Wind, Load, Sway

Gravity is the easy load. It goes straight down through the columns, and the columns are sized for it. Wind is the hard one.

The problem

A tall building is a cantilever stuck in the ground with the wind pushing sideways at the top. The taller and lighter it is, the more it moves. A 300-metre tower may sway a metre at the top in a design storm. Structurally that is fine; the steel is nowhere near yield. The problem is people: occupants feel acceleration, not displacement, and above a few milli-g they feel seasick, hear the building creak, and watch the water move in the glass. The design limit for a tall building is comfort, not collapse.

Resisting it

Braced frames put diagonals in the steel. Shear walls and a stiff concrete core turn the building into a hollow tube. Outriggers tie the core to the perimeter columns so the whole plan works together. Belt trusses collect the columns. The tube-in-tube and bundled tube (Sears/Willis Tower) push the structure to the outside where it does the most good. And shaping matters: tapering, twisting, notching the corners, and opening vents through the building all stop the wind from organising into the regular vortices that make a building sway in resonance.

Damping

When stiffness is not enough, add mass that moves the other way. Taipei 101 hangs a 660-tonne steel sphere on cables near the top; when the building sways, the sphere lags and pulls it back. Citicorp Center (1977) used a tuned mass damper on a film of oil — and became the subject of a famous engineering ethics case when a student's question revealed the bracing was vulnerable to quartering winds.

Testing

A scale model in a boundary-layer wind tunnel, with hundreds of pressure taps, measured against the wind climate of the site. The structural engineer's finite-element model does the rest.

The trades

Ironworkers and welders build the bracing and the outriggers; the concrete crews pour the core; industrial-maintenance technicians service the damper; the glazier's curtain wall has to take the pressure and the movement without leaking or cracking. The crane operator, of course, works in the same wind, and stops when the anemometer says so.

Try it: the sway model on the table. Push, and watch the damper answer.