Spectral response analysis

A spectral response analysis calculates the effect of an earthquake on a structure. The properties of the earthquake are defined in code-specific spectral curves (response spectra) that are supplied with SPACE GASS. Each spectral curve is a graph of acceleration in terms of g's versus period in terms of seconds, and is derived from the time-history record of a ground vibration for a specific level of damping. They are not dependent in any way on the properties of the structure being analysed.

A built-in graphical spectral curve editor allows you to modify or create your own response spectra and save them to a spectral curve library.

You can define spectral load cases, each of which references a list of modes, a spectral curve, a mass load case and a direction vector. It is common practice to have a spectral load case for each of the two horizontal orthogonal building directions, plus various combinations of them with the use of combination load cases.

The spectral analysis module has code-specific inputs for the latest Australian, New Zealand and Indian loading codes, however it can also be run in a General mode that can be applied to any loading code.

For an accurate spectral analysis, it is important that the spectral load cases have been defined correctly and that appropriate combinations of the spectral load cases have been specified. For more information, refer to "Spectral load data".

The analysis considers the vibration of the structure and identifies the maximum values that result from it. Generally, the maximums at different points of the structure occur at different times during the dynamic event. Consequently, the spectral results do not represent an equilibrium state of the structure, but rather an envelope of the maximums. Furthermore, because the earthquake action has no sign (ie. its accelerations are both positive and negative), the maximum values have no sign and hence the sign of the results is indeterminate. Usually, the results are dominated by one of the mode shapes which SPACE GASS can identify and apply its sign to the results. Alternatively, you can select which mode shape the sign should be taken from.

If horizontal base shear scaling is activated, the results will be scaled up if the total reaction in the vector direction is less than a user defined proportion of the total static force or a user defined percentage of the total mass.

The results of the spectral analysis include deflections, forces, moments, stresses and reactions that can be displayed graphically, printed or used in a design in the same way as the results from a static analysis. It is also possible to combine spectral load cases with static load cases in combination load cases.

An equivalent static seismic load generator tool is also available in SPACE GASS.

Important points

1. A spectral response analysis is linear only and therefore cannot be performed if your model contains cable elements.
   
   
2. Because it is linear, a spectral response analysis treats tension-only and compression-only members as normal members that can take tension or compression.
   
   
3. P-D and P-d effects are not taken into account during a spectral response analysis, however you can specify a user scaling factor that lets you simulate P-delta and other amplification effects.
   
   
4. Because the maximum response at different points on the structure occur at different times during the earthquake event, the spectral results do not represent an equilibrium state of the structure, but rather an envelope of the maximums. For these reasons, any post-analysis calculations (eg. overturning moments from plate cuts or user calculations) that depend on the structure being in equilibrium may not be reliable.
   
   
5. A buckling analysis cannot be performed with spectral load cases and therefore compression effective lengths from a buckling analysis are not available when doing a steel member design/check on spectral load cases. If you are performing a steel member design/check on combination load cases that contain a mixture of static and spectral load cases then the spectral load cases will not contribute to the calculation of the compression effective lengths. This may not be correct and so you should consider specifying your compression effective lengths manually in those cases.
   
   
6. A spectral response analysis requires a dynamic frequency analysis to be conducted first.
   
   
7. The spectral response analysis must be repeated after a dynamic frequency analysis because its results will have been deleted.
   
   
8. No automatic provision is made for extra torsion due to accidental eccentricity of the horizontal earthquake actions as required by most earthquake loading codes, however the diaphragm tools include options for generating the extra eccentric mass cases required to model accidental eccentricity. It is up to you to include these in the model. Further discussion is included in the "Accidental eccentricity" section of this chapter.
   
   
9. AS1170.4-2007 clause 6.2.3 imposes a lower limit on the period when doing the base shear scaling calculations, however this lower limit is not applied in SPACE GASS due to the inconsistent results it can produce. If the calculated period used in the base shear scaling is less than the clause 6.2.3 limit and you wish to apply the limit then you should select "Scale using fundamental period" and specify the period manually.
   
   
10. NZS1170.5 clause 5.2.1.1 imposes a lower limit of 0.4s on the calculated period for the purposes of calculating kμ. It is imposed in SPACE GASS for all modes (not just the fundamental mode) for the base shear scaling calculations and for the modal response spectrum calculations when calculating kμ.
    
    
11. The accuracy of the spectral response analysis depends on the accuracy of the dynamic frequency analysis on which it is based. It is therefore important that you set up your model correctly to achieve accurate dynamic frequency analysis results. For example, if master-slave constraints are used then positioning of the master nodes is particularly important for correct mass distribution. For more information, refer to "Dynamic frequency analysis".

Refer to "Spectral response analysis results" for details and interpretation of the results of a dynamic spectral response analysis.