Iterative Response Spectrum

Intro

This plugin allows user to perform linear analysis of Response Spectrum while considering soil non-linearity. It automates iterative stiffness updates for response spectrum analysis in MIDAS Civil NX. The plugin processes point springs, extracts displacement results, and updates stiffness iteratively until convergence. Users can track stiffness evolution across iterations and compare results efficiently.

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Benefits of this plugin

This plugin automates the iterative response spectrum analysis process by updating spring stiffness based on displacement results until convergence is achieved. It removes the need for manual stiffness adjustments and rerunning analyses, saving significant time and reducing errors. Users can easily track stiffness changes and export all results in Excel. It’s especially useful for nonlinear soil-structure interaction cases in MIDAS Civil NX.

How to use this plugin?

Note

Refer to below assumptions considered in the plugin

1. Force-deformation Function- is considered symmetric in +ve and -ve direction.
Eg. Spring defined in +Dx/-Dx direction will be converted to +Dx.
Also, F-D function should not contain any negative slope.

2. Linear Spring Stiffness- If a node has a multi-linear spring definition only in one direction, then after the first iteration, a minimal stiffness (0.001 only for translational dof) will be applied to other directions. 

3. Uniform capacity- If a deformation exceeds beyond the definitions provided in the Force-deformation function, the ITR script will consider constant force from the last available location until the new obtained deformation.
e.g. new deformation is 0.26 ft (refer below)
 

4. Node local axis- If node local axes are defined at the multi-linear spring definition, the node local displacements will be used for secant stiffness calculation. If not defined, global node displacements will be used.

5. Construction Stages (CS)

Without construction stages- If CS are not defined, then all the nodes with multi-linear definitions will be consider for the ITR script iteration.
With construction stages- If CS are defined, then software filters the nodes to be considered in following manner:
Checks the boundary groups that are active in last constructions stage -> Identifies the point linear/multi-linear springs from those active boundary groups -> These nodes are considered in ITR Script iteration.
Note: Boundary parameters under ‘Default’ option/group will not be considered for the plugin.
 

6. Tolerance - Tolerance should be entered in ratio i.e 0.05= 5%. It will be checked against all translational dof (local/global). It checks displacement of nodes relative to previous iteration.
 

Tolerance check example:
Tolerance: 0.01
Old displacement = 5 ; New displacement = 6
Tolerance = (6-5)/5 = 0.2 > 0.01.
Therefore, plugins interpolates force for ‘new displacement’, calculates stiffness and runs analysis again.
 

Conclusion

The Iterative Response Spectrum plugin streamlines nonlinear response spectrum analysis by automating stiffness updates until convergence. Users can perform accurate soil-structure interaction analyses with minimal input, reducing manual work and improving reliability.