Feel the vibration? There’s an app for that

It can strike in offices, apartments, shopping malls, and even on bridges: the dreaded bouncy floor. One minute you’re strolling across a reassuringly stable surface and the next you’re jolted into remembering that it’s not solid ground, but part of a man-made structure that can move and vibrate. And while this sensation (typically) doesn’t mean that your safety is at risk, it can be annoying and disconcerting. In sensitive environments like surgical wards and science labs, the effects can be more serious.

Human-induced vibration

Footfall vibration happens because each step we take applies what engineers call a dynamic impact force to the floor. Usually this results in very minor vibrations that building occupants don’t notice. Sometimes, however, a person or group can walk in sync with the floor: in other words, the footsteps match the frequency at which the structure naturally wants to vibrate.

This leads to a buildup of vibration called a resonant response that can cause the floor to move. (In ancient Rome troops used to break step when crossing a bridge to avoid this scenario.)

testing Footfall vibration app outside

Arup has a long history of designing new structures to mitigate this effect. We also provide advanced measurement of vibrations for existing structures, often through our Advanced Technology and Research (AT&R) team.

Accurately measuring floor vibrations typically requires highly calibrated (and very expensive) equipment, but from time to time it would be useful to take a quick measurement of vibration before embarking on a full-blown investigation.

The problem was, there just hasn’t been a tool out there for the job.

iPhone investigations

A few months ago my colleague Mark Nelson, an AT&R consultant in the New York office, visited Seattle to conduct an assessment of an existing building. Looking at the professional equipment he had brought with him, we started talking about how the iPhone’s accelerometer might compare. We ran some tests of our pro equipment side by side with the iPhone, then uploaded the results to his laptop to see how the recordings compared.

The verdict: pretty well. While the iPhone results aren’t as accurate as the pro equipment (which we expected), in terms of providing a quick, user-friendly estimate, their value was clear.

Seeing an idea worth pursuing, we applied for a chunk of Arup’s internal research funding to develop an app capable of using the iPhone’s internal accelerometers to measure floor vibrations.

An app for all

Working with a team of specialists in Arup offices around the world, we created the Footfall app, now available as a free download on the iTunes store.

Footfall Arup

 

Its simple interface allows users to measure the vibration of any floor (or stair) and obtain information that will help them understand and communicate about the movement. The app also includes background information explaining the basics of vibration measurement to help non-engineers interpret what they’re seeing.

We’re hoping that people around the world will try it (Android users, your turn is coming soon!) and provide feedback — and that the floors of the future will be less bouncy as a result.

 

Questions or comments for Will Watson? Contact will.watson@arup.com.

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