 # RFEM – Online Manual

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# 8.16 Surfaces - Principal Internal Forces

To control the graphical display of the principal internal forces, select Surfaces → Principal Internal Forces in the Results navigator. Table 4.16 shows the principal internal forces of surfaces in numerical form.

The table shows the principal internal forces sorted by surfaces. The results are listed in reference to the grid points of each surface.

The Grid Point and Grid Point Coordinates table columns correspond to those of the previous results table 4.15 Surfaces - Basic Internal Forces.

Moments / Shear Forces / Axial Forces

The Basic Internal Forces described in the previous chapter refer to the more or less freely defined coordinate system xyz of a surface. Conversely, Principal Internal Forces represent the extreme values of the internal forces in a surface element. For this purpose, the basic internal forces are transformed in the directions of both principal axes. The principal axes 1 (maximum value) and 2 (minimum value) are arranged orthogonally.

The principal internal forces are determined from the basic internal forces:

 m1 Bending moment in direction of principal axis 1 m2 Bending moment in direction of principal axis 2 αb Angle between local axis x (or y) and principal axis 1 (or 2) mτ,max,b Maximum torsional moment vmax,b Maximum resulting shear force from bending components βb Angle between principal shear force vmax,b and local axis x n1 Axial force in direction of principal axis 1 n2 Axial force in direction of principal axis 2 αm Angle between axis x and principal axis 1 (for axial force n1) vmax,m Maximum shear force from membrane components

The principal axes directions αb (for bending moments), βb (for shear forces), and αm (for axial forces) can be displayed as trajectories in the work window.