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Demand module (Ground Motion Selection)
The ground shaking is known to be one of the main sources of uncertainty in structural vulnerability assessment. For this reason, it was considered necessary to include in the VMTK a module to assist the vulnerability modeller in the selection of suitable ground motion records.
The current implementation of the VMTK requires the user to provide a collection of ground motion record from where to perform selection and/or numerical analyses. An important note to consider is that each input ground motion file must be formatted as two comma delimited column arrays where the first column contains the time series (in seconds) and the second column contains the corresponding ground acceleration (in the units of g).
In the current implementation of the VMTK, there are three options to select and scale ground motion records.
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The first is to simply use all the records provided by the user. This option leaves the record selection to the user’s criteria (i.e., it is assumed that the user has already selected a collection of ground motion records that fulfills the requirements of the vulnerability analysis)
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The second option uses a list of intensity measure levels (defined by the user) to create bins of target ground motion intensity levels and populate them with ground motion records. The algorithm initiates by computing intensity measures for the complete collection of ground motion records provided by the user. For convenience, a list of IMs has already been pre-programmed in the GUI (PGA, SA(0.2s), SA(0.3s), SA(0.5s), SA(0.6s), SA(1.0s) or SA(2.0s)) and can be selected by the modeller. When using the GUI for setting the target intensity levels, the user should input a series of float numbers separated by commas in units of g. Once the intensity measures for each ground motion record are computed, for each intensity bin the algorithm randomly selects suitable records until each bin contains a predefined number of ground motion records. In case there are not enough ground motion records to reach the minimum number of records for a given intensity bin, the algorithm will attempt to fill the missing number of records by scaling, conditional on the minimum and maximum scaling factors input by the user. If after scaling there are still not enough records to comply with the minimum number of records per bin, the algorithm continues to the next intensity bin.
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The third selection is based on the conditional spectrum method (CSM). The first stage of CSM involves performing a seismic hazard disaggregation to understand the distribution of magnitude, distance and epsilon for a given location. For convenience, the VMTK was made fully compatible with the hazard disaggregation analysis outputs (i.e. magnitude, distance and epsilon distribution) of the OpenQuake-engine.
Once the selection is finished the outputs are stored as csv files in the directory specified by the user. In the GUI is possible to plot the spectra of the selected records for inception being only necessary to input the range of periods of interest and the number of steps to consider.