Vibrations caused by human footfall, also known as human induced vibration, causes structures to vibrate to some extent whether due to sitting, walking, jumping, bouncing, or running — walking has been found to be the most common cause of vibration. Although this may create terrifying visions of Millennium Bridge-style swaying, or fragile flooring collapsing dramatically, the damage from these vibrations can actually be miniscule, causing discomfort in us. This might sound like a minor concern, but it’s highly important for any good engineer to make their buildings and bridges safe and comfortable.
For example, slender and lightweight structures have been found to be particularly responsive to human footfall. Plenty of research has been conducted to understand the behaviour of floor vibration. So, here, we’ll discuss why human induced vibration is a very important consideration in the design process.
The difference between resonance and fluttering vibrations
Surprisingly, there are numerous ways that human induced vibrations can have an impact on structures. Two of the main ways are resonanceand aeroelastic fluttering.
In simpler terms, 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.
However, aeroelastic fluttering is slightly 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.
If an object is resonating, it is technically 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.
When considering the effect of human induced vibrations, given that human movement is applying force and causing the structure to vibrate, it would be classed as fluttering. Some instances would also see resonation happening too, but it wouldn’t be a certainty.
Therefore, it’s important for engineers to reduce the damage or discomfort caused by either fluttering or resonating in the design process.
What are the potential impacts on structures from human induced vibration?
Human induced vibration and the fluttering and resonation than can occur can have several effects on the structure and those who walk through. These include:
- 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, resulting in a cycle of increasing and amplifying the swaying effect.
- 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.
- 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.
- Human health suffering. 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. Structural design software can be effective in mitigating this.
How to avoid resonance and fluttering
As touched on previously, modern designs that favour thinner slabs and wider column spacing are particularly susceptible to all forms of vibration, 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.
So, there we have the reasons why engineers must always consider human induced vibrations in their design process. Although yes, vibrations are natural, however considering the causes of vibrations in a structure and how this can impact the overall decision will make your structure a safe, comfortable experience.
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/