The power and stability of two derrick cranes combine to lift a metal frame in these photographs. Among construction buffs and some workers, the rare "two crane pick" is a hallowed event. Below, the cranes 'DB General' and 'Manson 24' lift a "wale frame," a large hollow metal square. The wale frame supports the corrugated sheet metal walls on the inside of a cofferdam. A cofferdam is a square wall of metal sheets for holding water back. When pumps drain water from inside the cofferdam, the bay's watery floor is transformed into dry land allowing work to proceed. This sequence of photographs, taken over sixteen minutes, shows the cranes lifting the wale frame from the future eastbound lanes support pier seven for movement to pier seven for the westbound lanes.
Learn more about cofferdams and the mechanics of building the bridge here.
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