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Short-span precast bridge works around the railroad.

The Toledo Zoo was south of the railroad tracks, but the much-needed land for expansion was on the north side of the tracks. A site plan that would relocate parking to the north side of the tracks, provide a new north entrance, and somehow get visitors across the tracks held the answer. A precast arch-box bridge to carry twin tracks of the Norfolk and Southern Railway over a pedestrian entry was a crucial feature of the zoo's expansion plan.

The zoo was able to secure an easement that permitted a pedestrian underpass beneath the tracks, subject to the proviso that one rail line must remain open at all times during construction. This stipulation led to an innovative two-stage construction process using precast arch-box sections from CON/SPAN Bridge Systems (Dayton, Ohio) to support the trains above the people. Once pedestrian access to the original south portion of the zoo was established, old parking lots there could be converted to new exhibit areas.


Specifications called for the bridge to be designed for Cooper E80 railroad loading, requiring that it be able to carry as many as four simultaneous 80-kip loads on its 16-ft span. This is several times the magnitude of an HS 20-44 typical highway loading for which the precast sections are more commonly used, and the site-specific design for the precast structure made appropriate changes for the added load. While maintaining the typical arch-box shape, the designers increased the wall thickness by two in. throughout the cross section, and enhanced the internal steel reinforcement.

The 60-ft wide bridge was made up of eight precast arch-box sections, having a clear span of 16 ft and a rise of 9 ft 2 inches. Precast headwalls and wingwalls with architectural finish were installed to retain nearly three ft of ballast and sub-ballast required for the railroad bed over the bridge. The pedestrian passageway under the bridge was lit using wired electrical conduit cast into the midspan of the precast during manufacture, and joined during erection. The electrical outlet boxes were also cast into the center of each arch segment.


The work site was tight, with only 15 ft 4 in. center to center of the two tracks, leaving less than 11 ft clearance between the rails. The work was staged to maintain rail traffic during construction.

First, working between normal train movements, sheet piling was driven midway between the two tracks for a distance of some 60 ft. For stability, additional piling was driven south of the south track, which was to remain in service during Stage I of the construction. With the piling in place, a section of the north track was removed and more piling was driven at right angles to the center-line piling, protecting the location in which the bridge sections would be placed. Then excavation and foundation construction could proceed for the first stage. The contractor, Rudolph/Libbe (Walbridge, Ohio), drove H-piling and poured a concrete cap that would serve as footings to support the bridge and wingwall sections.

Use of precast sections was favored by the Norfolk and Southern Railroad because it would shorten the time that rail service would be restricted. But precast plant manufacturing also gave the railroad extra dimensions of quality control.

BridgeTek-Central Ohio (Columbus, Ohio), supplied the bridge sections that were fabricated by United Precast (Mt. Vernon, Ohio). They were delivered to the site ready for installation as soon as the foundations had cured. Actual precast erection time for each stage of the pedestrian underpass was only five hours. As soon as necessary joint sealing and waterproofing were installed, backfill and restoration of rail traffic could proceed at once since the concrete was already at its design strength. When Stage I was complete and traffic restored on the north track, a similar procedure was followed for the south track. Piling between the tracks remained in place, but an opening was cut through so that the second part of the precast could be joined directly to the first half.


The Toledo Zoo now has a well-lighted pedestrian underpass that complements its new entry plaza leading from the relocated parking lot to the zoo exhibits. Freight trains thunder by over people on their way to see the animal life. The concrete was given a plain protective coating, but the walls of the underpass have since been filled on both sides with a more decorative touch - brightly colored, near-life-size caricatures of zoo staff members, and even a few of their captive friends.

Acknowledgment: In addition to the firms mentioned in the text, others involved in the project included: engineer/architect, SSOE (Toledo); landscape architect, The Collaborative, Inc. (Toledo); and construction manager, The Lathrop Company (Toledo).

M. K. Hurd is an engineer and writer specializing in concrete building methods. She is a former editor of Concrete Construction magazine and author of Formwork for Concrete, published by the American Concrete Institute.
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Title Annotation:Toledo Zoo expansion
Author:Hurd, M.K.
Publication:Public Works
Date:Sep 1, 1999
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