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4.15 Member Eccentricities
The length of a member corresponds to the distance between two nodes defined by the member line. However, for connections of cross-sections or downstand beams, reality is represented only to a certain degree. With member eccentricities, you can connect members eccentrically due to special member end sections. You can, for example, reduce design moments on horizontal beams for frames with large column cross-sections. Member eccentricities are taken into account through a transformation of the degrees of freedom in the local element stiffness matrix.
To check the entered eccentricities, use the photo-realistic imaging of the 3D rendering.
The member eccentricity can be related to one of the following axis systems:
- Local member axis system x,y,z, in consideration of member and cross-section rotation
- Global coordinate system X,Y,Z
Use the Display navigator or the shortcut menu of the member to display the local member axes xyz (see Figure 4.165).
In the Absolute Offset dialog section or in table columns B to G, define the eccentricities for the Member start i and the Member end j. The distances refer to the selected axis system, indicated by the upper- and lower-case indexes, which are also shown in the dialog graphic.
In the dialog box, you can use the and buttons to transfer the values from one side to the other.
In the Relative Automatically Offset dialog section, use the nine selection options to define the cross-section point relevant for determining the eccentricity. In the table, specify the position of the point in columns H and I. The point defines the distance by which the cross-section is moved on the start or end node.
By defining the point in the middle of the top flange, as shown in the figure on the left, you can, for example, attach a horizontal beam with its top edge to a column with a flush connection (without extension).
With a Transverse offset, you can arrange a member in a particular distance parallel to an object (member in surface, member in same direction). Select the number of the relevant object - a Member or a Surface - from the list or use in the work window. The eccentricity is determined from the Cross-section alignment defined above and the Axis offset (cross-sectional geometry or surface thickness) that you define by selecting one of the available nine or three check boxes. In the table, define the axis offset in the columns L and M.
For example, by defining the points on the edge of the top flange and on the bottom side of the surface as shown in the figures on the left, you can arrange a steel cross-section on the edge under a plate with a flush connection.
The last option in the Relative Automatically Offset dialog section allows you, for example, to easily connect a member eccentrically to the flange of a column. The offset can be arranged separately for Member start and Member end. The eccentricity is automatically determined from the cross-section geometry of the adjacent members. In the table, assign the axial offset in the columns N and O.
The Axial Offset dialog graphic is interactive with the input, illustrating the effectiveness of the selected check boxes.
You may prefer the input in the Relative Automatically Offset dialog section, because you can directly adjust the eccentricities when cross-sections are changed. RFEM automatically takes modified surface or cross-section dimensions into account.
Furthermore, eccentricities can be graphically assigned to members in the work window. Use the menu option
- Insert → Model Data → Member Eccentricities → Assign to Members Graphically
- Edit → Model Data → Member Eccentricities → Assign Graphically to Members.
First, define the reference system and the eccentricities.
After clicking [OK], members are divided graphically at one-third division points. Then you can click the member sides you want to apply the eccentricity to (see Figure 4.142). To assign an eccentric connection to both member ends, click the member in its center area.