ТОП просматриваемых книг сайта:
America's Covered Bridges. Ronald G. Knapp
Читать онлайн.Название America's Covered Bridges
Год выпуска 0
isbn 9781462914203
Автор произведения Ronald G. Knapp
Жанр Книги о Путешествиях
Издательство Ingram
The Boston and Maine Railroad built and owned most of the covered rail bridges in New England. This one at Bennington, New Hampshire, was built in 1877, probably by David Hazelton (1832–1908), chief bridge engineer for the railroad. Until it was destroyed by fire in 1965, it was the oldest remaining covered rail bridge in the United States. (NSPCB Archives, R. S. Allen Collection)
The Rouge or Sainte-Agathe Bridge spans the Palmer River over a rocky gorge next to Parc de la chute Sainte-Agathe in Québec, Canada. Built in 1928, it is typical of the province’s Town lattice variation. (A. Chester Ong, 2012)
Typical of the covered bridges in New Brunswick, Canada, the Tynemouth Creek Bridge near St. Martins in Saint John County was built in 1927 using a standardized Howe truss. Because it is near the Bay of Fundy, the area is often shrouded in fog. (A. Chester Ong, 2012)
chapter two
THE EVOLUTION OF COVERED BRIDGE DESIGN
The Philadelphia and Baltimore Railroad’s great rail bridge crossing the Susquehanna River between Havre de Grace and Perryville, Maryland, was near completion on July 26, 1866, when a powerful storm swept down the river valley and completely destroyed the 3,195-foot 13-span crossing. The series of tall Howe trusses reinforced with massive quadruple arches carried each of the 250-foot spans on piers set in water as deep as 40 feet. Because many nearby structures were untouched, the winds—whether straight line or a tornado—apparently lifted the bridge at least three feet before dropping it into the water. Immediately, work crews began securing the remains in preparation for rebuilding the spans in part with salvaged timber. (Library of Congress Prints and Photographs Division, Washington, DC, 20540/Frank Leslie’s Illustrated Newspaper, December 22, 1866, p. 217)
THE PLANNING, PREPARATION AND DESIGN OF A COVERED BRIDGE
In Canada, just as in the United States, old covered bridges today are being rebuilt—and even new bridges being constructed—using designs and methods that harken back to those employed in the nineteenth century. However, sophisticated engineering and methods of assembly now have reduced the hardships once endured by carpenters and masons. Québec’s Powerscourt Bridge had deteriorated badly by 2009 and was closed to traffic when it was decided to refurbish the entire structure. The photo shows the renewed trusses for one span back on the abutment and pier. (Gérald Arbour, 2009)
During the last years of the colonial period and into the first decades of the nineteenth century—roughly from 1760 to 1820—the relatively few men with knowledge of timber framing were called upon to meet unprecedented engineering challenges. None had formal training in civil engineering generally nor bridge construction in particular, and the science of stress analysis in bridge design was still decades away. What they had was common sense, experience, and a bold, creative streak. To that we might add generous doses of bravery and what some might even call “chutzpah.” A few, like Thomas Pope, emphasized the “bold, creative streak” over common sense—he proposed to span the East River with a monumental 1,800-foot wooden arch that would join the cities of New York (now Manhattan) and Brooklyn and even mentioned one over the Hudson River—but most men recognized both the limitations of the materials and their own abilities.
The previous chapter documented what is known of the early history of American timber bridge building, introducing many of the key players. The present chapter explores the process of planning the construction of covered timber truss bridges, the builders, and their many truss designs.
BUILDING A COVERED BRIDGE INVOLVES AT LEAST THE FOLLOWING STEPS.
1 Proposing and financing a bridge
2 Engaging builders for both abutments/piers and superstructure
3 Designing all aspects of the structure: abutments, piers, trusses, deck system, lateral bracing, roofing, siding, and roadway
4 Steps in the construction of the foundations and superstructure
5 Maintenance
6 Obsolescence and decline: the changing fate of covered bridges
Proposing and Financing a Bridge
Essentially, there were two ways to get a bridge built, either by private initiative or by a government body. Only gradually during the nineteenth century did governments take on the responsibility for building and maintaining bridges. Before that became common, bridge building was mostly left to private businessmen hoping not just to solve a transportation problem but also to turn a profit from tolls. Although we know little about bridge building before the advent of companies, we find pieces of evidence here and there. Frances Manwaring Caulkins’ History of Norwich, Connecticut (1874) provides an unusual amount of information on early bridge building: “The earlier bridges were built and kept in order by the inhabitants as highway work. In April, 1717, a petition was presented to the General Assembly ‘for assistance in building a cart bridge over Showtucket [Shetucket] at the falls’ . . . it is probably that this first bridge over the Shetucket was built in the usual way, by a general turn-out of the inhabitants” (p. 343).
Later, she refers to another incident—to be explored more fully further on because it involved a bridge building accident—and writes: “A large party of the inhabitants had assembled to assist in raising the bridge, which was 20 feet high and about 250 feet in length” (p. 343). If these practices were widespread, this pattern probably prevailed throughout the colonies, reminding us of today’s tradition of “barn raisings” among the Amish. Such bridges had to have been simple if ordinary citizens could construct them.
As the century progressed and the growing towns and cities required ever more substantial bridges to allow the transport of goods and mail, and travel for people over the colonies’ major rivers, it was necessary to form companies that in turn hired reputable builders, often selected from several competing proposals. After independence, virtually all of the substantial bridges spanning, for example, the Hudson, Merrimack, Schuylkill, Delaware, Mohawk, and Susquehanna rivers, required the formation of companies and approval by the state legislature. Similarly, the legislatures had to approve the formation of companies for canals, turn-pikes, and other public improvements. The fact that a bridge company was authorized, however, did not mean the bridge was actually built. The bills included toll rates for passage. Those for the Permanent Bridge in Philadelphia ranged from one cent per foot passenger, riderless horse, or head of cattle to twenty cents per four-wheeled pleasure carriage drawn by four horses to $1.35 for a “carriage of burden” weighing the maximum load allowed of six tons.
Philadelphia’s other great bridge, the Lancaster-Schuylkill Bridge, began with an Act of Incorporation approved by the legislature and governor on March 28, 1811, the company beginning with 800 shares at $50 each, for a total of $40,000. By June, the company had elected officers and began advertising for proposals. Eventually, they contracted with Lewis Wernwag to build a single-span bridge later known as “The Colossus.” All of Theodore Burr’s bridges were financed and built by companies, though sometimes there was a considerable time delay between formation of the