Below is the uncorrected machine-read text of this chapter, intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text of each book. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.
10 PROTECTIVESYSTEMS Bridge Protection Protection to bridge piers against ship collision can take either of two forms. One is provision of independent structures that will encounter the ship and deflect it or stop it, while absorbing the energy of impact before the ship hits the pier. This category includes embankments and berms (protective islands), moored cable arrays, independent structural barriers, dolphins and protective cells, and moored pontoons. The second category is that of deflecting and energy-absorbing devices affixed to the pier itself, designed to deflect the ship from exerting its full impact force or to absorb the energy of impact through deformation, reducing the maximum force exerted on the pier to acceptable levels. This category includes fenders of timber, rubber, and steel; sliding blocks of large mass; and hydraulic-type or mechan- ical fenders (referred to collectively as "fenders" in this report). A detailed review can be found in Saul and Svensson (1981, 1982a). Choice of Systems Decisions about which of these many types to employ depend on the available space, bathymetry, soils, types and sizes of ships, type of bridge piers, seismicity, ice, fog, tides, and many other factors. There has been some confusion whether primary consideration should be given to protecting both pier and vessel from damage during grazing- typ~ i~pacts in normal service or to protecting the bridge pier against the extraordinary event of direct collision from a high-energy vessel. Fenders are obviously suited to the first approach, whereas separate structures are best suited to the second. Consideration must be given to the damage to the ship in the extreme case: Will the ship sink in the channel and block it? Will oil tanks rupture and cause heavy pollution? In deciding the type of protective structure, the energy-absorbing mechanisms of the ship may also be considered: crushing, buckling, etc., of ship plates and bow. 63
64 Independent Structures Protective Islands These may consist of sand and gravel embankments, suitably armored (riprapped) against erosion by waves, currents, or propell~x scour. While they need not extend above water for protective purposes, ship pilots will prefer that they be visible. Moreover, consideration must be given to the very shallow drafts at the bow of large ships riding in ballast on a high tide. Protective embankments or islands exert a load on seafloor soils, leading to potential long-term settlement. The effect of this "downdrag" on the bridge pier and its piling must be considered. The colliding ship tends to plow into the embankment and ride up on it. Resistance comes from the passive pressure of the soil against the ship's bow and sides, from friction of the bottom against the embankment, and from the lifting of the ship as it rides up on the embankment. This system of protection was proposed for the main piers of the Great Belt Bridge in Denmark. Moored Cable Arrays A number of systems have been developed using cable arrays, supported by buoys at intervals and moored to anchors in the bottom. The buoys counter large differences in water level. The concept is that the bulbous bow of a tanker will engage the cable and be brought to a gradual stop by the stretch in the cable system and the dragging of the anchors. Such systems are being installed to protect the bridge piers of the Parana River bridges in Argentina. The great uncertainty lies in whether the bow will engage. Many ships still have a clipper bow (no bulb) and even bulbous bows encoun- tering the cable at an angle may fail to engage. Ships may ride over the cables with little resistance. Other potential problems are that the cable may capsize a smaller vessel and that a snapped cable is a lethal weapon. Independent Structural Barriers Pile-supported platforms, similar to concrete-wharf structures, protect the main piers of the Carquinez Bridge in California. Horizontally trussed frames of timber, steel, and concrete that are supported by vertical and batter piles have been used extensively: these resemble the nose dolphins of ferry slips a_nd have been designed to permit extensive deformation and local failure- as the ship penetrates successive resisting frames. The concept is to mobilize an extensive system of piles and structure to resist an extreme concentrated force applied at any point. The independent structural barriers tend to become rather large and expensive structures in themselves, with relatively high costs for repair after damage. The potential for fire during collision or from an unrelated cause must be considered. A fire in the timber protectors for the Richmond-San Rafael Bridge almost caused the loss of the steel span. Dolphins and Protective Cells Another type of independent structural protective device is that of dolphins or cells, placed so as to inter- cept a potentially colliding ship. These may be formed of steel sheet
