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BOUNCE BUSTER

Revolutionary new software could see the end to bouncy floor syndrome, a phenomenon which has cursed long span lightweight structures since the 1960s.

Consultant Arup has produced one of the first finite element analysis programmes which can demonstrate how vibration caused by people walking on lightweight structures such as open plan office floors affects an entire area.

The problem of vibration is common in office buildings. It can be felt by staff, but perhaps more importantly, might prevent sensitive equipment from working properly.

The analysis tool, GSA Footfal,l produced by Arup's software house Oasys can do much more than just map out vibration levels on a floor plate, it can also predict how the vibration is perceived, so that an engineer can advise on the positioning of sensitive equipment, partitions or even people more cost-effectively.

The technique is used as part of regular design of new structures at Arup. Peter Young, one of its structural engineers, explains how the software can be used to advise owners of existing buildings.

"We have projects where the occupants are complaining that the floors are too lively and the landlord has asked us to have a look at it,"
says Young.

"We install monitoring equipment under the false floor and record data for a week. "We then may find 10 occasions during the day [where vibrations] were at a level where people might complain.

"We decide whether it's consistent with anecdotal data and suggest mitigation."

Mitigation measures could include tuned mass dampers in the floor voids or dampers between the floors.

"However it's often simplest to change the layout," says Young.

This could be by shifting routes commonly used as thoroughfares through an office from the centre of a bay to an alignment closer to where support positions are.

"Alternatively you could just move the individual [complaining about the vibration]. Some people are more sensitive."

The software and accompanying Design Guide for Footfall Induced Vibration of Structures published by the Concrete Centre was made available at the start of this year.

But the technique has been standard at Arup for the last five years. Now they are pushing for it to be the industry standard.

The technique supersedes a Steel Construction Institute guide last updated in the 1980s. This was conceptually sound but published in the days before finite element analysis.

It catered for several different floor types, but could not be easily adapted for atypical floors.

"This method allows you to do a lot more that you couldn't previously do," confirms Young. Young urges that although it is possible to retrofit it is better to design for it.

"If you're going for a lightweight floor, you need to think about designing in damping," says Young.

"Lack of partitions and open plan offices, trapezoidal decking [composite floors] and lightweight concrete makes the problem worse."

How it works

A finite element model shows how vibrations are experienced by a particular structure by transposing the data into contour lines which take into account how the vibrations are perceived by a person.

This "response factor" is then used to compare whether the level of vibration is acceptable.
A response factor of one represents the magnitude of vibration that is just perceptible by a typical human; a response factor of two is twice that.

The software produces contours of the response factor, which gives an indication of which areas of the building are most prone to dynamic excitation. This can then be used to plan where to put vibration sensitive equipment.

Red zones are the most vibration prone, purple the least.

Commercial
on lively floors computer users complain because their screens wobble

Hospital floors
operating theatres require the utmost stability for delicate operations

Bridges
need to comply with BS5400, the British standard for bridge design loads

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