Considering Member and Surface Eccentricities

Technical Article

In the case of a parallel offset of the structural plane of members and surfaces, or in the case of applying an axial offset to members, the function of eccentricities may be useful.

Figure 01 - Eccentricities

Eccentricity of Surfaces

Surfaces can be displaced into the local z-axis using the "Support / Eccentricity" tab of the "Edit Surface" dialog box. For example, this would be necessary if there is a plate thickness jump within a plate and the surface must remain planar. In this case, a new surface with a greater thickness is entered and integrated into the thinner plate, and the eccentricity is defined.

Figure 02 - Surface Eccentricity

Eccentricity of Members

Members can be defined in the "Options" tab of the "Edit Member" dialog box, or you can create a new member. In the "Edit Member Eccentricity" dialog box, absolute and relative offsets are available.

You can select the absolute offset in reference to global or local (member axis) axis system and enter the eccentricity as the absolute distance to the previously used system line.

In the case of the relative offset, you can define the member eccentricity depending on the particular cross-section and the adjacent structural component (member or surface).

Furthermore, it is possible to select the automatic axial offset from the adjoining members.

Figure 03 - Member Eccentricities

Considering Eccentricities in Calculation

Eccentricities should not only be used for a realistic representation since they also affect the deformation and internal forces.

If the eccentricity is entered for a surface, imagine the analytical model as an open shoebox. Quasi invisible rigid surfaces are formed at all boundary lines, which are rigidly connected to the shifted parent surface. The height of the shoebox corresponds to the defined eccentricity. Therefore, the eccentricity should only be applied on the interior surfaces and not on the surfaces with free edges. Otherwise, restraining moments occur on the free edges. The results are displayed in the original altitude.

Figure 04 - Analytical Model of Eccentric Surface

Similar to the analytical model of the surfaces, a connection to the original node is created in the background in the case of member eccentricities. Therefore, members can also get boundary moments due to the eccentricity.

Figure 05 - Analytical Model of Eccentric Member

Especially in RFEM, it is possible to control the location of any existing member hinge in combination with the eccentricity. By default, these hinges are located in the shifted axis system. However, if the hinge should apply to the original node, you can set this separately for the member start and member end.

Figure 06 - Member Hinge Location

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