Analysis
In the Analysis tab window some options related to the analysis can be defined. In particular, it is possible to select the solver type, whether to perform eigenvalue analysis at every step in nonlinear dynamic and pushover analysis and to account (or not) for geometric nonlinearities.
Solver
Apart from the linear equation solver, users are able to select whether the initial loading, i.e. structural static loads, will be applied in one or more steps in the nonlinear analysis types. The default option is to apply it one single step.
Further, the option of executing eigenvalue analysis at every step in nonlinear dynamic and pushover analysis is available. Users may select to run an eigenvalue analysis at the end of the nonlinear analysis or to perform eigenvalue analysis multiple times during the nonlinear analysis by specifying after how many steps the eigenvalue analysis will be performed.
Users may currently choose between the following different solvers:
- A Skyline solver (Cholesky decomposition, Cuthill-McKee nodes ordering algorithm, Skyline storage format);
- A Frontal solver for sparse systems, introduced by Irons [1970] and featuring the automatic ordering algorithm proposed by Izzuddin [1991];
- A Sparse/Profile Solver for sparse systems, introduced by Mackayet al. [1991] and featuring a compact row storage scheme using elimination trees proposed by Liu [1986];
- A Parallel Sparse/Profile Solver for sparse systems, which is the parallel version of the Mackayet al. algorithm. The method was introduced by Law and Mackay [1992].
Users may select between these four options, or let the program select the most appropriate solver, depending on the characteristics of the structural model. It is noted that generally the Sparse/Profile solvers are considerably faster, especially in larger models. In particular, the parallel version is more efficient for larger structural models of 500 nodes and more. In contrast the Skyline method is usually more stable and is capable of accommodating zero diagonal stiffness items. When the automatic option is selected, which is the default option, the program performs a stability and size check prior to the analysis. If the model is not very small (i.e. smaller than 25 nodes), and if it can run with a Sparse/Profile solver without stability problems, this method is employed, otherwise the Skyline solver is chosen.
Readers are obviously advised to refer to the existing literature [e.g. Cook et al. 1989; Zienkiewicz and Taylor 1991; Bathe 1996; Felippa 2004] for further details on these and other direct solvers.
Finally, irrespective of the serial or parallel version of the selected solver, user may select to execute different operations of the structural analysis (initial checks, assembly of stiffness matrix, code-based checks and checks of the performance criteria) in parallel or not. Parallelizing these operations can be significantly faster in larger models, and this is the default option.
Geometric Non-linearities
Unchecking the option 'Include Geometric Non-linearities' will disable the geometric nonlinearity formulation described in here, rendering the analysis linear, from a displacement/rotation viewpoint, which may be particularly useful for users wishing to compare analysis results with hand calculations, for verification purposes. By default this option is active for frame elements and deactivated for masonry elements.
Run with Elastic Linear Properties
Checking this option will disable both material inelasticity and geometric non-linearities, leading to a totally linear, elastic analysis. By default this option is inactive, with the exception of Response Spectrum Analysis, when it is the default option.
Note: When users decide to run an analysis considering the linear elastic properties of materials (see the option described above), they should keep in mind that, if the elements are modelled using RC sections and 'infrm' elements, the infrm elements will account for the reinforcement; on the contrary, if 'elfrm' elements are employed, their properties are calculated using the concrete modulus of elasticity and the section dimensions, thus neglecting the effect of the reinforcement.
Calculate Support Forces from Rigid Links
Checking this option enables the calculation of the support forces in the cases when some DOFs of a constraint (rigid link, rigid diaphragms or equal DOF) are fixed with restraints. By default this option is inactive, because this calculation can cause minor numerical instabilities.