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Document Type: FAQ Product(s): RAM Structural System Version(s): Any Original Author: Bentley Technical Support Group
Document Type: FAQ
Product(s): RAM Structural System
Version(s): Any
Original Author: Bentley Technical Support Group
Any vertical or horizontal brace member may be assigned to be tension-only. The assignment is made in RAM Frame using the Assign menu - Braces - Tension-Only command. Assigning a brace to be tension only does not automatically release the brace for moments or torsion, however. That is controlled through the Assign menu - Braces - Fixity command.
If no assignment is made, then the braces will resist tension and compression elastically. Beams, columns and walls are always capable of resisting compression.
In the steel design mode, you might see the warning "T-O in comp" on the View/Update dialog box or "Member with Tension Only Shape is in Compression" on the design report. There are some cross sections that the RAM Frame Steel postprocessor can only check when they are in tension. If a single angle, rod or flat bar goes into compression, then the design will always give these warnings. You can manually check the design of such using the member force envelope results.
The same error message, "T-O in comp", can also occur for a member that is defined as a tension-only brace if there are custom load combinations utilizing negative terms on lateral load cases. It's important for models with any tension-only braces that the lateral loads be applied in all directions so that the load combos can use positive terms exclusively.
If a brace that was specifically assigned to be tension only is going into compression, that usually indicates a problem with the load combinations. For models with tension only members, the load combinations should never include negative terms in any load combinations (generated or custom). If there is a combo with a negative term, the resulting brace force could be compression and the warnings above will occur.
Consequently, any model with tension only members requires one lateral load case in the positive direction and another load case in the opposite direction if all the braces are to be correctly evaluated. For program generated loads, there is a check box for "Generate Additional Load Cases for Analysis with Tension-Only Members" that can be selected to do this automatically, but for user defined story forces or nodal loads, an additional case for the reverse direction must be manually input.
For any dynamic, response spectrum load case, all braces are assumed to be 50% effective. This generally accounts for an appropriate amount of stiffness for a symmetrically braced frame, but does not prevent the braces from showing compression results. This approach is also used for the eigensolution.
In the RAM Frame analysis, braces that are designated to be tension-only are completely ignored during the analysis of the gravity load cases (dead, live, roof and snow). For the evaluation of lateral loads the program uses an iterative approach to determine which braces are in tension under each lateral load case. Then the program superimposes the results based on the defined combinations in order to code check the braces.
In structures where all the braces are tension-only, a global instability can often result for gravity loads. If you get an instability warning or if the deflections are significantly large under gravity load cases there are several things you can do.
RAM Instability In Finite Element Analysis
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