The main function of the ACS SASSI MOTION module is to SSI response in terms of motion time histories, transfer functions or in-structure response spectra. For seismic problems, the ACS SASSI MOTION module reads the acceleration time history of the control motion from the input file and transforms it to the frequency domain using the Fast Fourier Transform technique. It then reads the computed transfer functions from File8 for selected output points, performs the interpolation and the convolution with the control motion, and returns to the time domain using the inverse Fast Fourier Transform algorithm. The resulting time histories of acceleration may be output directly or converted to output response spectra. The MOTION module applies unconditionally baseline corrections to all the nodal output acceleration time histories. File 13 contains the baseline corrected acceleration, velocity, and displacement time histories.

IMPORTANT NOTE: If the incoherent SSI analysis option is selected in the HOUSE window dialog, then the saved MOTION input file (with extension .mot) using AFWRITE command will include a request for applying the complex transfer function phase adjustment. If the MOTION input (.mot file) is saved with the coherent motion option in the HOUSE window dialog, then no phase adjustment is applied to either coherent (or incoherent) transfer functions. We recommend the use of the phase adjustment for incoherent SSI analyses.

The following options allow you to specify the analysis options for ACS SASSI MOTION module:

• Operation Mode - Select the operation mode from Solution and Data Check.
• Type of Analysis - Select the type of analysis from Seismic and Foundation Vibration. This option is the same as set in the Analysis Options - ANALYS dialog box.
• Baseline Correction - Select if the motion module will generate a baseline correction. The baseline correction is based on Hudson-Housner time-domain procedure. It computes only approximate absolute displacements. The user has to check if the ground displacement reasonable by checking if it is zero at the end of earthquake motion. The relative displacements between different locations can be computed more accurately by the difference absolute displacements.
• Output Control - Select the type of response from Output Only Displacements, Output Only Velocities, and Output Only Accelerations. Only accelerations can be requested for seismic problems.
• Output Only Transfer Functions - If this option is set, only transfer function printed plots can be requested at nodal points and time history of input motion need not be supplied.
• Save Complex Transfer Function - If this option is set then amplitude and phase of the computed transfer functions are saved in TFU and TFI files. This option is not recommended for typical SSI analysis since the information on the Fourier phase is often difficult to interpret from engineering point of view.
• Save File 12/13 - User can save FILE12 or FILE13 produced by MOTION module.
• Total Duration to Be Plotted - Input the total duration of time history to be plotted.
• Smoothing Parameter - The smoothing parameter is used to filter the potential spurious spectral sharp peaks and valleys that could be artificially introduced by the original SASSI interpolation scheme for transfer functions. Using the ACS SASSI spectrum plot capability, the analyst could investigate the effects of smoothing on the SSI results by comparing for selected nodes the computed and the interpolated amplitude transfer functions that are saved on hard-drive in text files with extension TFU and TFI. To use smoothing between any pair of the computed frequencies has to be at least one interpolated frequency.
• Incoherent SRSS - Allows the user to create a file “SRSSTF.txt” that will be read by the ACS SASSI HOUSE module.

Nodal Output Data The Node List contains lists of nodes having the same output request. Output requests in constrained directions are ignored. Press the Add button to add a new node list, the Edit button to edit the nodes of the selected list, or the Delete button to delete the selected list.

You may use blank, tab, ‘,’ , or ‘;’ as separators for the node numbers. You may use ‘-’ to add a set of nodes to the node list.

For each node list and for each direction (buttons X, Y, Z, XX, YY, ZZ), select the requested output from the check boxes placed on the right side of the Node List:

• Printed Plot of Transfer Functions *
• Save Time History of Requested Response **
• Plot Time History of Requested Response **
• Plot Acceleration and Velocity R. S. ***
• Save Acceleration and Velocity R. S. ***
• Print Maximum Requested Response **

* The transfer function for seismic problems is defined for total acceleration response while for foundation vibration problems, it is defined for total displacement response.
** The requested response for seismic problems is acceleration; for foundation vibration problems, it is determined by the selected option from the Output Control group box.
*** The response spectra are computed independent of the option from the Output Control group box. Therefore, displacement response spectra cannot be requested.

• Response Spectrum Data - The user can compute the response spectra for selected nodal locations using the setting described next.
• First Frequency - Type the first frequency used in response spectrum analysis (Hz).
• Last Frequency - Type the last frequency used in response spectrum analysis (Hz).
• Total Number of Freq. Steps - Type the total number of frequency steps used in response spectrum analysis. If the first frequency, the last frequency, and the total number of frequency steps are all zero or blank, the following default values are assumed: first frequency = 0.4, last frequency = 40, total number of frequency steps = 40. This leads to a plot which fills one page.
• Damping Ratios - Type the damping ratios used in response spectra. You may use blank, tab, ‘,’, or ‘;’ as separators between damping ratios.
• Nr. of Fourier Components - Type the number of values to be used in the Fourier transform. The value is the same as set in the Analysis Options - SITE dialog box.
• Time Step of Control Motion - Type the time step of control motion (sec). The value is the same as set in the Analysis Options - SITE dialog box.
• Multiplication Factor - Type the multiplication factor for scaling the time history. Use only if Max. Value for Time History is blank.
• Max. Value for Time History - Type the maximum value of time history to be used. The values of the time history will be scaled to this value. Use only if Multiplication Factor is blank.
• First Record - Type the first record from the time history file to be used (default: first).
• Last Record - Type the last record from the time history file to be used (default: last).
• Title - Type the title of the acceleration time history.
• Acceleration History File - Type the full path and name of the time history file. To view the time history file, use the Plot / Time History command.
• File Contains Pairs Time Step-Acceleration - Select this option if your time history file contains pairs of time step and acceleration values on each line. Otherwise, the file is expected to contain the time step on the first line, and on the following lines one acceleration value.

Convert Time History to Response Spectrum

This option is for the user convenience. Using it the user can compute response spectra for an external time history file (not for a selected nodal output). This option is convenient to be used after we compute new acceleration time histories based on algebraic summation outside the code. The resultant time histories can be input to the code to compute their response spectra. The setting for response spectrum calculations are assumed to the same as for the nodal outputs (same calculation frequencies, damping and number of time steps).

To use this option the user needs to check the option. Then user needs to browse for the acceleration time histories he wants to select by using for computing their response spectra.

• Post Processing Options - The three options in this box are to allow the user to generate data for Bubble , Vector Tf and Displacement plot in all nodes. These options can be used in tandem with the options in the Node Output data fields.