Hydrostatic loading is a common special case of spatial variation. In this case, there is often a fluid surface, above which there is no load. Such a load you can describe with an expression like if(Z<ZSurf,rhoFluid*g_const*(ZSurf-Z),0). Here, ZSurf and rhoFluid are assumed to be parameters containing the Z-coordinate of the fluid surface and the mass density of the fluid respectively.

If a load has a spatial variation which is fast relative to the element size, you may need to increase the accuracy of the numerical integration used to compute the load contribution. As a default, a load which varies no faster than the polynomial order of the displacement shape functions can be integrated exactly. To change the integration order, enable Equation View. In the Equation View node under the current load node in the Model Builder tree, you can then increase the integration order.

Loads that are moving along the structure with time can be modeled using an expression X-v*t, where v is the velocity of the load. The mesh independent point load of the type Point Load, Free is particularly well suited for this type of modeling. If a distributed load is modeled using this approach, it is often necessary to increase the integration order as discussed in the previous section, since the load patch will typically cover partial element faces.