Vibration tests of a steel-frame building

Abstract

The natural frequency and damping of a steel-frame building were experimentally determined for the first two modes of vibration from a steady state resonance curve obtained with a variable frequency mechanical oscillator. The value of the first natural frequency was 3.5 cps, which may be compared with the 2.9 cps frequency which was determined from calculations of the response to the transient excitation of a quarry blast. The difference in these frequencies is explained by the fact that somewhat different modes of vibration were excited by the two tests, and by possible differences in structural damping at the high and low amplitude levels of the two tests. The maximum accelerations set up during the steady state tests were of the order of 0.005 g., whereas the accelerations during the blast reached values around 0.1 g. The damping measured for the low amplitude steady state vibrations was 3.416 of critical damping.