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Seismic analysis

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.

GSA has support for a variety of seismic codes including

  • ASCE 7 (USA)
  • Eurocode 8 (Europe)
  • FEMA 356 (USA)
  • GB50011 (China)
  • IBC (USA)
  • IS1893 (India)
  • Ordinanza PCM 3274 (Italy)
  • UBC (USA)

Modelling implications

The basic model can be built as any other GSA model but the following points should be considered for seismic analysis.

The vertical direction should be global Z.

Define storeys for the structure. Most codes make reference to storeys at some point and for simple structures they can be defined automatically by the Model > Create storey grid planes menu command. A storey is defined by a height in the Z direction along with appropriate storey tolerances.

Codes give the situations where floors are allowed to be treated as rigid diaphragms. Where this is the case create rigid constraints (xy-plane type) either graphically or if you have defined storeys using the Model > Create rigid membranes from storeys command.

In most cases seismic design will be carried out with reference to a particular design code, GSA supports to a lesser or greater extent UBC 1994, UBC 1997, IBC 2000, FEMA 356 (rehabilitation code) Eurocode 8 (1994 and 2003) and Ordinanza PCM 3274 (Italian). The seismic data is stored along with the response spectrum data as the two are closely interrelated.

In all seismic analysis the first stage is to determine the levels of ground motion that are to be considered. This can be expressed in different ways and varies from one code to another and will typically vary depending on the type of building. When working to a particular code, the various parameters required will be defined in the code. Values are either supplied by the code or have to be found by reference to external sources. For example the US geological survey gives detailed information of SS and S1 values for all of the USA.