CAN LONG-DURATION VIBRATIONS DAMAGE STRUCTURES AT SUB-CRITICAL PARTICLE VELOCITIES?

Homeowners in a seismically quiet area of the upper Midwest, USA, noticed unusual vibrations whose onset in 1997 correlated with the appearance of cracks in walls and foundations that continued to develop over ~18 months. Vibrations reportedly lasted for up to 8 hours, beginning and ending abruptly. In an industrial building nearby, large industrial machines had been assembled and tested since 1997, bolted to a ~2 ft thick ~97,000 lb concrete pad in direct contact with the ground and mechanically isolated from the building. Vibrations were efficiently transmitted into a dense clay unit ~100 ft thick, which was overlain by the Holocene fluvial sands upon which the distressed structures had been built. This case illustrates the importance of duration, in addition to frequency and peak particle velocity, in determining whether vibrations will result in damage to a structure. Peak particle velocity recorded in one of the distressed houses during a test of the machine was ~0.0147 in/s in a narrow frequency range of ~7-10 Hz. The record was collected in a wood-frame house >50 years old with cinder-block basement, stucco exterior, and old plaster or new drywall on interior walls. Plants, a computer monitor and the springs in sash windows in the house began to move during the test, demonstrating that the machine vibrations were transmitted ~0.75 km to the house with sufficient energy to excite macroscopic particle motion. Correlation between the initiation of machine-source vibrations in 1997 and structural damage is based on testimony. Ordinances patterned after U.S. Bureau of Mines standards for maximum peak accelerations related to construction blasting are common, generally holding that at frequencies below ~40 Hz, short-duration peak particle velocities below ~0.75 in/s are safe for modern homes with drywall interiors, and velocities below ~0.5 in/s are safe for older homes with lath-and-plaster construction. These standards do not address the effect of long-duration vibrations. We infer that the critical peak velocity at which deformation occurs may decrease with increasing duration of vibration, given a vibrational frequency near the resonant frequency of the structure. This is analogous to the decrease in strength with decreasing strain rate observed in rock-deformation experiments. Duration is also an important factor in damage associated with earthquakes.