Hydraulic and earth engineering – Marine structure or fabrication thereof – Dock
Reexamination Certificate
2000-12-05
2002-09-17
Bagnell, David (Department: 3673)
Hydraulic and earth engineering
Marine structure or fabrication thereof
Dock
C405S218000, C405S220000, C114S266000, C114S267000
Reexamination Certificate
active
06450737
ABSTRACT:
FIELD OF THE INVENTION
This invention relates generally to concrete floating docks and methods for making the same.
BACKGROUND OF THE INVENTION
Floating dock structures are widely used in marinas and boat harbors as a means for docking and mooring water craft in tidal waters or in other bodies of water subject to changing water level. Typically, floating docks structures are constructed by interconnecting individual or modular float sections. A common arrangement for a floating dock includes a central walkway comprised of float sections connected end-to-end and a series of dock sections projecting perpendicularly from the walkway so as to form a series of boat slips.
Individual float sections are commonly formed by completely or partially encasing a buoyant flotation core, such as polystyrene, in a concrete shell. Over time, traditional concrete float sections have shown their inability to consistently resist the environmental conditions and dynamic forces acting upon them without damage. Traditional concrete floats have also shown their inability to consistently float with adequate freeboard without listing or relative twisting between dock sections. Maximizing freeboard is necessary to ensure that the dock components comprising material susceptible to corrosion and deterioration, such as steel or wood, generally reside above the splash zone of the dock.
Generally, the design of concrete docks is influenced by three factors: (1) the limitations of lightweight aggregate-based concrete and thin concrete sections; (2) the limitations of the strength, flexibility and compatibility of the connections linking dock sections; and (3) the complexity of manufacturing dock sections. The shortcomings of traditional concrete docks are further described with respect to these factors as follows.
Limitations of Lightweight Concrete and Thin Concrete Sections
Prior art concrete floats utilize thin concrete sections or lightweight aggregate-based concrete to minimize dead weight and thereby increase freeboard. Some float systems utilize both thin sections and lightweight concrete simultaneously.
Lightweight aggregate-based concrete, however, is generally not a preferred material for float sections because of its weak structural properties and porous nature. Thin concrete wall sections are also deficient in that they do not provide sufficient coverage for the embedded steel reinforcing which is susceptible to corrosion when placed in seawater. Corrosion of the steel reinforcing reduces the overall strength of the float and expands and spalls the concrete, further weakening the float and exposing the reinforcing to the elements, which in turn causes additional corrosion.
Floats made from lightweight concrete or with thin wall concrete sections are also vulnerable to freeze-thaw deterioration. This phenomenon occurs when water enters voids in the concrete via superficial cracks and expands upon freezing, thereby enlarging the cracks and spalling the concrete. Enlarged cracks and spalling allow additional water to penetrate the concrete, leading to yet larger cracks and more spalling. In a relatively short time, concrete surfaces can become so damaged by freeze-thaw deterioration that the entire float module must be replaced. The effects of freeze-thaw deterioration are even more destructive in floats made from lightweight concrete because the porous nature of such floats facilitates the absorption of water.
Since it is desirable to cast a concrete float that is both lightweight (to increase freeboard) and durable, it is necessary to increase the thickness of the concrete shell where it is most needed while eliminating other concrete sections to compensate for the added weight. For example, the durability of the protective concrete shell can be improved without increasing weight by eliminating the concrete bottom of the float, which is not required for structural support, so that the thickness of the side walls and deck may be increased. In such a configuration, the exposed portion of the flotation core is typically covered with a protective coating such as polyurethane.
Thicker concrete sections can also be achieved by eliminating redundant end walls in a series of dock sections coupled end-to-end. Conventional floats are typically cast in lengths of 8 to 12 feet. In contrast, monolithic concrete floats can be cast up to 60 feet in length. By increasing the overall length of the individual dock sections, the number of dock sections and thereby the number of end walls adding weight to the dock assembly is reduced. Removing weight in the form of end walls therefore allows for thicker concrete walls and decks.
Limitations of the Strength, Flexibility and Compatibility of Dock Module Connectors
Concrete float sections are customarily connected to each other by elongated, rigid timber members, or wales, that extend along the upper side edges of the floats. The wales are typically fastened to the floats by tie rods extending transversely through the float and projecting through the wales. The ends of the rods are threaded to receive conventional nuts and washers which are torqued against the wales to compress the wales against the float. Alternatively, U.S. Pat. No. 3,967,569 to Shorter, Jr. (Shorter) discloses float units having horizontally projecting structural flanges that extend along the length of each side wall. The float units are interconnected end-to-end with wooden wales placed on the bottom and top surfaces of the structural flanges and bolted vertically therethrough. Each wale is underlain with a thin steel strap for additional structural strength.
When set in motion by wind or wave action, the heavy mass of traditional concrete floats causes flexing of the wooden wales which in turn, transmits substantial forces to the bolts or tie rods that fasten the wales to the floats. This leads to excessive wear or fatigue of the metal fasteners and as a result, adjacent float modules will eventually come in contact with each other and gradually beat themselves apart. It is difficult to keep the tie rods or bolts sufficiently tight so as to prevent movement of the wales relative to the float units because of the moisture incompatibilities of the wood-steel-concrete connection materials. Once bolt slip or compression of the wood occurs, the holes in the wales enlarge and allow for additional movement of float units, causing failure of the metal fasteners and contact between the concrete surfaces of adjacent floats, which in turn, causes wear and breakage of those surfaces. Further, bolt slip and wood compression causes the fasteners to work loose in the concrete, occasionally pulling out of the float.
Dock attachments, such as cleats and utility stands, are usually bolted directly to the wales because the concrete sections are too thin to accept bolts and are prone to cracking. Attachments subjected to substantial forces, such as mooring cleats, cannot develop their rated capacity because attachment strength is limited by the relatively thin timber members. In addition, to prevent damage to vessels using the dock, connections made with tie rods must not protrude past the mooring face of the float and therefore must be recessed within the side of the wale, which further weakens the connection.
Timber wales are also not capable of resisting the torsional twisting or listing of the assembled float units. The relative twisting between float units is partly due to the inconsistent structural properties of the different materials comprising the float connections and the high water absorption characteristics of the wooden wales. Over time, as the wales continue to creep and the float units become unevenly saturated, the twisting and listing of the float system becomes worse. Twisting and listing of floats is unsightly and is a safety concern in wet or icy conditions.
Flexible joints or hinges are typically used to couple longer, monolithic floats because the flexure forces, concentrated at the joints between float units, would quickly overpower a traditional timber wale-style, rigid connection. One such hi
LaFave Kelly D.
Rytand David H.
Bagnell David
Klarquist & Sparkman, LLP
Mitchell Katherine
Rytand David H.
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