Difference between Truss, beam, and frame element

[JCdzx]

Hi

My name is John, currently working as a structural engineer. I have had some discussion with my college and couldn't get an ultimate conclusion about some issues in FEA. I would like to raise some questions here and would be really appreciate if someone could answer them for me.

1. As much as I concerned, I have understood that truss elements can take only axial forces (tension/compression),no moments can be supported, while beam elements possess only shear and bending capacity (no axial deformation is allowed). As for frame element , which is the combination between truss and beam elemens, is capable of supporting all axial, shear, bending loads. Does my understanding conform with the concept of FEA, basically is it correct?

2. Could you illustrate the differences between these 3 elements in FEA?

3. As I have experienced using Abaqus, I could tell that I am quite confused the difference between beam and frame elements. In Abaqus, both element types can support axial, shear, bending, and torsional loads. As far as I know, beam elements do not support axial deformation. However, I know that I can apply axial forces to a beam element and obtain correct stresses and deformation in Abaqus. Could you illustrate the significant discrepancies of such usage in Abaqus?

4. In the assumption of plane frame element, it states that the axial and bending effects are uncoupled. What does it imply by saying "uncoupled"? I do not get it with the term "uncoupled"?

Feel sorry for such long questions, however hopefully these questions are also useful for other members.
Thanks!

 

[transmissiontowers]

A truss member is a pin ended 2 node member that only has axial load. A beam member is the same as a frame member. They are both a 2 node member and have all 6 DOF at both ends so they can carry axial, shears, moments and torque. If you are working as a structural engineer, these concepts should have been covered in very early statics courses.

In computer programs a plane frame is a special case that throws away the out of plane moments and shears of a 3D frame structure. Back before there were computers, the computational effort was so great that engineers assumed away the 3rd dimension and only solved a 2D plane frame with the rationalization that the forces were distributed to another 2D plane frame perpendicular to the plane frame they were analyzing at the time. This is also true for trusses where they assumed a plane truss would carry the loads as axial forces and they made assumptions that forces were distributed by members so they could do a method of joints graphical analysis to solve the indeterminate truss problem.

As computers became available, programs were written to solve truss and frame problems in 2D and 3D.

_____________________________________
I have been called "A storehouse of worthless information" many times.

 

[rb1957]

beam elements can react axial and bending loads; I've never seen a beam element unable to react axial load, but then I don't use Abaqus.

if an Abaqus beam element can only react bending loads, then I'd back it up with a rod element to take the axial loads, and sum the two elements in a hand calc.

I don't get the "decoupled" comment either ... ask Abaqus directly.

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

 

[Denial]

I'm not an Abaqus user either, and never have been.[ ] But I suspect that the "uncoupled" comment refers to the standard situation with a (thin) beam element's element stiffness matrix for linear, small-displacement behaviour.[ ] It is merely a mathematician's way of saying that, for a beam fully clamped at each end, if you move one end axially you do not induce any bending, and if you move the end transversely you do not induce any axial force.