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A new bridge and a protected bridge to fill towns' needs: engineering firm tackles sensitive design issues.

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Manchester-based Hoyle, Tanner & Associates Inc.'s bridge and transportation groups have finished design and construction oversight for the replacement of the Lyndeborough Road Bridge over the South Branch of the Piscataquog River in New Boston.

The previous bridge was constructed in 1940. Over the past 70 years, time and environmental elements deteriorated the existing bridge resulting in the bridge being classified as "structurally deficient," thus requiring complete replacement. The town administered the project through the state Department of Transportation's Municipally Managed Bridge Aid Program, with the town paying 20 percent of all project costs and the DOT paying the remaining 80 percent. (Hoyle, Tanner has designed over 70 bridge projects for New Hampshire communities through this funding program.)

The existing bridge was a narrow 52-foot-long, single-span steel beam structure posted for 15 tons and supported by concrete-faced stone abutments. Existing roadway geometrics were poor, with a "Y" intersection and another road immediately north of the bridge. Goals of the project included a proposed bridge that met current statutory loading requirements, carrying two lanes of traffic, and improving the "Y" intersection configuration.

Originally, the bridge and approach guardrail was specified as standard DOT metal guardrail. During the design phase, a community meeting was held at the site, and Hoyle, Tanner presented the scope of construction to abutters and other interested neighborhood citizens.

The outcome of the meeting was to change from the metal guardrail to a timber guardrail system in order for the new bridge to blend into the existing woodland environment of the site.

According to project manager, Jason, T. Lodge, "the community meeting was a success as it allowed all abutters to be informed about the project and to provide valuable input."

As a result, Hoyle, Tanner specified a proposed bridge type that closely resembled the existing bridge. The new bridge is a steel beam structure with a concrete deck on concrete abutments. By maintaining the same look of the existing bridge, specifying timber guardrail and improvements to the "Y" intersection, Hoyle, Tanner was able to design a new bridge that met the town's goals, but preserved the natural viewscape of the surrounding area.

The construction cost was $820,000. Contractor was Cold River Bridges LLC of Walpole.

A bridge in Swanzey

The Thompson Covered Bridge has served the town of Swanzey since 1832, but removal of the downstream deteriorated Homestead Woolen Mill Dam resulted in the need for renovations to protect the bridge from the effects of "scour" from the Ashuelot River.

Scour is the erosion of foundation soils around or under bridge abutments and piers > caused by water action. If left unprotected, scour of the foundation materials can lead to collapse of bridge structures.

Scour protection was accomplished through underpinning of the center stone pier and the installation of stone stream barbs upstream of the bridge. Stone stream barbs are engineered stone structures that channel the river flow away from the bridge abutments and reduce the potential for scour.

The underpinning was accomplished by installing steel rods through pier base stones, then ringing the pier with steel sheeting driven into hard glacial till.

Next, the area between the sheeting and pier was excavated and filled with concrete. Finally, keyed stone fill was added around the pier and shaped to deflect water and ice floes away from the pier. The bridge was further protected against arson through the installation of a deluge sprinkler system.

In addition to scour protection measures, the center pier was cleaned, re-chinked and repointed. Chinking involves adding small stones between uneven joints in the large pier stones while pointing includes adding a cement-based mortar to the joints for water protection. A small tree was also removed from the downstream end which was causing damage to the stonework in this area.

The project did not materially affect the appearance of the bridge, and most of the repairs will be covered by water, which is an important aspect of the project due to the historic importance of the bridge.

According to Hoyle, Tanner project manager Sean lames--who coordinated the project with town officials, state agencies and other consultants--"the process was extensive, but it was very important that all interested parties were able to contribute to the project to make it a success. In the end, the project benefited from the input of many different people with varied expertise. It was a joint effort, and the result was a great project."

Subconsultants on the project were Haley & Aldrich Inc. and SFC Engineering Partnership Inc.
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Title Annotation:ENGINEERING
Publication:New Hampshire Business Review
Geographic Code:1U1NH
Date:Feb 11, 2011
Words:752
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