Modeling and Design of Trusses in RAM Structural System

 The modeling of trusses in RAM Structural System can be done easily, but there are some important design issues to consider. Below is an image of a truss modeled using long beams for the chords on two separate levels.

If the truss needs to have a pitched top chord, that can also be done by modeling a column at the apex (on the top chord level between nodes 3 and 8 in the figure below), then model the top chord using two beams rather than one and modify the elevation of the column top to create the slope.

The diagonal and vertical members of the truss are modeled as vertical braces using the add-special brace command with an offset distance consistently measured from one reference column. If the braces do not meet at a common work point multiple nodes will results in RAM Frame creating eccentricities or abnormal analysis results.

The resulting truss will, by default, have internal nodes that are disconnected from the rigid diaphragm in the RAM Frame analysis. The end nodes, however, will still be connected to a rigid diaphragm. The presence of this rigid diaphragm has the effect of inhibiting the natural deflections of the truss. The top chord will not compress and the bottom chord will not elongate. Consequently, the net vertical deflection of the truss and the axial forces in the chord beams will be unconservative. For this reason, it is recommended not to use a rigid diaphragm when analyzing a truss for gravity loads. Similarly, the semi-rigid diaphragm stiffness will adversely affect the chord forces, some "Flexible/None" is the best diaphragm setting. 

If the same model is to be used for lateral analysis, then a decision has to be made about which frames are effective in resisting the lateral loads. If there is a rigid diaphragm connected to other braced frames, excluding the trusses, then the model can be set up using rigid diaphragms on all levels, then manually disconnect the trusses from the rigid diaphragm using the assign - nodes - diaphragm connection command. If the truss is required to be part of the lateral force resisting system, consider only attaching a single node to the diaphragm. The axial force in the chords will change depending on which node is connected to the rigid diaphragm.

There are a few additional limitations to consider in the design of trusses. Since the chord is generally modeled as a single beam, the section size must remain constant over the length. Similarly, since the chords are beams, they are limited to web vertical applications using W shapes, channels or tubes. The unbraced length of the chords can be specified in RAM Frame - Steel mode, but the default method will assume the chords are braced in both axes at each panel point. It is not possible to assign an axial release to the bottom chord at the support which is generally the way a top chord bearing truss with gravity loads is detailed. since the top and bottom chords are modeled on two separate levels they cannot converge to a point, there must always be at least a small separation between the chords.

See Also

Structural Product TechNotes And FAQs

RAM SS Semirigid Diaphragms

RAM Instability In Finite Element Analysis