B21 and B31

[mj345]

Hello All,

Thank you for everybody for quick replies.

I have couple of questions using ABAQUS

1. What is the difference between B21 (Planar beam) and B31 (Beam in Space)? What are the applications of them?
2. Why to add membrane elements as a skin on Tetrahedral elements to get more accurate results?

Your help is appreciated.

Thanks

 

[Erik Panos Kostson]

1. B21 is a plane beam element with active dof 1,2,6 (ux, uy and rotate_z if it is defined in xy plane), while B31 is a general 3D beam in space with all 6 dof active (1,2,3,4,5,6).
This info can be found in the help manual section 29.3.8.

2. This is used to get surface stresses on bricks, the membranes should be weak so not to influence the overall stiffness of the structure.
(I never used this though-try to code, teach, coach and support more rather than doing FEA these days , so not sure how common this approach is - in my 5 years at Strand7 I have never seen it, but than again it is used mostly for structural analysis of beam/plate structure)

 

[FEA way]

1) Both are 2-node linear beam elements but the difference lies in the number of degrees of freedom - B21 has 3 DOFs(1,2,6) while B31 has 6 DOFs (1,2,3,4,5,6). Of course this also means that some output variables are not available for B21 (for example SF3 and SM2 or SM3).

2) Skins are used to model thin reinfirving layers for example in sandwich beams. Their advantage is that you don’t have to use solid elements thus reducing the number of degrees of freedom.

 

[mj345]

Thank you so much for quick response