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Is posible to set point restrictions in beam coordinates?

  1. Alberto Fernandez

    From what I’ve read in the manual and what I’ve seen in the examples the answer seems to be no, but I’d like to be sure. Is it possible to set restrictions on a point in the member coordinate system?

    For loads it is not a problem as they can be rotated and expressed in a1,a2,a3 anyhow. However, when setting restrictions on a three-dimensional structure there are cases where the orientation of restrictions do not match the global axis. It would be very useful to have a way of setting those restrictions in the element frame (b1,b2,b3)

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    1. Qi Wang

      Hi Alberto,

      "From what I’ve read in the manual and what I’ve seen in the examples the answer seems to be no, but I’d like to be sure. Is it possible to set restrictions on a point in the member coordinate system?"

      -- I don't understand this question. But prescribed values, including displacements and rotations, can be applied to key point and they can be either constant value or functions (through time function).

      For loads it is not a problem as they can be rotated and expressed in a1,a2,a3 anyhow. However, when setting restrictions on a three-dimensional structure there are cases where the orientation of restrictions do not match the global axis. It would be very useful to have a way of setting those restrictions in the element frame (b1,b2,b3)

      --If the "element frame (b1,b2,b3)" refering to the undeformed local/member frame, you may rotation those loads to global (a1, a2, a3) frame by the direction cosine matrix (DCM).

      Qi

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    2. Alberto Fernandez

      That is what I meant, that there is no problem for loads because they can be rotate from one frame of reference to another. But for prescribed values that is not possible as far as I see. If we prescribe displacement of degree of freedom 1 to 0, to put an example, we are defining that degree of freedom to not move in direction a1. If that beam is angled in 3d space I don’t see a way of prescribing a degree of freedom in a local direction (b1, b2, or b3). Following the same example, if we want to set to 0 displacement in direction b1.
      Is this the case?

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    3. Qi Wang

      Alberto,

      I think the constrains/prescribed degree-of-freedom (DOFs) can be converted to global frame (a frame) using the same algorithm for the loads. Displacements are also vectors  so that should follow the same transformation rules as loads.

      Qi

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    4. Alberto Fernandez

      There are situation in which I don’t see how. A rotation is going to couple a degree of freedom when it may not always be wanted.
      I attached a toy problem (2d) to illustrate the situation. I know it would be very easy to align the beam with a1, but let’s imagine that there are some other beam that are aligned differently in the structure. If we set a BC of displacement 0 in either a1 or a3 at the bearing position we are also restricting it from moving axially.
      Is there a way to set up a problem like this in GEBT? Maybe I’m missing something

       

      toy-problem-example.png

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