Dynamics: Free Vibration

Measurement of the fundamental natural frequency of a building through free vibration generated using vibrators

Fig. 15-17: The test building (Courtesy of Building Research Establishment Ltd)

Fig. 15-18: The free vibration record of the building

When the free vibrations of a structure can be measured as accelerations, velocities or displacements, the natural frequencies of the structure can be determined from their vibration time histories.

Fig. 15-17 shows an eight-storey steel framed test building in the Cardington Laboratory of the Building Research Establishment Ltd. Different dynamic test methods were used to determine the natural frequencies of this building. One of the methods used was to record the free vibrations of the building [15.8].

Four vibrators mounted at the corners of the roof of the building were used to generate movement of the structure. Once movement was initiated the vibrators were turned off and the resulting free vibrations of the structure were measured by accelerometers. Fig. 15-18 shows the acceleration-time history of the decayed vibrations. The frequency of the oscillations, which is a natural frequency of the structure, was determined from the time history as the inverse of the time interval between two successive acceleration peaks.