For a skyscraper caught in a powerful wind, standing perfectly still is not an option. The taller the building, the more it tends to sway.
Engineers have traditionally fought this movement by making buildings heavier and more rigid. But a research team from Imperial College London and the engineering company Arup has explored a very different idea: instead of fighting every movement, why not put some of it to work?
Their inspiration came partly from traditional Japanese pagodas. In these wooden structures, different parts can make small movements relative to one another. This flexibility helps the structure release energy rather than simply resisting it.
The researchers adapted that principle for a modern skyscraper. Their design allows several usable floors near the top of a tower to move slightly apart from its central core. Springs and devices called dampers connect the two sections and keep the movement under control.
The basic idea is surprisingly simple. When wind pushes the tower in one direction, the movable floors respond differently from the central structure. Their enormous weight then helps absorb some of the energy behind the tower's motion.
It is a little like bending your knees when you land after a jump. A completely rigid body would receive the full impact. A small, controlled movement makes the landing much easier to handle.
Many skyscrapers already use a device called a tuned mass damper. This is usually a giant weight — sometimes weighing hundreds of tonnes — installed near the top of the building. As the tower sways, the weight moves against it and reduces the motion.
Such systems can work well, but they occupy valuable space and require extra structural support. They are also usually designed for a particular pattern of movement, which means a separate solution may be needed for earthquakes.
The new approach does not add another enormous weight. It turns part of the building's existing weight into the damper. Better still, the moving floors can remain normal, usable parts of the building.
To test the concept, the researchers studied a hypothetical 65-story tower standing 300 meters tall. They built a 1:300 scale model and placed it in a wind tunnel. They also used computer simulations to examine how the full-sized design might respond to earthquakes.
In the wind-tunnel tests, the system reduced peak acceleration — the motion most likely to make occupants uncomfortable — by as much as 71%. It also cut some of the forces acting near the base of the tower by up to 50%.
The earthquake simulations produced encouraging results as well. On average, movement at the top of the building fell by 42%, while acceleration across the tower dropped by 34%. The movable upper section experienced even larger reductions in some measurements.
Despite the name, "movable floors" would not mean desks and chairs sliding across an office. Under ordinary wind conditions, the difference in movement between the floors and the core remained below about five centimeters in the physical tests. The connections could be handled with construction methods already used to accommodate small building movements.
Reducing these forces could allow future towers to use less steel and concrete in their cores, columns and foundations. That, in turn, could lower both construction costs and carbon emissions.
There is one important limitation: no full-sized building has used the system yet. The results come from a scale model and numerical simulations. Larger tests and a real pilot project will be needed before engineers know how well the idea works in practice.
Still, the research offers a useful lesson. Strength does not always mean refusing to bend. For the skyscrapers of the future, learning how to move may be another way of learning how to stand.