Railway Track & Structures - April 2008 - (Page 42)

Bridge inspection Among the unique access issue, at Martin’s Creek Viaduct, during inspection the field crew found that the railroad right-of-way access road to the site had been severely rutted during the winter and early spring due to recent, heavy rains, such that the inspection equipment could not gain access to the bridge. The remote and mountainous terrain prohibited alternate access means. CP Railway crews were not available to respond to the situation, because of emergent conditions on other railway bridges within the territory, also caused by the recent heavy rains and severe flooding in the Susquehanna River Valley. This situation threatened costly delays to the project. The H&H inspection team leader at the site was experienced with operation of small construction equipment. Without delay, the inspector rented a small bulldozer for the day, performed the necessary grading work of the access road. All work was outside the limits of the FRA fouling envelope for the active tracks. After the minor but necessary roadway repair work was done, the inspection team accessed the site, and the inspection team leader informed the home office of the completed work. The viaducts have a height of up to 240 feet above the valley floor. The inspection plan required coordinating a variety of access equipment to effectively access portions of the structure for hands-on inspection. As a safety measure, the local fire and rescue departments were notified prior to the inspection of the work being performed, so that they might be prepared in the unlikely event of the need for emergency rescue. To access the lowest portions of the structures, a 135-foot man-lift was used where ground access permitted. To access the upper portions of the structure, an Underbridge Inspection Unit (UBIU) with a 60-foot reach was used. Rail-mounted units are available. However, where an access road is available, using road-based vehicles is vastly preferable, to avoid scheduling and other access issues with track time. Track time availability was very limited for our use at Tunkhannock and Martin’s Creek Viaducts. To access the mid-height portions of the structure, a spider basket attached to a truck-mounted crane was used. Safe climbing using harnesses was also employed, along the top and center of 42 Railway Track & Structures April 2008 the arches where the edge falling hazard was more than one body-length distant. The detailed, site-specific work inspection plan was particularly important for efficiently coordinating the use of crews and equipment at these two structures. The comprehensive inspection of these massive concrete structures was accomplished in just a few weeks time. Due to the difficulty in accessing some areas from the top of deck or from ground level (a result of structure size, site topography and obstructions), it was determined to be economically unfeasible to inspect every surface of the structure hands-on. The best use of inspection resources within the allotted schedule was to perform a complete and thorough visual inspection of the structures using reasonable access methods, and then to focus on close-up inspection of a representative span at each structure, and to extrapolate these inspection data to other spans. By following this methodology, the inspection teams focused more on the behavior of the structure and on overall rehabilitation needs. The similarities in deterioration conditions and locations throughout the structure were noted. The Tunkhannock and Martin’s Creek Viaducts are composed of Portland Cement concrete, with steel reinforcement. However, the design for the structures is not based upon modern reinforced concrete principles, but rather on unreinforced masonry design practices of the 19th Century using mass concrete in place of stone masonry. Additionally, loading conditions used for the original design differ from current AREMA recommendations. For live loads and impact loads for Tunkhannock Viaduct, the 1911 stress sheets show a non-Cooper series of locomotive wheel concentrations. The wheel concentrations were positioned on the spans to produce the maximum stresses in the arch rib for two live load cases, namely 1) live load plus impact applied to the full span length, and 2) live load plus impact applied to one-half the span length. The original stress sheets apply the live load to the arch rib as concentrated loads at the spandrel columns. Since the concentrated loads are proportional to the spacing of the spandrel columns rather than the magnitude and location of the wheel concentrations, it is apparent that an equivalent uniform live load was used as part of the original design. It appears that the original stress sheets calculated the concentrated loads at the spandrel columns as follows: Locomotive axle loads were positioned on the span for maximum stress at the arch rib. Once positioned, the concentrated loads were summed and divided by the loaded length to generate a uniform load. A distribution factor was applied to the live load. The live loads were then divided by the width of the arch rib to generate an equivalent uniform load per unit width of arch rib. The concentrated loads at the spandrel columns were then determined by calculating the reactions of the equivalent uniform live load, considering the tributary area to be one half of adjacent concrete deck span lengths. From review of the concentrated live loads tabulated in the stress sheets and working backwards using this procedure, uniform track loads were derived. Temperature loads are calculated on the 1911 stress sheets based on a temperature rise of 30 degrees F and a temperature fall of 30 degrees F. 1911 Load Combinations were: • Dead Load + Temperature. • Live Load + Impact on half the arch span + Dead Load + Temperature. • Live Load + Impact on full arch span + Dead Load + Temperature. No other loads or combinations were used in the original design. The main arch ribs of the Tunkhannock Viaduct were constructed by the alternate block or “voussoir” method. With this method, the arch rib is constructed in transverse blocks of such size that each block can be completed at one pouring, or within about a day’s work. The blocks are poured in such an order as to give a uniform deflection of the centering, and also prevent the crown of the arch from rising as the lower arch loads are placed. This method has the advantage of reducing shrinkage stresses in the arch ring to a minimum. Steel centering was employed in the construction of the Tunkhannock viaduct. From 1918 Concrete Engineers Handbook by Hool and Johnson, the use of three hinged steel centering had the following advantages: • Crown deflection using steel centers is usually much less than that obtained by employing timber falsework. • It is possible to compute the deflection of each point of a steel center with some degree of accuracy while, in the case of a wooden center, the probwww.rtands.com http://www.rtands.com

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Railway Track & Structures - April 2008

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