Skyscrapers are designed to sway, and this movement is a fundamental and intended aspect of their engineering. This flexibility is not a structural flaw; rather, it is a deliberate and necessary feature that ensures the safety and resilience of these towering structures. The ability to move allows skyscrapers to manage the dynamic forces they encounter, preventing damage and maintaining stability.
Understanding the Forces
The primary environmental force causing skyscrapers to sway is wind, which exerts significant pressure on their vast surfaces. As wind flows around a building, it can create a phenomenon called vortex shedding. This occurs when alternating vortices are shed from opposite sides, generating oscillating forces that push and pull the building, leading to its characteristic sway. The shape of a skyscraper significantly influences how it interacts with wind, with certain designs, like twisted or tapered forms, helping to disrupt the organized shedding of vortices and mitigate these forces.
Seismic activity is another force that can induce skyscraper movement. Earthquakes cause ground motion that transmits kinetic energy into the building’s base. This energy travels upwards through the structure, causing it to move and deform. Skyscrapers in seismically active regions are specifically engineered to absorb and dissipate these horizontal forces, allowing them to flex rather than resist rigidly.
Designing for Dynamic Motion
Engineers ensure skyscrapers can sway safely by incorporating flexibility into their fundamental design. Buildings are constructed to bend and deform under external loads, dissipating energy and preventing stress buildup. This flexibility is a deliberate choice over rigid construction, as a completely stiff building would be more prone to cracking or breaking under extreme forces. Structural damping is another important principle, designed to absorb vibrational energy and reduce the amplitude of oscillations.
A common technology used to mitigate excessive sway is the tuned mass damper (TMD), a large mass, often near the top, that moves out of phase with the building’s natural sway. This counter-movement absorbs kinetic energy, reducing overall oscillation. For instance, Taipei 101 features a 660-metric-ton steel pendulum that can reduce sway by up to 40%. Other damping systems include viscous dampers, which act like large shock absorbers, and tuned liquid dampers, where liquid sloshing in tanks helps counteract motion.
Building materials and foundation design also play significant roles in dynamic stability. Modern skyscrapers utilize high-strength, flexible materials like steel and reinforced concrete, which can withstand bending and stretching without permanent deformation. Deep, robust foundations anchor the structure securely, yet the overall design allows for controlled movement of the superstructure above ground.
Perceiving the Movement
The movement of skyscrapers is often imperceptible to occupants under normal conditions. Engineers prioritize occupant comfort by designing buildings to minimize acceleration, which is what humans primarily feel.
However, in extreme wind events or at very high floors, some individuals may perceive the movement. When movement is felt, it can sometimes induce sensations similar to motion sickness. This indicates the building is performing as designed to safely dissipate forces, rather than signaling a structural issue.