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Note: I corrected a (very) minor issue in Jayrod's response when I quoted him. That's why we often add an additional rotational restraint about the global vertical axis (Y in your case). But, the program isn't quite smart enough to know that all it sees is a zero in the main diagonal of the stiffness matrix and it wants to find a way to correct that instability. You and I (as reasonable engineers) know that there isn't any way for torsion to get into the column. Just to elaborate on item number 2: In a 3D analysis a column that is supported by a "pinned" boundary condition at the bottom and is connected to beams with pinned end is technically unstable for torsion. You can look at the end to see if it's restraining any torsion and address that situation as required. I'd bet once you did those things, your model would be stable and run without even a stability warning (different than a stability error) RE: New to risa3d instability JoshPlumSE (Structural) 12 Jan 21 22:51Ģ) change all pins at the bottom of the columns to include Y-axis rotation as a reaction. You would need to ensure your diaphragm performance matches that assumption. So along grids b, c, d, e and f I would provide Z translation reaction and at nodes N2 and N14 I would provide X translation reaction.ģ.1) In lieu of the reactions at the top of the columns being changed, you could put a diaphragm in place, but that would act as a rigid diaphragm.
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You can look at the end to see if it's restraining any torsion and address that situation as required.ģ) provide lateral restraints for the tops of the columns that don't currently have a brace in the line. So, My steps to try and correct your model would be as follows.ġ) change all column end releases to fixed-fixedĢ) change all pins at the bottom of the columns to include y-axis rotation as a reaction. Usually any modelling errors will be abundantly clear just from that deflected shape. Run this load case and look at the deflected shape. So I also often model all columns as fixed end releases (and all beams pinned unless they are intended on being fixed) and I also will model the boundary condition at the bottom of the column from a true pin, to a translation and y-axis rotation fixed, but leave the x and z axis rotation free.ģ) I often will create a load combination of just dead load, with no P-delta analysis.
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If only a single member has a fixed end, the stability errors go away and when you look at the analysis it doesn't actually end up taking any additional load because all the other members at the node are pinned.Ģ) I find many instability warnings are a cause of the columns being able to spin on their axis when modelled how you have it. There needs to be at least one member there that has a fixed end, otherwise the program will throw instability warnings. I should add three more things now that I've opened the model:ġ) I never have all members going to a node as pinned. Otherwise you may have convergence issues as the model will throw out any solution where a tension only member takes any compression load, regardless of how minor. If you are having tension only bracing, I find you'll have better luck if you specify the members as Euler Buckling, allowing them to take a minor amount of compression without throwing the entire solution out. Also ensuring that the total expected load going into the bracing is correct. Then for each of the beams that is acting as a collector I'd provide a uniformly distributed axial load so that they are properly analyzed as beam columns. And then I apply the appropriate lateral loads to the members that are intended to receive them.įor example, a line of columns has one braced bay in the x-direction, I would then provide Z translation restraints to the tops of the columns that would be normally be provided by the flexible diaphragm. Again, haven't opened your model yet, but when I have a flexible diaphragm with discreet bracing, often I need to provide lateral restraints at the top of the columns that prevents lateral movement. You wouldn't be able to get the gravity cases to work if your columns aren't appropriately restrained at the top. You need to keep this in mind when using the diaphragm function and if you have a flexible diaphragm, that it won't properly attribute the lateral loading around. In Risa3d, without pairing it with RisaFloor, creates a rigid diaphragm only that essentially attaches all the nodes together via rigid links. All nodes in that plane would be locked such that they would move together. If the nodes at the tops of all of the columns are at the same elevation, you just specify one of those nodes to have a diaphragm in the correct plane.