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sábado, 5 de março de 2011

ACTION: Evaluation of Fendering Systems - Attachment

Summary Of Information From NTSB/SIR/98/02


(Adopted May 5, 1998)
[Full report is available at as a pdf file (SIR-98/02)]

Abstract: The 560-foot-long Liberian tankship Julie N, carrying a cargo of heating oil, collided with the south bascule pier of the Portland-South Portland (Million Dollar) Bridge (see photo) in Portland, Maine, about 11:05 on September 27, 1996. There were no injuries, but the collision resulted in a 33-foot-long hole in the vessel's hull beneath the waterline. About 4,000 barrels of oil spilled into the harbor. The vessel sustained about $660,000 in damage, and the cost for cleanup of the oil was approximately $43 million. Repairs to the Million Dollar Bridge were about $232,000. Because of the continuing problems encountered in conducting postaccident testing for alcohol and drugs, the full report contains additional sections devoted to the discussion and analysis of postaccident testing.

NTSB/SIR/98/02 Photo: Million Dollar Bridge

Bridge Damage: The Maine Department of Transportation (MDOT) conducted a postaccident condition survey on September 28, 1996, to ascertain the damage to the bridge. (See sketch next page) The survey indicated that pier 18 (South Portland side) had temporarily shifted over 4 inches horizontally and came to rest with a permanent horizontal set of 1 ¾ inches. The pier had also apparently moved upstream by a distance of 1 ¼ inches. The MDOT also inspected the underwater portions of the pier 18 channel face and initially found no evidence of distress or disturbance of the marine growth covering the masonry. Further underwater surveying on September 30, 1996, between piers 18 and 19 found "definite scraping and a section loss of concrete on the shaft [lower pier] below waterline exposing No. 9 [1 1/8 inch in diameter] reinforcing bar in outer corner [northeast corner] approximately 6 inches below stone course." The spalled (chipped) concrete measured about 6 inches by 6 inches. (Superimposing of the frame of the vessel revealed that this corner of the pier had contacted the vessel, resulting in the approximately 33-foot-long tear in the vessel's underwater hull. In addition, a section of granite pier cap was broken off the easterly end, channel side, of pier 18.)

NTSB/SIR/98/02 Schematic: Million Dollar Bridge

Damage was also sustained by the superstructure. On pier 18, the live-load shoe was displaced several feet, along with the live-load girder and the pedestal-shaped shoe underneath the girder, which had rested on the concrete pedestal on the pier. (The live-load girder frames back into the transverse floor beam at the next panel point on the outside truss. The girder is stabilized by a brace attached to the end point of the truss. The end of the truss also bears on the concrete pedestal). In addition, the truss experienced movement of 1 to 2 inches, and the center lock of the leafs at the channel centerline was misaligned about 3 inches. The MDOT hired the contractor building the new bridge to make the repairs, and the bridge was functioning again on September 29, 1996. The cost of repairs to restore the bridge to normal operation was approximately $32,000. In October 1996, the MDOT replaced the damaged wooden fender system at pier 18 with a steel structure that provided extra shielding for the corner of the pier that the vessel's hull contacted to create the long underwater opening. The cost of the new fender system was about $200,000, for a total cost of $232,000.

Bridge Fender System (Past and Present and Repairs): The original fender system protecting piers 18 and 19 consisted of wooden timber construction and afforded 100 feet of horizontal clearance. The original fendering system consisted of vertical oak buffer piles, about 43 feet long, placed from just below the mean low water line to about 5 feet below the top of the pier, with 5- by 12-inch horizontal timbers (wales). The bottoms of the piles were wedged into 10-inch-diameter round holes formed in the concrete footing. A review of the MDOT's bridge files indicated that the fendering system has been changed many times since the initial construction. A major change occurred in 1949, following a change in channel alignment. The purpose of the 1949 alteration was to make the face of the fenders parallel to the revised channel axis. This resulted in decreasing the horizontal clearance to 97 feet 4 inches. MDOT records show that from 1949 to 1992, changes were made in the design of the structure, but the faces of the fender system remained parallel to the channel. No channel width data were given with the changes.

An underwater survey conducted by the MDOT after the Julie N accident indicated that the vertical piles along pier 18 were not in the sockets in the footing and that the fender system along the pier did not reach the bottom. Instead, the verticals of the fender system went below low tide while the bottommost wale remained above the mean low water line. It could not be determined when the full vertical timbers had been removed. According to a 1992 plan, the corner of the rectangular plinth (lower section of the pier) extends a short distance in front of the fender. (This was the corner which the vessel's hull contacted to initiate the 33-foot-long tear.) The 1992 plan shows the approach fender system flaring away from the piers; it consisted of a series of "bents," with a cluster of "dolphins" at the end of six bents. An additional four bents increased the flare. Some of these bents had been removed before the accident to provide room for a barge being used for the construction work on the new bridge. The new (temporary) steel fender system constructed soon after the accident and attached to pier 18 was designed to be stronger and stiffer than the wood fender system that had been in place at the time of the accident. The design was intended to provide better buffering of the corner that had pierced the hull of the Julie N.

Fender System for the New Bridge: The designers of the fender system protecting the new bridge elected to use an independent steel pile-supported system and rubber fenders mounted on the bridge piers. The design also included four 60-foot-diameter steel cells filled with gravel, located upstream and downstream of each bascule pier. The independent pile-supported system prevents contact with the piers for most collisions, while the rubber fender system on the piers is designed in anticipation of more serious events, in which a vessel would penetrate and likely demolish part of the pile-supported system. The pile-supported fender system consists of nine horizontal steel I beams (wales) attached to 36-inch-diameter steel piles spaced 9 feet apart along each pier face. The I beams are faced with protective timber strips. The fender system flares away from the channel alignment at an angle of 20° on the upstream and downstream side of each pier, passing directly in front of each of the 60-foot-diameter cells. The pile-supported fender at each pier face was designed to absorb the energy caused by the collision of a 25,000-deadweight-ton vessel (Julie N had a deadweight tonnage carrying capacity in long or in metric tons of 29,994 tons.) moving at 5 knots at an impact angle of 7°. The substructure and piers supporting the rubber fender system are designed to absorb the energy caused by a collision with a 50,000-deadweight-ton vessel moving at 5 knots and impacting at a 15° angle. The fenders were designed to allow for up to 13.25 feet of flare (overhang) at the bow of a vessel. Thus, the superstructure of the bridge would not be contacted unless the flare of a passing vessel exceeded 13.25 feet. Vessels can have as much as 20 feet of flare at the bow, making contact with the superstructure inevitable for such vessels.

Updated: 11/17/2006

Fonte: US Department of Transportation/Federal Highway Administration

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