Truss members are modeled in CSI Software using frame objects which are assigned end releases. A designated truss member is not available to modeling because CSI strives to keep the interface and controls simple and practical, using fewer objects and features which are more versatile. Hence frame objects are suitable to represent truss members.

Modeling a truss member

To model a truss member, draw a frame object without end or joint offsets, then assign M2 and M3 releases to both ends, and a torsional release to one end. When multiple truss members are connected to form a truss system, end releases should be coordinated to prevent numerical instability.

An alternative modeling technique, which eliminates the need to specify end releases for each member, is to make only translational degrees-of-freedom (DOF) available to analysis by selecting Space Truss from the Analysis Options menu, available through Analyze > Set Analysis Options > Set Solver. Space Truss and Space Frame are compared in the Joint DOF article.

Truss stability

Stability considerations which must be taken into account when modeling truss systems include:

1. Stability is dependent upon the load carrying capacity of members.
   
   Distributed loading (including self weight) must not be applied to truss members because they are released from flexural capacity, and are therefore unable to resist the moments generated by distributed loading. When distributed load is applied to a truss member, moment will find no reaction, and a condition of instability will arise. By providing translational stiffness, and not rotational stiffness, truss members transfer load only through axial behavior. Only concentrated point loads should be applied to the nodes at either end of a truss member.
2. Stability is dependent upon the relationship between truss geometry and support conditions
   
   The structural system must be statically determinate such that equilibrium conditions are maintained, and numerical formulation may resolve internal forces and external reactions. A 2D truss is statically determinate when:

Before interpreting results, verify stability conditions by checking for instability warnings in the log file, available through File > Show Input Log Files.

See Also

• Frame FAQ

• Joint DOF

SAP2000 does not have a designated truss element, but truss elements can be modeled using frame elements with end releases as described in the following sections.

Approach 1

The frame element becomes a truss if there are moment releases at both ends and a torsion release at one end, and there are no end offsets or joint offsets. This approach works well for isolated truss members. When multiple such members are connected together, the end releases must be coordinated to prevent numerical instabilities or the approach described in the next section should be used.

Approach 2

If your structure is a true truss (i.e. all member are truss members that carry only axial forces), it may be convenient to use the following modeling approach:

• Use only a single member between connected joints.
• Specify M2 and M3 releases at both ends of all members and torsional release at one end.
• Make only translational DOFs available for the analysis by specifying the Space Truss option on the Analysis Options form available via Analyze > Set Analysis Options > Set Solver options.

This approach will eliminate the need to specify different end releases separately for each member. Also please note that even members with end releases still resist their own self-weight by a flexural resistance.

See Also

• Frame FAQ, section "Is there a truss element available in the SAP2000 program?"
• Available DOF

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