February 12, 2020 Human Induced Vibration Guide for Engineers

Vibrations caused by human footfall, depict the image of Millennium Bridge-style swaying or buildings collapsing. But is this the reality?

Well, the reality is that the ‘damage’ caused by human induced vibrations are less likely to ruin a structure and more likely to cause discomfort in people. Despite not being as dramatic as a structural failure, any good engineer that takes pride in their work wants to make sure the people using their structures, whether it be bridges or buildings or anything in between, can do so safely and comfortably without having to threat. Because of this, it is vital human induced vibration must be set as a priotiy throughout the design process.

Fluttering vs Resonance

Vibrations can affect infrastructure in many different ways. Two of the main ways are resonance and aeroelastic fluttering.

When Object A and Object B simultaneously vibrate at the same natural frequency, resonance occurs. Object B resonates with the vibrations of Object A and begins to vibrate too. If you think about singing to break a wine glass, it is a similar process! Although the person singing isn’t actually touching the glass, the vibrations of their voice are resonating with the glass’s natural frequency, this ultimately causing the vibration to get stronger and stronger and eventually, break the glass.

When a force is applied to Object B and causes it to shake, aeroelastic flutter occurs. It’s not necessarily at the same frequency as Object B’s natural vibration, but the motion in which Object B moves is all the same.

Speculation as to whether the collapse of the Tacoma Bridge bridge was caused by fluttering or resonance.

Technically speaking, if something is resonating, it is considered to be fluttering 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. The distinction between these two have been confused by engineers throughout history.

Since human induced vibrations are caused when human movement is applied to forces that causes structures to move, this can be classed as fluttering. Some instances would also see resonation happening too, but it wouldn’t be a certainty.

Ultimately, it is important that engineers use the correct structural design software in order to create designs that help reduce damage or discomfort that fluttering, or resonating creates to help with the safety and congenial of individuals

Human induced vibration: what are the potential impacts?

Both the structure and its users can be affected by human induced vibrations, fluttering or resonations in numerous ways:

• Damaging or interfering with 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.
• Swaying 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, ultimately resulting in a cycle of increasing and amplifying the swaying effect.
• Human health suffering. According to research findings, it is thought that 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.
• Jeopardising 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.

Ways to avoid this

The likes of thinner slabs and wider column spacing that are typically found on more modern buildings are especially vulnerable to being affected by all types of vibrations, human-induced or otherwise. Using structural design and analysis software at the design stage is an effective method for engineers to test footfall on a design and see the resulting vibrations.

It’s essential that engineers consider all potential catalysts of vibrations when creating structures. Vibrations are natural; however, the likes of human football vibrations could drastically impact the design of the structure overall.

Amy Hodgetts

Amy Hodgetts is a content writer on behalf of Oasys, a leading commercial developer of engineering software. A content writer and web content optimiser, Ms Hodgetts is a graduate from the University of Glasgow, with an undergraduate MA (Hons) in English Language.

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://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/

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