When we think about human-induced vibrations or vibrations caused by human footfall, our minds will probably jump to an image of Millennium Bridge-style swaying or even a building collapsing. However, the damage that is caused by such vibrations is often less visible than you might think, and more likely to result in disruption than cause a building to topple. Even if the consequences aren’t too dramatic, it is still important that engineers consider human vibrations in the design process to ensure that structures are safe and comfortable to use.
Impulse vs Resonance
Human vibrations can affect structures in multiple ways. These include resonant, and impulse or transient response.
Put simply, resonance occurs when Object A vibrates at the same natural frequency as Object B.
Resonance is created when one object (object A) vibrates at the same frequency as another (object B). 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. In the case of a structure, resonance occurs when the pedestrian’s feet land in time with the vibration.
In some cases, such as when a structure is light or stiff, natural frequencies can be too high for resonance to occur. Here the discomfort is caused by the initial “bounce” of the structure caused by the footstep and is a concern on light or stiff structures.
Because of this, it is important that engineers design structures in such a way that the vibration effects will be reduced.
What can human-induced vibration do?
Vibrations induced by humans can impact structures and their users in several ways. These include:
- Interfering with sensitive equipment. Depending on the building’s purpose, what it houses 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.
- Swaying bridges. One of the most famous examples of human-induced resonance impacting a structure occurred with the Millennium Bridge. As people walked across the bridge, the 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.
- Human discomfort. 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 older buildings are.
- Jeopardising structural integrity. The build-up of constant vibrations on a structure can, eventually, lead to structural integrity being compromised. A worst-case scenario would be the complete collapse of the structure and is the reason some bridges insist that marching troops break step before crossing. 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.
What can be done?
Every form of vibration can have a serious impact both on designs with thin slabs and wide columns, and designs with short spans (due to their low mass). Using sophisticated structural analysis software is an effective method for engineers to test for and mitigate footfall and other vibrations at the design stage.
