The paper presents the results of an investigation into excessive local vibrations in a tabletop foundation for a 120 MW steam turbine generator (STG) located in northern Iran. These vibrations have persisted since the 1990 Rudbar-Manjil (Mw 7.3) earthquake. The investigation involved measuring the tabletop vibrations at several locations during normal operation of the STG unit. The measured data were analyzed in the frequency domain to extract the dynamic properties of the soil-foundation system. These properties were then utilized to calibrate a finite element model of the foundation.
The results obtained from Fourier analysis of the vibration data and dynamic analyses of the foundation indicate that the excessive vibrations were primarily caused by the extremely low damping of the soil-foundation system. This low damping was attributed to the significant injection of slurry cement grout into the subsoil after the earthquake. The dynamic analyses also revealed that the excessive vibrations occurred due to near-resonant responses of local modes of vibration with frequencies close to the operating frequency of the STG unit. To mitigate the vibration problem, the installation of tuned mass dampers (TMDs) at critical locations of the foundation was proposed. A dynamic analysis of the tabletop foundation with eight TMDs demonstrated that this solution effectively mitigates the excessive vibrations.