The BASSIN methodology has the following features: It computes the three-dimensional dynamic response of an arbitrarily configured, elastic, bridge/abutment/backfill system. It assumes this system to be underlain by a soil continuum that is represented as an elastic half- space. It represents seismic input motions as being harmonic and induced by planar P-, SV-, SH-, or Rayleigh waves with arbitrary wavelength, amplitude, and direction of incidence. The methodology uses a substructure approach in which the structure (which comprises the bridge, abutment, and backfill) is represented using a three-dimensional finite element model, and the underlying soil continuum is represented using a boundary element approach. The finite element model defines the stiffness matrix, mass matrix, and fixed-base nodes for the structure. The boundary element approach characterizes the underlying soil medium using frequency- dependent impedance matrices and driving force vectors that also incorporate the free-field wave motions. Compatibility and equilibrium requirements at the soil/structure interface are used to couple these substructures. The steady-state response of the soil/structure system is then computed.