CLASSI (Continuum linear analysis of soil structure interaction analysis) uses a combination of finite element and boundary element analysis techniques. By modelling the soil using boundary elements and the structure using finite elements, CLASSI achieves an efficient representation of the soil structure interaction problem. Furthermore, the time history for the input motion is reduced using a fast fourier transform and the whole problem solved in the frequency domain to couple together the components of the soil and structure analysis. The result is a fast and efficient analysis of multiple detailed 3D structures in the time domain. This results in a level of approximation not dissimilar from a finite element model, but with far greater computational efficiency due to the method employed.

The solution for soil is carried out using various Green’s function solution procedures (integral equation methods) for arbitrary surface or embedded foundations in a horizontally layered elastic or viscoelastic halfspace. The integral equation procedures used approximate the stress distribution over the interface by a number of uniform stress patches. The following is a list of capabilities and limitations for the program

  • The foundation medium can be defined as an elastic or viscoelastic, uniform or horizontally layered continuum halfspace. Damping within the foundation medium is defined using constant hysteresis damping ratios for each of the soil layers.
  • The structural foundation is assumed to be rigid and bonded to the soil.
  • Multiple structural foundations can be analysed.
  • The wave environment can be defined using either vertical or inclined P, SV, and SH body waves with R and L surface waves, all within a three dimensional seismic incidence wave environment.
  • The program is linear and does not include an automatic iteration procedure. Therefore the strain dependency of soil properties has to be considered using an succession of equivalent linear analyses.

CLASSI uses several steps to achieve this goal:


The soil is represented as infinite horizontal layers (this limits the solution to surface mounted foundations), which are represented using Green’s functions to compare motions at one point with motions at radii from that point.

These are computed for vertical and radial motions at several frequencies.


The foundations are represented on the surface of the soil beneath each structure by defining a number of patches of uniform stress.  The patches form foundations which are considered rigid.  By using the green’s functions generated by GLAY the interaction between all the regions can be calculated, gathered by foundation then used to compute frequency dependant complex impedances for the group of foundations modelled.

The foundations are modelled with adjacent rectangular patches and symmetry can be invoked to speed up modelling


This module is used to import the finite element model for each structure.  The original code was written for SAP IV.  We have extended this to run with ANSYS.  If you use a different finite element code for your structural analysis please contact us since we are always extending the capability and will be happy to discuss writing a convertor.

The features of the model imported by INSSIN are: mass matrix, mode shapes and nodal geometry.  Other details of the model do not need to be imported since the output generated is a series of time histories at the nodes.


This module takes the frequency dependant complex impedance matrices generated by the CLAN module and the finite element details generated by INSSIN for each structure along with a series of time histories then solves the dynamic SSI problem in the time domain to generate time histories and response spectra at each node.


GLAY, CLAN, INSSIN and SMAX are the original SMACS command line programs updated to run on a windows platform and extended to improve the quality assurance tracability of files and the size of problem which can be analyzed.

Although all these components can be run from the command line Win-SSI includes a graphical user interface to guide the user through the solution and permit building projects with strict quality assurance tracking.