How to solve an unstable structure in calculation

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1

I am a beginner and I am starting the exercise on the metal hall.
When I run the calculation, I get an error message saying that the structure is unstable and it points me to a nodal support.
How can I solve this problem?

Exercice 1.rf6 (1.4 MB)

3

Hi Justin,

the instability stems from how some members are defined and connected.

  1. Columns are defined as truss members
    In your model, the columns in the first and last frame are currently defined as truss members (tension/compression only).

    • Truss members can carry only axial forces, but no bending moments or shear forces.

    • For columns in a hall frame, you normally need beam members because they must transfer bending and shear to stabilize the structure.

    :backhand_index_pointing_right: Change these column members from truss to beam (e.g. “Member Type: Beam”). This already improves the stability a lot.

  2. Unwanted “node on member” at the crossing of tension-only members
    It seems that during modelling the option “automatically connect lines/members” was active.
    As a result, at the crossing points of your tension-only members, an extra “node on member” was created and the members were connected there.

    • For tension-only members, this additional connection is problematic because they cannot provide the necessary compression stiffness at that node.

    • This leads to a mechanism and the solver reports an instability.

    :backhand_index_pointing_right: Delete these unintended “node on member” connections at the crossing points of the tension-only members.

After these two changes

  • columns as beam members, and

  • removal of the unwanted nodes-on-members at the tension-only crossings,

the static analysis should run without stability errors.

Best regards,
Ann-Kathrin Dannwerth

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