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Middlebury's Pulp Mill Bridge Reopened. (Aug. 23     Work on the Pulp Mill Bridge Nears Completion. (Aug. 14)
Work Continues on the Pulp Mill Bridge. (July 11)     Pulp Mill Bridge Closed For repairs.(June 24)

Some Commentary On The Pulp Mill Bridge. [WGN 45-0-04]
by Jan Lewandoski

       The Pulp Mill Bridge in Middlebury is possibly Vermont's oldest bridge and certainly one of its most heavily traveled. We reopened it to traffic in late August after 2 1/2 months of work rebuilding the center truss and re-footing the arch.
       The Pulp Mill Bridge has a long history of problems, beginning with crucial errors in its original construction. Built around 1820 (or perhaps 1850) as a double barreled multiple kingpost with a nearly 200 foot single span, it began to fail shortly afterwards.
       H. W. Congdon reports that reinforcing laminated arches were added around 1859, and R. S. Allen says it was subdivided into 3 spans long ago. The crucial flaw is that the posts were not shouldered into the bottom chord but were merely necked down to a very small size and allowed to pass through the chord. Most of the resistance to the horizontal thrust of the main braces, which is great in a span this large, was carried on a single square bolt that transfixed the chord and post at this point, and on the check braces. Not shouldering the posts also eliminates the post as a shear block to help keep the bottom chord lamina from sliding by each other in tension, and there were relatively few shear blocks elsewhere in the chords.
       The subdivision of the bridge, whenever it happened, involved reversing the direction of half the main braces in the bridge and bolting new bearing shoulders for them on the opposite sides of the posts. If you walk to the center of the middle span, you can view the bridges original center kingposts, with their entasis.
      While all three of the trusses in the Middlebury span (as well as the middle span and the two that I haven't yet repaired) are dramatically in need of work, there were only funds available this year for work on the center truss with its obvious problems. We swung 63-foot steel I-beams under the bridge, placed cribbing and jacks upon them and gained control of the center truss. We could only lift it to barely level because the two outer trusses were (and are) hanging below it and dragging it downward. We cut the arches out of the concrete, took the chord apart, discarded the bad work of the 1980's, introduced chord members and sisters between 38 and 49 ft. in single stick lengths with staggered joints, shear blocked all the un-shouldered post positions, replaced a damaged major post (using a 8" x 16" x 22 ft. hemlock timber), and re-footed the arch on steel thrust blocks. In addition we changed the hanger and tie rods for that truss and rebuilt much of the far too thick and heavy floor that burdens the bridge. The only thing that keeps this bridge standing are the shortness of its spans and the fact that there are three trusses. According to Gil Newbury's analysis a few years ago, only the center arches are tall enough to contribute anything to the structure. The outer ones have large negative areas and are mostly weighing down the bridge. Lets hope funds become available to fix all of this beautiful structure.
      Most of the timber for my repair was spruce (the long sticks), beech and white oak. The work was carried out by Paul Ide , Don Estes, Clark McKenzie and myself, with help from Brown's Welding and Crane in Bristol.

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Joe Nelson, P.O Box 267, Jericho, VT 05465-0267, jcnelson@together.net
This file posted September 18, 2002