Copyright 2019: New Hampshire Department of Transportation, 7 Hazen Drive, Concord, NH 

Welded plate .

The individual beams were connected together with a plate welded to the tops at each bent as shown in the photo.  The bent is the location where the piles and pile cap work together to support the bridge.

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Combined Simple and Continuous Beam System

This unique design feature called for a combined simple and continuous I-beam stringer system which was rarely used by the New Hampshire Highway Department (NHHD).

 

The system utilized individual steel beams with a specific curvature, each with specific angles at their ends. The stringers, or support beams, were made continuous by field-welding steel splice-plates to the top flange over the bents.

 

The beams were specifically designed to function as simple beams under dead load, which is the constant weight of the concrete deck, sidewalks and railings.  The beams were also designed to function as continuous beam under live loads, which is the variable weight from the vehicles, bicycles, pedestrians and the accumulation of snow.

 

This was a sophisticated and uncommon design for the time. This unique continuous beam design was not evident in photographs and required examination of the original plans to be certain.

To view the 1954 design plans for both bridges as well as additional historic information go to the Historic Documentation page of this website by clicking on the link below.

Historic Documentation

Four individual beams make up the part of the exterior girder that is visible in this photo.