Human Induced Vibrations
|The human body is a fluid driven mechanical system
that exhibits a number of natural rhythms and resonances. The rhythms vary from
the regularities of sleeping patterns, alpha brain waves, heart rhythms,
hormonal regularities, walking and running, blinking, speaking, singing ...
Many humans are sensitive to external low frequency vibrations that must be taken into account when designing tall dwellings (that respond to wind gusts) and long bridges. So called "infra-sound" can be uncomfortable for humans. The air in the interior of a car travelling at 60 mph on a motorway can be readily excited to exhibit low frequency infra-sound (even if all windows are closed) due to the transmission of vibrations from between the tyres and road surface. Sound pressure with frequencies below 100Hz from automobile engines can excite the air in rooms of adjacent houses via the motion of windowpanes. This can in turn induce a structural response in the walls and floor of the room.
The mechanism of walking is very complex, particularly over irregular terrain. No robotic device has yet sufficient processing power to drive a vertical linked bipedal structure that can manoeuvre efficiently up a garden path let alone climb a mountain!
If a pedestrian walks with a stride of about 0.9m across a bridge he behaves like a series of moving point loads. Depending on how many paces are required to cross the bridge and the nature of its design and damping characteristics it is possible for the pedestrian to drive the bride into resonance. This is the reason for the custom of asking troops to break step when marching across a bridge. In 1831 a cast iron bridge at Broughton collapsed under the resonance effect of 60 soldiers who marched across it. In the era of the steam train a similar phenomenon arose from the pulsating forces generated by the balance weights on the driving wheels of steam locomotives. The design of old railway bridges owes much to the problem of "hammer-blow" induced by railway traffic.
The constituents of the standing human body can be excited into resonance at frequencies between 3 and 100Hz. At the lowest of these frequencies the abdominal viscera can be set into (uncomfortable) oscillation. The torso and pelvis have excitable modes of oscillation in the 5-7 Hz range. The upper torso and spine begin to oscillate between 10-14 Hz while the head and shoulders can be excited between 20-30 Hz. Finally there is an eyeball resonance in the 60-90 Hz range. Because of the relative displacements of body parts resonances below 10Hz are particularly uncomfortable and can lead to serious damage if sustained.