Allows the user to specify a pre-load to be applied to the elements. This pre-load must refer only to prestress beam or 2D loads. The internal loads are used to calculate a geometric stiffness ( matrix) for the element which is added into the normal element stiffness . By providing an “out-of-plane” stiffness this allows pre-tensioned bars to be analysed in the linear solver.
Note that for a modal P-delta analysis, the geometric stiffness is calculated as a result of solved displacements including the effect of the pre-load. It follows that the derivation of the matrix is to include the effect of the pre-load as well as the supplied P-delta force.
