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No.27 Loma Prieta earthquake

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San Francisco was rocked by a 7.1 magnitude earthquake on 17 October 1989. The epicentre was Loma Prieta, about 100km outside the city.

At least 60 people died and damage to property was estimated at about $6bn (£3.7bn). Several large pieces of infrastructure suffered significant damage, despite the relatively weak force of the quake on the notorious San Andreas Fault. Failure of a single span on the double deck Oakland Bay Bridge was the biggest structural surprise. Engineers said eastwest shaking caused a 125mm longitudinal motion that slammed expansion joints shut and split the fixed joints of the deck with a force of 900t

But the most damaging failure was the collapse of an upper section of the double deck Interstate 880 (I880) viaduct during rush hour. Commuters were crushed in
their cars.

The 1950s viaduct road failed where it ran over landfill. The top deck’s collapse was caused by brittle shear failure at the top end of the lower half of the columns.

Main rebar in the bottom transverse beams was insufficiently anchored or tied horizontally at the ends, while vertical column bars had too few stirrups. Four 12mm diameter bars formed a movement joint between the upper and lower halves of the column, acting as a chisel during shaking.

A further 16km of similar viaduct in San Francisco and Oakland received structural stress to reinforced concrete portal frames.

Interstate 280, designed and built in the 1960s, suffered severe shaking and shear failures where it crossed areas of landfill. Ground level road paving was damaged on the I80 east of Oakland Bay Bridge.

The Embarcadero skyway in northern San Francisco was also badly damaged.

Masonry buildings cracked and failed in shear, including a four storey warehouse near the I280 which lost the tops of its end walls, causing the deaths of five people.

There were immediate and widespread calls to upgrade structures built before California introduced competent building codes in 1933 and seismic bridge design rules 40 years later. Many modern structures stood up to the earthquake relatively well though.

A tunnel used for the Bay Area Rapid Transit system survived undamaged, as did water tunnels in downtown San Francisco and many service pipes.

Downtown office buildings of up to 50 storeys, most of which were founded on piles more than 50m long, survived. Lowrise structures in the China Basement area were virtually undamaged, although several large office buildings in Oakland sustained significant damage.

Many timber framed houses near the main shock area lost chimneys but most stood up well. Serious failures resulted where buildings had been rocked off stub timber columns.

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