When building structures on soft soils experience strong motion (e.g. earthquakes, construction work, explosions) the response of the structure is modified by the response of the soil.

Since building structures tend to consist of a series of discrete elements such as beams and shells it has long been established that the  finite element method can model the response of these very accurately.

However the finite element method has significant shortcomings when modelling the propagation of vibrations through soil.  Vibrations in soil travel as waves and in order to model the propagation and reflection of the waves in the soil medium a relatively fine mesh of elements is required which extends over a significant volume.  If the mesh is not adequately fine higher frequencies will be lost and if the mesh does not extend far enough wave reflections at the boundaries of the model lead to interference and can produce incorrect results.

Although many elaborate solutions have been created to create appropriate boundaries to dynamic soil models these still require large finite element models which take a lot of time to produce, validate and run.

Since the soil below a structure is an infinite and often layered medium it is far more efficiently represented using a continuum model with boundary elements.

In the late 70s the CLASSI method was created by Prof. H.L Wong and Prof. J.E Luco in California.  They managed to construct a way of combine finite element models of the structures with a continuum model of the soil in the complex frequency domain.  For structures which have foundations which can be considered essentially rigid compared to the soil and substrata which can be modelled with horizontal layers their method is very computationally efficient and very accurate.

The CLASSI method is a stable solution and so well suited to fast paced projects where reliable solutions are required.

In the early 80’s the US nuclear regulatory commission conducted a seismic safety margins research program (SSMRP) which used the CLASSI method to investigate the margins in seismic analysis of nuclear structures with respect to such factors as soil, structural stiffness, time history input.  As a result of this research programme the SMACS code was produced which forms the basis of our Win-SSI program.