When people walk they naturally create human induced vibrations which, although often insignificant, can impact in various ways on structures i.e. bridges and buildings. Although not in the league of structural failure, it is still important that engineers factor in and minimise the potential effects of these vibrations, in order to make structure users safe and comfortable.
Main Effects – resonance and fluttering
The two main effects of human induced vibrations are resonance and aeroelastic fluttering.
In simpler terms, resonance happens when Object X vibrates at the same frequency as Object Y’s natural frequency. Object Y resonates with this and begins to vibrate too. Think singing to break a wine glass! Although the person singing isn’t touching the glass, the vibrations of their voice are resonating with the glass’s natural frequency, causing this vibration to get stronger and stronger and eventually, break the glass.
Aeroelastic flutter happens when a force is exerted to an object which in turn makes it shake. It’s not necessarily at the same frequency as Object B’s natural vibration, but it makes Object B move all the same.
When something resonates, it also flutters. But not everything that flutters is necessarily resonating. This is how confusion over disasters such as the Tacoma Bridge collapse occur — for a long time, and to this day, the event is used as a textbook example of resonance. However, it’s been argued that the bridge’s collapse wasn’t caused by resonance, but by fluttering.
When human force is exerted and causes an object to vibrate, this is fluttering. Some instances would also see resonation happening too, but it wouldn’t be a certainty. Engineers must, of course, design to reduce the damage or discomfort caused by either fluttering or resonating.
Vibrations from human footfall, and the fluttering or resonation it can cause, can have many effects on a structure. These include:
· Hindering sensitive equipment. Depending on the building’s purpose, what it houses can be affected by the vibrations of people using the building. Universities, for example, may have sensitive equipment whose accuracy and performance could be damaged by vibrations.
· Effecting the structural integrity. The build-up of constant vibrations on a structure can, eventually, lead to structural integrity being compromised. A worse-case scenario would be the complete collapse of said structure. · Effecting human health. According to research, vibrations in buildings and structures can cause depression and even motion sickness in inhabitants. Buildings naturally respond to external factors such as the wind or human footfall within. This low-frequency vibration can be felt, even subconsciously, by people. It has been argued that modern designs featuring thinner floor slabs and wider spacing in the design of columns mean that these new builds are not as effective at dampening vibrations as older buildings are.
· Wobbling bridges. One of the most famous examples of resonance, human induced vibrations, and fluttering all impacting a structure occurred with the Millennium Bridge. As people walked across the bridge, the vibrations and swaying caused oscillations in the bridge. Everyone crossing the bridge would then sway at the same time to avoid falling over, resulting in a cycle of increasing and amplifying the swaying effect.
Compared to old designs, contemporary structures tend to integrate thinner slabs and columns that are spaced widely, making them vulnerable to vibrations. Using structural design software at the design stage is an effective method for engineers to test footfall on a design and see the resulting vibrations.
Although vibrations are something that happen naturally, engineers must consider reducing the effects of them in their designs to ensure security and comfort.