| Applies To | |||
| Product(s): | RAM Structural System; Ram Frame | ||
| Version(s): | Any | ||
| Area: | Modeling; Analysis | ||
Modeling and Design of Trusses in RAM Structural System
Trusses can be modeled in RAM Structural System, but there are limitations and important items that must be considered during modeling, analysis, and design of the truss members. The purpose of this tech note is to address these issues.
Modeling
Trusses are modeled using a combination of beams, columns, and vertical braces. RAM Steel ignores the presence of braces in its analysis. As a result, all truss members and any member supporting the truss must be modeled as a frame member, and the truss must be analyzed and designed in RAM Frame.
There are two common ways to model the truss. Both approaches will yield similar results.
1. Model the top and bottom chords as single, continuous members and model the diagonal and vertical web members as vertical braces using Layout - Brace - Add Special. The braces would be modeled with an offset distance consistently measured from one reference column. If the braces do not meet at a common work point, there will be multiple closely spaced finite element nodes in RAM Frame. This will create eccentricity, which can lead to abnormal analysis results and may produce meshing warnings if a semirigid diaphragm is used.
2. Model the truss verticals as columns supported by a single, continuous bottom chord below, model the top chord as a series of beams spanning between the columns, and model the diagonals as vertical braces using Layout - Brace - Add Standard. The top chord can be sloped using this approach by modifying the elevations of the columns (Layout - Columns - Modify Elevation).
RAM Structural System does not allow a beam on one layout to be connected to another beam that is modeled on a different layout. As a result, the top and bottom chords must be modeled on separate layouts. This limitation prevents the chords from converging to a single point like the truss shown below. These truss must always be modeled with a small separation between the chords.
Beams in RAM Structural System can only be modeled as I-shapes, HSS (tubes), or channels. It is not possible to model the top chords with any other section, including WT or double angle sections.
Analysis
Rigid and semirigid diaphragm can inhibit the natural deflection of truss and restrict elongation of the chords. This can result in unconservative net vertical deflections and axial chord forces. Rigid diaphragms are especially problematic. In RAM Frame, the end nodes of the physical chord members are assumed connected to the rigid diaphragm. The internal nodes (any node between the ends of a physical beam member) are disconnected to the rigid diaphragm by default. When both ends of a member are connected to a rigid diaphragm, elongation of the member is prevented and there is 0 axial force. For this reason, it is recommended not to use a rigid diaphragm when analyzing a truss for gravity loads. Similarly, the in-plane stiffness of shells associated with semirigid diaphragms can affect the chord forces. The adverse effects associated with vertical deflection and chord forces do not occur with "Flexible/None" or "Pseudo-Flexible" diaphragms.
See the web page below for more discussion on the diaphragm types available in RAM Frame:
RAM Frame - Criteria - Diaphragms
If the truss is not part of the lateral force resisting system, then all of the nodes can be disconnected from the rigid diaphragm. This will prevent lateral force from being directed into truss and prevent the adverse diaphragm effects discussed above.
If the truss is part of the lateral force resisting system, consider using rigid diaphragms and disconnecting every node but one end node to the diaphragm. Nodes can be disconnected from a rigid diaphragm using RAM Frame - Assign - Nodes - Diaphragm Connection. Note, however, that the axial force in the chords will depend on which node is connected to the diaphragm. If you are not sure which end node to disconnect, try each node, compare analysis results, and use engineering judgement to determine which node to connect to the diaphragm.
Design
The unbraced length of the chores is determined internally by the program. Beams framing into the chords are assumed to brace both the top and bottom flange for minor axis bending. Joists framing into chords are assumed to brace the top flange only for minor axis bending. A major and minor axis brace point is assumed at locations where a brace frames into a beam and where a frame column is supported by a beam below. The unbraced length can be assigned in RAM Frame - Steel mode - Standard Provisions - Assign - Beams - Unbraced Length.
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.
See Also
Structural Product TechNotes And FAQs
RAM Instability In Finite Element Analysis