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.