Human-induced vibrations — the vibrations caused by human footfall — can spark images of unstable structures swaying and in some cases collapsing. But, in reality, the vibrations caused by human footfall are unlikely to result in anything as dramatic.  They are, however, likely to cause discomfort for structure users.  Any good engineer will want the people using the structures they have helped to build to be safe and comfortable. In essence, human-induced vibrations is an important topic of consideration in the design process.

Resonance and impulse affects

There are two different ways human-made vibrations can affect a structure: resonant or impulse and transient.

Resonance occurs when Object A vibrates at the same naturally occurring frequency as 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.

Impulse or transient vibration responses are a problem on structures that are too high for resonance to occur within them, like if the structure is stiff or light. 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.

Engineers must take into account which vibration effects will occur when they are in the designing process. Using sophisticated structural engineering software can assist in this.    

Possible impacts of human-induced vibration

Human-induced vibration can have a number of effects on a structure on those who use it. These include:

• Interference on 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.
• Moving 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.
• Discomfort for humans. 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.
• Risking the strength of a structure. 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 to combat it?

As highlighted in this article, modern designs are open to all forms of vibrations due to their thinner slabs and wider columns, but short spans can also be a victim due to their low mass. Using structural design and analysis software is an effective method for engineers to test for and mitigate footfall and other vibrations at the design stage.