When a system is subjected to an external sinusoidal force it oscillates at the external frequency and not at its natural frequency. This is called forced oscillation. Vibrations and built structures Vibrations in built structures can just be a nuisance, or cause illness, or even structural damage. Vibrations come from two main sources, internal or external. Inside buildings, vibrations come from machines such as air conditioning units, pumps, elevators, fans, front end loaders, presses, etc, or from people walking, jumping, or running around.
External vibrations come from road and rail traffic, subways, construction, earthquakes and winds.
The Tacoma Narrows Bridge had shallow plate girders instead of the normal deep stiffening trusses. Damping The damping capacity is the ability of the structure to absorb energy. Elastomeric pads can be placed at the basement level of every column to reduce the magnitude of vibrations in the remainder of the building. When an external source sends vibrations through the soil into the foundations of a buiding, the vibrations move both sideways and upwards. Soft soils give larger amplitude vibrations than hard or rock-like materials, but the amplitude of those vibrations decreases more rapidly with distance.