ACS SASSI Documentation

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====== STRESS Module Options ======

The main function of the ACS SASSI [[sassi:STRESS]] module is to compute and output maximum stresses, forces, or moments in the elements. The user may also request time histories of those components to be printed and saved.
The program reads the acceleration (displacement) transfer functions from File8 and information about elements from File4. Then, for each requested elements, the STRESS module computes the stress, force, or moment components at each frequency, performs interpolation and convolution with the control motion, and finds the corresponding time histories by returning in the time domain using the inverse Fast Fourier Transform algorithm.

Time history of octahedral shear stress (strain), which is a measure of maximum shear stress (strain), is calculated in time domain for each time interval defined by the time step. From this, the effective shear strain (0.65 * maximum shear strain) for the soil elements can be estimated and used to define new strain-compatible soil properties.

{{ :sassi:user_interface:stressopt.png |}}

The following options allow you to specify the analysis options for ACS SASSI STRESS 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 - [[sassi:user_interface:options:ANALYS]] dialog box.
  * **Auto Computation of Strains in Soil Elements** - Set this option if you want STRESS to automatically compute the strains in all soil elements (if secondary non-linear effects are considered). This option should be used together with the save option of transfer functions of forces and moments in beam elements (see next two save options). 
  * **Save Stress Time Histories on File15** - Set this option if you want STRESS to save the stress time histories on File15. This save option should be used with the next save option.
  * **Save Transfer Functions of Forces and Moments in Structural Frame** - Set this option if you want STRESS to output transfer functions for beam element nodal forces and moments. Transfer function values at frequency solution points (uninterpolated) for beam element nodal forces are written to the formatted File14. The transfer functions are computed for all the 12 degrees of freedom of the beam elements, i.e. six degrees for each end node. 
  * **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 - [[sassi:user_interface: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 - [[sassi:user_interface:options:SITE]] dialog box.
  * **Frequency Set Number** - Type the number of the frequency set. The value is the same as set in the Analysis Options - [[sassi:user_interface:options:SITE]] dialog box.
  * **Multiplication Factor** - Type the multiplication factor for scaling the time history. The value is the same as set in the Analysis Options - [[sassi:user_interface:options:MOTION]] dialog box.
  * **Max. Value for Time History** - Type the maximum value of time history to be used. The value is the same as set in the Analysis Options - [[sassi:user_interface:options:MOTION]] dialog box.
  * **First Record** - Type the first record from the time history file to be used. The value is the same as set in the Analysis Options - [[sassi:user_interface:options:MOTION]] dialog box.
  * **Last Record** - Type the last record from the time history file to be used. The value is the same as set in the Analysis Options - [[sassi:user_interface:options:MOTION]] dialog box.
  * **Title** - Type the title of the acceleration time history. The title is the same as set in the Analysis Options - [[sassi:user_interface:options:MOTION]] dialog box.
  * **Acceleration History File** - Type the full path and name of the time history file. The file name is the same as set in the Analysis Options - [[sassi:user_interface:options:MOTION]] dialog box.
  * **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. The option is the same as set in the Analysis Options - [[sassi:user_interface:options:MOTION]] dialog box.
  * **Element Output Data** - The list box contains groups and lists of elements having the same output request. Press the Add button to add a element list, the Edit button to edit the elements of the selected list, or the Delete button to delete the selected list. 

{{ :sassi:user_interface:options:elementlist.png |}}

You may use blank, tab, ‘,’ , or ‘;’ as separators for the element numbers. You may use ‘-’ to add a set of elements to the element list. Select the group from the Groups list box.

For each element list and for each component (buttons from the Components group box), select the requested output from the buttons placed in the Component Request group box.

If the nonlinear soil SSI analysis option is used, then the nonlinear soil element group (including all solid elements) has to be selected by the user. The nonlinear SSI results are saved at each iteration in FILE74.

**SOLID elements**

The available components are:
  * Stress / Strain XX Direction
  * Stress / Strain YY Direction
  * Stress / Strain ZZ Direction
  * Stress / Strain XY Direction
  * Stress / Strain XZ Direction
  * Stress / Strain YZ Direction
  * Octahedral Shear Stress

The stresses in the 3D solid elements are computed at the centroid of the element and are referred to in global axes. The stresses are shown in the following figure:

{{ :sassi:user_interface:options:solidstress.png?400 |Stresses in SOLID elements}}


In addition to the user-requested response, ACS SASSI [[sassi:MOTION]] may also output other components of the response if calculation of these components is necessary in order to determine the requested response. For example, in order to output maximum octahedral shear stress for 3D solid elements, all six components of stress  must be computed. Therefore, ACS SASSI MOTION also outputs the maximum response of these components. However, the corresponding time histories are not to be saved on File12 unless they are specifically requested by the user.

**BEAMS elements**

The available components are:
  * Force 1-Direction - Node I
  * Force 2-Direction - Node I
  * Force 3-Direction - Node I
  * Moment 1-Direction - Node I
  * Moment 2-Direction - Node I
  * Moment 3-Direction - Node I
  * Force 1-Direction - Node J
  * Force 2-Direction - Node J
  * Force 3-Direction - Node J
  * Moment 1-Direction - Node J
  * Moment 2-Direction - Node J
  * Moment 3-Direction - Node J

The forces and moments in beam elements are computed at the end and are reference in local beam axes:

{{ :sassi:user_interface:options:beamaxis.png?400 |Local axes for BEAMS elements}}

**SHELL elements**

The available components are:
  * Force XX-Direction (S<sub>x’x’</sub>)
  * Force YY-Direction (S<sub>y’y’</sub>)
  * Force XY-Direction (S<sub>x’y’</sub>)
  * Moment XX-Direction (M<sub>x’x’</sub>)
  * Moment YY-Direction (M<sub>y’y’</sub>)
  * Moment ZZ-Direction (M<sub>x’y’</sub>)

The membrane forces and bending moments are computed in respect to the local element coordinate system. The forces are in units of force/length (F/L) and the moments are in moment/length (FL/L). The location of the infinitesimal elements where the forces and moments are computed are shown in the following figure:

{{ :sassi:user_interface:options:membrstress.png?400 |Membrane stresses and bending moments in SHELL elements}}

**PLANE elements**

The available components are:
  * Stress / Strain XX-Direction
  * Stress / Strain ZZ-Direction
  * Stress / Strain XZ-Direction

The stresses in 2D PLANE elements are computed at the center of the element and are referred to in global axes. These stresses are shown in the following figure:

{{ :sassi:user_interface:options:planestress.png?400 |Stresses in the 2D PLANE elements}}

**SPRING elements**

The available components are:
  * Force X-Direction
  * Force Y-Direction
  * Force Z-Direction
  * Moment XX-Direction
  * Moment YY-Direction
  * Moment ZZ-Direction


The available requests for each component are:
  * No Request
  * Print Only Maximum Response
  * Print Maximum and Save Time History of Response to File12 - if this option is selected, time histories of the requested response are stored to File12. This file can later be used to recover the time histories for plotting purposes.

**Post Processing Options**
The Options in this box to allow the user to save the stress data in all of the node so it can be plotted in the contour plot. These Options also allow the user to specify a restart of motion to calculate soil pressure after the motion has initially been run.  The Frame Selection button opens a file “Frames.txt” which is used by the stress module.
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