What is Beam Element Section Offset and How Does It Affect Calculation Results?

First, let’s explain the basic concept of section offset:

Section offset refers to the situation in finite element analysis where the structural center and the section center do not coincide. The structural center here can be understood as the line connecting the end nodes of the element, and the section center can be understood as the line connecting the centroid points of the first and last cross-sections of the element. Not only beam elements have section offset, but shell elements also have section offset situations.

Why does section offset exist?

There are many reasons for section offset, which can be mainly categorized into two causes:

1. Design reasons: In some cases, for practical considerations, the theoretical finite element model differs from the actual situation. For example, in common frame structures, when we model floor beams and slabs, if we use the slab elevation as the modeling elevation and the default node and section center are aligned, the beam element section will protrude above the floor, which obviously does not match the actual situation. Therefore, the beam element needs section offset so that the top of the beam element is flush with the top of the slab, matching the actual condition.

2. Construction reasons: In industries with large product tolerance, design and construction are not always perfectly matched in many cases. During verification, it is no longer possible to follow the design drawings, but rather the actual objective conditions should be followed. That is, the section position may not match during actual construction. For civil engineering, especially in the reinforcement industry, special attention should be paid to section offset.

Impact of section offset on member forces:

Since we typically apply loads based on nodes, i.e., the center of the structural member, when section offset occurs, there is a certain distance between the load and the section center. In this case, the load becomes an eccentric load, and the impact is that in addition to the original load, the beam may also be subjected to a certain eccentric bending moment or eccentric torque.

Below, a cantilever beam is used to briefly illustrate the impact of section offset.

A certain cantilever beam with a length of 3m, cross-section dimensions of 200X400, concrete material C30, subjected to a vertical concentrated force of 20KN at the free end, as shown below:

Consider the following three working conditions:

Condition 1: No section offset

Condition 2: Section offset of 200mm in the short axis direction;

Condition 3: No section offset, but consider an eccentric torque of 20KN*200mm.

The calculation results for each condition are as follows:

Condition 1 result, maximum displacement is 5.712mm

Condition 2 result, maximum displacement is 5.846mm

Condition 3 result, maximum displacement is 5.864mm:

From the calculations, it can be seen that after considering section eccentricity, the calculation results show a certain difference. When the load is large, this difference cannot be ignored! At the same time, if you do not want to consider section offset during modeling, you can also apply eccentric loads or use rigid links to apply loads.

The above is for beam elements; the same applies to shell elements!