Why engineers need to account for human induced vibration…
by Natasha Christou Digital Marketing ConsultantThankfully, incidents like the Millennium
Bridge ‘wobble’ are few and far between, but it’s clear that engineers working
on a project – whether that be a building or bridge structure – need to make sure
that the end result is safe and comfortable for use.
Human footfall isn’t likely to result in
serious structure failure, but this doesn’t mean that human induced vibration isn’t
something that needs serious consideration.
Read on…
Resonance and vibration explained
Vibrations can affect structures in a wide range
of ways. Two of the main ways are resonance and aeroelastic fluttering.
When Object A vibrates at the same natural
frequency as Object B, this is known as resonance. As a result, object B resonates
with this and will begin 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 differs slightly; for
example, a force is applied to Object B, causing 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.
When an object resonates, 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.
Fluttering also occurs with human induced
vibrations, and an example of this is when human movement is applying force,
causing the structure to vibrate. Some instances would also see resonation
happening too, but it wouldn’t be a certainty.
Engineers have to ensure that their designs
reduce the damage or discomfort caused by either fluttering or resonating.
Possible outcomes
The structure and users within a building
can be compromised when fluttering or resonation occurs, and it can have a
range of impacts:
·
Jeopardising structural
integrity. Eventually, the build-up of constant vibrations
on a structure can lead to structural integrity being compromised. A worse-case
scenario would be the complete collapse of said structure.
·
Swaying bridges. The Millennium Bridge is one of the best examples of resonance
caused by human induced vibrations and fluttering. 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.
·
Jeopardise integrity of
sensitive equipment. Depending on what the building
is used for, what is within and what can be affected by the vibrations of those
inside the building. Universities, for example, may
have sensitive equipment whose accuracy and performance could be damaged by
vibrations.
·
Human health suffering. Research has found 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.
Resolution?
Modern designs that favour thinner slabs
and wider column design and spacing tend to be susceptible to all forms of
vibration, whether it is human-induced or otherwise. Using advanced structural
design software at the design stage is an effective method for engineers to
test footfall on a design and see the resulting vibrations.
Vibrations are inevitable, but engineers
must account for a wide range of catalysts that trigger vibrations in a
structure, such as human footfall, adapting their overall design accordingly.
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Created on Dec 10th 2019 07:43. Viewed 291 times.