August 16, 2012 The new Bay Bridge East Span’s
marquee self-anchored suspension span (SAS) is getting tauter and taller
this week. Early in the morning on Tuesday, August 14, crews began the
laborious and delicate process of tightening the vertical suspender
cables as the first step in transferring the SAS’ weight from
temporary supports underneath the bridge decks to the cable suspension
system above the decks. And on Wednesday, a crane perched 500-plus
feet in the air started to crown the span’s signature tower with
four pieces known as tower heads, which will bring the tower to
its full 525-foot height.
At a press briefing this morning at Aquarium of the Bay Theater in San Francisco, the spotlight was on the load transfer operation, which Bay Bridge spokesman Bart Ney called the biggest “lift” on a project that has seen many lifts of massive steel deck and tower pieces. The initial phase under way this week involves the tightening of eight pairs of suspender cables, which together are supporting 10 percent of the bridge’s load. “It’s an amazing, historic day for the Bay Bridge,” Ney said.
Over the next three months, hydraulic jacks at the edge of the twin bridge decks will sequentially tighten all 200 vertical cable strands. Also in play now is the centerpiece of the suspension system: the nearly mile-long main cable that loops around the decks and tower, and from which the suspender cables hang. Whereas bridge cable systems are typically anchored into the ground, the SAS main cable is anchored into the deck itself, hence the term self-anchored suspension span.
As the suspension system is tightened and adjusted, it gradually will lift the 35,200-metric ton steel structure off the underlying temporary trusses. In the process, the main cable will push out to the sides, the tower will be released from its tilt toward Yerba Buena Island and stand upright, and the SAS’s twin decks will compress and shrink a bit.
“Things are moving and the forces are huge,” said Brian Maroney, Caltrans’chief engineer for the East Span project, who also was on hand at the press briefing. “This is a series of complex engineering processes that we’re going to have to watch closely.” — Brenda Kahn