Large Axial Forces in Beams Due to Offsets?

All,

I have modeled a gate, basically consisting of frame members comprised of beams, and skinplate, comprised of plates.  I have attached the plates to the beams using rigid link members.  The plates are loaded with a distributed pressure load and transferred to the beams through the rigid link members.

The beam force summary is reporting large axial loads in the beams, which seems strange.  All of the loading in the model is in the global z direction, which is perpendicular to the local x axis of the beams.  I'm not sure what is causing these axial forces, since there is no loading in that direction.  Does anybody have any idea of what may be causing this?

Thanks in advance,

Ben

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  • As you mentioned in the title, offsets lead to axial forces in members. Usually, an offset that is transverse to a member's longitudinal axis causes that.



  • It is not understood as to why lateral offset should cause axial force in the beam. I would request Ben to advise the direction of load applied on to the plates and  also whether any other load is also applied to the structure

  • sureshprsharma,

    Consider a simply supported beam. Apply a uniform distributed load. You will get shear and bending, but no axial force.

    Next, apply a local Y offset (or a global Y offset) at both ends. Apply the same load. The beam will have an axial force.

    The reason is, when you apply an offset, a rigid member is created from the beam face to the support node. Those rigid members resemble very stiff columns. So, instead of just a beam, you now have a portal frame with one beam and 2 very stiff columns. Just as you get an axial force in an ordinary portal frame with a uniform load on the beam, there will be an axial force on this offset beam too.

  • Dear Lose.

    At the outset thanks to you for your valuable and worthy contribution in this forum. The case cited by you is not similar to the one in this posting. You are applying the load in the global Y direction, that is why axial force

    will be in the vertical member of the portal.I suppose you do not mean the horizontal member of the portal. In this case in question the load is applied in local Z direction on a vertical plate i.e. in global Z direction. Under this circumstance it does not appear  feasible that the beam will be subjected to internal axial force in local or in global X direction.

    Please clarify.

  • Sureshsharma,

    Thanks for the kind words.

    In the question you posted on Oct 22, you wrote "why lateral offset should cause axial force in the beam.". I presumed that by lateral, you meant that the offset is along a direction that is perpendicular to the longitudinal axis of the beam. That is why I provided an answer involving a simply supported beam.

    In general, if the offset, no matter what direction it is along, induces axial deformation in a member, and the ends of the beam are not allowed to deform axially in an unrestricted manner, it will generate axial forces.

Reply
  • Sureshsharma,

    Thanks for the kind words.

    In the question you posted on Oct 22, you wrote "why lateral offset should cause axial force in the beam.". I presumed that by lateral, you meant that the offset is along a direction that is perpendicular to the longitudinal axis of the beam. That is why I provided an answer involving a simply supported beam.

    In general, if the offset, no matter what direction it is along, induces axial deformation in a member, and the ends of the beam are not allowed to deform axially in an unrestricted manner, it will generate axial forces.

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