Roark's vs FEA and Simply Supported vs Fixed Edges

[KarlParks]

thread727-255244

Starting a new topic since the previous was closed.

https://www.eng-tips.com/viewthread.cfm?qid=255244

I'm looking at a flat plate that will be subjected to a pressure load just like the previous forum thread.

I am trying to design a vacuum box and I'm analyzing one face. I am new to using Roark's 7th edition and new to FEA. I'm trying to use Creo Simulate and have been able to compares Roark's "Rectangular plate,all edges fixed" with my FEA model with good accuracy. My issues is comparing Roark's "Rectangular plate; all edges simply supported" with my FEA model... my deflection and stress values are way off if I fix the bottom edges of my plate instead of the outer surfaces.

Couple of questions:
- What's the difference between all edges simply support and all edges fixed in a FEA model (Creo Simulate)?
- What are the deflection values so different in Roark's Calculations for fix edges and simply supported?

12x10x0.125" steel plate
6.7psi pressure load

All Edges Fixed!
sigma_max = 16440.192 psi
y_max = 0.0222 in

All Edges Simple Supported!
sigma_max = 16131.456 psi
ymax = -0.0729 in

Thanks... still a student and have not worked with many pressure loads on flat plates (only cylindrical tanks).

Karl

 

[rb1957]

something doesn't smell right ...
SS edges deflects down, but fixed edges deflects up ?

Never heard of "Creo Simulate". I'd try some simple test cases that you can solve by hand ...
free the plate on two (parallel) sides ... now SS plate should deflect like a beam and fixed plate like a double cantilever.

If you are a student please read the signs. This is a site for professional engineers.

If you're new to FEA ... try simple cases and read texts ... whether you know it or not, you're walking in a minefield.
Applying incorrect boundary conditions is the most common source of error/failure in FEMs.
Not knowing exactly what your code is doing with your definition of boundary conditions is the most common precursor to the above !

another day in paradise, or is paradise one day closer ?

 

[tigny]

Hi,

- What's the difference between all edges simply support and all edges fixed in a FEA model (Creo Simulate)?
- What are the deflection values so different in Roark's Calculations for fix edges and simply supported?

Get a good read of the definitions of Roarks 11.1.4. Edge Conditions.
You may find a clue why the FEA model differs from rules calculations.

Best regard,