Editing Harvard Bridge (section)

===Engineering study, 1971–1972===
[[File:Harvard Bridge Art.JPG|thumb|The Harvard Bridge is decorated with both serious and whimsical works of art.]]
An engineering study was performed by the Metropolitan District Commission (later merged into the [[Department of Conservation and Recreation (Massachusetts)|Department of Conservation and Recreation]]) in 1971–1972 due to complaints by bridge users of excessive vibration.<ref name=haer5 /><ref name=leet2>Leet, phase 2</ref>  The bridge was found to be understrength for its load.  Before the final study was complete, the recommendation was to place a load limit of {{convert|8|ST|MT|lk=on|sp=us}} per axle and a total of {{convert|15|ST|MT|sp=us}} per vehicle, or to restrict trucks to the interior lanes, where the bridge was stronger.  A {{convert|25|ST|MT|adj=on|sp=us}} limit was imposed.<ref name=leet3>Leet, phase 3</ref>

Suggestions made included strengthening the existing structure by adding either struts or plates to make the existing four beams along the length of the bridge into a stiffening truss, or to replace the superstructure with a new one, made of either steel or concrete, which would be up to current standards.<ref name=leet2 />  The recommendation was to replace the superstructure with one weighing approximately the same in order to reuse the piers, which were in good condition.<ref name=leet3 />

The reasoning was that the cost of a new structure could be predicted much more easily than the cost of repairing and reinforcing the existing bridge.  The resulting new bridge would be of known materials and quality, such as ductile structural steel rather than brittle wrought iron, and rated at [[AASHO]] HS-20.  Repairing the existing structure would leave old wrought iron of uncertain quality and condition standing, and would not bring the design up to (then) current standards.<ref name=leet3 />  Detailed engineering calculations were included.<ref name=leet32>Leet, phase 3, appendices</ref>  The price was estimated at US$2.5 million to US$3 million<ref name=leet3 /> (US${{formatnum:{{Inflation|US|2500000|1972|r=-6}}}} to US${{formatnum:{{Inflation|US|3000000|1972|r=-6}}}} with inflation<ref name=inflation-US />).

The action taken based on this study was to establish load restrictions on the bridge, {{convert|15|ST|MT|sp=us}} in the outer lanes, {{convert|25|ST|MT|sp=us}} on the inner lanes.  This was expanded in 1979 to a flat limit of {{convert|15|ST|MT|sp=us}} on the whole bridge.<ref name=haer8>HAER, p.8</ref>