A new damped outrigger system to stabilise tall buildings has cut the cost of a 200m tall residential tower in the Philippines by around £2M, the designer has claimed.

The system uses small viscous dampers, mounted between the structure's outriggers and columns to introduce a degree of redundancy to the frame and allow it to perform better under wind loads.

"With tuned mass dampers, there is only one of them and there is a lack of redundancy," explained Arup associate, Rob Smith who led the Philippines projects.

"(Tuned mass dampers) can be used for serviceability conditions, but you wouldn't want to rely on them for strength, to resist ultimate wind loading," he said. "With our system, because there are many different viscous dampers, if one failed there would not be much difference."

He pointed out that the redundancy meant the system could be designed to take ultimate loads, (loads that might cause yielding or overturning) as well as service loads (loads that might cause deflection and vibration). Design loads on the rest of the structure could therefore be reduced and the structure reduced in stiffness.

"The cost of the dampers is offset by the saving in the structure," said Smith.

The usual method of reducing lateral building movement is to add significant extra structure to stiffen the building or to install tuned mass dampers, which act as a pendulum at the top of the building, swaying in the opposite direction to the building.

Viscous dampers act as energy absorbers, like car shock absorbers and damp out vibrations. Although not as commonly used in structures in the UK they have been used in seismic areas such as California. This is the first time they have been used to reduce a building's response to wind.

Source : New Civil Engineer