ExactInternalForces incorrect with partial BarElement end releases

[NickN-Eng]

Hi epsi1on,

First of all, a big thank-you for the work you have done on this project!

I was wondering if the BarElement Start/EndReleaseConditions were supposed to be working? Alternatively, it may be an issue with the internal forces (I am using GetExactInternalForceAt()).

In the problem below, I have defined a simply supported beam with 1kN/m uniformly distributed load. However, instead of fixing the Start/End NODES, I am setting the EndReleaseConditions. I have fixed the Rz of one of the beam ends so that the bar is fully constrained.

The internal forces are incorrect. The support reactions are correct.

I want to set the EndReleaseCondition on the bar, instead of the node constraint because I want to torsion ally restrain the beam in it's own axis. For example, if I had a diagonal beam in 3D, I cannot restrain the beam in pure torsion (local Rz) through nodal restraints - nodals restraints are in global coordinates.

Code is below:

        var l = 10;
        var model = new Model();
        model.Nodes.Add(new Node(0, 0, 0) { Label = "n0" });
        model.Nodes.Add(new Node(l, 0, 0) { Label = "n1" });
        //Fixed nodes
        model.Nodes["n0"].Constraints = Constraints.Fixed;
        model.Nodes["n1"].Constraints = Constraints.Fixed;

        var bar = new BarElement(model.Nodes["n0"], model.Nodes["n1"]) { Label = "e0" };
        //Pinned bar releases
        bar.StartReleaseCondition = Constraints.MovementFixed;
        bar.EndReleaseCondition = new Constraint(dx: DofConstraint.Released, dy: DofConstraint.Fixed, dz: DofConstraint.Fixed, rx: DofConstraint.Fixed, ry: DofConstraint.Released, rz: DofConstraint.Released);
        model.Elements.Add(bar);

        var sec = new UniformGeometric1DSection(SectionGenerator.GetISetion(0.24, 0.67, 0.01, 0.006));
        var mat = UniformIsotropicMaterial.CreateFromYoungPoisson(210e9, 0.3);

        (model.Elements["e0"] as BarElement).Material = mat;
        (model.Elements["e0"] as BarElement).Section = sec;

        var u1 = new UniformLoad(LoadCase.DefaultLoadCase, -Vector.K, 1, CoordinationSystem.Global);

        model.Elements["e0"].Loads.Add(u1);

        model.Solve_MPC();

        var n0Force = model.Nodes["n0"].GetSupportReaction();
        var n1Force = model.Nodes["n1"].GetSupportReaction();
        Console.WriteLine("support reaction of n0: {0}, n1: {1}", n0Force, n1Force);

        var elm = model.Elements[0] as BarElement;
        var fnc = new Func<double, double>(x =>
        {
            try
            {
                var xi = elm.LocalCoordsToIsoCoords(x);
                var frc = elm.GetExactInternalForceAt(xi[0]);
                return frc.My;
            }
            catch
            {

                return 0;
            }

        });

        FunctionVisualizer.VisualizeInNewWindow(fnc, 1E-6, l - 1E-6, 50);

[epsi1on]

I've just commented line 1028 through 1043 of file EulerBernoulliBeamHelper.cs and this issue is fixed. Not sure, but i think did add those lines to fix another issue. Anyways this is the result for your code:

[NickN-Eng]

Thanks for the quick response! Have had a look at the version history and you made this update in Commit 872e35b2 07/11/2019 13:37:47 "update". I dont fully understand how GetLoadInternalForceAt() works, but as part of this update, you removed this: ...and added: Do you remember why you made the change? I hope commenting out lines 1028->1043 doesnt break something else!!

[epsi1on]

I've added the unit test for this issue:

BriefFiniteElementNet.Tests.BarElementUniformLoadsTests.LoadInternalForce_uniformload_eulerbernoullybeam_endrelease()

This way if any other changes break this issue, we'll understand.

[NickN-Eng]

Cheers epsi1on!