Seismic Soil-Structure Interaction Analysis. FLUSH is a development of the complex response finite element program LUSH for seismic soil-structure interaction analysis. It includes the following features: 1. Plane strain quadrilateral elements for modeling of soils and structures. 2. Beam elements for modeling of structures. 3. Multiple nonlinear soil properties for equivalent linear analysis. This allows for different damping in each element. 4. An approximate 3D ability which makes it possible to perform certain structure-soil-structure interaction analyses at essentially the same cost as 2D analysis. 5. Transmitting boundaries. 6. An out-of-core equation solver which reduces core-size problems. 7. Internal deconvolution. This feature eliminates the need to perform an independent site response analysis for determination of the rigid base motions. 8. Convenient file handling features which provide restart ability and a permanent record for later recovery of details of the solution. 9. Printed, plotted or stored output time histories of acceleration and bending moments. 10. Computation of maximum shear forces in beam elements. 11. Printed or stored acceleration velocity response spectra. 12. Plotting of Fourier amplification functions. 13. Optional baseline correction of input and output motions.