====== NONLINEAR Module Options ======

The EQL module is used to analyze structural components using a equivalent linear material property analysis. The main components that can be analyzed are shear walls which are comprised of shell elements, and springs which are modeled using translation springs. The analysis is an iterative procedure similar to the equivalent linear analysis which is also used to analyze nonlinear behavior of soils.

For the shear walls, the material properties that are altered are the elastic modulus and damping ratio for the shell elements, these properties are defined in Young's modulus versus displacement and equivalent hysteretic damping versus displacement curves, similar to G-D curves for soils. The equivalent hysteretic damping values of this curve are calculated by the EQL module for a given hysteresis model. The EQL modules uses the backbone curve from the user and then uses either the Cheng-Mertz shear hysteretic model for reinforced concrete, Cheng-Mertz bending hysteretic model for reinforced concrete, Takeda hysteretic model, or a general Masing rule hysteretic model to generate Young's modulus-damping versus displacement curves. The user may also directly input the Young's modulus-damping versus displacement curves with using the EQL module. The springs are analyzed in a similar way, except the spring stiffness is changed opposed to the Young's modulus value used in shells.

The degraded Young's modulus values of the curve do not depend on the hysteretic model chosen because secant stiffness is used. This secant stiffness only depends on the backbone curve. The hysteretic model chosen controls the loading and reloading path which only changes the equivalent hysteretic damping values.

**Note**: The backbone curve input by the user is generally in terms of a force-displacement, shear-shear strain, or moment-rotation relationship. Thus when the secant stiffness of the curve is found this value of stiffness is normalized with the elastic stiffness (Kelastic). This normalized curve is then scaled with the elastic young's modulus value (Eelastic), this allows the stiffness-damping curves to be in terms of Young's modulus and damping. 

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

The EQL module requires the input files: modelname.sit, modelname.hou, modelname.EQL, and the corresponding nodal displacement files (xxxxxTR_[X,Y,Z].THD, xxxxxR_[XX,YY,ZZ].THD) for the nodes in the shear walls and springs. The modelname is the name of the active model. The .EQL file is written from ACS-SASSI User Interface . The EQL module reads the corresponding .hou, .sit, .EQL, and corresponding .THD files to generate several output files and a new updated .hou file that can be re-run for the next iteration. This iteration analysis using  ACS SASSI can be very time consuming and laborious. To help the user with this issue the equivalent linear analysis can be executed by running the provided equivalent linear analysis batch file. 

The EQL module generates a Panelxxxx.crv, Panelxxxx.thd, Panelxxxx.ths,  Panel_EQL_Matl_Prop.txt, and Panel.fmu files as outputs for the shear wall elements, and SPRINGxxxx.crv, SPRINGxxxx.thd, SPRINGxxxx.ths,  SPRING_EQL_Matl_Prop.txt, and Panel.fmu files as outputs for the spring elements. As well as a general output text file which is named by the user and contains nearly all the input and output data from the program. 

The Panelxxxx.crv file contains the Young's modulus-damping curves for each panel, and this curve contains the Young's modulus - damping versus displacement curve. The Panelxxxx.thd and Panelxxxx.ths files contain the panel displacement and force time histories respectively. The Panel_EQL_Matl_Prop.txt file contains the new material properties of the shear walls. The Panel.fmu file contains the ductility (μ) and force reduction factors (Fμ) of the shear walls. The ductility value of the shear walls is found by dividing the maximum value of displacement by the cracking displacement point, or the displacement point on the backbone curve where the elastic region ends. The force reduction factor is found by dividing the elastic analysis force by the final nonlinear force. The EQL module provides similar files for the SPRING output files.

The equivalent linear analysis performs an analysis similar to the used in equivalent linear analysis of soils, (SHAKE methodology). Given the displacement time history of a shear wall, and the elastic modulus-damping curves an equivalent displacement is found by multiplying the displacement factor times the maximum displacement in time history. The displacement factor is defined by the user and is typically 0.6-1. This equivalent displacement can be used to find new values of elastic modulus and damping ratio that is used in the material properties for the shear wall. 

This analysis is performed for all shear walls, once the new material properties are known a new HOUSE file is written and ACS SASSI is re-run. This process is repeated until convergence of material properties is achieved. 

===== Global Modeling Options =====

  * **Disp. Factor**  - Equivalent Displacement Factor. (Typically between 0.6-1) For equivalent linear analysis this factor determines the equivalent displacement level (DF time max value in time domain) that is used to determine the new material properties
  * Use Non-linear (Element) Options – Select nonlinear element types to be used. This is controlled by the check boxes in panel.
  * **Damping Cutoff** – User specified percentage of damping cutoff.
  * **Damping Scale Factor** - User specified Scale factor for damping.

===== Backbone Curve Data =====    
  * **Backbone Curve** - Backbone Curve number being displayed
  * **Type** – backbone curve type
    * 1 - Cheng-Mertz shear
    * 2 - Cheng-Mertz bending
    * 3 - Takeda
    * 4 - General Hysteresis Model 
  * **Yield Number** – number of the curve point that is the yield point of the curve
  * **Curve Data Grid** - From this panel the user can view and edit backbone curves. This window is not capable of adding new or removing data points from the curve. Curve size and curve points must be initially defined using the BBC commands, however the user can modify a point on the BBC curve from this box.

===== Panel Data =====
 
  * **Panel** - Panel Number in model
  * **Group Num.** - Corresponding group number for the shear wall panel
  * **BBC Num.** - option to define which backbone curve to use for shear wall
  * **Disp. Opt.** - displacement calculation option
    * 1 – Shear strain
    * 2 – Vertical Bending Rotation
    * 3 – total displacement
  * **Force Opt.** -  force calculation option = option to define which hysteresis model to use to calculate nonlinear force output
    * 1 - Cheng-Mertz shear
    * 2 - Cheng-Mertz bending
    * 3 - Takeda
    * 4 - General Hysteresis Model

 ===== Spring Data =====

  * **Spring** – Spring Number 
  * **Group Num.** - Corresponding group number for the non-linear spring
  * **Elem. Num.** - Corresponding element number for the non-linear spring
  * **BBC Num.** - option to define which backbone curve to use for spring
  * **Disp. Opt.** - displacement calculation option 
  * **Force Opt.** -  force calculation option = option to define which hysteresis model to use to calculate nonlinear force output
    * 1 - Cheng-Mertz shear
    * 2 - Cheng-Mertz bending
    * 3 - Takeda
    * 4 - General Hysteresis Model 

===== Beam Data =====

  * **Beam** – Beam Number
  * **Group Num.** - Group of the Nonlinear Beam
  * **Spring Gr.** - Spring group for the beam ends
  * **BBC Num.** - option to define which backbone curve to use for beam
  * **Force** -  force calculation option = option to define which hysteresis model to use to calculate nonlinear force output
    * 1 - Cheng-Mertz shear
    * 2 - Cheng-Mertz bending
    * 3 - Takeda
    * 4 - General Hysteresis Model
  * **Beam End 1** – element that defines one end of the beam 
  * **Beam End 2** – element that defines the other end of the beam