SITE Module Options

The ACS SASSI SITE module has two basic operation modes:

  1. Form and solve transmitting boundary eigenvalue problem - Mode1
  2. Solve the site response problem - Mode 2

Mode 1: The ACS SASSI SITE module reads the soil layer properties and for each specified frequency forms the transmitting boundary submatrices for Rayleigh and Love wave cases. Then it solves the two eigenvalue problems, as described in the Theoretical Manual, from which the eigenvalues and eigenvectors of the soil layering are obtained. The results are then written to File2. The halfspace condition is also simulated at this stage. ACS SASSI SITE automatically generates a specified set of sublayers whose thickness vary with frequency attached to viscous dashpots at the base. The generated sublayers and dashpots are then added to the fixed top layers.

File 2 provides the information needed to run ACS SASSI SITE in Mode 2 as well as to compute the transmitting boundary by ACS SASSI POINT. Since the eigenvalue problems to be solved for an arbitrary three-dimensional horizontally layered site are the same as those to be solved for a plain strain model, the information from File2 can be used for both two- and three-dimensional cases as well.

Mode 2: The ACS SASSI SITE module recovers the soil layer properties and the eigen solutions for Rayleigh and Love wave cases from File2. Then, according to the existence of each wave type, ACS SASSI SITE computes the mode shapes and wave numbers for each wave type in the defined coordinate system. Then, once the composition of the wave types causing the seismic environment and the nature of the control motion is known, ACS SASSI SITE will scale and superimpose the results of all the wave types. These results are then stored in File1, which is used later for seismic analysis. Thus, this file will not be generated for foundation vibration analysis. If the seismic environment is the same for a two- and three-dimensional case, the information from File1 can be used for both problems.

The following options allow you to specify the analysis options for ACS SASSI SITE module:

For a time history analysis, the time step and number of Fourier components for the selected time history of the control motion must both be defined, and the frequency step may be left blank. ACS SASSI SITE computes the corresponding frequency step = 1 / (time step * number of Fourier components) . This frequency step may then be used to set up the frequency numbers in the specified frequency set.

For a single harmonic analysis the frequency step must be given and may be directly used to set up frequency numbers in the specified frequency set. In this case, the time step of control motion and the number of Fourier components are not used and therefore may be left blank.

The seismic environment may be assumed to consist of one single wave type or several wave types. The basic wave types are P-waves and S-waves, which are also called body waves. When these waves impinge on the ground surface or layer interfaces, surface waves which include R-waves and L-waves may be generated.

P-waves involve motions in the direction of wave propagation. S-waves involve motions perpendicular to the direction of wave propagation. S-wave motions in the vertical plane are called SV-waves. Horizontal S-waves are called SH-waves. R-waves involve horizontally propagating elliptical motions in the vertical plane and L-waves consist of horizontal motions perpendicular to the horizontal direction of wave propagation.

With the above definitions, ACS SASSI SITE sets up a coordinate system so that P-waves, SV-waves, and R-waves involve particle displacements in the x’z’ plane while SH-waves and L-waves involve particle displacements along the y’ axis. Therefore, the z’ axis is always vertical up, x’ is in the vertical plane of wave propagation, and y’ is perpendicular to x’ and z’ following the right-hand rule.

Wave propagation and associated parameters

In the case of seismic environment composed of two or more wave types, the ratio of participation of each wave type must be given. This ratio in general may be frequency-dependent and is defined at two discrete frequencies for each wave type. These frequencies must cover the frequency range of analysis. The ratio values for intermediate frequencies will be obtained by simple interpolation and therefore need not be given at exact frequencies for which complete solution is required.

In the case of seismic environments consisting of one simple wave type, the two frequencies (one in the beginning and the other at the end of the frequency range of analysis) with assigned ratio values of 1 are enough to define the ratio curve.

All ratio values are positive decimal numbers less than or equal to 1, and the addition of the ratio values of all the participating wave types at each frequency must be 1.