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Repair & Strengthening
By Jan Lewandoski
More than one thousand wood truss highway bridges, mostly 19th century in origin, continue to carry vehicular traffic in North America. The repair and strengthening of these bridges is made challenging by the need to carry increased highway loads and satisfy modern engineering criteria while retaining the historic form and, in the majority of cases, the original structural system of the bridge. Solutions, worked out on a case by case basis, between contractors, engineers, and State Historic Preservation Officers have ranged from restoration of the original structural system, to improving the strength of the bridge by methods compatible with its historic form, to augmenting or by-passing the historic structural system by means of steel girders or additional piers.
Introduction: During the second half of the 18th century and the first years of the 19th a number of substantial wooden truss bridges, with single spans exceeding 150 feet were constructed both in Europe and North America. Notable among them were the Schaffhausen Bridge (1758) in Switzerland, Timothy Palmer's trussed arch across the Merrimac in Massachusetts (1794), and Louis Wernwag's 340 foot (104 m.) Colossus in Philadelphia (1812). In spite of their success, these bridge designs were little imitated, perhaps due to their complexity and consequent expense. Rather, there emerged between 1804 and 1840 four patented trusses named for their American designers: the Burr Arch (1804), the Town Lattice (1821), the Long Truss (1834) , and the Howe Truss (1840), which established models which dominated North American wooden bridge building until the mid 20th century. Bridges using one of these four trusses account for approximately 80% of the surviving spans longer than 60 feet in North America today. The other 20% use a wide variety of apparently successful, but less popular, trusses, such as the Paddleford, McCallum or Pratt. Wooden bridges 60 feet and shorter are usually of king or queenpost truss type, designs that probably originated in the roof systems of the public buildings of antiquity.
Approximately 20,000 wooden truss bridges were built in the U.S. and Canada between 1794 and 1958. After 1820 the majority were "covered bridges". i.e. roofed and sided, to protect the woodwork from weather. At least 1100 are still in use for vehicular traffic on public highways today.
While four truss types, along with the king and queenpost. dominated design. they were built by a multitude of builders over a vast geographical area, with consequent variation in detail and scale. A Burr truss might have a pair of arches clasping a single line of posts. as in the Village Bridge in Waitsfield, Vermont. or pairs of posts clasping an arch. Some Burrs and Long trusses were double barreled, i.e. a tall third truss, rising to the height of the ridge of the roof, divided the two lanes of traffic. as at the Schoharie Creek crossing in North Blenheim. New York . Town Lattice trusses were typically 14 ft. 6 in. tall and built up out of 6 layers of 3 inch plank, but the 100 foot span double lattice railroad bridge at Wolcott, Vermont (1908) has trusses 25 feet tall and 12 lamina thick. Towns, Longs and Howes were all occasionally assisted by arches. The design of these bridges became very refined. Sophisticated systems for varying the size of posts and braces to reflect loading conditions at different points in a span were developed for the Long trusses at Guilford. Maine and North Blenheim, New York by builders lacking the capacity to analyze unit stresses ( Quantitative engineering analysis of wooden trusses was only just being developed in the 1850's by Herman Haupt in his General Theory of Bridge Construction and Squire Whipple in A Work on Bridge Building.(1)
Length of span was experimented with widely in 19th century wooden bridges. The Columbia Bridge across the Susquehanna River in Pennsylvania was 5690 feet long and supported by 28 piers for an average span of 196 feet. Theodore Burr's McCall's Ferry Bridge, also across the Susquehanna, was a single span of 360 feet. It was unfortunately destroyed by ice in 1815, when only 2 years old. While the average clear span of the surviving wooden bridges in North America is slightly under 100 ft., there are some notable exceptions. The double barreled Long Truss with arch at North Blenheim, New York, built by Nicholas Powers in 1854, has a single span of 228 ft. and currently has 1 in. of positive camber. The Cornish-Windsor Bridge, a Town Timber Lattice built in 1866 across the Connecticut River between Vermont and New Hampshire has two spans, each 204 ft. in the clear, and carries a traffic volume of 2500 vehicles per day.
