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Preliminary beam problem

M

msiva1981

Member
I am trying to verify Analytical results for a simply
supported beam with a point load in the middle as a bench mark test. I get two
different results using idealization and solid beams for the same beam section,
load, material and BC . Could anyone tell me why?

Iam applying load on negative Y Axis. My BC on one end of the I BEAM Are

Dx=Dy=DZ=0

RX=RY=0 Rz=free

The other end DY=0 DX=DZ=free

Rx=Ry=Rz=free

For these conditions my
idealized beam gives me theoretical results but the 3D beam does not.

Please let me know as to why this is happening



Siva
 
Siva,

My first guess would be that the 3-d Beam (solid elements?) is over constrained at the ends. Can you post the model?
 
Mloew,

Thank you for willing to help

I have attached two files

1.
Simply supported beam with Beam idealization

2.
Simply supported beam with solid elements

I am trying to verify
displacements with hand calculations. The one with Beam idealizations is very
close. I am happy with that. But the one with solid elements is nowhere close. Both
these files have same cross section and both have same BC. I am concerned about
displacement.

Please let me know as to what I am
doing wrong

Thanks

Siva


2007-10-08_131008_Frame_Analysis.zip
 
Solid elements which Mechanica uses are not accurate for bending. The only way to obtain accurate results is to use the beam idealization. Other FEA software, such as ANSYS, allow the user to define another type of element (quadratic elements) to 3D solids whichare capable of calculating bending more accurately. In Mechanica, the quadratic element can only be applied to shell. Hope this helps.
 
Hi JRay,
Thanks for your reply. yes i obtain results pretty close to theory using beam idealizations.i want to run an optimization study to select the lightest shaped beam for my study. Is that possible with beam idealizations? Also iam not sure how to apply uniformly distributed load using beam idealizations (pressure load). Would you know?
Thank you for your time
Siva
 
Siva,


Sorry, I can't help you with the design optimization using Mechanica. I'm not too familiar with conducting those.


For a distributed load (WF2): Select "New Force/Moment Load; In the References drop down menu select "Edges/Curves"and pick the edge/curve used to define the beam; Click the "Advanced" button, this will show two more drop down menus, the first is distibution. Within there is "Force per Unit Length."


Good Luck,


Josh
 
I don't agree with the comment from JRay. Solid elements in Pro/MECHANICA are perfectly well suited for capturing the stress distributions typical of bending. The minimum polynomial order is 3 and therefore the equivilant of a quadratic element. With increasing edge order due to convergence, more calculation points are added. This is not the issue.

I still believe that there is a potential problem with the constraints and that care needs to be taken to create an equivalent BC set to the beam model; especially since solids do not have rotational DOF.
 
Hi all,
Thanks for all your help. As Mleow pointed out i had my constraints wrong. I selected the entire surface of the beam on both ends. When i selected just the bottom edges on both sides i could obtain results in comparison to theory
So i guess Pro Mechanica solid elements work on beams
Thanks
Siva
 
I tired to do the same analysis, but it is wrong:


100x100x1000 squared beam,


Dx=Dy=Dz=0 on both the BOTTOM edge of the beam.


Load per unit area = -1 (Y)


The result isWRONG if compared to theory (75 MPa): Proe gives 50 MPa on the top, 30 MPa on the bottom.


The analysis isRIGHT if I put Dx=Dy=Dz=0 onMIDDLE edge (corresponding to the neutral plane).... just 75 MPa.
 
hello JRay.


From where do you get this information about bending and solid elements in mechanical?...could you give any practical example?..
 
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