MasterFrame non-linear elements
The use of the following elements in a MasterFrame model will require the modification of the stiffness matrix discussed above and so will require the use of an iterative solving process to account for the non-linearity of the model. The non-linearity which arises with the use of these elements is due to geometric non-linearity, rather than to material non-linearity. Material non-linearity is not considered in MasterFrame.
Tension only members resist only tensile axial forces – they have no bending stiffness. In models with tension only members, an initial analysis is carried out to determine the nature of the axial force in all members in the model. Where a tension only member is found to have an axial force, the member axial stiffness in the stiffness matrix is set to zero and the analysis is repeated. The result of the 2nd run of the analysis determined if any other tension only members need to be taken as zero in the stiffness matrix. The process is repeated until no further changes to the stiffness matrix are required. The full process is repeated for all load cases.
Similar to the tension only member, the compression only member will resist only axial forces, with no bending stiffness. A similar analytic process is required to modify the stiffness matrix after each iteration within a load case.
Nodal Spring supports
MasterFrame models spring supports using a linear elastic spring model. In this case, the force in the spring is proportional to the distance, with no limit imposed on the magnitude of the force or displacement. While the spring element is modelled as a linear elastic element in the analytical model, the displacement of the spring will require a modification of the stiffness matrix of the structure and the modified structure may result in a different displacement and so a different force in the spring. Hence a non-linear analysis is required. The process is similar to that outlined above.