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 and to account for geometric nonlinearities.

Solver
Apart from the linear equation solver, users, 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 in one single step. Further, the option of executing eigenvalue analysis at every step in nonlinear pushover analysis is available. Users may select to run an eigenvalue analysis only at the end or at multiple times during the nonlinear analysis.

Users may currently choose between the following different solvers:

• A Skyline Method (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 treesproposed 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 Sparse/Profile solversare 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, parallel for more than 1000 nodes or the serial otherwise.

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.

Pushover Parameters
The number of the analysis steps of Pushover and Dynamic Analysis is defined by the user in this tab, as well as the maximum interstorey drift of the structure; the default option for the maximum interstorey drift is 2%, whereas the number of pushover analysis steps depends on the selected predefined settings scheme. For the default settings scheme (i.e. No 4) the default value is 50, which is reasonable for the majority of cases. Different values may be employed for the analysis steps and the maximum interstorey drift in X and Y direction.

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.

Run with Linear Elastic 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.