Roark's Flat Plate Equations vs. FEA... what's the deal?
Roark's Flat Plate Equations vs. FEA... what's the deal?
(OP)
I started a new topic since this one was closed:
http://www .eng-tips. com/viewth read.cfm?q id=171673& amp;page=4
I'm looking at a flat plate that will be subjected to a pressure load. The finite element model results were ~15% off of my hand calcs so I'm back to a simple case and I still can't figure it out.
Basic problem I'm looking at in Roark's:
10" x 12" x 0.125" thick steel plate
6.7 psi pressure load
All edges fixed
Roark's results:
stress along edge = 16440 psi
stress at center = 5943 psi
deflection = 0.016 in
MSC Nastran results:
14100 psi (14% off of Roark's)
6933 psi (16% off of Roark's)
0.021" (24% off of Roark's)
Nastran model was run with various mesh sizing, CQUAD4 elements, and all nodes along edges fixed in 123456 directions.
I don't think I'm violating any of the assumptions presented for these tables so why is there such a big difference in results?
I'm not a structural analyst by trade so sorry if I'm overlooking something simple.
http://www
I'm looking at a flat plate that will be subjected to a pressure load. The finite element model results were ~15% off of my hand calcs so I'm back to a simple case and I still can't figure it out.
Basic problem I'm looking at in Roark's:
10" x 12" x 0.125" thick steel plate
6.7 psi pressure load
All edges fixed
Roark's results:
stress along edge = 16440 psi
stress at center = 5943 psi
deflection = 0.016 in
MSC Nastran results:
14100 psi (14% off of Roark's)
6933 psi (16% off of Roark's)
0.021" (24% off of Roark's)
Nastran model was run with various mesh sizing, CQUAD4 elements, and all nodes along edges fixed in 123456 directions.
I don't think I'm violating any of the assumptions presented for these tables so why is there such a big difference in results?
I'm not a structural analyst by trade so sorry if I'm overlooking something simple.





RE: Roark's Flat Plate Equations vs. FEA... what's the deal?
RE: Roark's Flat Plate Equations vs. FEA... what's the deal?
Thanks for the reply.
RE: Roark's Flat Plate Equations vs. FEA... what's the deal?
Just click on "Thank namklof for this valuable post!" under the post that was helpful. Another screen will pop up to confirm.
It's a nice way of saying 'Thank you'.
RE: Roark's Flat Plate Equations vs. FEA... what's the deal?
It's unusual to see an FEM softer than real world.
Cheers
Greg Locock
SIG:Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips.
RE: Roark's Flat Plate Equations vs. FEA... what's the deal?
ymax -2.22E-02
Sig side -16440.192
Sig cent 7692.672
Results from my FEA program (Strand7) were:
in the same order as above
4 node 12x10 grid
-2.35E-02 105.5%
-16370 99.6%
7788 101.2%
8 node 12x10
-2.30E-02 103.5%
-16190 98.5%
7686 99.9%
8 node 24x20
-2.301E-02 103.5%
-1.637E+04 99.6%
7.558E+03 98.2%
9 node 24x20
-2.301E-02 103.5%
-1.637E+04 99.6%
7.558E+03 98.2%
Doug Jenkins
Interactive Design Services
http://newtonexcelbach.wordpress.com/
RE: Roark's Flat Plate Equations vs. FEA... what's the deal?
Tri3: -5.3%
Quad4 +4.7%
Tri6 -12%
Quad8 +0.48%
Quad9 +0.48%
So it seems the quad elements are a little softer than the analytical solution in this case.
Doug Jenkins
Interactive Design Services
http://newtonexcelbach.wordpress.com/
RE: Roark's Flat Plate Equations vs. FEA... what's the deal?
Cheers
Greg Locock
SIG:Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips.
RE: Roark's Flat Plate Equations vs. FEA... what's the deal?
Doug Jenkins
Interactive Design Services
http://newtonexcelbach.wordpress.com/
RE: Roark's Flat Plate Equations vs. FEA... what's the deal?
RE: Roark's Flat Plate Equations vs. FEA... what's the deal?
IDS, yes, there is a problem with my Roark's calculations posted above. Not sure what I did, but what you posted is the correct deflection & center stress.
Did a little reading and have a rested mind so things are much clearer now.
Thanks again for the help.