Why Should Engineers Consider Human Induced Vibration
Human induced vibration is a phenomenon caused by vibrations of human footfall in buildings and structures – although this might sound concerning and make us think of collapsing buildings and rickety bridges, the damage caused is actually minimal. Of course, this damage is still considered by engineers when bridges and buildings are designed, so to make people who are passing through feel comfortable and safe. Here, we’ll talk about why human induced vibration is an important consideration in the design process.
What is resonance and impulse?
There are two effects of human induced vibration: resonant, and impulse or transient response.
In simpler terms, resonance occurs when Object A vibrates at the same natural 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 can be a problem on structures where its natural frequencies are too high for resonance to occur, such as where the 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. Engineers must, of course, design to reduce the vibration effects caused by either impulse or resonance.
What are the potential impacts from human induced vibration?
There are numerous effects on structures and people. These include:
- 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.
- 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.
- 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 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.
How to avoid human induced vibration?
As touched upon above, modern designs that favour thinner slabs and wider column spacing are particularly susceptible to all forms of vibration, human-induced or otherwise, but short spans can also suffer due to their low mass. Using sophisticated design and analysis software and utilising retaining wall solutions is an effective method for engineers to test for and mitigate footfall and other vibrations at the design stage .