What if skyscrapers could not sway?

Skyscrapers are very, very tall. When strong wind blows, they sway a tiny bit.

What if a skyscraper could not sway at all?

Stand at the foot of a skyscraper and it looks perfectly still. But very tall buildings do move. In a strong wind, a tall tower sways back and forth. During an earthquake, the whole building moves with the shaking ground.

So what if a skyscraper could not sway at all? Would it be stronger and safer? Or would something go wrong? In this lab we follow three clues: the push of the wind, giant swinging weights, and bending without breaking.

FEMA's guide to earthquake-resistant design describes what happens when a structure is pushed sideways by a lateral force, such as a strong gust of wind, and released: it vibrates back and forth. The time for one full cycle is its natural period, a basic property of every building.

This thought experiment imagines a skyscraper that could not sway at all: infinitely stiff, with no vibration. To reason about it, we look at why tall buildings move, how engineers calm that motion with dampers, and why the ability to bend without breaking matters in earthquakes.

Wind pushes on a tall building, like it pushes on a tree.

The building bends a tiny bit and swings back. That is called swaying.

Wind pushes on the side of a tall building. The building bends very slightly, then springs back, and swings to and fro, like a ruler you flick while holding one end on a table.

Every building has its own natural swing time. FEMA, the U.S. emergency management agency, explains that tall buildings take several seconds to swing back and forth once, while short buildings take only a few tenths of a second. Too much swaying can be a problem: researchers at Trinity College Dublin note that wind-driven motion in tall buildings can make people inside feel uncomfortable, even sick.

A building's natural period is the time for one cycle of free vibration. FEMA notes that high-rise buildings tend to have natural periods of several seconds, while short buildings have periods of a few tenths of a second.

Researchers at Trinity College Dublin describe wind-induced motion in tall buildings as a serviceability and habitability issue that can cause occupant discomfort and nausea; they study tuned liquid dampers to reduce the accelerations in slender modular towers. The damper firm Motioneering adds that tall buildings are more flexible than shorter structures, so they can sway significantly in strong winds or earthquakes.

Imagine a skyscraper that never moves, not even a tiny bit.

It would have to be super stiff. And it might get a harder shake in an earthquake.

Now imagine a skyscraper that could not sway at all. In the wind, nobody inside would feel any motion. That sounds good!

But to never bend, the building would need to be enormously stiff, with much bigger, heavier walls and columns. And in an earthquake, a building that can't move at all must take the full shaking of the ground. FEMA's example from one earthquake shows that stiff buildings with short swing times would have been shaken much harder than tall, flexible ones. These are ideas, reasoned from how buildings move.

A perfectly rigid tower is impossible, but its tendencies are instructive. Its natural period would fall toward zero, so in an earthquake it would follow the ground motion directly rather than filter it. FEMA's response spectrum from a recording of the 1940 Imperial Valley earthquake shows that structures with periods of about 3 seconds or more would have experienced 0.1 g or less, while structures with periods of 1 second or less would have experienced about 0.7 g.

That comparison is for one earthquake and one site, and spectra differ. A rigid tall building might also lose the ductile behavior that lets real structures absorb energy. These consequences are reasoned, not tested.

Some skyscrapers have a giant heavy ball hanging inside!

When the building sways, the ball moves the other way. It calms the swaying.

Some skyscrapers calm their swaying with a giant weight called a tuned mass damper. A famous one is in Taipei 101, a skyscraper in the city of Taipei. According to Motioneering, the company that designed it, it is a huge golden steel ball, 5.5 meters wide and weighing 660 tonnes, hanging on cables high up in the building.

Motioneering explains that a tuned mass damper moves out of step with the building's sway, absorbing energy and reducing the motion. In Taipei 101, hydraulic cylinders help steady the ball. Motioneering also says the damper has come through several earthquakes and a typhoon in 2015 with gusts of 145 miles per hour.

Motioneering describes a tuned mass damper as a large mass on springs and dampers, usually near the top of a building, that moves out of phase with the building's sway, absorbing energy and reducing motion. Taipei 101's damper was designed by Motioneering, and the figures here are the designer's own: a steel sphere 5.5 m in diameter and 660 metric tons, located at the 87th floor, suspended from cables anchored on the 92nd floor and stabilized by hydraulic cylinders.

According to Motioneering's 2024 newsletter, the damper has withstood multiple earthquakes and a major typhoon in 2015 with wind gusts of 145 mph. A damper works only because the building can move; a tower that could not sway would have nothing for it to calm.

Steel can bend a lot before it breaks. That helps buildings stay up.

Bending a little is safer than snapping.

In a very strong earthquake, engineers want buildings to bend and stay standing, not snap. FEMA calls this ductility: the ability to be overloaded and damaged while still holding up the load.

Steel is very good at this. When it is pulled too hard, it stretches a lot while still carrying the weight. That's why steel bars are put inside concrete walls and columns. A building that couldn't bend at all might lose this way of surviving big shocks.

FEMA lists ductility and toughness among the key features for seismic performance: the ability of a structural element to sustain damage when overloaded while continuing to carry load without failure. Masonry and concrete crush in compression and crack in tension; steel, by contrast, yields when overloaded in tension, stretching a great deal while continuing to carry load.

So reinforced concrete uses steel bars that yield and protect the concrete, and structures are proportioned so that some members yield to protect the rest. Building codes reflect this: FEMA describes a response modification factor, R, from 1 for systems with no ductile capacity to 8 for highly ductile ones. A building that could not deform at all would forgo this protective behavior.