A unique covered bridge design has been built in the United States, The North Carolina Department of Transportation project will provide a pedestrian overpass over Highway 52 at Old Salem, site of North Carolina's first Moravian community. Moravian settlers arrived in North Carolina from Bethlehem, Pennsylvania, in 1753.

To enhance and blend with the cultural landscape and historic architecture of Old Salem, several types of covered bridge structures were considered. The final choices were the Burr-arch truss and the Town lattice. The Burr-arch truss is a two hinged arch combined with a multiple king post truss. The arch affords great stiffness. The town lattice has many redundant members providing a truss with great toughness. Though Theodore Burr's (1771-1822) patent of 1817 claimed nothing but the arch combined with the multiple king post truss, it became the most popular covered bridge system in the United States.

This paper will discuss the development of the design for the Old Salem Bridge and the application of covered bridge technology to both the preservation of historic structures and the construction of modern transportation structures.

The Concept

Why build a covered bridge in Old Salem. As a cultural tourism destination, a pedestrian, overpass structure was needed to enhance and blend with the cultural landscape and historic architecture of Old Salem by conforming with the traditional technology of covered bridge building in North America. At the same time, it was important to Old Salem planners such as John Larson that the structure not he confused as an original artifact. To differentiate, the bridge is clad with transparent panels which convey a modern appearance while exposing the traditional interior construction. The bridge will serve as a gateway for vehicular traffic entering Old Salem as the museum expands westward.

Description

The bridge is 120 foot span Burr-arch covered bridge which spans Highway 52 through the City of Winston-Salem enabling pedestrians to gain access to Old Salem from a visitor parking lot. Pedestrians using this bridge will enter Old Salem adjacent to the Museum of Early Southern Decorative Arts.

There is no historical precedent for a covered bridge at this location or in the vicinity of Old Salem. Certainly, North Carolina had many covered bridges which spanned nearby rivers such as the Deep, Catawba, Haw. Dan and the Yadkin.

History

A small group of five men left Pennsylvania traveling down the coast past Chesapeake Bay to Edenton, North Carolina. Their leader was August Gottlieb Spangenberg, whose task it was to search for the 100,000 acres of land for the church to purchase from the Earl of Granville. Accompanied by the Earl of Granville's chief surveyor and three local inhabitants, they traveled westward (2 ).

After a wrong turn which led them into the North Carolina Mountains, the group found the tract of land they were searching for. They had surveyed several plots which proved to be unsuitable prior to locating a large tract of land which they named Wachovia (2). In 1755, the first married couples arrived to take up residence in the temporary town of Bethabara, while the permanent settlement of Salem was established in 1766.

For many decades overland travel between the two Moravian communities was common. This historical link between Pennsylvania and North Carolina is important in the justification of the type of truss system for the new Salem Bridge.

Recycled Timber

During the process of material acquisition, BRTW revealed that they were unable to locate posts of a sufficient size in sufficient quantity, They offered several solutions for mechanically attaching blocks to the posts. The designer considered the mechanical attachment, but rejected it as inadequate. Alternatively, to fabricate the large posts from new material would require extensive drying, if timbers so large could be found at all.

It appeared at one point that the bridge would be furnished in reverse of the plans, which require new material with only the posts of recycled timber.

Material Grading

It was agreed that first all timbers should be graded in accordance with SPIB Grading Rules. Drill holes would be considered to be open knots with the same criteria applied regarding size, location and frequency, After grading, all suitable timbers would be load tested by applying a known load and measuring the deflection across a given span in order to compute the Modulus of elasticity. This would provide a measure of stiffness for each piece, presumably enabling us to discard pieces which fell below the published values for the grade. Certainly, knowing the Modulus of Elasticity for compressive members is particularly valuable. For tension members, it was thought that the recycled material, being extremely dense, would be a least as strong as published values for the same grade of new material.

