Aluminium - economical installation, enduring strength

Aluminum - economical installation, enduring strength

Marina World Sept/Oct 2003

Aluminum has a wealth of advantages in pontoon and breakwater applications. Dennis Tuel * discusses the benefits.

Aluminum conjures images of soda cans, crushable in the grip of a six-year-old - a mental picture that, when applied to aluminum docks, translates to piles of soft metal mangled by the wake of a canoe. But his thinking misguided, akin to assuming that a house of cork is as sturdy as a house of oak since, after all, wood is wood. Just as trees produce diverse qualities of wood, alloys (combinations of elements) produce variants of aluminum. When a waterfront installation calls for affordable custom design and enduring strength, aluminum is an attractive alternative.

One of Earth's most abundant metallic elements, aluminum is economical to produce and easily customized. While one aluminum alloy forms thin, recyclable beverage containers, another combination of elements produces a lightweight metal strong enough for use in supersonic aircraft. Since 1852, when the first all-aluminum seagoing vessel cast off from the coast of France, waterfront equipment manufacturers have generated an evergrowing number of marine applications for this corrosive resistant metal. Today, aluminum marine alloys are used throughout the marine industry, from small outboard motors and fishing boats to multi-ton passenger ships and ferries. Give its years of service on the water, aluminum deserves a place on the shoreline as well.

Aluminum docks and wave attenuators benefit from a number of inherent qualities. Naturally rustproof, aluminum looks better, longer than other materials. Its ease of fabrication is well-suited to custom construction. As it is light in weight, it is easy to remove and install where docks are seasonal and, when used properly, it woks well in brackish coastal locations. Unlike steel, which decays from rust if untreated, aluminum gains strength from oxidation. When aluminum oxidizes, it forms a film even more resistant to corrosion than the aluminum itself. Offering further protection, marine specific alloys, such as ShoreMaster's 6061-T6 and 6005-T5 alloys, enhance the metal's strength and natural resistance to corrosion by fusing aluminum with fortifying elements during the manufacturing process.

In any construction, choosing the right material is only part of the equation. A dock or wave attenuator's strength depends equally on the system's design. Whether made from aluminum, steel or concrete, a system's durability rests on the shape and size of its members and how well the construction fits the system's specific location. When planning a dock or attenuation system, a competent engineer first completes a thorough site design analysis. Then, using precise computer programs such as AutoCAD or Pro E, the engineer manipulates the design to meet site requirements. Without a site-specific design, the dock or attenuation system may lack structural integrity.

For example, a design may work in freshwater, which shows little impact on aluminum, but may be inappropriate for a saline environment. When submersed in salt water, aluminum is vulnerable to galvanic corrosion if it encounters an electrical current. Therefore, docks and wave attenuation systems in saline environments are best constructed to minimize the aluminum's submersion. A dock system currently under construction in Charleston, South Carolina, demonstrates how innovative engineering can prevent potential damage.

Concrete was beyond this riverside marina's budget, and steels' rust potential concerned the marina owners, so aluminum was the best choice for a dock system strong enough for the Charleston site. Located within the Atlantic inter coastal waterway, the marina is susceptible to saltwater, wind and wave action, as well as occasional hurricanes. To accommodate the owner's preferences and site's severe conditions, engineers at ShoreMaster designed an aluminum dock system with over-sized structural members and a wider construction. Attached to this dock, a built-in wave wall employs, as both floatation and protection, a double row of polyethylene-encased floats. The outer row of floats extends 24 inches into the water and is balanced with concrete, which adds to the wave-dissipating mass of the structure. This prevents the aluminum from becoming submerged and, in turn, minimizes the risk of galvanic corrosion. The result is a strong, attractive and economical attenuator dock that ensures tough protection for years to come.

The clean lines of the World Class Rail system create an attractive floating dock system at Jackson Hole, Wyoming.

Proper engineering also requires selecting a construction style appropriate to the site. Aluminum docks are generally built in two styles: single-plane or truss. Single-plane systems, such as ShoreMaster's World Class Rail, are manufactured with an outer frame composed entirely of aluminum tubing or rails. Depending on the site conditions, this system may be constructed with sidewalls or varying sizes - the rougher the water, the larger the rail. The popularity of single-plane systems stem partly from economy, as these docks are generally less expensive than a truss style. The appearance of a single-plane system also appeals to marina owners who prefer the clean lines of the World Class Rail, which can be finished with a variety of decking materials to match docks already existing on the property.

Better suited to lighter duty environments, single-rail systems can be used in both salt and freshwater. At well-protected Mystic Pointe Marina in Aventura, Florida, also located on the Atlantic intercoastal waterway, the site's conditions allowed for a light duty dock system. In 1989, Mystic Pointe Marina selected ShoreMaster's World Class Rail for aluminum's resistance to corrosion and for the metal's low maintenance requirements. While the marina's harbor is sheltered, however, the site is hardly exempt for severe weather. Fourteen years and multiple hurricanes - including the infamous Hurricane Andrew - have proven well-engineered 'light duty' aluminum to be more 'duty' than 'light.'

When a dock or attenuation system must accept significant loads, a heavy-duty truss system is in order. Truss systems are composed of a tube frame with a reinforcing rod welded in repeating 'V' shapes along the water walls. Because the open 'V' construction of a truss creates more room for utility raceways, this style is popular in marinas that provide such amenities to their boating clients. Roofed systems also require the support of a truss, since a roof adds to the dock's weight and wind load transfer.

To deflect waves, ice and debris expected at this excursion boat dock in St. Paul, Minnesota, ShoreMaster engineers angled the aluminum dock and attached an attenuator system ot the dock's deep truss.

Another situation that calls for truss-style construction is the need to provide moorage for larger vessels. Because truss-style construction is stronger and stiffer than single-rail, it allows for longer finger piers than single-rail systems. Harriet Island in St. Paul, Minnesota, needed extra-long dockage to accommodate a floating bed and breakfast and an old-fashioned riverboat that ferries tours along the Mississippi River. No standard dock on the market would survive this challenging location, which is battered each winter by ice and floating debris. A custom-designed ShoreMaster aluminum truss dock was the solution. To accommodate large passenger loads and the additional freeboard necessary for the tour boat, ShoreMaster customized the height of the truss. An attached attenuator deflects waves, ice and debris. Once again, easily-customized aluminum provides the engineering solution to a challenging site.

If you still think aluminum is only for soda cans, toss out the old idea. Better yet, recycle it. Aluminum is an economical and versatile solution to challenging marine environments north and south, in freshwater and salt, from icy lakes to coastal waters. In the water or on the shoreline, aluminum offers uncompromising strength and clean look that lasts.

*Dennis Tuel, Jr., P.E., is senior vice president at US Company, ShoreMaster. He has served ShoreMaster clients for 18 years and overseen the installation of hundreds of docks and wave attenuation systems.