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Pressure loads are applied to the surface of the geometry and are defined by the *surface_load *element with the type attribute set to *pressure*:

<surface_load type="pressure" surface="surface1"> <pressure [lc="1"]>1.0</pressure> <value surface_data="pressure_value"/> <symmetric_stiffness>0</symmetric_stiffness> </surface_load>

The surface must be defined in the *Geometry *section. If the pressure value differs per facet, it must be defined (as in the example) using the *value *parameter and the *surface_data *attribute (Section 3.9.2↑).

These pressure forces are also known as *follower forces*; they change direction as the body is deformed and, in this case, are always oriented along the local surface normal. The sign convention is so that a positive pressure will act opposite to the normal, so it will compress the material. The *pressure *element defines the pressure value [**P**]. The optional parameter *lc* defines a loadcurve for the pressure evolution. If *lc* is not defined a constant pressure is applied.

The optional *symmetric_stiffness* parameter (0 or 1) determines whether to use the exact form of the stiffness matrix for this surface load (0=non-symmetric), or the symmetric form (1=default). The non-symmetric form is numerically more robust, but it may also require using a globally non-symmetric stiffness matrix (see Section 3.5.2↑).