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About alwaeli

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    2nd Lieutenant

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    AREX consultant
  1. Negative Stiffness Eigenvalues

    Additional suggestions which may resolve negative-stiffness errors include: Remove auto line constraints. Select all deck/shell objects in the model, then select Assign > Shell Elements > Auto Line Constraint and uncheck the Apply to Full Structure option. This will remove auto line constraints and possibly resolve instabilities. Dear Umar, The above suggestion works! thank you. However, I would love to understand the engineering behind it. How line constraint is affecting the stability of the floors in P-Delta analysis when the slab stiffness modifier is reduced from 1.0 to 0.5 for instance? Do you have some sort of technical explanation to that? Best regards, Wael
  2. Negative Stiffness Eigenvalues

    Dear Umar, Your response is appreciated. I will try the suggestions in your quote and update you with the results. Meantime, I need to ask you about the best way to mesh floor slabs in multi-story building ETABS model? Will it program default AutoMesh or manual meshing? Keep in mind that on each floor, I have different values of superimposed dead/live loads on different locations in the slab. I really need to know your perspective on that as most of the problems I normally face with the results are caused by the slab meshing. Regards, Eng. Wael
  3. Dear members, Using ETABS 16.2 version, when setting the stiffness modifiers for my slabs (shell-thin) to 1.0, the run goes smooth without warring and I will get reasonable mode periods. However, when I modify the m11, m22, and m12 stiffness to 0.5 for my slabs and run the model again, I got some negative stiffness eigenvalues in my P-Delta analysis and no modal analysis is made! When I tried 0.9 instead of 0.5, no warnings! Tried 0.8, warnings came back! By the way, there is no warnings and or errors when perform "check model". Everything is ok. Attached is my ETABS file for your reference. Please advice if you have faced similar problem? ETABS Model.rar

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