Railway Track & Structures - April 2008 - (Page 43)

able settlement at each bent is much less definitive. • The advantage of allowing the deflection to be quite accurately computed makes it possible to give the centers a preliminary camber so that when the concrete is in place and the centering withdrawn, the arch ring will assume its true position. • The unique feature in the steel centering used in constructing the Tunkhannock Viaduct was an adjustable panel at the crown of the steel arch trusses. The above information on construction method is important to the proposed load rating analysis methods for the following reasons: • The alternate block method of construction tells us concrete shrinkage stresses during construction were kept to a minimum. • The arch ribs are not monolithic, but are composed of a series of concrete blocks, with a series of construction joints along the entire rib length. It is assumed that if a particular section of the rib is subjected to high bending moments, these construction joints will crack, effectively forming a hinge to redistribute the forces. • Since steel centering trusses were used, it is assumed that deflection of the centering was accounted for during construction and arches were constructed in their “true” position. Analyzing the structures using finite element computer programs provided many opportunities for deep thought, review of assumptions, re-analysis and further, iterative computational processes, until rational analysis results were obtained. Some of the modeling assumptions that were found to have significant effects on distribution of loadings and structural stresses included the following: Amtrak’s Connecticut River Bridge in the Northeast Corridor. • How is the arch spring restrained? Is the main arch best modeled as a two-pin, three-pin or fixed structure? The answer to these questions required research into the original construction method employed for the arch, as explained above, and as further explained below. • Do the spandrel walls sit atop the arch simply as dead load? For Tunkhannock Viaduct’s dead loads, the 1911 stress sheets apply the design dead load from the ballasted deck and spandrel walls to the arch ribs as concentrated loads at the base of the spandrel walls. The self weight of the arch rib is applied as a series of concentrated loads along the length of the arch rib. If the spandrel walls interact with the arch, is their connection pinned or fixed? The arch ribs were modeled as a series of continuous segments or members with fixed end supports at the skewbacks. During the initial analysis of the 180-foot spans, the “dead load plus tempera- Lucin Rail Cutoff under construction, courtesy of Salt Lake City Chamber of Commerce. Salt Lake City - September 21-23 REMSA World Rail Expo 2008 Held in conjunction with AREMA Annual Conference The largest Maintenance-of-Way exhibition in North America! Join the world’s finest display of equipment, tools, materials and technology. Meet the industry’s high-profile decision-makers from around the globe. Reserve your exhibit space now! Visit www.remsa.org or email contact@remsa.org REMSA, 417 West Broad Street, Suite 203, Falls Church, VA 22046 email: contact @remsa.org • fax: (703) 241-8589 www.rtands.com Railway Track & Structures April 2008 43 http://www.rtands.com

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Railway Track & Structures - April 2008 Contents On Track Industry Today Supplier News AREMA News NRC News TTCI R&D Railroads Seeking More Advanced Systems to Handle Materials SKOL Bounces Back From Summer Flood Landmark Railroad Bridges Need Inspection, Too Products and Literature People Calendar Sales Representatives Advertisers Index Website Directory Professional Directory Classified Advertising Chicago Perspective

Railway Track & Structures - April 2008

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