Railway Track & Structures - April 2008 - (Page 40)

Bridge inspection Normal rating per AREMA Chapter 15 is based upon 0.55 Fy, and this is sufficient to explain the less-than-E-60 capacity of the low-rating members. Increasing the allowable stress for normal rating to an amount very little above 0.55 Fy will increase the rating capacity from E-57 to above E-60. For pin-connected members, allowable tension stress is K = 0.45 Fy, per AREMA Section 15-1.4 2004 update, Table 15-1-11. In Table 3-11 of the 1994 update, no special provision for pin-connected truss members was indicated. The 2004 commentary for this section references a single, 1939 ASCE transaction paper as the source for the moreconservative allowable stress applied to pin-connected members. We obtained a copy of the paper and found that forged eye bars are specifically excluded from the investigation, as stated in the first paragraph of the paper: “The present investigation is concerned with pin-connected plates that do not have reduced width in the body, as is the case of the forged eye-bar…” Therefore, basic allowable stress for the pin-connected eye-bars at Little Hell Gate were assumed the same as for other tension members, or 0.55Fy. CPR’s Tunkhannock Viaduct in Pennsylvania. calculated rating capacity of slightly less than the basic acceptance criteria of Cooper E-60, between E-57 and E-59. We investigated why these members would rate less than what the bridge was originally designed for, namely Cooper E60 live load. This was especially puzzling since the original design drawings specifically stated that two full tracks of load are applied per truss, as compared to a computed maximum 1.6875 tracks tributary to the trusses for current conditions and design standards. Also, the original design would have likely included higher steam impact. So why would the bridge rate less than E-60, especially with lighter, current design loadings? In researching the possible reason for these lower than anticipated initial ratings, the answer was found in an ASCE transaction paper that had been written by designer Othmar Ammann.1 Ammann stated that unit stresses used for steel materials were taken as “from five-eighths to three-fourths of the minimum elastic limit,” i.e., 0.625 Fy to 0.75 Fy. Tunkhannock and Martin’s Creek Viaducts The Tunkhannock Viaduct is a 12-span reinforced concrete arch bridge that, at the time of its completion in 1915, was the largest concrete bridge in the world. The structure was designed by Abraham Burton Cohen and constructed by the Delaware, Lackawanna and Western Railroad as part of the Hallstead Cutoff. Tunkhannock (also known as the Nicholson Viaduct) was constructed to eliminate the difficult grade changes that the railroad encountered along its original path on what is now Pennsylvania Route 11. The construction of the bridge also shortened the travel distance of trains between New Milford and Clarks Summit by 3.5 miles. It was originally designed to carry two tracks, but today carries only one along its eastern side that services both northbound and southbound freight trains from Binghamton, N.Y., to Scranton, Pa. The bridge stretches 2,375 feet across the rural valley over which it is sited, and soars 240 feet above the east branch of Tunkhannock Creek at its highest point. The Tunkhannock Viaduct is a designated historic civil engineering landmark of the American Society of Civil Engineers. The structure has 10 visible spans, each measuring 180 feet in length, and is flanked by two abutment spans that were buried during construction. Each of the piers is founded on bedrock, which at the time of construction was up to 138 feet below grade. Nearly half of the structure’s mass is buried underground. There are reportedly an average of 10 freight trains per day carried on the line at present. The closed western track line area now serves as an access road along the full length of the bridge. The track is supported on timber ties that rest in a ballast and gravel bed. The ballast is supported by a reinforced concrete bridge deck, which was constructed with an applied waterproofing system. Three-foot-thick reinforced concrete parapets extend the length of the bridge along each fascia. At the visible spans, the concrete deck was poured atop reinforced concrete spandrel arches, which are then supported by reinforced concrete spandrel walls. In each span, there are 12 spandrel walls, 10 of which are supported on the arch ribs themselves. The two end walls comprise a portion of the tower at each support pier. The spandrel walls apportion the span into 11 equal-length panels. The main 40 Railway Track & Structures April 2008 www.rtands.com http://www.rtands.com

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Railway Track & Structures - April 2008

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