Forced Vibration Test

A small eccentric-mass shaker was used to excite the retrofitted structure with harmonic forces. The shaker is comprised of a welded steel frame that is bolted to a steel baseplate, four main bearings, two main shafts, and six attached weights. Eccentricity of the vibrator can be varied between zero and 100% in several increments. The shaker has an operating frequency range of 1-25Hz and a maximum force output of 88 kN. The response of the structure was recorded using tri-axial accelerometers. By developing a frequency response function (FRF) from the data, the modal frequencies could be directly estimated from peak-picking, and the modal damping ratios could be found using the half-power bandwidth method.