Editing Harvard Bridge (section)

==Engineering==
[[File:Harvard Bridge postcard, 1910.jpg|thumb|300px|Postcard showing Harvard Bridge looking toward Boston in 1910, from the roof of the Riverbank Court Hotel (now [[List of Massachusetts Institute of Technology undergraduate dormitories|Maseeh Hall]], an MIT dormitory)]]
[[File:Harvard Bridge postcard 1920ish.jpg|thumb|300px|Postcard showing Harvard Bridge, looking toward Cambridge and MIT sometime between 1916 and 1924]]

Originally projected as a wooden pile structure with stone pavement for the first {{convert|200|ft|m sm|sp=us}} (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.{{sfn|Alger|Matthews|1892|pp=13-14}}  The engineers were [[William Jackson (engineer)|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).{{sfn|Alger|Matthews|1892|p=32}}

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 {{convert|200|to|300|ft|m sm|-1|sp=us}} below existing ground, is a very dense till composed of gravel and boulders with a silt-clay matrix.  Above that to approximately {{convert|30|ft|m sm|0|sp=us}} below the surface is Boston blue clay (BBC).  Over this are thin layers of sand, gravel, and fill.  The BBC is [[Consolidation (soil)|overconsolidated]] up to a depth of approximately {{convert|70|ft|m sm|-1|sp=us}}.<ref name=haer4 />

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 [[cantilever bridge|cantilevered fixed spans]] and suspended spans, of plate girders with one [[Swing bridge|swing span]].{{sfn|Alger|Matthews|1892|pp=18-26}}  The Boston abutment rests on vertical piles, while the Cambridge end is directly on gravel.<ref name=haer4 />

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 (Boston)|Massachusetts Avenue]] on both sides of the river.  As originally built, the total length between centers of bearings on abutments was {{convert|2164|ft|9|in|m sm|sp=us}} with a draw {{convert|48|ft|4|in|m sm|sp=us}} wide between centers.  The width of the bridge was {{convert|69|ft|4|in|m sm|sp=us}} except near and on the draw.{{sfn|Alger|Matthews|1892|p=17}}

The bridge as built was composed of fixed and suspended spans roughly {{convert|75|ft|m sm|sp=us}} long and piers {{convert|90|ft|m sm|sp=us}} apart, center to center.{{sfn|Alger|Matthews|1892|p=18}}  The span lengths alternated between {{convert|75|and|105|ft|m sm|sp=us}}.  The longer spans were cantilevered, while the shorter spans were suspended between the cantilevers.<ref name=haer4 />

The original roadway contained two lanes for horse-drawn vehicles and two street car tracks, for a total width of {{convert|51.0|ft|m sm|sp=us}}.  There were also two {{convert|9|ft|2|in|m sm|adj=on|sp=us}} sidewalks.<ref name=haer3>HAER, p. 3</ref>  The original roadway and sidewalk stringers were of wood, with an approximately {{convert|1.25|in|mm sm|adj=on|sp=us}} thick covering of asphalt on the sidewalk and a {{convert|2|in|mm sm|adj=on}} spruce wearing surface on the roadway.<ref name=haer3 />

The exception was at the swing span, which was {{convert|48|ft|m sm|sp=us}} wide.  This span was approximately {{convert|149|ft|m sm|sp=us}} long, and sat on a wooden pier.  It was a double-cantilevered, electrically-driven structure also carrying a bridge caretaker's house.<ref name=haer3 />

The bridge opened on September 1, 1891.{{sfn|Alger|Matthews|1892|p=15}}  The original cost of construction was $511,000,{{sfn|Alger|Matthews|1892|p=29}} ${{formatnum:{{Inflation|US|511000|1891|r=-4}}}} in current dollars.<ref name=inflation-US />