How skyscrapers survive earthquakes

Why height helps

Every building has a natural period, the time one full sway takes. A stiff low-rise house of brick or block has a period of roughly a tenth of a second; a 60-story tower takes around six. Shaking close to a fault arrives in sharp pulses of a tenth to half a second, the rhythm the low-rise building answers to, and a push at a building's own rhythm builds motion the way a push on a swing does. That mismatch is why unreinforced masonry kills people in earthquakes and tall frames generally do not.

The shaking that does reach a tower

Long-period waves survive long distances, where sharp ones fade within tens of kilometers, so a very large earthquake hundreds of kilometers away can deliver slow ground motion to a city that feels almost nothing else. Deep soft sediment makes it worse, re-radiating the arriving waves at its own slow period. Mexico City in September 1985 is the case every engineer learns: the rupture was about 350 km away on the Pacific coast, and the old lake bed under the center of the city turned the waves into a regular shaking with a period near two seconds. In the worst-damaged district most buildings lost stood between about 7 and 15 stories, the range whose period matched. Around 412 collapsed citywide.

Base isolation

Base isolation puts bearings between foundation and structure, so the ground moves while the building above moves much less. A lead rubber bearing is a stack of thin rubber sheets bonded to steel plates around a lead core: soft sideways, stiff vertically, the lead yielding to absorb energy. A friction pendulum bearing does it with geometry, sliding on a curved steel dish whose radius sets the isolated period while gravity recenters the building.

Either device pushes the period clear of the sharp shaking, and the price is displacement: the base can travel tens of centimeters, so it needs a moat and flexible services across the gap. A supertall already has a long period, so isolation goes mostly into hospitals and mid-rise structures while tall towers rely on the frame.

Ductile frames, and why detailing beats strength

No frame can take a large earthquake elastically at a sensible cost, so codes ask for the opposite quality. A ductile frame deforms past the point where it yields and keeps carrying load, turning motion into damage in places the engineer picked. Steel beams hinge near the column face, following the strong column, weak beam rule: beams give first, because a column that fails takes everything above it.

Ductility only exists if the connections deliver it, which the Northridge earthquake of January 1994 made unavoidable. Welded steel moment frames in Los Angeles were thought the most reliable seismic system there was, and inspectors found brittle cracks at the welds between beam and column flanges in building after building, some with no visible exterior damage. The FEMA-funded investigation rewrote the detail by 2000, with tougher weld metal and beam sections reduced so the hinge forms away from the weld, and steel detailing changed worldwide on the back of it.

Dampers

Dampers take energy out of the motion. The tuned mass type, a weight near the top tuned to the building's period, is mainly a wind device and helps less in an earthquake, because it needs several cycles to come into step. Taipei 101 carries the visible example at 660 tonnes. The devices that do seismic work sit in the frame instead: Japanese practice fits oil and viscous dampers into the bracing on many floors, where they resist in proportion to how fast the frame is deforming and work from the first cycle. The wind side of damping is on how far skyscrapers sway.

Where the real risk sits

Almost none of a city's earthquake risk is in its new towers. A soft-story building has one floor far more flexible than those above it, typically ground-floor parking on thin columns under solid walls; the displacement concentrates there and the upper structure comes down on it. San Francisco began a mandatory retrofit program in 2013 covering roughly 5,000 wood-frame buildings permitted before 1978, and Los Angeles followed with an ordinance reaching about 13,500. The fix is usually steel frames or plywood shear walls at the weak floor, and non-ductile concrete frames carry the same problem at larger scale. Fire and evacuation are on whether skyscrapers are safe; the ground is on skyscraper foundations.

The record so far

No modern code-compliant high-rise has collapsed in an earthquake. The clearest test is the magnitude 9.0 Tohoku earthquake of March 2011, whose long-period waves reached Tokyo from about 400 km away. Towers in Shinjuku swayed for around 13 minutes and tall buildings in Osaka, 770 km away, for more than ten. None came down; what failed was the contents, from ceilings to elevators.

A code does not promise survival at a given magnitude, because magnitude measures a rupture's energy and a building feels only the shaking that reaches its site. Design works to two levels of site shaking: no damage worth repairing in the moderate case, no collapse in the rare severe one. Japan drew that line on 1 June 1981, when its revised code took effect after the 1978 Miyagi earthquake, and the 1995 Kobe earthquake showed that 97 percent of the buildings that collapsed predated it. Skyscrapers in Japan follows that history, and how skyscrapers are built covers the frame.