Frame Analysis

[GeorgeBru]

Hello,

I design a truss structure using the Frame analysis. In truss structures the forces acting on the members are tensional and compressive in theory.

When I use the frame analysis on the truss which result should I take care? Smax or Axial stress? I am trying to have my results inside the elastic limits (less than Smax), but in some cases Smax is very high; for example 450 MPa while Axial stress is 150 MPa.

[henderh]

Hi George,

  As you mentioned, in theory Truss structures are two force members due to the pinned constraints relieving any moment loads.  However, if you add gravity there will be bending moments.  Similarly, certain types of constraints added to beams / nodes will restrict the rotational DOFs such as Fixed.

  In order to analyze a frame as a truss, you'll need to add releases to the beam to enable zero rotational stiffness.  This is done by changing from fixed to something else like uplift none (example image attached).

  I hope this answers your questions.  Please let us know if you have any additional questions, comments or suggestions.

  Best regards, -Hugh

[GeorgeBru]

Perfect! thank you for your answer, I really appriciate it.

But if I want to apply gravity as well, due to approach a realistic view of the truss (it is truss boom of a STS gantry crane), what results should I notice? Smax? I know that Smax=MXmax + MYmax + Axial.. whats the purpuse of Smin? Smin = MXmin + MYmin - Saxial.. what information can I obtain by this? More specificaly, could you please explain me the differences between them? Smax, Smin, Axial, and what info each one provides?