In September of 2009, the latest bridge to connect the neighboring New Hampshire communities
of Bristol and New Hampton officially opened. The new $4.7 million, Central Street Bridge—
the fourth to span the Pemigewassett River at this location—features high-strength steel girders
eight feet deep and spanning 240 feet, making it the longest single-span bridge of its kind in New
Hampshire.
Designed to resist floodwaters
The new Central Bridge was designed to resist floodwater forces from a breach of the upstream
Ayers Island hydroelectric dam. Dangerous floodwaters have been a regular occurrence at this
location in years past. Even though the bridge is located well above the Pemigewassett—the
structure is supported on abutments up to 40 feet high (placing it 10 feet higher over the river
than the previous bridge)—the design called for a steel bracing system between the girders
along their full length. A 3,000-pound structural steel member is embedded in each abutment to
restrain both ends of the bridge from washing downstream.
The weight and depth of the steel plate girders was reduced through the use of high strength,
Grade 70 steel. This increased the clearance above the river, created a more slender and
streamlined appearance, and allowed for smaller cranes to erect the girders.
New approaches
The approaches to the old bridge were narrow and dangerous, containing steep grades and tight
turns. When opposing cars came across the bridge, one vehicle would usually stop for the other
to pass. With the new Central Street Bridge, vehicular and pedestrian safety has been greatly
increased, thanks to realigned approaches, wider travel lanes (28 feet between curbs), flatter
grades, softened curves, and a six-foot-wide sidewalk, with only minimal impacts to adjacent
properties along both approaches.
Minimizing impact
Granite from the old abutments was re-used for 240 feet of retaining wall along the west
approach. In order to minimize impacts to the very steep slope of the western river bank, the
abutment was designed with tall and curved wingwalls that step up into the hillside. The walls,
60 feet in length and up to 42 feet in height, are curved to parallel the roadway approach and
minimize side impacts. The footing is stepped in increments of 13 feet, which significantly
reduced the cost of reinforced concrete, rock excavation, and temporary earth retention. The steel
sheet piles used for the west abutment cofferdam were designed to remain in place by anchoring
to a six-inch thick reinforced concrete sub-footing, and serve as permanent scour protection for
the abutment’s footing.
Level spreaders comprised of stone fill were added to naturally treat stormwater run-off before it
reached the Pemigewassett. The design included plantings throughout the rip-rap stone slopes to
minimize the impact of absorptive heat transfer from the rocks to the river’s waters.
Wade Brown, P.E., is a Principal Engineer with Kleinfelder/S E A Consultants of Manchester,
N.H. Brown has more than 23 years of structural engineering experience, with expertise in
seismic design, analysis, and retrofit of structures made of concrete and steel. He can be reached
at wade.brown@seacon.com.