Specifics of Using Tension Members

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Tips & Tricks

001053 24 June 2015 Tips & Tricks Sebastian Hawranke Modeling | Structure RFEM RSTAB

Shoring braces usually obtain the ‘tension member’ type. There are a few specifics to note because in the case of uniform, symmetrical structures and solely vertical loads, an error message often appears as follows: ‘The model is unstable in Node 20 relocatable in the Y‑direction’.

The reason is that the supports and beams are compressed. In this context, all shoring braces absorb pressure and fails at the same time, which eventually leads to the instability of the structure. You can easily retrace this by converting tension members into trusses (only N) and calculating them according to the linear static analysis.

The ‘Failing members to be removed individually during successive iteration’ option under the ‘Global Calculation Parameters’ tab can prevent the complete failure of tension members. In most cases, this is the solution.

Alternatively, there is the second option ‘Assign reduced stiffness to failing members’. For this, minor compression forces may occur in the tension members.

In addition, applying an initial prestress can be helpful. It is consistent with general building practice because many braces can be readjusted. Determination of the appropriate prestress is an iterative process. However, it should not exceed five percent of the ultimate tension resistance of the bracing.