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I created a structural model of a laced strut using plate elements. To study second order effects of bow imperfection, I applied a joint displacement of 20 mm in a SAP 2000 v18 model. When the analysis was completed, I double checked the results and found a large joint reaction in the range of 300,000 kN at the joint with applied displacement. Elsewhere, joint reactions do not exceed 1,000 kN.

The model ran with no warnings or errors. I expected the sum of all forces to be zero for equilibrium, but that seems not to be the case. The global displacement looks fine with the laced strut bending to the specified displacement.

  1. Should all joint reactions sum to zero, if nodal displacement is applied?
  2. Should the joint reaction at the node with specified displacement be disregarded?
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  • $\begingroup$ Please edit your question with a screenshot of your model to help us diagnose what's going on. $\endgroup$ – Wasabi Jun 15 '17 at 15:19
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The sum of all joint reactions (resolved in any direction) should be equal and opposite to the sum of all applied forces (resolved to the same direction). An applied displacement is not an applied force. Therefore, assuming no other loads are present in the loadcase with applied displacement, the joint reactions should sum to zero.

A reaction at the node with applied displacement is expected. Something needs to hold that node in its displaced position; the force will do that. But as the sum of all reactions should be zero, other supports should have reactions to balance the reaction at the displaced node.

You should not disregard the joint reactions from your model. For some reason or another your model is not behaving as you expect. You need to determine what is causing that behaviour, so that you can either realise that it is the correct behaviour, or fix it.

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  • $\begingroup$ What steps should I take to determine what is causing the behaviour? I have tried cutting sections across and the sectional forces output looks normal. $\endgroup$ – Jennifer Jun 15 '17 at 8:26
  • $\begingroup$ Define "normal". If there is a reaction at a node of 300,000kN, then the sum of all attached members' forces (resolved in the direction of the reaction) plus externally applied forces should equal -300,000kN. You should also check what other loads are applied in this loadcase, to check that e.g. self weight of members isn't affecting the total reaction. $\endgroup$ – AndyT Jun 15 '17 at 8:40
  • $\begingroup$ Normal means within 1000kN. The only place where abnormal force is found is the joint where i applied the displacement. I am looking at one load case where only one joint displacement is applied and nothing else. $\endgroup$ – Jennifer Jun 15 '17 at 8:54

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