FAQ 001448 EN

Calculation RFEM RSTAB

Can I also design a cable truss in RSTAB?

Answer

We recommend you to prestress the cable members. In this way, you prevent compressive forces that would result in a failure. Cables should be used only if the initial deformations represents a significant part of the changes of the internal forces, that is, if large deformations can arise. In case of a simple linear stay, for example a projecting roof, tension members are completely sufficient.

When evaluating the initial deformation mode, the display factor in the control panel is set to "1," so that the tightening effects appear realistic.

The large deformation analysis, also known as "cable theory," takes into account the analysis of the longitudinal and transversal forces. If you select the large deformation analysis, all surfaces and members are subject to this calculation method.

The Newton-Raphson procedure is used, in which the nonlinear equation system is solved numerically by means of iterative approximations with tangents. The convergence behavior can be influence in the "Options" tab by means of the number of load increments.

When using the large deformation analysis, the stiffness matrix is created after each iteration step for the deformed system.

RSTAB

Locally defined member loads are now handled in such a way that the load direction is used as applied to the undeformed system. All internal forces are transformed to the member axis system.

If members of the type 'cable member' are used in the system, they are always calculated according to the large deformation analysis. The rest of the members are treated according to the selected calculation method.

RFEM

Note that there is a considerable difference between locally and globally defined loads. Think of a horizontal surface subject to a uniform surface load. If the load is defined globally in Z, the load keeps its direction, when the FE elements are twisted. However, if the load is applied in the direction of the local surfaces axis z, the load on the element is rotated corresponding to the twist of the element.

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