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Why it’s Important to Reduce the Effects of Human-induced Vibrations in Engineering

Human induced vibration is when footfall causes vibrations
in structures. This might sound dangerous to a structure’s integrity, but the
effects from human induced vibrations are more likely to cause discomfort in
people rather than ruin the design. It’s important for engineers to make their
structures secure and comfortable for people to pass through, so in this
article we’ll explore what can actually happen from these vibrations.

Resonance and Fluttering Vibrations

Two of the main effects of vibrations on structures are
resonance and aeroelastic fluttering.

Resonance vibrations happen when two objects vibrate at the
same natural frequency as each other. 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 flutter is slightly different. A force
is applied to an object, which causes it to shake. It’s not necessarily at the
same frequency as the object’s natural vibration, but it makes the object move
all the same.

When an object resonates, it flutters 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.

Human induced vibrations are categorised as fluttering
because human movement is applying force which causes the structure to vibrate.
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. 

Human-induced Vibrations Effects

Human induced vibration can lead to several affects upon the
structure and its users. These include:

  • Having damaging effects on 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.
  • 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 human heath to suffer. 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. 
  • Threatening 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.

Reducing the Effects

Unlike older designs, modern designs have an affinity for
thinner slabs and wider column spacing, which makes them prone to vibrations. Using
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 always going to happen, so it’s important for
engineers to make sure the effects are reduced as much as possible.

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