The Harvard Bridge (also known locally as the MIT Bridge, the Massachusetts Avenue Bridge, and the "Mass. Ave." Bridge) is a steel haunched girder bridge carrying Massachusetts Avenue (Route 2A) over the Charles River and connecting Back Bay, Boston with Cambridge, Massachusetts. It is the longest bridge over the Charles River at .
After years of disagreement between the cities of Boston and Cambridge, the bridge was built jointly by the two cities between 1887 and 1891. It was named for Harvard University founder John Harvard. Originally equipped with a central swing span, it was revised several times over the years until its superstructure was completely replaced in the late 1980s due to unacceptable vibration and the collapse of a similar bridge in Connecticut.
The bridge is known locally for being marked off in the idiosyncratic unit of length called the smoot.
Conception
In 1874 the Massachusetts Legislature authorized construction of a bridge between Boston and Cambridge, and in 1882 follow-up legislation set out its location.
The bridge was to have a draw with an opening of at least . Boston interests opposed the bridge, mainly because it did not provide for an overhead crossing of the Grand Junction Branch of the Boston and Albany Railroad. Further legislation in 1885 changed the draw to a clear opening of at least and no more, until the other bridges below the proposed location were required to have a larger opening. There was still no substantive progress until 1887, when Cambridge petitioned the Legislature to compel Boston to proceed; the resulting act required each city pay half the cost, and allowed Boston to raise up to $250,000 (US$ with inflation) for this purpose, in excess of its debt limit. This implied an estimated cost of US$500,000 (US$ with inflation) for the bridge.
The Legislature provided for a bridge commission, to consist of the mayors of Boston and Cambridge plus a third commissioner to be appointed by the mayors. The mayors of Boston and Cambridge, Hugh O'Brien and William E. Russell, appointed Leander Greeley of Cambridge as the third commissioner, though this appointment changed over time.
The commission's 1892 report claimed:
Name
The bridge is named for the Reverend John Harvard, for whom Harvard University is also named, rather than for the university itself. Other names suggested included Blaxton, Chester, Shawmut, and Longfellow.
Engineering
Originally projected as a wooden pile structure with stone pavement for the first (because the Charles River Embankment extension was expected to take that space) the design was changed to be entirely iron spans on stone piers. The general plans were approved on July 14, 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 below existing ground, is a very dense till composed of gravel and boulders with a silt-clay matrix. Above that to approximately 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 .
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 with a draw wide between centers. The width of the bridge was except near and on the draw.
The bridge as built was composed of fixed and suspended spans roughly long and piers apart, center to center. The span lengths alternated between . The longer spans were cantilevered, while the shorter spans were suspended between the cantilevers.