Vibrations caused by walking is known in engineering as human induced vibrations. This is less like the collapse of a structure and more like a bit of a wobble that can cause discomfort. Although less serious, it’s vital that engineers minimise vibrations for security and comfort. In this article we’ll explain why vibrations are important for engineers to consider.

Resonance and Fluttering

Structures can be affected by vibrations through resonance and aeroelastic fluttering.

Resonance occurs when Object A vibrates the same as Object B’s natural frequency. 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.

Although similar, aeroelastic flutter is different in that when force is applied to Object B it shakes. 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 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.

In regard to human induced vibrations, when force is applied and causes a structure to vibrate, it’s also categorised 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. 

Possible Effects from Human Induced Vibration

Resonation and fluttering can result in several effects upon the structure and its users. These include:

Ways to Avoid it

Contemporary designs tend to include thinner slabs and wider column spacing sensitive to vibrations. 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.

Vibrations are a natural effect of movement, so it’s imperative that engineers’ designs reduce the overall impact.

Sources:

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/