Why it’s Essential for Engineers to Minimise the Effects of Human Induced Vibrations

  • Guest Blog
  • 12 months ago

Vibrations caused by human movement is also known as human-induced vibration. The effects of human-induced vibrations are minimal in that they’re unlikely to cause a structure to collapse but are likely in causing discomfort to people passing through. Although the effects are small, in this article, we’ll explain why engineers must make their structures comfortable and safe by reducing vibrations.

Different Types of Vibrations

Vibrations can affect structures in many ways, the two main ways are resonance and aeroelastic fluttering. Put simply, resonance occurs when Object A vibrates at the same natural frequency as Object B. Object B 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.

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.

When an object resonates, it also flutters too. 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.

As human movement causes human-induced vibrations, when 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

Resonation vibration and aeroelastic fluttering can cause a number of effects 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 column design 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 worst-case scenario would be the complete collapse of said structure.
Minimising Effects

Structures that are modern are usually designed with thinner slabs and wider column spacing than other structures, which makes them susceptible to vibrations. Using building design software at the design stage is an effective method for engineers to test footfall on a design and see the resulting vibrations.

In order to make people feel safe and comfortable, it’s essential that engineers work to reduce human-induced vibrations when designing buildings.

Sources:

https://www.oasys-software.com/news/analysing-vibration-with-gsa/

https://www.oasys-software.com/case-studies/footfall-analysis-singapores-helix-bridge/

https://www.oasys-software.com/case-studies/princeton-university-frick-laboratory/

http://homepage.tudelft.nl/p3r3s/MSc_projects/reportRoos.pdf

https://www.forbes.com/sites/startswithabang/2017/05/24/science-busts-the-biggest-myth-ever-about-why-bridges-collapse/#1b9e3b001f4c

https://phys.org/news/2017-03-impact-bridges-skyscrapers-human-health.html

https://www.quora.com/Whats-the-difference-between-resonance-and-aeroelastic-flutter

https://www.telegraph.co.uk/science/2017/03/19/wobbly-skyscrapers-may-trigger-motion-sickness-depression-warn/