ANALYSIS OF TURBO-GENERATOR FOUNDATION WITH SOIL-STRUCTURE INTER-ACTION

Abstract

With the advent of large capacity turbogenerators, to cater the present demand, the heavy R.C. frame foundations should be regarded as serious engineering problem to choose a solution which is technically sound and economical. Machine foundations with dynamic loads confront the designer with a much more complex problem. Therefore, this study is taken up on an actual pile-supported frame-foundation under a turbogene-rator of 110 MW capacity running at 3000 rpm. The study incorporates the rigorous analysis and full-scale testing. The analysis carried-out with usual two-dimensional approach and more precise three-dimensional formulation, for (a) considering the base rigid and (b) considering the subgrade flexibility of soil-pile system. Comparison of 2-D and 3-D analysis have been made and results substantiated with actual observations. Limitations of 2-D analysis have been brought out. The basic difference between 2-D and 3-D due to continuity of the longitudinal beams and base slab is emphasized. Testing programme incorporates (a) static load deflection test on a beam to find out the modulus of elasticity of concrete, (b) horizontal load deflection test at top deck level to find the horizontal stiffness, (c) free-vibration test for natural frequency and damping, (d) vibrations in steady-state condition at several operational speeds of machines for resonant frequen-cies and responses, and (e) vibrations in run-down test to isolate various frequencies of different modes of vibrations. With the increased size and output of turbogeneratcxs together with their heavy frame fouhdations usually require stronger subgrade, so that subgrade stiffness significant. For the same reason piles are provided. This results in substantial interaction between the foundation and subgrade. bubgrade flexibility is found to have a very significant effect on the vibration levels. Particularly where the soil is stiff or the piles are provided, the consideration of base flexibility is essential to model the actual picture. Also the significance of coupling between the foundation and the rotor is indicated as an area of further investigation as it becomes important if bearing stiffness and/or that of oil film are inadequate.