Cable FAQ

Should a load which represents initial cable conditions be applied before subsequent nonlinear analyses are run for additional external loads?

Answer: Yes, in most cases, it is a good practice to assign a load which represents initial cable conditions. This is done using the Cable Geometry menu. Initial cable tension is dependent upon initial geometry and applied loading. Given distributed loading on a single cable with fixed supports, the cable drawing tool can calculate this tension. Full analysis must be implemented for systems of cables, cables connected to flexible structures, and conditions where additional loads are applied. The best approach depends on what information is known and how the structure is to be built. If cable length is known, a simple nonlinear analysis procedure should be sufficient. Otherwise, initial-geometry iteration or target-force loading may be necessary.

Once the initial shape and tension of a cable is determined, further analysis is necessary when additional loading is applied. Nonlinear analysis is also recommended. Video 16 of the SAP2000

video series provides excellent instruction on this topic.

In conventional static-linear analysis, does calculation consider cable stiffness?

Nonlinear analysis is necessary to generate meaningful results concerning cable behavior. Users may perform linear analysis, though cable stiffness values should be taken from a nonlinear static-analysis load case where cables are subjected to all loading conditions, including self-weight, transverse, etc. When a cable with slack is not subjected to any form of transverse loading, a negligible self-weight is imposed to enable the formulation. However, it is best to apply this self-weight load manually such that it is of realistic magnitude.

Why are the cables of my model returning nonzero moment values?

Objects intended to be cables would generate bending response when modeled using straight frame objects. If moment values are returned, users should model cables as catenary elements by unchecking the Model Cable Using Straight Frame option on the Cable Geometry menu.

How can I display the wind load applied to cables?

Users may display wind load on cables by using the Display > Show Load Assign > Frame/Cable/Tendon menu command. To assign wind load to cable elements, the auto load pattern should be defined using the Exposure from Frame Object option.