I need to specify an imperfection covering several members lying on top of each other, but it is not possible.
What is the reason for this?
At the tree members, you cannot specify a continuous imperfection, because the members differ in their direction. The imperfections, however, follow the directions of the local axes.
A good way to see this is to activate the local member coordinate systems in the "Display" navigator (see Figure).
The problem with the different directions of the local z-axis is due to the general location of the members. You can see this, for example, in window 1.7 (RFEM 1.17) in the column Location of the member. If a member has an inclination, the location of the local axes y or z first are automatically specified by RSTAB. The z-axis is orientated in such a way that the Z component of the directional arrow always shows in the positive global Z-direction relative to the global coordinate system. The axis is thus obtained according to the right-hand rule. For more information, see the RSTAB manual, chapter 4.7, or the RFEM manual, chapter 4.17.
The inclined location of the member is usually due to an imprecision of the nodes. This is also the case in this example (see figure), as the Y-coordinate of the node 1 shows (0.002 m instead of 0.000m).
Furthermore, for the member display, it is possible to consider the tolerance as vertical. To do this, select on the menu "Tools" -> "Regenerate Model."