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# 8.21 Surfaces - Contact Stresses

If the model has surface supports (see Chapter 4.9), Table 4.21 shows the contact stresses ("soil contact pressures") of surfaces in numerical form. To control the graphical display of results, select Surfaces → Contact Stresses in the Results navigator.

For 2D slabs, only the results column σz is displayed.

The table shows the contact stresses sorted by surfaces. The results are listed in reference to the grid points of each surface.

Grid Point

The numbers of the grid points are listed by surface. For more information about grid points, see Chapter 8.13.

Grid Point Coordinates

Table columns B to D show the coordinates of grid points in the global coordinate system XYZ. When you click into a table row, the corresponding grid point is indicated in the work window by an arrow, provided that the synchronization of selection is activated (see Chapter 11.5.4).

Contact Stresses

The stresses are related to the directions of the local surface axes. When you analyze curved surfaces, they refer to the axes of the finite elements (see Figure 8.41).

The contact stresses have the following meanings:

 σz Contact stress ("soil pressure") in direction of surface axis z where Fz: Contact force in the FE node in direction z A: Application area for FE node τyz Shear stress from surface support where Fy: Contact force in the FE node in direction y A: Application area for FE node τxz Shear stress from surface support where Fx: Contact force in the FE node in direction x A: Application area for FE node

In the table, positive contact stresses are visualized by blue bars. Negative stresses are correspondingly characterized by red bars.

### Note

The table shows the stresses as forces per surface that is passed into the support. Thus, with regard to signs, the table does not show reactions on the part of the foundation. If the local surface axis z is orientated downwards, a load in the direction of the z-axis results in a positive stress σz, for example. Thus, the signs result from the direction of the surface axis z (see Figure 4.76).

The orientation of the local surface axis z can be changed quickly for 3D models:

Right-click the surface to open the surface shortcut menu (see left figure above) and select the Reverse Local Axis System option. Please note, however, that a potential failure criterion changes the direction of action.