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The rapid expansion of the American railroads in the past century was made possible by the extensive use of timber in bridge and track construction. The fact that even today about 40 per cent of our total bridge mileage is of timber is evidence that this type of construction is safe, economical, and practical. If all the timber railroad bridges were placed end to end, they would extend for about 1800 miles, which is about the same as the mileage of steel bridges and twice that of concrete bridges. Timber bridges are generally renewed in kind; one large western railroad is renewing timber trestles at the rate of 7000 ft per year. A typical timber trestle consists essentially of piles, caps, stringers, and ties. Six piles are often used in each bent with the outside piles battered to provide additional lateral stability. The bents may be spaced at 14 ft and have 14- by 14-in. by 14-ft caps to distribute the live load to the piles. The ties carrying the rails rest on two chords, each chord made up of five 8- by 16-in. by 28-ft timber stringers. The stringers are interlaced so that at least two of the stringers are continuous over one bent. The structure is braced laterally and longitudinally with 3- by 8-in. timbers bolted to the piles. On high structures, additional horizontal timbers, called sash bracing, are used. There are many variations. In many cases a solid floor is placed on the stringers to carry a ballasted deck. Many trestles on secondary lines have four- or even three-ply chords supported on four-pile bents. Another interesting and economical timber structure, shown in Fig. 1, consists of 60-ft laminated timber girders, carries heavy logging trains of the Weyerhaeuser Timber Co., and is located near Longview, Wash. This is the longest timber girder span in this country carrying railroad loading. Incidentally, the railroads have had several experimental timber trestles in service for 17 years where the stringers and caps are of laminated timbers. An inspection of these trestles has shown that the laminated timbers were in excellent condition with no indication of checking.
Ruble, E. J.
Executive research engineer, Association of American Railroads, Chicago, Ill.
Drew, F. P.
Research engineer, Association of American Railroads, Chicago, Ill.