Direction and Effect of Section Resultants

Tips & Tricks

000953 7 January 2015 Tips & Tricks RFEM Gerhard Rehm Results

As already mentioned, it is possible to display resultants of a section. This post explains which part of a cross‑section area is referred to. The easiest way would be to apply the resultants to a section edge of the area. However, since a section may continue through several areas with different local coordinate systems, the determination by means of a section edge is not possible.

The section ‘crosses’ one or more surfaces, thus creating two parts. One part is referred to as the ‘original part’, to which the resultants take effect, and the other part is referred to as the ‘detached’ part. Now you need to determine which part is the ‘original’ and which is the ‘detached’ part.

For this purpose, you can set the ‘section coordinate system’ (SCS) in the section, which is oriented similarly to the global coordinate system. It is important to note that the global Z‑axis of the SCS is always oriented upwards, even if the global Z‑axis is oriented downwards. The detached part is the one that is more distant from the respective SCS axis.

All three axes are checked one by one: first X', then Y' and finally Z'. If the section is parallel to the global X-axis, then the X' allows no determination and you need to check the Y', and so on.

Example 1

X' does not allow any determination as the section is parallel to the X‑axis.
Y' does not allow any determination as the surface is in the XZ plane.
Z' is governing here, upper part is detached in +Z'.

Upper part = detached part
Lower part = original part

Force always acts on the original part.

Example 2

X' is governing here, right part is detached in +X'.

Right part = detached part.
Left part = original part.

Force always acts on the original part.
It is not necessary to check Y' and Z'.

Example 3: Section through a wall

X' does not allow any determination as the section is parallel to the X‑axis.
Y' does not allow any determination as the surface is in the XZ plane.
Z' is governing here, upper part is detached in +Z'.

Upper part = detached part
Lower part = original part

The detached part is only virtual here as the section is at a boundary line of the area.
Force always acts on the original part.

Example 4: Section through a ceiling

X' does not allow any determination as the section is parallel to the X‑axis.
Y' is governing here, right part is detached in +Y'.

Right part = detached part
Left part = original part

The original part is only virtual here as the section is at a boundary line of the area. Therefore, the resultant is directed against the X'‑axis or it acts on another ‘section edge’.

Force always acts on the original part.
It is not necessary to check Z'.

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Structural engineering software for finite element analysis (FEA) of planar and spatial structural systems consisting of plates, walls, shells, members (beams), solids and contact elements