ORIGINAL EAST SPAN DEMOLITION
Crews have begun to carefully dismantle the 77-year old East Span section-by-section, in roughly the reverse order of how it was built in the mid-1930s.
Demolition of the nearly 2-mile long original East Span will take place in three phases over three to five years. Crews will first work in the westward direction toward Yerba Buena Island, taking apart the cantilever section and demolishing the S-Curve, before moving east to dismantle the 504’ and the 288’ truss spans. The demolition team will then head to the waterline to remove the piers, pilings and foundations that support the span. When the entire demolition project is complete, over 58,000 tons of steel and 245,000 tons of concrete will have been removed.
PHASE ONE: SEPARATING AND DISMANTLING THE CANTILEVER TRUSS TO YERBA BUENA ISLAND
The first phase of the demolition process: Cut the cantilever truss at its center in order to begin dismantling it. Crews have already begun work, removing the 1,400-foot upper deck of the cantilever section and 600-feet of the lower deck to lighten the bridge’s load.
A cantilever span is constructed using cantilevers, which are horizontal structures supported only on one end. Cantilevers must be firmly anchored in order to hold up the necessary weight. Between the two sides of the cantilever is an independent bridge called the suspended span. The weight of the suspended span causes the two sides of the cantilever to want to tip toward the center. In order to keep this from happening, the outside arms must be held down by anchor spans. On the west end, the concrete anchorage on Yerba Buena Island secures the bridge; the east end is anchored by the rest of the bridge that extends towards Oakland.
Before the demolition team can cut through and separate the cantilever, preparations must be made to reduce structural tension and pull the suspended span on each end toward the rest of the bridge. Otherwise, the two halves of the suspended span would collapse and fall into the water when it is cut.
Workers are neutralizing these stresses by tying back the upper chords of the suspended span to the cantilever spans. Jacking frames, pins, and hydraulic jacks are then installed, and tension is applied to reduce the compression forces at work.
Once these forces are controlled, the cantilever structure will be cut in two – right down the middle – and the contractor will begin dismantling the steel pieces.
This section of the original span along with the temporary bypass are being cleared first in order to allow contractors to complete the bicycle and pedestrian path and to construct the permanent eastbound on-ramp from Yerba Buena Island.
PHASE TWO: DISMANTLING THE TRUSS SPANS
During the second phase of the demolition process, crews will remove the rest of the cantilever section and dismantle the five 504’ and 14 288’ truss spans to the Oakland shoreline. This contract will be advertised for bid in July 2014.
PHASE 3: REMOVING UNDERWATER FOUNDATIONS
Crews will head beneath the structure to remove the underwater foundations that support the original span. Demolition contractors will remove the piers and pilings to the waterline and then extract the foundations down to the mud line.
Removing one member from a truss bridge can change the loads on other members, possibly creating a dangerous spring-action effect.
To figure out the best way to remove high-tension pieces, engineers are using a 3-D finite element computer program model based on structural analysis and historical records that shows how the forces are distributed.
During demolition, the team will monitor the state of the span via a system of 90 retroreflector prism targets installed at key locations on the bridge. As steel members of the bridge are removed, the updated target locations will be determined and entered into a computer system. This data will be compared up against the predicted locations in the finite element model software.
Length of East Span – 1.97 miles
Total steel – 58,209 tons
Total concrete – 245,470 tons
Bridge steel – 51,687 tons
Pier/foundation steel – 6,522 tons
Bridge concrete – 66,962 tons
Pier/foundation concrete – 158,470 tons