Why Beam Segments Matter in SUMO

Investigating torsion on a concrete beam fully supported by a wall had me scratching my head. I modeled a concrete beam in SUMO with a pinned line support along its length and fixed supports at its ends to prevent it from rotating around its axis. I also included a Rigid Link perpendicular to the beam for applying loads that would induce torsion, along with the beam’s self-weight.

But when I ran the simulation, the results showed bending moments and shear forces in the beam. How could that be? With a line support, this should have been impossible. It didn’t make sense—until an unrelated question shed some light: “How does SUMO space the support points along the beam?” For plane elements, this is straightforward—it’s based on the FE mesh size. But for a beam… it’s all about THE SEGMENTS!

By default, beams or columns are divided into just one segment. While this is adequate for calculating bending moments and shear forces, it can create significant issues for deflection, buckling—and, in this case, for an accurate analysis. Increasing the number of segments adds more nodes along the beam’s analytical line. As I increased the segments, the bending moment and shear force values approached zero (as they should).

Essentially, each segment acts like a small beam with support on each end. The line support only functions as intended when more segments are added.
Check out the images below to see the progression from 1 segment to 100—and the dramatic change in results.

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