65 piles, filled with sand and capped with concrete, and given marginal protection against low-energy collisions by timber fendering. Sheet pile cells, when ruptured by ship collisions, tend to rip and burst completely but have been successful in stopping a 35,000 owr tanker in Philadelphia (Ostenfeld, 1965) and a 45,000 DWT tanker at the Outerbridge Crossing, New York (Hahn and Rama, 1982). A more favorable form of cell is the concrete caisson, with multi- directional reinforcing, designed to experience punching shear locally without full failure. Steel cylinder shells can be suitably strength- ened to prevent ripping and progressive collapse. Such dolphins (cells) yield to the impact force by sliding and tilting. In softer soils, the cells may have to be pile-supported and may resemble smaller bridge piers. The dolphins proposed for the Zarate-Brazo Largo bridges in Argentina (Figure 10) consist of concrete caissons on piles with projecting fender-protected concrete platforms. Moored Pontoons A number of cleverly designed floating barriers have been developed that fail progressively, thereby engaging adjoining units. The force is ultimately transferred to the seafloor by cables and anchors. Moored pontoons may be designed as box beams and arranged in sawtooth fashion to deflect the bow of a vessel, or may be arranged as successive beams. These systems must be designed to resist tidal currents and storm waves and still remain effective. They require maintenance and after each incident, of course, repair. Moored pontoon systems may interfere with the operation of small vessels. They appear to have serious limitations and questionable applicability to the protection of major bridge piers against very large vessels. Fenders A detailed review of fender systems can be found in Derucher and Heins (1979). Fenders are designed to absorb the energy of small and moderate collisions and to reduce the force transmitted to the pier. Ideally, the reaction will be largely nonelastic (to dissipate energy rather than store it); otherwise, a ship hitting a pier on one side may be thrown with greater impact against the opposite pier. Timber fenders (using both piles and timbers) are designed with multiple elements arranged to bend and deflect, and ultimately to fail in horizontal shear and crush. Rubber fender units are designed to deform by bending, shearing, or buckling, thus maintaining a relatively constant force through a substantial deformation. Steel fender units are designed to fold like an accordion when their elastic strength is exceeded, or to fail in controlled buckling. Massive concrete blocks may be supported on the basic concrete pier to slide under heavy impact, with the resisting force provided by friction. They may have rubber or other buffers to cushion their final impact on the pier. Large concrete masses have been hung from pile supports in such a way as to swing upward in collision.
66 .e·igure 10 Dolphins propo1:>ed ~or-.za.rate-Brazo:Largo River bridges, * Ai".gentlna lrnb" fenders Q2Sâ¢Q2S ⢠01s1once 026 l 91)8TOPof P.lo\lorrn I- - Stfflplote i. 20mm :/! HAT⢠4J.' ~;a ,[. '1 l "lS.L ⢠uo l I f , LAT ·0.50 '' Timber fender g L _;aoo ~) ' ~- Plan ..- Scour protection · - Filter layer Presentriver bed ·JIDQ._ Section A-A *SOURCE: R. Saul and H. Svensson (1982), "Means of Reducing the Conse- q.uences of Ship Collisions with Bridges and Offshore Structures," IABSE Colloquium, Introductory Report, p. 175.
67 Hydraulic dampers have been used on offshore terminals to accept overloads with controlled force and relatively large deformation. They are expensive and difficult to repair1 however, the travel (allowable deformation) at high force may be larger than that available with other , systems. ~ Integral fender systems are generally well designed to resist impacts of the small and moderate collisions typical of normal opera- tions. Because their total travel is limited to a few feet, their ability to absorb large amounts of energy, as, for example, in the direct collision of a major ship, is severely limited.
68 Unprotected pier (in 8 ft of water) of new highway bridge over Houston Ship Channel Photograph: John Herbich, Texas A&MUniversity
