Longfellow Bridge Approach Structure for the MBTA
Location: Charles MGH/Redline Station Boston, Massachusetts
Contract Value: $36,456,600
Project Dates: May 2003 – February 2007
Description of the Structure
The Longfellow Bridge Approach Structure, built in 1911, carries two MBTA Rapid Transit Red Line tracks over eleven (11) spans between the Beacon Hill Tunnel and the Longfellow Bridge. Each of the eleven (11) spans consist of three (3) 60” to 96” deep longitudinal riveted plate girders along with transverse floor beams and longitudinal stringers and a reinforced concrete deck. The Bridge structure is approximately 34 feet in width.
The substructure consists of a granite block abutment at one end and a concrete reinforced abutment at the opposite end along with ten (10) intermediate piers which support the Bridge structure via either two (2) 36” deep transverse plate girders or one (1) 72” deep transverse plate girder.
Description of Shoring Performed
Two locations required shoring of the Bridge structure. The first were the outer two longitudinal riveted plate girders at the 55 foot long bridge span 3. The outer girders at this location were within the original Charles/MGH Station, constructed in 1934, which had to be demolished. The midpoint of the longitudinal riveted plate girders were supported by a steel column which had to be removed in order to construct a road below the span 3 Bridge structure. In order for the structure to be sound after the removal of the support columns, the plate girders had to be longitudinally stiffened by adding additional plates to the top and bottom of the plate girders along the middle third of the plate girders. To accomplish this 16 foot shoring towers with jacks were installed at the underside of the plate girders on each side of the existing support columns. The plate girders were then jacked vertically to transfer the bridge plate girder load from the support column to the shoring/jacks. The support columns were then removed and replaced with the stiffener plates that were bolted to the bridge plate girders. Once the stiffener plates were installed, the jacks were lowered and the load was transferred from the shoring/jacks back to the bridge plate girders.
The second bridge structure location which required shoring was at bridge pier 5. The new Charles/MGH station would encompass bridge spans 5 and 6, with bridge pier 5 directly in the center of the proposed station lobby. The design required replacing the single concrete pier with four offset concrete piers, two below each longitudinal plate girder with each of the two approximately 6 feet away from the centerline of the original pier 5. A combination of transverse and longitudinal plate girders and beams along with diaphragms, had to be installed on top of the new piers to shore the bridge once the original pier 5 was demolished. After the four new piers were constructed and a portion of the structural steel shoring was installed, the bridge load was transferred with the use of jacks from the existing concrete pier to the new structural steel and concrete piers. The original concrete pier and the transverse plate girders that were seated on the pier were then demolished and the remaining structural steel was installed. The jacks were then lowered and the bridge structure was now solely supported by the four new piers and structural steel. BHD contracted the design team of Simpson Gumpertz and Heger to assist us with this intricate work. BHD and SGH worked hand in hand in determining the best sequence of installation for the various structural steel members.
Description of Waterproofing Performed
The portion of the existing bridge structure, span 5 and span 6, which would be encompassed by the new Charles/MGH Station would essentially create a roof over the new Station’s lobby. Since there is no roof above the structure, the 130 foot long by 34 foot wide section which included two bridge expansion joints and would roof the Station lobby had to be waterproofed. BHD again contracted the design team of Simpson Gumpertz and Heger (SGH) to design a long lasting waterproofing system.
During a weekend train diversion, BHD removed the two tracks, rail ties and ballast in this section. The existing deteriorated bridge deck waterproofing was then removed and the concrete deck was scarified. SGH specified the Stirling Lloyd Eliminator waterproofing system which was then applied to the concrete deck. The new ballast, rail ties and rail were then installed and train service resumed.
Description of Concrete Repairs Performed
All seven (7) concrete piers and the one (1) concrete abutment were cracked, spalled and/or delaminated and in need of repair. A combination of 500 sf of surface patch (less than 4” in depth), 500 sf of deep patch (between 4” and 8” in depth) and 500 lf of crack injection were performed. Areas of deteriorated concrete were removed and saw cut back to sound concrete and a combination of drilled in reinforcing and welded wire fabric were installed and the areas were patched. In addition, cracks in the piers and abutment were epoxy injected.