Human induced vibrations caused footfall, often paint pictures of collapsing buildings or swaying bridges. But the likelihood is, that it will cause a discomfort for people, rather than any kind of serious damage to structures. Still, good engineers want to make sure that anyone using their structures, such as bridges or buildings, can do so as safely as possible. To achieve this, human induced vibration must be given serious consideration before, during and after the design process.
Resonance and impulse
Resonance and impulse are the two ways in which human-induced vibrations can affect structures.
Resonance happens when two objects vibrate at the same natural frequency. 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.
Impulse (or transient vibration) responses can cause structural problems when natural frequencies are too high, for example, when a structure is light or stiff. 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.
To help prevent it, engineers must consider the reduction of vibrations and the effects they can have when designing structures.
Human induced vibration – the potential impacts
Human induced vibration can have several impacts on structure the people using them. These include:
- Depending on the purpose of the building, interference can be caused by vibrations of people in 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.
- A famous example is 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.
- Jeopardising integrity. The build-up of vibrations on a structure can,lead to structural integrity being compromised. A worse-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.
- 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.
An effective way to avoid it
Designs that include thinner slabs and wider column spacing are vulnerable to all sorts of vibrations, human-induced or otherwise. Using structural design software is an effective method for engineers to test for and mitigate footfall and other vibrations at the design stage .