Skip to main content

Forces

Forces are calculated at the end of beam, bar, tie, strut, cable and spring elements, from the displacements at the nodes.

Beam element forces may only be output in Local axis directions.

The most commonly used beam force results: Fx axial force diagram, Fz shear force diagram and Myy bending moment diagram are available from the Diagrams toolbar.

Intermediate Forces​

Forces, and other force based results, along elements are calculated from the end conditions on the element and any load on the element. The positions at which intermediate forces are reported can be specified either in terms of a number of equidistant points or by selecting intermediate forces at interesting points.

Interesting points are determined from the applied loading, as follows.

For moments: an interesting point is located at every point load on the beam and at the ends of every distributed load. It also places points at a number of equidistant points along distributed loads. In Graphic Views the number of equidistant points along distributed loads is dependent on the number of elements currently being displayed: if more than 50 elements are displayed then the number is 3, if less than 5 elements are being displayed (even if there are many more in the whole model) then the number is 19. In Output Views the number is always 3.

For shear forces: an interesting point is located immediately to either side of every point load on the beam and of each end of every distributed load.

Sign Convention​

The force in the beam at any point is the force required to maintain equilibrium if the beam is cut at that point and the end 2 part of the beam is discarded. Thus:

  • Axial forces and stresses are positive when tensile.
  • For a beam viewed horizontally with its Z-axis pointing vertically up, a positive Myy is hogging.
  • For a beam viewed horizontally with its Y-axis pointing vertically down, a positive Mzz is hogging.

This is demonstrated for the following two beams:

beam-force-sign-convention

Drawing a graph of the numerical output of moment and shear force gives:

bendingmoment-shearforce-graph

Drawing bending moment and shear force diagrams in graphics gives:

bendingmoment-shearforce-graphics

Sign Convention Definitions​

  • Moment: positive/negative depending on direction of moment relative to axis direction (rotational direction).
  • Force: positive/negative depending on direction of force relative to axis direction.
  • Bending moment diagram: drawn on tension side
  • Shear force diagram: positive corresponds to negative slope of bending moment diagram where a bending moment diagram reads from left to right (as viewed) or bottom to top, if vertical.