Human-induced vibrations affect structures in different ways, causing discomfort for occupants and users
Human-induced vibrations – why engineers should always consider it – By Amy Hodgetts
The concept of vibrations caused by human footfall – commonly known as human-induced vibrations – can sometimes conjure up images of buildings collapsing or Millennium Bridge style swaying. But in reality, the ‘damage’ caused by human-induced vibrations is less likely to ruin a structure and more likely to cause discomfort in those using it.
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 vibrations must be considered within the design process.
Human-induced vibrations affect structures in two ways – impulse or transient response, and resonance. The latter occurs when a vibrating system forces another system around it to vibrate with greater amplitude. 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 the glass breaks. In the case of a structure, resonance occurs when the pedestrians’ feet land in time with the vibration.
Alternatively, when a structure’s natural frequencies are too high for resonance to happen, impulse or transient vibrations can be a problem. Here, the discomfort is caused by the initial ‘bounce’ of the structure caused by footsteps and is a concern in light or stiff structures.
To help reduce the effects of vibration caused by either impulse or resonance, engineers must look into the design of structures in great detail.
Structures can be affected in numerous ways by human-induced vibrations.
One of the most famous examples is the Millennium Bridge in London. As people walked across the bridge, their footsteps caused the bridge to sway, and everybody had to walk in time with the sway because it was difficult not to. Thankfully, this feedback can only occur with horizontal vibrations, so building floors are safe from it, but footbridges need careful checking to prevent it.
According to research, vibrations in buildings and structures can cause depression and even motion sickness in inhabitants. Tall buildings sway in the wind and footsteps can be felt, even subconsciously, by the occupants. It has been argued that modern efficient designs featuring thinner floor slabs and wider spacing in column design mean that these new builds are not as effective at dampening vibrations as are older buildings.
Interference with sensitive equipment
Depending on the building’s purpose, they can be affected by the vibrations of people using the building. Universities and laboratories, for example, may have sensitive equipment whose accuracy and performance could be damaged by vibrations. Even in ordinary offices, the footfall vibration can wobble computer screens, upsetting the workers.
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 the structure, and is the reason why marching troops may break step before crossing a bridge. Crowds jumping in time to music or in response to a goal in a stadium are also dynamic loads that might damage an under-designed structure.
Avoiding the issue
Some modern design structures are particularly susceptible to forms of vibration, human-induced or otherwise, such as designs with thinner slabs and wider column spacing. However, the likes of short spans can also suffer due to their low mass.
Using sophisticated structural design software is an effective method for engineers to test for and mitigate footfall and other vibrations at the design stage.
Amy Hodgetts is a content writer for Oasys, a leading commercial developer of engineering software