Harvard Bridge - Engineering

Engineering

The Acts of 1887 declared the bridge to be a wooden pile structure with stone pavement for the first 200 feet (61 m) because the Charles River Embankment extension was expected to take that space, but that was changed such that the whole distance would be of iron spans on stone piers. The general plans were approved on 14 July 1887. The engineers were William Jackson (Boston City Engineer), John E. Cheney (assistant Boston City Engineer), Samuel E. Tinkham (assistant engineer), and Nathan S. Brock (assistant engineer at bridge).

The subsurface conditions at the bridge location are extreme. Much of Boston is underlain with clay, but the situation at the bridge is exacerbated by a fault which roughly follows the path of the Charles River itself. From a depth of approximately 200 to 300 feet (60 to 90 m) below existing ground, is a very dense till composed of gravel and boulders with a silt-clay matrix. Above that to approximately 30 feet (9 m) below the surface is Boston blue clay (BBC). Over this are thin layers of sand, gravel, and fill. The BBC is overconsolidated up to a depth of approximately 70 feet (20 m).

The substructure originally consisted of two masonry abutments and twenty-three masonry piers, as well as one pile foundation with a fender pier for the draw span. The superstructure was originally twenty-three cantilevered fixed spans and suspended spans, of plate girders with one swing span. The Boston abutment rests on vertical piles, while the Cambridge end is directly on gravel.

Originally, the bridge was built across the Charles River connecting West Chester Park, in Boston, with Front Street, in Cambridge. This is now called Massachusetts Avenue on both sides of the river. As originally built, the total length between centers of bearings on abutments was 2,164 feet 9 inches (659.82 m) with a draw 48 feet 4 inches (14.73 m) wide between centers. The width of the bridge was 69 feet 4 inches (21.13 m) except near and on the draw.

The bridge as built was composed of fixed and suspended spans roughly 75 feet (23 m) long and piers 90 feet (27 m) apart, center to center. The span lengths alternated between 75 and 105 feet (23 and 32 m). The longer spans were cantilevered, while the shorter spans were suspended between the cantilevers.

The original roadway contained two lanes for horse-drawn vehicles and two street car tracks, for a total width of 51.0 feet (15.5 m). There were also two 9-foot-2-inch (2.79 m) sidewalks. The original roadway and sidewalk stringers were of wood, with an approximately 1.25-inch (32 mm) thick covering of asphalt on the sidewalk.

The exception was at the swing span, which was 48 feet (15 m) wide. This span was approximately 149 feet (45 m) long, and sat on a wooden pier. It was a double-cantilevered, electrically-driven structure also carrying a bridge caretaker's house.

The bridge opened on 1 September 1891. The original cost of construction to 1 March 1892 was US$510,642.86. This is equivalent to US$13,210,000 with inflation.