Mass. Dept. of Conservation and Recreation
Iconic 58-ft tall bridge towers will be repointed, cleaned and seismically braced.
The 100-year-old Longfellow Bridge, spanning the Charles River between Boston and Cambridge, is scheduled for an estimated $200-million restoration that could start in 2010. Work on the historic 11-arch steel girder structure with its signature ‘salt and pepper shaker’ towers could include the lost art of riveting.
The 2,135-ft long, 105-ft wide four-lane bridge was extended in 1956 with two approach spans and underwent major repairs in 1959. Now, it suffers from cracked decking, separating granite and tilting towers. The original bridge rests on ten timber-pile supported hollow granite block piers and two hollow abutments. The central piers support four, 58-ft tall granite towers resembling salt and pepper shakers.
The bridge is an iconic element in the Charles River Basin Historic District and carries about 30,000 vehicles and 90,000 Massachusetts Bay Transportation Authority Red Line subway riders daily on two tracks running down the middle. It also carries a substantial amount of foot and bike traffic. “The major problem is to rehab the structure while maintaining traffic,” says Michael J. O’Dowd, Massachusetts Highway Dept. project manager. “What we are proposing is a ‘shoo-fly,’ a temporary relocation of one of the track lines onto the vehicular portion of the roadway. We will rehab one lane of traffic on either side of the tracks first, then finish the outer lanes. After that, we will relocate the westbound rail to the adjacent fast lane and the eastbound rail to the existing westbound. We will repair the eastbound track, then move service back to its current position and then repair the westbound track and move it from the roadway back to its current position.”
MHD and the state’s Dept of Conservation and Recreation are jointly managing work on the famous bridge, which is named after Henry Wadsworth Longfellow. A $4-million preliminary design contract has been awarded to the local office of Jacobs Civil Inc. Work includes environmental permitting, seismic study, civil traffic, site work, railroad and structural engineering. “Work is about 90% complete and we are awaiting MHD review of the preliminary engineering. Once complete, the environmental process can start,” says Jonathan Taylor, Jacobs project manager.
O’Dowd notes the 9-in. to 12-in. diameter piles are in good shape. About 1,000 piles, each 30-ft to 40-ft long support each pier. The cast-in-place concrete pile caps and granite piers also are in good condition but require repointing and cleaning. The interiors of each pier will be retrofitted with steel bracing or concrete stem walls for seismic resistance.
The steel arches under the rail lines are deteriorated due to water infiltration and will be rehabbed in place by renovating and replacing corroded sections. Steel posts, varying in length from 2-ft to 8-ft at the top of the arches and supporting the roadbed also will be rehabbed.
There are about 90,000-to-100,000 steel members in the structure and all will be cleaned and painted. Floor beams and stringers will be replaced but in a more conventional method. “The bridge uses steel buckle plates between the stingers and many have already been replaced with concrete deck slabs and we want to remove the remaining plates,” says O’Dowd. “The arches and posts will be historically maintained and the railing and lamps will be restored to period fixtures. The towers will be rebuilt and steel-framed seismically braced.”
Melding state highway standards with historic considerations could be a major challenge. Facia steel may be riveted in keeping with period construction. “We have retired members that were part of rivet gangs,” says Jim Brown, treasurer, ironworkers’ union Local 7, South Boston. “They still have the knowledge although rivets have been obsolete since the 1950s.” The local’s apprentice book still has a chapter on riveting.
Riveting is done by four-person crews. A heater tends the forge that turned the rivets cherry hot and uses tongs to toss the nugget to a catcher who makes the grab in a catch can, knocks off the scale and inserts it in the hole. A bucker-up uses a cupped-head dolly to hold the rivet in place as a riveter working from the other side with a pneumatic hammer would form a head on the shank, locking the steel members in.