Human induced vibrations caused by footfalls, won’t cause any major damage to structures, but they will more than likely cause discomfort for people. Good engineers will want to make sure that everyone using a bridge or building can do so with peace of mind for their safety. To achieve the right level of safety for users, human induced vibrations must form part of the design process, before and after a structure is created.
Impulse and resonance
Impulse and resonance impulse are the two main ways in which human-induced vibrations have an impact on structures.
Impulse responses cause structural problems when natural frequencies are too high – commonly where structures are 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.
Two objects vibrating at the same natural frequency cause resonance. Think singing at a pitch strong enough to break a 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.
Considering the reduction of vibrations will help to lower the impact human induced vibrations can have.
Human induced vibration – what are the impacts?
Human induced vibration can have several impacts on bridges, buildings and people using them. These include:
- Swaying structures. 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. Interference from people. 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 decrease in structual 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.
- Discomfort and depression. 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.
Avoiding the problem – a potential solution
Designs that include thin slabs and wide spacing can pick up a variety of vibrations, human-induced or otherwise. Using software to create good column design is an effective method for engineers to test for and mitigate footfall and other vibrations at the design stage .