SUMO: Member Fixities Explained

Fixed or Pinned? This is an incredibly important question when it comes to Structural Engineering, and the effect of either is massive. It can also lead to a lot of confusion if not understood properly and software conventions can add to the confusion.

When defining a member/element in SUMO, its’ parameters include its’ Start and End Fixity (amongst others). Clicking on one of these dropdowns provides you with the following options:

  • Fixed
  • Pinned (xyz pin)
  • Torsion (xy pin)
  • x pin
  • y pin
  • z pin
  • yz pin
  • xz pin

Allocating these correctly can mean the difference between a smooth and accurate analysis versus that dreaded MKL Pardiso Error. Understanding what the mean is important and, luckily, much simpler than it may seem.

Fixed is simple. The member start/end is fully fixed translationally and rotationally. When it comes to the pin’s:

  • A pin indicates that there is no rotational fixity along that axis.
  • xyz pin means that there is no rotational fixity along the x-, y- and z axes.
  • xy pin means that there is no rotational fixity along the x- and y axis, but there is along the z axis.

The member axes DO NOT correspond to the global axes. As with normal structural conventions, the x-axis is the strong axis, y-axis is the weak axis and the z-axis is the longitudinal axis. Consider the example below:

These two I-Beams are exactly the same size, the one has just been rotated by 90°. For both beams they are loaded, globally, in the y-direction. Locally, however, the first beam (on the left) is loaded ALONG its’ y-axis and the second beam (on the right) is loaded ALONG its’ x-axis. This will cause bending about the x-axis and y-axis respectively.

In the Results workspace, if you switch to the X-Moment display, you will see the following:

When you switch to the Y-Moment display, you will see the following:

I trust that the above clearly displays the axis naming and difference between the global and local axes.

With the above in mind: If you set a member start and end as pinned (xyz pin), you are allowing that member to rotate around its’ longitudinal axis. Setting it to torsion pinned (xy pin) restrains it from rotating around its’ longitudinal axis.

More than not, applying pinned fixities can very easily result in numerical instability and cause the analysis to abort. The content above highlights why that is the case. Where will you apply Pinned fixities then? More so with braces than anything else.

In conclusion, the above content should explain the conventions for member fixities in SUMO and allow you to manage this particular parameter much better.

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