Moisture Content

It became obvious that moisture measurement using meters would be problematic. Because of accumulated salts in the recycled timbers, moisture contents varied widely, The history of the timbers, according to BRTW, included some which were taken from a marine structure and others which came from a building which stored agricultural chemicals. Several samples were oven dried, establishing that the average moisture content was much less than what the hand held moisture meters had indicated. Storage at the job site was critical to the moisture content of these materials. Timbers were stored at the site off the ground, but uncovered. Rain affected the top surface of the timbers. Water filled mortise holes and areas around stakes and checks resulted in high moisture readings in various locations.

Through numerous conversations between the North Carolina Department of Transportation, the timber frame subcontractor BRTW, and the designer, materials were evaluated and approved using a combination of hand held moisture readings verified with oven dry tests.

Conclusion

The Salem Bridge revives traditional limber framing and bridge building as it was applied to the construction of covered bridges in North America during the first half of the nineteenth century. The project required the application of modern timber engineering to the design and construction of a cultural heritage structure.

Project Justification

The following time line is provided to show that the Burr-arch truss is the most logical choice for this particular span.

Chronology

1753 The first Moravian settlers arrive from Bethlehem, PA. They establish Bethabara as a temporary community.

1771 Theodore Burr, a Harrisburg. PA engineer and contractor is born in Connecticut.

1772 The residents of Bethahara move to Salem.

1784 Ithiel Town, a New Haven, Connecticut architect and bridge promoter is born.

1804 Theodore Burr builds a 400 foot span bridge across the Hudson River at Waterford, New York.

1817 Theodore Burr's patent of 1817 claims nothing but the arch combined with the multiple king post truss. This became the most popular covered bridge structural system.

1820 Ithiel Town's plank-lattice design patent.

1822 Financial failure and death of Theodore Burr.

1835 Ithiel Town (1784-1884) secured a patent for a lattice bridge with double webs and secondary chords.

1835 The Humpback Bridge is built near Covington, Virginia. The 120 foot long multiple king post bridge has an eight foot rise.

l842 The pamphlet "Hints on Bridge Construction by an Engineer" is published by Herman Haupt of Pennsylvania.

1844 Ithiel Town dies.

1894 Bunker Hill Covered Bridge built in Claremont, North Carolina after Herman Haupt's improved lattice.

1970 The oldest covered bridge in the United States, built in 1812, is a Burr located near Lewisburg, Pennsylvania.

1971 Nearly 300 Burr-arch trusses are still standing with 178 located in Pennsylvania, more than any other state.

1980 Of 893 historic covered bridges remaining in the United States, 231 are in Pennsylvania and 6 in North Carolina.

1989 Of the one remaining covered bridge in Northampton County and the six remaining covered bridges in Lehigh County near Bethlehem. PA, all are Burr-arch trusses.

Bishop Louts Werwag (1770-1843), who was born in Germany, and Timothy Palmer of Newburyport, Massachusetts built many great timber bridges. Their bridges varied in design and construction, each being designed for a specific span, utilizing only one, of a kind trussed arch. Because of their complexity, a pedestrian bridge built after the designs of Werwag and Palmer were judged not practical for this project. For these reasons, it was decided that the Burr-arch truss was the logical choice for a covered bridge to be built at Old Salem.

This is the bridge that Moravian settlers would have built during the first half of the 19th century as their community matured. During this period many Burr-arch trussed bridges were built in Northampton and Lehigh Counties in Pennsylvania near the Moravian settlement of Bethlehem. In 1970, nearly 300 Burr-arch trusses were still standing with 17 located in Pennsylvania. Of the seen surviving covered bridges in Lehigh and Northampton counties, all are Burrr-arch trusses (3). Although a Town lattice bridge was proposed to be constructed on the plank road near Old Salem, this type of bridge was not desirable as a pedestrian bridge because it does not provide sufficient open space between chords to produce an open appearance. The Town lattice is distinctive in its use of simple sizes of lumber, and the small amount of framing. The resulting simplicity of framing requires less skilled labor but produces a close tunnel-like structure which has to overshoot its supports in order to distribute its end reactions. The Burr-arch truss requires more skill in timber framing, similar to the large Dutch barns found in many German communities.