## Advanced Preferences

The Advanced Preferences dialog is available from the Miscellaneous page of the Preferences. This is where various low level settings can be set including those relating to threading and the behaviour of various view types.

The Advanced Preferences dialog is available from the Miscellaneous page of the Preferences. This is where various low level settings can be set including those relating to threading and the behaviour of various view types.

Different element types and analysis types are available in GSA. The types of analysis may be limited or modified by the elements selected.

The most common analysis in GSA is a linear static analysis. If a model is set up and loading defined a linear static analysis can be initiated using the 'analyse' (sigma) button.

For bridge analysis the live load requirements are very different to those for ordinary structural analysis. A number of tools are available in GSA to simplify the generation of these load cases. Three different ways of using the tools are available for different design codes and different situations:

Bridge load optimisation is selected in the Analysis wizard.

A buckling analysis is required where it is important to investigate the potential effects of buckling on the structure. These effects can be significant well before a structure actually buckles.

Modal analysis is carried out ignoring damping, however much of the dynamic post-processing relies on damping to get a solution.

Dynamic analysis covers a range of analysis options from determining the dynamic characteristics of a structure, through looking at linear response, to full nonlinear dynamic analysis.

Element types

When the model is nonlinear the only option for dynamic analysis is a time-history analysis which steps through the solution one time step at a time. There are two basic schemes for this time stepping: implicit and explicit. The time-history option implemented in GSA is explicit.

Footfall induced vibration analysis is to evaluate the responses of structures subjected to the actions of human footfalls. The structural responses include nodal accelerations, velocities and response factors etc. The human footfall loads are considered as periodical loads which are represented by a number of harmonic loads according to Fourier series theory. The detailed descriptions of human footfall loads can be found from references 35, 29 and 1 in the Bibliography which are also listed below. Footfall induced vibration analysis utilizes dynamic analysis results (frequencies, mode shapes & modal masses etc) to calculate the structure responses . The outputs of footfall analysis are the maximum responses of the structure for the given ranges of walking frequencies etc. The following three design guides of footfall analysis can be considered:

Harmonic analysis of structures is to calculate the maximum elastic response of a structure subjected to harmonic loading at steady state. The structure responses include nodal displacements, velocities and accelerations as well as element forces and moments etc. The harmonic loading is the load that varies sinusoidally along with time. Harmonic analysis is based on modal dynamic analysis results (frequencies, mode shapes and modal masses etc). It calculates the maximum structure responses for the given harmonic loads using modal superposition method.

Real structures are not the perfect structure envisaged by the designed but contain imperfections. The design codes try to ensure that imperfections are considered in the design process. This feature allows imperfections to be included in the analysis.

For most users the most common use of GSA is for linear static analysis. The Getting started manual guides a new user through the stages of building and carrying out a linear static analysis. In all cases a knowledge of linear static analysis is a pre-requisite for the more advanced analysis options.

Linear time history analysis is to calculate the linear responses of structures that are subjected to dynamic loads (force excitation) or base accelerations defined by the combination of the applied loads or base acceleration and load curve. The structure responses include nodal displacements, velocities and accelerations as well as element forces and moments etc at the chosen time intervals. If it is force excitation, the magnitude and locations of the dynamic loads are defined in the same way as that for static analysis and the variation of the dynamic loads along with time is defined by load curve in the unit of time versus load factor. If it is base acceleration, the load curve should be acceleration recordings of the ground motions in the unit of time versus acceleration, the ground accelerations varying with time given by the load curve can be scaled by a scaling factor that can be defined in the analysis wizard.

This allows various key parameters to be included in the LS-DYNA export. Refer to the LS-DYNA manuals for a detailed description of these parameters.

The GSA analysis options are focused on linear and mildly nonlinear problems. In some cases it is necessary to use a more advanced analysis. This can be provided by LS-DYNA. This analysis option allows the GSA model to be used to create an LS-DYNA analysis

There are several different types of nonlinear analysis, but there are two different effects that need to be considered.

For some structures it is important to be able to take account of the changes in the stiffness of a structure due to the load. When a column is subjected to an increasing compressive axial load its ability to carry transverse load is reduced until the Euler load is reached when it can no longer carry any transverse load.

Periodic load analysis is to calculate the maximum responses of structures that are subjected to periodic loads. The structure responses include the maximum nodal displacements, velocities and accelerations. The periodic loads are generic and the dynamic load factors of each of the harmonic components of the periodic loads at a given frequency are defined by the users through dynamic load factor table.

The raft analysis option analyses vertical soil-structure interaction for raft and both vertical and horizontal soil-structure interaction for piles. It provides a means of linking the GSA static analysis of a structural model (typically a raft or piled-raft) with PDisp soil settlement analysis. PDisp is a program which calculates the soil settlements (and stresses if Boussinesq analysis method is used) under the normal and/or shear pressure loads on vertical and/or horizontal planes within the soil. PDisp has been embedded in GSA as the soil settlement analysis engine, so standalone PDisp program is not needed in GSA raft and piled-raft analysis. The data modules in GSA relating to soil properties and analysis can be imported from or exported to the PDisp program.

When modelling a raft supported on soil some information is needed to clarify how the soil is considered.

Response spectrum analysis is a method for estimating the seismic response of a structure from a set of modal dynamic or Ritz analysis results along with a response spectrum which capture the frequency content of an expected earthquake.

Analysis – simple static

There are many different approaches to seismic analysis. The main approaches implemented in GSA are Equivalent Static Procedures, Response Spectrum Analysis, Linear Time History Analysis, and Nonlinear Time History Analysis. In general the response spectrum method is recommended in the various seismic codes as this is based on a dynamic analysis of the structure. However at times the simpler equivalent static method which ignores the actual dynamic response of the structure may be useful.

Analyse