Railroad Bridge Reuse Evaluation Crossing the Androscoggin River

Berlin, NH

As a major cost saving initiative to save the Cascade Mill, the Fraser Paper company looked to convert their facility on the west bank of the Androscoggin River to natural gas by connecting across the river to an existing Portland Natural Gas Transmission distribution pipeline and to a proposed pipeline to supply landfill gas from the nearby Androscoggin Valley Regional Refuse Disposal District landfill further east. Reuse of an existing former railroad bridge, owned by Fraser upstream of the Mill, with only limited repairs for a reasonable lifespan (about 20 years) was a vital component of the financial feasibility of the overall project.

Through research of historical sources, SA deduced that the bridge was likely constructed between 1875 and 1903 and is a skewed, two-span, pin-connected Whipple through truss, also known as a double intersection Pratt truss, Murphy-Whipple truss, and/or Linville truss and likely constructed of wrought iron or combination of wrought and cast iron. The Bridge is one of two known remaining Whipple truss bridges in New Hampshire, and thus is considered to be historically significant. Whipple trusses were commonly used for railway bridges in the United States between 1860 and 1890.

SA visually inspected each of the twin 130-ft. long spans, cataloging bridge members, dimensions and conditions as well as visual inspection of the abutments and center piers. SA field evaluated scour at the abutments and piers by probing. After our initial inspection determined that further structural analysis was warranted (i.e. the condition was not significantly deteriorated to be unusable), SA developed a 3-D finite element model of the bridge to evaluate capacity to support loads from the new pipeline crossing and temporary construction equipment required to construct the pipeline. Based on our visual inspection and analysis results, SA recommended maintenance and repairs considered necessary to utilize the bridge as pipeline crossing for lifespan of 20 years.