Risa 3D Finite Element Beam Analysis 4

[wijgeng]

Hi,

I am getting familiarized in using Risa 3D for plates and it got me wondering. Can I use plates to analyze beams? Specifically, I am interested in modeling a plain concrete beam in bending with a distributed load. This will allow me to compare my results to those obtained from beam theory so I can get a better understanding of the results Risa provides for plate elements. I having a couple issues.

1. Are plate elements the correct elements to use?
2. Can plate elements be stacked? For example, if I have 12" thick by 9" wide elements and I want a 24" deep beam, can I put two layers of elements?
3. Am I correct that applying a 1 ksf plate load over a 9" wide beam will give me 0.75 k/ft load? It seems that my resultant moment is consistently associated with a 1 k/ft load regardless of the beam width I use. This doesn't seem right to me..

Any other pointers anyone has would be appreciated, thanks!

 

[klaus.]

yes...but it depends on the modelling

No

 

[JAE]

RISA has solid elements which might be better than plates.

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[jayrod12]

Agreed with JAE.

 

[wijgeng]

Thanks JAE, this is exactly what I was looking for!

 

[JoshPlumSE]

Some notes on solid elements, from when I did the validation on them some years ago:
a) You need at least 4 elements through the thickness of the beam in order to accurately capture the stress variation you will get from bending stresses.
b) The solid element formulation that RISA uses is more sensitive to distortion. Cubes are best. Elements that look like bricks should be fine. Elements that are more distorted than that could be problematic.
c) The FEM model will not exactly match hand calcs for deflection and stresses. If you look at it closely you may realize some of the assumptions (plane sections remaining plane) that we make for beam theory do not 100% hold when done in FEM. Though it should be pretty close.
d) There are verification examples that come with the RISA install for bending and torsion of a rectangular beam made of solids. You might want to start there.

Hopefully, this helps.

Sincerely,

A former RISA employee who is always happy to provide assistance!

 

[WARose]

1. Are plate elements the correct elements to use?

To represent beams? No. Why not use stick/beam elements (with cracked section properties)? It's been a while since I used RISA (thank God), but IIRC you can do that.

A note on solids (to build on what Josh was saying): they can (in some software) over estimate stiffness. So I'd check that first to see what you've got before you start anything else.