The Chelsea Street Bridge was originally built in 1936. It crosses Chelsea Creek and connects East Boston in Suffolk County with Chelsea in Middlesex County via Chelsea Street. The Bridge is a rare Strauss heel trunnion type Bascule Bridge. The Bridge has had many repairs over the years. The latest repairs being done in the mid-1990s.
The riveted steel frame Chelsea Street Bridge is 446.5’ long and 70’ wide with two, 10’ sidewalks. The bridge has six spans; 4 steel stringer approach spans, one on the East Boston side and 3 on the Chelsea side with a single leaf Strauss heel trunnion bascule span. The approach spans are 66’ long, the tower span is 42’ long and the draw span is 140’ long from the heel trunnion to the center of the landing bearing. The moveable pan is a through Warren truss with verticals.
The Strauss heel trunnion draw moves by means of two operating struts. The struts are attached to the moving leaf and attached to a rack that engages the pinon which is rigidly fastened to the fixed tower. The main pinons are driven through a train of reducing gears by two 50-horse power motors with direct current. A differential automatically equalizes the loads on the operating struts. The heel trunnions are 21” in diameter, the counterweight trunnions are 38” and the counterweight weighs 1100 tons. The draw pivots around the heal trunnion which is located at the base of the bascule truss. As the moving leaf rises, the rocker arm lowers which causes the concrete counterweight to move downward. When the bascule span is in the full open position it provides a vertical clearance of 112’9” above the high tide. Additional motors power the end locks which can also be operated manually.
The Chelsea Street Bridge was being replaced because it was declared a hazard to navigation by the US Coast Guard. The 96 foot wide channel had its own class of tankers, the Chelsea Class. These single hull tankers are being phased out of existence for environmental reasons.
The difficult part of deconstructing this Bridge would be to do it safely and efficiently. The overhead concrete counterweight weighs over 2,000,000 pounds. The Bridge had to be held in the open position during demolition without the help of its motor or brakes, both of which were not operational during demolition. Our first task was to design a method to restrain the rack and pinion so that the bridge stayed in the open position without moving while we changed the weight distribution through deconstruction. The idea is to maintain as much of a balance as possible between steel superstructure removal and concrete counterweight removal.
The next task was to define this balancing act. We started by removing the steel bascule span at the toe end about 160 feet above mean high water. If we removed 25 tons of steel we had to remove about 25 tons (about 12 cubic yards) of concrete counterweight. A major part of this balancing act was to make sure the Manitowoc 2250 crane on a barge was able to make the picks. So, we went through the process of determining how much steel could come off the bridge based on the capacity of the crane on a barge. With this piece of steel removed, we checked the stability of the remaining portion of the bascule span, a Warren through truss. We then checked the strength of our restraint at the rack and pinion for holding the bridge in position until that amount of concrete counterweight was removed. Then we repeated the process for removing the next piece of steel.
With the bridge in the open position, the counterweight span, which is the steel structure that supports the counterweight, reached up about 90 feet in the air above the roadway. The steel in the counterweight span weighed about 500,000 pounds and it must come down without collapsing and possibly injuring the new bridge’s East Boston Tower which is built around it. Again we had to check the capacity of the crane on the barge to make the steel picks and then make sure the remaining counterweight steel was still stable. We had a structural model of the entire bridge and counterweight span. After each piece was removed the computer did an analysis and we confirmed that no remaining steel was overstressed and the remaining steel was stable. Finally, all 2,000,000 plus pounds of steel was safely on the ground and the 2,000,000 plus pounds of concrete counterweight was also demolished.
The contractor then removed all of the substructure (bridge piers and abutments), dolphins, fender system, and bridge tenders house. The standard plans and procedures for this work was also done by GZA.
All of this work was done to make room for the new bridge and the new Panama Canal Class tankers. The new bridge is a 400’ span vertical lift bridge which will provide a 175’ of vertical clearance and 200’ of horizontal clearance.