Comprised of high-strength steel girders 8 feet deep and spanning 240 feet, the Central Street Bridge is the longest single-span bridge of its kind in the state of New Hampshire.

A number of prominent bridges span the Pemige-wasset River in New Hampshire. Some of the more noteworthy include the Blair Covered Bridge in the town of Campton, Clark’s Bridge in North Woodstock (one of the last Howe truss railroad bridges left in the world), and the Pumpkin Seed Bridge at Livermore Falls (the only bridge in the Granite State to feature a double-bow truss formed by curved upper and lower chords).

Several notable bridges have also connected the neighboring communities of Bristol and New Hampton. The first structure — a roofless stringer bridge supported by two piers in the Pemigewasset — opened to traffic in 1823. The second — a two-span, covered truss bridge with a single stone pier — opened in 1836 and lasted until 1928. The third structure opened that same year and was comprised of a single-span High Parker truss with a polygonal top chord. It withstood floodwaters and the elements for decades before finally being deemed “functionally obsolete” by the New Hampshire Department of Transportation in 2001.

That’s when Bristol and New Hampton began the process of replacing the bridge with a structure equal in standing to the many others spanning the Pemigewasset. Today, after much planning and construction, residents of both communities feel they have accomplished that goal. Comprised of a single-span High Parker truss with a polygonal top chord, the new $4.7 million Central Street Bridge features high-strength steel girders 8 feet deep and spanning 240 feet, making it the longest single-span bridge of its kind in the state of New Hampshire.

One of the structure’s most unique design elements is its steel bracing system, which will help the bridge resist floodwater sources, particularly if there is a breach of the upstream Ayers Island hydroelectric dam. Floodwaters brought about the collapse of the second bridge at this location in 1928.

The structure’s force-resisting system includes several interior steel diaphragms designed to transfer the floodwater force from the upstream girder to the concrete bridge deck. The bridge deck, which acts as a rigid horizontal diaphragm, would collect the force from each interior diaphragm and transfer it equally to each end of the bridge. The steel end diaphragms, located between the girders, would collect the total force and transfer it down to a 3,000-pound structural steel restraint member, which is embedded in the concrete abutment. This restraint member is located just downstream from the third of four bridge girders. A small gap separates the girder from the steel restraint block. This gap would close as the girder’s steel-reinforced elastomeric bridge bearing deforms transversely from the floodwater force.

Another unique design element includes the weight and depth of the new bridge’s girders, which were reduced through the use of high-strength (Grade 70) steel. Besides giving the structure a more streamlined appearance, the girders allowed designers to increase the bridge’s clearance above the river. The new bridge is supported on abutments as high as 40 feet, which increased the clearance above the Pemigewasset by approximately 4 feet, while the elevation of the roadway was raised by approximately 10 feet.

Designers also addressed a frequent complaint locals had regarding the former structure. The approaches to the old Central Street Bridge were narrow and dangerous, containing tight turns and steep grades. It wasn’t uncommon to see a vehicle stopped on the approaching road, allowing an oncoming vehicle to pass. On the new bridge, realigned approaches, wider travel lanes (28 feet between curbs), flatter grades, softened curves, and a 6-foot-wide sidewalk mean vehicular and pedestrian safety has been greatly increased.

The new Central Street Bridge also features a number of design elements aimed at minimizing the impact to its natural setting. Some of these include:

• Re-using granite from the old abutments of the former structure. This granite was used for 240 feet of retaining wall along the west approach, as well as for a pedestrian walkway and steps leading to the river on the east.
• Designing the abutment with tall and curved wingwalls that step up into the hillside. This minimizes impacts to the steeply sloped land of the western river bank. Furthermore, the walls (60 feet long and as high as 42 feet) are curved to parallel the roadway approach and minimize side impacts. Also, the footing is stepped in increments of 13 feet, which significantly reduces the cost of reinforced concrete, rock excavation, and temporary earth retention.
• Adding level spreaders comprised of stone fill. This was done to treat stormwater runoff naturally before it reached the Pemigewasset. Also, plantings were added throughout the rip-rap stone slopes to minimize the impact of absorptive heat transfer from the rocks to the river’s waters.

One obstacle encountered during the bridge’s construction period was the procurement of steel. Because one of two U.S. steel mills was engaged in tank production for the U.S. military, the other mill was faced with an overload of domestic orders, which resulted in a delay to the project. The high-strength (Grade 70) rolled plates required to fabricate the steel plate girders became available seven months late, delaying steel fabrication and extending the steel erection phase of the project to beyond the following year’s high-water season. Even with the delay, good management of the construction phase helped keep the total construction cost below the 2007 bid price.

The new Central Street Bridge is serving as not only a transportation link, but also as a source of pride for two closely connected New Hampshire communities. The attractively designed structure has taken its rightful place alongside the various other prominent bridges spanning the Pemigewasset.

Wade Brown, P.E., is a principal engineer with Kleinfelder/SEA 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 contacted at wade.brown@seacon.com.

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