PEPPERELL — Unlike its predecessor, the Chester A. Waterous bridge, which has been likened to a wooden dog house sitting atop steel beams, the new landmark covered bridge is designed as a totally wooden structure that includes reinforcements for the wooden decking under the road travel surface.
A high-tech design totally different from its forebear, the new bridge will be maintained in an up-to-date manner, according to one of the subcontractors for general contractor, S & R Inc. of Lowell.
A key element is the strength and fit of two diameters of reinforcing steel rods hidden within the massive girder walls. They strengthen the structure to allow modern-day car and truck traffic.
The rods will be stress-tested every six months, using electronically-read strain gages put in place by High Tech Corp. of Littleton, a subsidiary of First Technology Corp. of Michigan.
“There are two sizes of reinforcing rod, one an inch-and-a-half thick. There are more than 30 of them,” High Tech assistant engineer Jason Clark.
“They’ll be tested for no torque and for shear load, using postage stamp-sized foil registers attached to the rods with polypenholic (epoxy) backing. When the material moves either by stretching or otherwise, the (electrical) resistance changes, which our equipment interprets as strain,” he said.
The foil-strain gage indicators are manufactured by Vishay Micro Measurements, constructed in a ladderlike configuration, Clark said.
High Tech’s president is Douglas Unkel and Domenic El Aschkar is senior application engineer for the Pepperell project. The company also has offices in other locations and employs about 50.
Clark said the firm does considerable strain measurement for compressor blades on the “hot side” of GE and Pratt and Whitney jet aircraft engines, limited testing of airframes, a considerable amount of medical equipment and hydroelectric plant components.
“If we find an issue, we’ll report results to S & R,” Clark said.”Our contract is for an indefinite period.”
The travel surface of the new bridge will be asphalt atop a wooden deck. The beams and trusses, bolstered by the reinforcing rods, form a very strong self-supporting structure.
The former bridge rested on steel I-beams, which deteriorated alarmingly over the years. Its load-bearing capability had been reduced to six tons years ago.