Human footfall causes a range of vibrations, all with varying degrees of effect on the overall structural integrity of a building. This concept might conjure images of Millennium Bridge-style swaying or collapsing buildings — but in reality, the ‘damage’ caused by human induced vibrations is less likely to ruin a structure and more likely to cause discomfort in people!
New York
Photo by: Michael Giugliano / Pexels
Though not as dramatic as a structural failure, any good engineer wants to make sure the people using their structures, be it bridges or buildings or anything in between, can do so safely and comfortably. This is why human induced vibration must be considered within the design process.
Resonance vs fluttering
Vibrations can affect structures in a range of ways, any many of the effects go unnoticed. Two of the main ways are resonance and aeroelastic fluttering.
When Object A vibrates at the same natural frequency as Object B, this is classed as resonance. 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 notably different; it happens when a force is applied to Object B, making it 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 is resonating, it is 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.
For human induced vibrations, given that human movement is the cause of the force and making the structure vibrate as a result, it would be classed as fluttering. Some instances would also see resonation happening too, but it wouldn’t be a certainty.
Engineers are presented with the challenge of designing structures that will experience minimal damage or discomfort caused by either fluttering or resonating. As such, structure design software has modernised a lot in order to tackle this issue.
Potential impacts from human induced vibration
Fluttering or resonation caused by human induced vibration, can have a range of consequences for the structure and its users, including:
- Interfering with sensitive equipment. Depending on the purpose of the building, what it houses can be affected by the vibrations of people within. Universities, for example, may have sensitive equipment whose accuracy and performance could be damaged by vibrations.
- Swaying bridges. The Millennium Bridge is a key example of resonance and fluttering caused by human induced vibrations. 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 suffering. Studies have found that structural vibrations in buildings 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. When a constant vibration builds up within a structure it might eventually lead to integrity issues with the building. A worse-case scenario would be the complete collapse of said structure.
How to avoid it
Many modern designs that favour thinner slabs and wider column spacing tend to be more susceptible to all kinds of vibration, not just human induced occurrences. 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 impossible to rule out vibrations. As a result, engineers have to consider the certain catalysts that lead to vibrations in a structure and account for factors such as human footfall, limiting the impact that it will have on their overall design.
Amy Hodgetts is a content writer on behalf of Oasys, a leading commercial developer of engineering software. Ms Hodgetts is a graduate from the University of Glasgow, with an undergraduate MA (Hons) in English Language.
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