in classic beam theory).
What I would like to do is to get the stiffness right so that I can model a 3-sided box correctly. Does any one know of a program that models the finite elements correctly for shear deformations, especially along non-orthagonal axis.
I need a way to do this, if by hand or by computer. So if ANYONE can give me guidence, it would help.
Thanks
Doug.
RE: Semi rigid diaphragm
RE: Semi rigid diaphragm