Why Engineers Need to Minimise the Effects of Human Induced Vibrations
Vibrations that happen through human movement are known as ‘human induced vibration’. These effects are so small that they’re unlikely to cause a structure to collapse but could potentially makes users feel uncomfortable. Even though the effects are small, this article looks at why engineers need to minimise vibrations to make sure structures are safe for the public.
What are the different types of vibrations?
Did you know that vibrations can affect structures in many ways? Well, there are two main vibrations that affect structures are resonance and aeroelastic fluttering. Resonance takes place when Object A vibrates at the same frequency as Object B. After that, Object B then resonates with this and starts 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.
The second main vibration is aeroelastic flutter, which is a little different. In this case, a force is applied to Object B, which causes it to shake. It’s not necessarily at the same frequency as Object B’s natural vibration, but it makes Object B move all the same.
Objects also flutters when they resonate. However, not everything that flutters is actually 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.
Moment that occurs by humans causes human induced vibrations and the structure vibrates when force takes place. Force is applied and the structure vibrates, it’s considered as 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.
Negative Effects of Vibrations
As we have already mentioned, resonation vibration and aeroelastic fluttering can cause a number of affects upon the structure. Including:
- Destructing 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.
- Bridges that swing. 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.
- Human health interference. 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.
- Ruining 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.
Modern structures are created with thinner slabs and wider column spacing compared to other structures, which makes vibrations occur more often. 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.
It’s extremely important that engineers work to decrease human induced vibrations when buildings are being designed, in order to ensure the public feel comfortable and safe.