Why is human induced vibration so important to engineers and builders?

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26/03/2020

Engineering and minimising the effects of human induced vibration

An important aspect of most engineering and building projects is to tackle the issue of human induced vibration, which occurs as humans walk. Although you might picture swaying structures (e.g. the Millenium Bridge wobble), the effects of human induced vibrations are unlikely to be serious (as in a collapsing structure) but more likely to result in human discomfort when using the end structure, whether that be a building or bridge.

In this article, we explore why human induced vibration is so important to engineers and builders.

Resonance and Fluttering

Vibrations can affect structures in numerous ways, namely resonance and aeroelastic fluttering.

Resonance is when Object A vibrates at the same 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.

Aeroelastic flutter is when 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’s also fluttering. 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.

Human induced vibrations cause structures to vibrate as human movement is applying force. Some instances would also see resonation happening too, but it wouldn’t be a certainty. Engineers must design to reduce the damage or discomfort caused by either fluttering or resonating.

The potential effects from Human Induced Vibration

Human induced vibration can affect structures in many ways:

Causing bridges to sway. 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.
Causing damage to 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.
Negative effects on human health. 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.
Endangering 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.

How to minimise the effects

Newer designs that incorporate thinner slabs and wider column spacing in the column design are vulnerable to all types of vibrations including human-induced. Using software for building design at the design stage is an effective method for engineers to test footfall on a design and see the resulting vibrations.

Although vibrations are a natural occurrence, it’s essential that engineers minimise the effects from human footfall through structural design.