Lightweight floor slabs deliver maximum ceiling heights and cost savings, but have a tendency to develop Millennium Bridge syndrome. Now a shock-absorbing solution – developed by Arup, of course – is set to put workers' feet back on firm ground.
The Coffee Slurps over the Rim of the Mug. The image on a computer monitor flickers and jumps. But the problem isn't an earthquake, subsidence, or even the ripple effect of a far-away bomb blast – it's the common problem of vibrating floors experienced in thousands of modern offices in the UK.

People may get used to vibrating floors, but they don't like them. The vibrations are simply caused by people walking across the floor, and are an increasing problem in today's lightweight structures. The simple solution – building heavier and therefore thicker floors – takes up valuable space and money.

But a system devised by consultant Arup and specialist contractor Richard Lees Steel Decking could minimise the problem in new developments. It comes in the form of a thin strip of elastic material sandwiched between two metal strips, and is an unobtrusive 3 mm thick.

The system, called Resotec, is used with composite steel and concrete floors, popular with developers of fast-track office buildings. Steel beams support corrugated metal decking, into which concrete is poured. The Resotec strip is sandwiched between the metal decking and steel beam. As the floor moves there is differential sideways movement between deck and beam. The visco-elastic material in Resotec dampens this movement so the floor vibrates much less.

Arup has been investigating the problem of vibrating floors for some time. "We've been drawn into this idea because of several complaints," says Michael Willford, a structural engineer and director at the firm. "In one building the corridor between the desks was down the middle, which is the worst place. If it had been along the edge it would have been much more difficult to excite the floor."

Current design and fit-out trends are compounding the problem. Engineers have optimised floor design to such an extent that floors have become very light, and office furniture is getting lighter too. Because the floor and its load weigh less, there is less mass to dampen vibrations, so the floor moves more.

Significantly, lightweight flat computer screens, which themselves contribute to the problem, are particularly prone to vibration. The screens are mounted on thin stalks so tend to move around much more if the floor vibrates. Willford says the occupants of one building using flat screens were very badly affected. "There were people who couldn't continue working at them because they were moving so much," he reveals.

Because of this, some developers have started to demand more resilient floors. Currently, the maximum level of acceptable vibration is eight times the minimum amount of vibration a person can detect. "Our experience is that at around eight, you can get adverse comment on floor vibration," says Willford. "Some developers have picked up on this and reduced the factor from eight to four." Unfortunately, this is expensive and space-consuming. "To achieve four you would need to double the mass of concrete and steel," warns Willford.

Although a shock-absorbing solution such as Resotec might seem obvious, the method is radical and innovative. Steel beams and metal decks are firmly locked together with fixings called shear studs, which transfer loads from the concrete to the steel beam. Putting a flexible strip in between these components breaks that rigid bond and theoretically makes the floor weaker, as the beam and concrete are not acting as a composite.

Arup calculated that some of the shear studs are not strictly needed. It found that steel beams alone could carry normal design loads along 25% of their length from the nearest structural support. Only the middle part of the floor – the part furthest away from structural columns – needed the additional strength provided by the concrete. The team has therefore taken out the shear studs around the perimeter of the floor and only connected the concrete and beams in the middle 50% of the floor. This means Resotec can be sandwiched between the beam and deck over half of the floor area.

Having come up with the idea, Arup needed somewhere to try it out. The spiral ramp at Foster and Partners' City Hall in London proved ideal: if if was fully unwound, it would be 100 m long. As it is supported very sparingly, it could be prone to vibration. It also happens to be a steel box girder supporting concrete steps, so is similar to a steel and concrete composite floor.

The only other way of stopping the ramp vibrating would have been to use tuned mass dampers, the system of heavy weights that have been successfully used to stop Arup's Millennium Bridge wobbling. "This would have been very difficult," says Peter Young, the Arup structural engineer responsible for practical implementation of the concept. Tuned mass dampers are difficult and expensive to install, and visually intrusive. Resotec, on the other hand, does nothing to alter structures geometrically or visually.

So, after rigorous testing, the Arup team got to try out the Resotec system. "The tests showed very clearly we were getting the damping we thought we were going to get," says Young. This proved the idea worked, and Arup could rely on its model for calculating the level of damping.

This positive result gave More London, development manager of City Hall, sufficient confidence in the system to try it out on its Plot One building next door. At this point Arup decided it needed a partner to turn the idea into a commercial reality. To achieve this, Skanska subsidiary Richard Lees Steel Decking, which specialises in the manufacture and installation of profiled steel decking, was brought on board.

The joint Arup–Richard Lees team built two test floor slabs with standard composite flooring, and fitted one with Resotec. Tests showed the Resotec reduced the vibration by 50%. After this positive result, the team then had to develop a production facility and work out how the material would be fitted on site.

Because More London's Plot One building had already been designed, compromises had to be made. "We came in late in the game," says Willford. "The steel sizes were fixed and were not necessarily the sizes we would have chosen, so we could have got a better result."

Willford says that beams should be selected so that Resotec can be fitted to half its length, but this wasn't always possible at Plot One. Also, the system requires four shear studs in each trough to attach the beam to the deck in the middle part of the floor, instead of the standard two. This means the beam flange has to be wider. Adrian Shepherd, associate technical director at Richard Lees, is keen to stress that Resotec has to be designed in at the concept stage.

After its high-profile trial run at Plot One, the Resotec system is now about to be launched onto the market. Richard Lees Steel Decking has bought the rights to manufacture and fit the solution from Arup, and is confident it will be a success. Shepherd says: "It doesn't affect the floor or construction cycle and creates an improved, dynamically performing floor within the constraints of modern construction." And that has to be very good news for office workers of a nervous disposition.

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