Norfolk Southern s Online Rebuild of the Coosa River Bridge

Transcription

1 Norfolk Southern s Online Rebuild of the Coosa River Bridge Author: Harold Hal E. Lewis, PE, Vice President, HDR Engineering, Inc. 200 West Forsyth Street, Suite 800, Jacksonville, Florida Telephone: Facsimile: AREMA 2002 ANNUAL CONFERENCE September 24, 2002 at 11:00 a.m.

2 ABSTRACT Bridges built in the early 1900 s frequently require complete replacement based on a combination of factors including new loading considerations, deteriorating material conditions, and functional obsolescence. The existing steel trusses on the Coosa River Bridge had reached the end of their useful life based on fatigue and material condition. However, the stone piers were still in good condition and could last for many years to come. In an effort to keep project costs down to a minimum and take advantage of the good pier conditions, the rebuild of the Coosa River Bridge adopted a rehabilitation strategy for the substructure and a replacement strategy for the superstructure. This paper outlines the key design features for the successful rehabilitation of the substructure and rolling in of new truss spans. The project s performance exhibits how a complex project can be executed on-time, on-budget, without change-orders. Introduction Norfolk Southern Railway (NS) crosses the Coosa River in Childersburg, Alabama, at Milepost P , providing a critical strategic link between Columbus, Georgia, and Birmingham, Alabama, that serves as an alternate route and relief valve for rail congestion in Atlanta. Built in 1906, the existing Coosa River Bridge was comprised of an approach trestle and three 150-foot and one 200-foot Pratt truss spans supported on narrow stone piers founded on rock. The existing bridge had several fatigue-related problems and operated under a permanent slow order. The line routinely carries a low average tonnage, but when congestion in Atlanta develops, the line s importance increases dramatically. The flexibility the line offers was deemed very important to NS and necessitated the bridge s rebuild after NS determined in June 1999 that the bridge was nearing the end of its serviceable life and full or partial replacement would be required. Minimized line outages were critical to Norfolk Southern In September 1999, NS hired HDR to design and assemble construction documents for the bridge rebuild. Due to cost and schedule considerations, rehabilitation of the existing bridge was selected over full replacement. Additionally, any line outages for the rebuild would have to be minimized and the rehabilitated bridge would have to include greater track clearances. The rebuilt bridge

3 was planned to be in service within 18 months, including design and construction efforts. Due to operational requirements, any line outages for the rebuild could not exceed 24 hours, although three of the four required track outages were actually less than 12-hours. The Contractor elected to use a time-proven and economical method for an on-line replacement consisting of a controlled demolition of the existing truss spans followed by roll-in of the fully erected replacement truss spans from adjacent falsework. Special post-tensioning and doweling integrated the new cap with the existing pier cap and shaft. Rehabilitated Piers Of the original bridge structure, only the existing abutments, piers, pier caps and foundations were retained for use in the rehabilitated structure. The requirement of greater track clearances necessitated the use of wider trusses, and consequently, wider caps. Because the existing piers and pier caps were determined to be in good structural condition, they were incorporated into the rehabilitation. Although incorporating the existing piers and abutments into the rehabilitation would save time and money over a complete reconstruction, widening the caps of these substructures would present some unique challenges for their construction while maintaining rail traffic. The existing caps were encapsulated inside new, wider caps to accommodate the wider trusses. The new concrete pier caps were constructed while the bridge was still in service with trains going overhead. New truss bearings were supported outside the limits of the existing caps, which meant that the new trusses would be entirely supported on the new cap areas. The bond between existing stone substructures and the new concrete would be critical to transfer the large shear forces created by the truss loads. A combination of reinforcing dowels and post-tensioning bars were selected to create a positive bond and moment transfer between the existing stone and new concrete caps.

4 The wider trusses also required extensions to the abutment backwalls and wingwalls. This was accomplished by using a short jump span to allow excavation and construction at the abutments. New Bolted Warren Trusses Early in the design phase, NS decided to use bolted construction for the new truss spans in lieu of welded construction. Bolted steel construction inherently has better fatigue performance, which increases the service life of the new trusses. However, the bolted construction generally costs a little more and takes longer to fabricate. The four bolted trusses used 2,800,000 pounds of A709 Grade 50W (unpainted weathering steel) and required 145,400-7/8 inch diameter high-strength bolts. Computerized numeric drilling yielded high accuracy. The new Warren Truss replacement spans were fabricated using computerized numeric drilling. In addition, truss spans generally require shop assembly prior to shipment to ensure proper fit up prior to arrival in the field. In this case, the first truss span was shop assembled and the resulting assembled truss confirmed the accuracy of the fabrication process. The remaining three truss spans utilized a virtual shop assembly by assembling key components to confirm fit up while the rest were modeled by computer. This process coupled with close coordination from the steel detailer, fabricator, contractor and engineer yielded highly accurate bolted truss spans that did not require any field reaming.

5 Controlled Demolition/Roll-In New trusses were erected on adjacent falsework and rolled into place. procedure required procurement of special permits for the demolition. The Contractor Scott Bridge Company had the construction option to either float-out/floatin or roll-out/roll-in the truss spans. The contractor selected a hybrid technique utilizing controlled demolition and a roll-in procedure based on the site conditions and its preference. The controlled demolition/ roll-in procedure for each of the four truss spans was conducted without delay or additional track time. The river depth was a concern for the demolition, because it did not provide a lot of extra freeboard for the new trusses to roll over the demolished truss spans lying in the river. The Additionally, the Contractor provided extra safety in their falsework design by securing additional temporary falsework clearances prior to detonation of explosives. The trusses were actually rolled to the outside of the falsework to provide additional blast clearances (25- foot clear versus 5-foot normal clear). The contractor still shielded the new trusses during demolition procedures, but this unique approach provided an extra factor of safety for the protection to this important route. Each existing truss span was demolished using controlled explosive procedures. CONCLUSION With a tight schedule, clear communication was the key to this successful project. As a result of the focused team effort, the online rebuild of the Coosa River Bridge P was completed on budget, on schedule and with zero change orders/claims throughout design and construction. Norfolk Southern can now operate the line at posted speed and preserve the line s relief valve capability.

6 ACKNOWLEDGEMENTS Technical Assistance and review by Willie Benton, NS Corp, Atlanta, GA, and Chuck Davis, Scott Bridge Company, Opelika, Alabama.