FEM background: load applied on beams with line support

Introduction

This chapter is to provide a closer information about the behaviour of the finite element model. There is certain feature in the background which might cause certain confusion, this chapter is to explain how it works.

Feature

Internal consideration of the load applied on the beam which is supported by line support

Any load applied on a 1D beam member which is supported by any type of line support, is internally considered as nodal loads applied on finite element nodes of the corresponding beam. Example is provided below.

Example 1 - simple supported beam

There are two beams, same material, same line loads are applied. The difference is in boundary condition in the GCS Y direction, in a direction no load is actually applied. One beam is supported by line support in Y direction constrain the translational motion, the other is not.

Number of finite elements per beam is important here. To check the actually considered mesh (nodes, beams), display options needs to be set as in the figure below. In this case, the beam with line support in Y direction consists of 4 finite element nodes and 3 finite element beam members.

After linear analysis of the corresponding load case, there will be a difference in the bending moment My. The beam which is not supported by the line support has a nice parabola shape moment, as expected. However, the beam with line support has bending moment in a shape of linear by parts. The reason is, in this case, the line load is actually internally considered as a nodal loads by the solver - hence, the FEM node 5 and 6 are both loaded by a force of 1 kN. This is the reason of the bending moment pattern.

Example 2 - structure

Another example is shown on more a complex structure modelled in one plane (same structures, the difference is only in line support in out-of plane direction). Loaded by the self-weight. The difference in bending moment might be significant in this case, if the mesh is not fine enough.

In order to get similar results, either mesh needs to be finer (the parabola on the structure right would however not be smooth, but linear by parts). Alternatively it is posisble to support the structure in the out-of plane direction only in selected nodes, not along the whole lines.

Note: This behaviour is feature.