By the 1870's wooden bridges were meeting with severe competition from iron and steel trusses and suspension designs. By the early 20th century reinforced concrete appeared as a rival as well. Nonetheless the construction of wooden truss bridges persisted, on a diminishing scale, into the middle of the 20th century. Many continued to be built by local bridge builders in rural towns as part of a continuing craft tradition, "unengineered" in the modern sense of the term, and based on sketches or a model rather than a complete set of plans. Others, such as the 108 ft. Howe truss on the Rutland Railway at Shoreham Center, Vermont, built in 1897, or the 100 ft. double lattice built in 1908 on the St. Johnsbury and Lake Champlain Railway in Wolcott, Vermont were designed by professional bridge engineers in distant offices. Quebec and New Brunswick in Canada, and Oregon in the U.S. all carried out provincial and state funded covered wooden bridge building programs for public highways that lasted into the 1940's and 1950's.
Repair And Strengthening of Wood Truss BridgesWooden bridges were frequently repaired or strengthened during the 19th and early 20th century. This was necessitated by rot or damage from floods and ice, heavier vehicles using the bridge, or the fact that the bridge as built as simply not as stiff or strong as it was intended to be. The most common methods used were: adding more timber to a truss, adding wooden arches to a truss. and shortening the span by means of additional piers.
Adding more timber to a truss generally increased its thickness, since it is very difficult to add height to a wooden truss. The Henry Bridge, a Town Lattice in Bennington, Vermont (c. 1840) had its lattice doubled within a decade of its construction to accommodate the wagons of iron ore that began crossing it. The Cornish-Windsor Bridge (1866) between Vermont and New Hampshire had 40 ft. spruce timbers bolted to its top chords over the central pier and at mid-span on the bottom chords, both high tension areas, in an attempt to arrest the alarming sag caused by its ambitious 234' spans. This work was carried out sometime before 1912.(2)
With lattice trusses it is possible to "sister" lattice,i.e. slip another lattice between the chords immediately alongside an existing lattice that is either damaged or located in a high stress area in need of strengthening. This was done at both the Paper Mill Bridge (1889) in Bennington and the West Dummerston Bridge (1872). both in Vermont.
The retrofitting of laminated arches to wooden trusses was a common method of strengthening bridges. The Pulp Mill Bridge (c. 1853) in Middlebury, Vermont was constructed as a double barreled Burr arch spanning 195 feet. Due to a misinterpretation by the Pulpmill's builder of Burr's post to chord connection, the bridge began to distort and sag soon after construction. Around 1860 10 layers of 2" x 6" plank were laminated into an arch that sat on top of the original arch (composed of naturally curved 4" x 12" timbers in series) and attached to the truss.
The addition of piers to shorten a span is a relatively simple upgrading solution for lattice trusses, in which all diagonals can function in either tension or compression. For most other truss types, however, the division into several spans requires that half of the main bracing be reversed. The aforementioned Pulpmill Bridge in Middlebury, Vermont, after being retrofitted with arches, was divided into three spans by two new piers. This involved reversing the direction of half the braces and bolting wooden shoulders to the rear of the posts to accept the new brace orientation. The 144 ft. Howe Truss at Jay. New York was subdivided early in this century by 3 piers. This subdivision produced such short spans that it was not thought necessary to reverse any of the bracing, leaving the single former counter braces to do the work previously carried out by pairs of main braces in half the panels.
Long trusses are equipped with large hardwood wedges where the vertical posts meet the top and bottom chords, designed, according to the inventor, for "trussing" or re-cambering the bridge. Howe trusses have vertical steel rods in place of posts and are described in Wm. Bell's Art and Science of Carpentry (1859) as permitting of re-cambering at a later date. (3) However, this author has come upon no account of these operations actually being carried out on a sagged bridge.
A final historic method of strengthening a failing bridge is to decrease its dead load by removing roof and siding, as was done on the Winooski River railroad bridge in Montpelier, Vermont in the late 19th century. To do so however, is a last desperate act, since the uncovered span is unlikely to last more than 20 years, while covered wooden bridges can persist almost indefinitely.
