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  • load application

    Hello, I would like to get some clarifications about the way the loads are applied in RFEM.


    Loads defined as "6.6 Free Concentrated Loads"

    Let's consider two loads applied as in the following figure, inside and near the border of the surfaces.
    Loads applied on the undeformed geometry
    Loads applied on the undeformed geometry

    One load is along the X direction and another along Z, both with magnitude 1 kN.


    The structure is subject to form-finding and large displacements. The displacement results are shown in the following figure: in practice none of the original load is transferred to the structure (see resultants in the next figure).
    Klicke auf die Grafik für eine vergrößerte Ansicht

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    Displacement results

    My question is the following: are the loads transferred to the structure as defined after form finding? This would explain the fact that the loads in the previous example are disregarded.

    From a preliminar test it seems so.

    Klicke auf die Grafik für eine vergrößerte Ansicht

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    Klicke auf die Grafik für eine vergrößerte Ansicht

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    Thanks in advance,
    Andy

  • #2
    Hello.

    Free loads will be distributed to the FE nodes before form finding. Then the form finding is performed and the FE nodes and the load move with the nodes. The rotation of the nodes does not matter in this case because the load is defined in the global coordinate system. Therefore it keeps its direction. If the load were defined in the local coordinate system of the surface, it would also rotate.

    Frank
    Support Team der
    Dlubal Software GmbH
    info@dlubal.com
    https://www.dlubal.com

    Kommentar


    • #3
      Hello, thanks for the reply, but your explanation is contrary to my previous findings. I'm probably missing something.
      In particular, from my tests, if the load is inside the FE nodes before form-finding (see load along X in the first figure on my first post, hereabove) but is outside the FE nodes after form-finding, it is NOT applied (see second figure, the X-resultant is zero).


      Another minimal test: a triangular planar membrane, with a load 0.1 kN along Z, applied at X=0.50 m, Y=0.10 m, i.e. above the lower border, at midpoint (see below).

      Klicke auf die Grafik für eine vergrößerte Ansicht

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      This load is well inside the undeformed mesh.
      After form-finding, the lower border translates about 130 mm, see below.

      Klicke auf die Grafik für eine vergrößerte Ansicht

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      If I solve the case with the applied load, I obtain no displacements nor forces (see figure below), indicating that the applied load, at the position (X=0.5 m, Y=0.1 m) is not transferred to the FE mesh.

      Klicke auf die Grafik für eine vergrößerte Ansicht

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      If I apply the same load just above the free-form displacement, i.e. at X=0.5 m, Y=0.13 m, I obtain a deformed solution, see below. It is also apparent, from the solution, that the load is applied at the border of the membrane (and not 0.13 m inside it).

      Klicke auf die Grafik für eine vergrößerte Ansicht

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      Yours sincerely,
      Andy

      Kommentar


      • #4
        Hello.

        You're right. I wrote nonsense in my first post.

        The free load is only projected onto the surface after form finding.


        I took a closer look at the problem again. In the attached file one can reproduce this well. The free point loads in load cases 2 and 3 are both within the original area. The load in load case 2 also lies on the surface after form finding.

        Formfinding

        The load in load case 3 no longer lies in the deformed surface after form finding.

        Formfinding
        Frank Faulstich
        Support Team der
        Dlubal Software GmbH
        info@dlubal.com
        https://www.dlubal.com

        Kommentar


        • #5
          Many thanks Frank, now it is clear.

          Best regards,
          Andy

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