Buildings, especially tall buildings, are susceptible to vibrations. The most dangerous vibrations come from earthquakes – however, we as structural engineers in Cornwall and Devon don’t have to worry about them too often!
Human footfall, though, can cause vibrations inside high offices or buildings which are annoying for the people inside at best, and seriously dangerous at worst. In this blog post, we examine how and why footfall can destabilise a building, and what structural engineers do to minimise that destabilisation.
How Footfall Produces Vibrations
Human footfall can produce unacceptably high levels of vibration for a number of reasons. Some of the most common examples of this are when many people engage in vigorous exercise in the building (such as running down corridors) or synchronised rhythmic activity (e.g. synchronised swimming in an indoor pool). A building can also be susceptible to vibration from footfall if the structure is designed poorly, such as if it has a flexible, long span.
One of the most famous examples of a failure of design to incorporate vibration from footfall is the Broughton Suspension Bridge. Constructed by Samuel Brown in 1826, the iron chain suspension bridge was built to connect Broughton and Pendleton over the River Irwell in the 1830s. However, in 1831, troops marching in close step attempted to cross the bridge. While the bridge maintained itself from the weight, the mechanical resonance (or more broadly vibrations) shook the deck so much that it collapsed. Two things were learned from that occasion: firstly, that the British army should not march in close step when crossing bridges, and from then on the military ‘breaks step’ when crossing. The second thing learned was that vibration needs to be carefully considered when constructing certain kinds of structure.
Vibrations can be either external or internally caused. An internal vibration might be from something like a washing machine or factory machinery: they are sourced from something inside of the building. External vibrations might be caused by cars on busy roads, nearby trains, construction activities or more. If a building is built by a busy street then, for example, or is built to house sensitive or large capital equipment, vibrations have to be taken into account in building design.
Footfall creates internal vibration, as it occurs inside of a building. Walking is usually the most significant source of floor vibration in above-grade floors in a building. Footfall causes the floor of a building to vibrate slightly, just like a guitar string being plucked: the floor vibrates at its natural frequency with each step. Walkers in the middle of a structure produce more vibration than ones closer to the columns, just like plucking the centre of a guitar will produce more vibrations than along the fret of its fretboard. Finally, the quicker someone walks inside a building, the louder the vibrations, just like the harsher someone plucks or strums a guitar string, the louder the noise.
How Structural Engineers Minimise Vibrations
There are many different ways one can minimise vibration from footfall in a building, but most can be broken down to one of two things: either architectural solutions or structural solutions.
Architectural mitigations most often involve moving around rooms of a building to minimise stress in weaker areas. For example, switching office space with staff rooms can help reduce footfall and change activity locations to where the building is less prone to vibrations.
Structural changes can include things such as making heavier or floating the floor, stiffening the beams, adding interstitial posts, or applying a supplementary damping system to dissipate energy more efficiently. There are many ways to implement each of these structural changes to a building as well, so as structural engineers, we have many tools at our disposal to reduce building vibration.
There are also some basic sound design principles which can be applied to try and reduce the amount of footfall-induced vibrations in a building. For example, by locating corridors near column lines, more footfall will occur next to where the structure is most durable, which reduces vibrations. Furthermore, by putting vibration-sensitive equipment away from the centre of structural bays and nearer the columns, there is less risk of something breaking or moving as a result of footfall vibration.
Generally, a building built with vibration problems will always have vibration problems. There are many steps that a structural engineer can take to reduce the degree of vibrations in a building, particularly those caused by footfall. However, retrofitting is never as ideal a solution as properly constructing a building with the goal of minimising vibrations in mind from day one.
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