Contemporary Repair and Strengthening of Wood Truss Bridges
I have been the framing contractor for the restoration or rehabilitation of eleven wood truss
bridges (as well as the construction of two new ones based upon historic models), and a
consultant on the repair of seven others.
The Waitsfield Village Bridge, Waitsfield, Vermont
The Waitsfield Village Bridge is a 108 ft. single span Burr Arch built in 1834, making it the
oldest surviving bridge of any material, in the state of Vermont. The trusses are in very good and
almost entirely original condition, even retaining 1 to 2 inches of positive camber. In the middle of
a busy tourist town, and posted for 16,000 lbs., it is crossed by an average of 1000 vehicles per
day, mostly cars and light to medium weight trucks. The height of this bridge's opening, and its
prominent mid-town location, precludes it use by tractor trailers, log trucks. concrete trucks, or
mobile cranes, which are the usual sources of overloading in this locality. A modern bridge with a
higher weight limit exists 2 miles downstream.
A careful examination of the bridge discovered that each bent post was missing a key original element of the truss, variously called the chalk, check, or kicker brace, specifically designed to aid the post in resisting this particular thrust (see Figure). The check brace from one post had been removed to make room for a walkway joist added during this century. The other two were removed in recent years during chord repairs at the end of the truss.
Restoration, in this case removing the effects of previous abusive repairs, was the appropriate course for strengthening the Waitsfield Bridge. The bridge was supported on cribbing from the river, the truss dismantled at three of it's ends. three vertical posts replaced, and check braces installed. The replacement posts were identical in size to the spruce (Picea var.) originals, but were made of white oak (Quercus Alba), a species stronger in bending, horizontal shear and compression perpendicular to the grain.
It was believed that this change of species would help upgrade the truss against heavier traffic loads while staying within the historic form of the bridge. White oak is also among the most naturally rot resistant species, a consideration for truss members near the abutment and the drainage of the road. The work was completed in 1992, with the bridge appearing no different than in 1834, other than a species change in three pieces of timber.
The Mill Brook Bridge. Fairfax, Vermont
The 60 ft. lattice truss in Fairfax was sagging noticeably. It had extensive areas of rotten
chord and lattice due to poor approach drainage and roof leakage. Many lattice bottoms had been
broken by ice and debris during floods. In addition, repairs, even to the bottommost tension
chord, had been made by bolting in short, 4 to 8 ft. pieces of plank, producing a multiplicity of
joints where the bridge could least afford it.
The Cornish-Windsor Bridge, between Vermont and New Hampshire
The Cornish-Windsor Bridge is a Town Timber Lattice built in 1867. 468 ft. in length, it is the
longest two span wooden bridge in the world. A timber lattice is a variation of the more common
plank lattice, using 6" x 8" timbers for the web rather than 3" x 11" plank, and depending upon
shouldered lap joints with a keeper bolt at all lattice and chord crossings, rather than transfixion
with hardwood pins, to resist flexure in the truss.
Extending the Service Life of Wooden Bridges
The covered wooden truss is undoubtedly, with the exception of stone, the most enduring of
bridges. The average age of in-service wooden bridges in North America today is 120 years, and
many are as much as 50 years older than that. The wooden bridge suffers from leaking roofs, bad
road drainage, collisions, and increased weight of traffic. Most of these problems can be solved by
restoration in kind or repairs within the tradition of the bridges structural system, i.e. using timber,
wooden pins, metal bolts, and joinery typical to the structure. The use of rot resistant species or
treated timber at and near the intersection of bridge and highway is also helpful. but they are not
needed elsewhere on a bridge, since the majority of the structure sits farther from sources of
moisture than do buildings sitting on foundations on the ground, and they are very well ventilated,
having neither interior sheathing nor a tight floor.
CONCLUSION. Over the last 13 years I've served as contractor or consultant on 19 historic
wooden bridge projects in 5 U.S. states and 2 Canadian provinces. Although no single policy
exists among the interested parties in these several locales, an evolving general approach can be