2.27Solver

spFrame has a two-pass solver using the stiffness method of analysis. The first pass can be performed after the model geometry is defined. The program computes the element stiffness matrices and builds the associated global stiffness matrix of the whole structure. The element stiffness matrix is a square matrix proportional to the member degrees of freedom (e.g. a plane truss element stiffness matrix is 4 x 4, whereas a space frame element stiffness matrix is 12 x 12). The element stiffness matrix is then multiplied by the applicable transformation matrices to account for member orientation and any special corrections.

The second pass can be executed after the applied loads and the corresponding load combinations are defined. The program builds the load matrix, inverts the global stiffness matrix, builds the displacement matrices, and solves for the joint displacements, joint reactions and member end forces. Therefore, the structure can be analyzed under different sets of loads and/or load combinations without having to recalculate the stiffness matrices again

When executing the solver for the first time for a particular problem, a first-order analysis is performed and the resulting joint displacements, joint rotations, support reactions, and member end forces are computed. To compute secondary P-Δ effects or to solve for tension-only members, a second run must be executed. This second run is necessary because the solver needs some results from the first run. In a second-order analysis, members designated as tension-only members will be removed from the model if they are subjected to any compressive loads.