Running a spectral response analysis

You can run a spectral response analysis by selecting "Spectral Response Analysis" from the Analysis menu.


image\ebx_-1773499217.gif Before a spectral response analysis can proceed, you must have created some spectral load cases and performed a dynamic frequency analysis.



Load case list

Leave blank if you want to analyse all spectral load cases, otherwise enter the load cases (separated by commas or dashes) that you want analysed.


When specifying the load case list, you can either list them directly or you can click the button to display and select from a list of the spectral load cases currently in the job.


Modes list

This field contains the list of modes that will be considered during the spectral response analysis. You can leave the list blank to include all the modes that were analysed in the dynamic frequency analysis (recommended) or you can list specific modes (separated by commas or dashes) that you want to be considered.


The spectral analysis must include enough modes so that the total mass participation factor in each direction being considered exceeds 90%. If it is less than 90% then more modes should be included in the mode list, otherwise the results may not be accurate. If you leave the modes list blank and still can't get 90% mass participation then you will have to re-run the dynamic frequency analysis with more modes included. Mass participation factors are calculated from the dynamic frequency analysis and are shown in the "Dynamic Frequencies and MPFs" report and the "Spectral Response" report.


It is important that you include the fundamental mode (ie. the mode with the longest period in the direction of the earthquake), together with all other modes that have significant mass participation. If you're not sure which modes to include then you should include them all by leaving the list blank. It will improve the accuracy of the analysis and won't significantly slow down the analysis.


Retain results of other load cases

Tick this box if you want to keep the analysis results of previously analysed spectral load cases. Otherwise, if they are not being re-analysed in the current session, they will be discarded.


Loading code

This allows you to select the loading code to be used. If you choose the AS, NZS or IS code, you should ensure that you have also selected spectral curves for that same code in your spectral load data.


The major difference between the "General" loading code and the other codes is that the spectral curve multiplier must be manually defined for General, whereas it can be calculated based on various code-specific factors for the AS, NZS and IS codes.


Limit state - NZS1170.5 only

For NZS, you must choose between serviceability or ultimate limit states together with an appropriate structural ductility factor as per NZS1170.5-2004 4.3.


Scaling of horizontal base shear

This is a code related parameter that instructs the program to scale the results so that the sum of the horizontal support reactions (the base shear) obtained from the spectral analysis is not less than a user defined proportion of the total static force or a user defined percentage of the total mass. For example, consider a case where the calculated total reaction in the horizontal direction under consideration is 35kN, the total static force in that direction is 42kN, the total mass is 300Tonnes and you have specified the minimum base shear to be 1.6% of the total mass. In this case the results must be scaled up so that the total reaction is at least 42kN or 1.6% of 300T (=47kN), whichever is greater. This equates to a scaling factor of 47/35 = 1.343 which is applied to all the displacements, forces, moments and reactions. If the total reaction already exceeds the total static force and percentage of the total mass then no scaling is done.


Base shear scaling seems to have been removed from the dynamic analysis of AS1170.4-2007 when it superseded AS1170.4-1993 (refer to AS1170.4-2007 Preface item (q)), however in SPACE GASS 12.65 and later versions we have made it available with AS1170.4-2007 for users who wish to apply it anyway. For AS1170.4-2007 it is turned off by default.


The total static force for the two horizontal directions is calculated from the total mass multiplied by the acceleration obtained from the spectral curve. The period used to get the acceleration is usually the fundamental period of the structure (ie. the one with the largest translational period in the direction being considered). You can control this via the base shear scaling settings from which you can choose between: (a) the period from the dominant mode (ie. the one with the largest mass participation factor), (b) the period from a user specified mode or (c) a user defined period. If base shear scaling is turned off then the period from the dominant mode is used.


For NZS1170.5 a lower limit of 0.4s is imposed on the period when determining kμ as per clause, however if "Scale using fundamental period" is selected then no lower limit is imposed on the user defined period. Note that in older versions of SPACE GASS the 0.4s lower limit was also imposed on user defined periods when calculating kμ however this was removed in SPACE GASS 12.85. For the other seismic loading codes, no lower limit is imposed on the period used in the base shear scaling calculations.


Note that AS1170.4-2007 clause 6.2.3 imposes a lower limit on the period when doing the base shear 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.


When calculating the static force using a code-specific spectral curve, the acceleration from the curve does not reduce as the period approaches zero. For AS1170.4-2007 refer to the non-bracketed values in table 6.4, for NZS1170.5-2004 refer to figure 3.1 or for IS1893.1-2016 refer to figure 2A. Note that for the spectral response analysis itself, the acceleration does reduce as the period approaches zero as per AS1170.4-2007 figure 6.4, NZS1170.5-2004 figure 3.2 or IS1893.1-2016 figure 2B.


Masses (ie. lumped masses and self-mass) applied to restrained degrees of freedom are included in the total static force and total mass calculations, and consequently may affect the horizontal base shear scaling. Masses applied to restrained degrees of freedom were excluded from the base shear scaling calculations in pre-v12.65 versions of SPACE GASS.


Base shear factor

If base shear scaling is activated then by default the base shear is scaled so that it equals or exceeds the total static force, however if you want to allow it to be less than the total static force then you can specify a base shear factor of less than 1.0. For example, if NZS1170.5-2004 clause applies then you should use a base shear factor of 0.8.


Minimum base shear as percentage of total mass (%)

If base shear scaling is activated and you want the base shear to be not less than a certain percentage of the total mass then you can specify it in this field. 1% of the total mass is assumed by default.



There are a number of code specific factors that can be typed in directly or calculated automatically based on descriptions of the structure location, structure importance and construction method. They are used to calculate the spectral curve multiplier and other parameters in the spectral analysis.


Site subsoil class - NZS1170.5 only

The site subsoil class is only used when non-NZS spectral curves are used. For NZS spectral curves the site subsoil class is always taken from the curve, regardless of what you specify in this setting.


User scaling factor

The user scaling factor can be used to scale up the results in order to take into account P-delta or other effects that would increase the deflections, forces, moments and stresses. The user scaling factor is applied first and then horizontal base shear scaling is applied afterwards if required. A user scaling factor of 1.0 has no effect on the results. For more information refer to AS1170.4 clause 6.7.3 or NZS1170.5 clause


Sign of the results

Because the results of a spectral response analysis are a combination of a number of dynamic modes, the final sign of the results has to be determined. Choosing "No sign" is of limited use and means that all deflections, forces, moments and reactions will be positive. Choosing "Auto Sign" means that the sign of the dominant mode shape (ie. the one with the largest mass participation factor) will be applied to the results. Choosing "Select Mode" tells the program to extract the sign from a nominated mode shape.


Spectral curve multiplier

The spectral curve multiplier is used to scale the spectral curve accelerations based on the location of the structure, the type of construction, the return period and the required performance. It can be typed in directly or, by clicking the button next to the spectral curve multiplier field, can be defined via various code-specific factors. For AS1170.4 it is based on probability, hazard, structural ductility and performance factors, for NZS1170.5 it is based on hazard, return period, near-fault and structural performance factors, and for IS1893 it is based on the zone factor, damping multiplying factor, importance factor and response reduction factor. Each of the code specific factors can be typed in directly or calculated automatically based on descriptions of the structure location, structure importance and construction method.


Mode combination method

The deflections, forces, moments and reactions calculated for each mode must be combined to produce a complete set of results for each spectral load case. The two commonly used methods are as follows:



When all of the information has been entered, the spectral response analysis module performs its calculations for each load case. If you want to terminate the analysis before it is finished, just press ESC or the right mouse button.