Design Considerations

When considering a structural system, architects will want to consider a number of factors to ensure a proper specification decision.

Aesthetics

Hot-dip galvanized steel offers a natural gray finish consistent with a clean, contemporary aesthetic. As galvanized steel weathers and the zinc patina forms, the material assumes a uniform matte gray color that blends unobtrusively in wooded areas and offers the non reflectivity desirable in more densely populated areas. When color is preferred, powder coating can be added—a scenario that achieves both visual effect and added protection. In fact, architects are increasingly recognizing the value of combining hot-dip galvanizing with a powder coating in what is known as a duplex system. In this arrangement, painting or powder coating is applied over hot-dip galvanized steel, offering a synergistic protection system that is far superior to either system alone. The paint provides an added protective layer that prevents degradation of the zinc and the galvanized steel maximizes the life of the paint by preventing underfilm corrosion and cracking.

European studies² indicate that the zinc-paint combination will last 1.5 to 2.3 times longer than either system alone, meaning that a 70-year galvanizing life and a 10-year paint life will produce coating with a life of 120 to 220 years. The synergistic effect increases the life of the paint 1.5 to 2.0 times vs. application over black steel. Eliminating or minimizing the need to regularly repaint saves significant costs, too. To ensure effectiveness of the duplex system, particular attention must be paid to proper surface preparation with coordination among the designer, fabricator, galvanizer, and painter/coater recommended throughout the process.

Duplex systems have been incorporated in many high-profile projects. One example is the Salvador Dali Museum in St. Petersburg, Florida, a work of art reflective of the artist's characteristic juxtaposition of classical and fantastic elements. The structure, designed by Yann Weymouth and Novum Structures, features an undulating, abstract 75-foot glass structure that flows from the plaza up and around the cubist “treasure box” museum. Located in the highly corrosive Floridian coastal environment, designers faced several challenges. While a durable, low-maintenance corrosion protection system for the steel was a given, the steel substructure had to meet the highest aesthetic standards without distracting from the glass structure. The scale and complexity of the architectural elements meant that the steel elements had to be fabricated, processed, and delivered to the jobsite in a well-thought-out sequence to minimize loss and downtime. Incorporating a duplex system of powder coating over hot-dip galvanized steel for corrosion protection proved to be the ideal solution for addressing these issues—the powder coated finish enabled color selection that would blend with the facility, while the galvanized steel beneath ensured long-lasting corrosion protection.

Designers of Boston's Chinatown Park also opted for a duplex system to afford corrosion-resistant protection and to incorporate brilliant colors that express a modern take on traditional village festival spaces, contemplative gardens, gateways, garden walls, flowing water, and Asian plantings. A stylized sampan sail, fabricated in stainless steel and LEDs, and the duplex coated steel gateway in vermillion are striking elements of the park.

Hot-dip galvanized steel also facilitates complex projects. Designers of the 3,000-space parking facility at the rapidly growing Charlotte Douglas International Airport, for example, wanted to depart from the traditional parking “box,” preferring a structure more like a curved airplane wing. Accordingly, architect and engineer formulated a design that attaches stainless steel cladding to a galvanized structural steel frame. Three hundred tons of hot-dip galvanized steel were used in the project including bow-string trusses, embed plates and anchors, stair towers, stairways, hand rails, and castellated beams. Galvanized steel was specified for its overall aesthetic appearance, its maintenance-free service life that will enable the structure to remain corrosion free well into the future, and its compatibility with stainless steel. When used together, galvanized and stainless steel provide a uniform appearance and an economical alternative to specifying stainless steel for the entire structure.

ASTM Standards

Three specifications pertain to the coating thickness, adherence, and finish for hot-dip galvanized coatings: ASTM A123, A153, and A767. Of the three, ASTM A123 is the primary specification, and references galvanized products except fasteners and small parts, which are governed by A153, and reinforcing steel bars, covered by A767. A few supporting specifications referenced in these specifications cover design practices, repair and touch-up, and painting over galvanizing, notably ASTM A143, ASTM A384, and ASTM A385. Further, ASTM A780 covers touch-up and repair and ASTM D6386 and ASTM D7803 refer to hot-dip galvanizing for paint or powder coating. A compilation of these specifications appears in Selected Specifications for Hot-Dip Galvanizing.

Profiles

In designing steel elements for hot-dip galvanization, architects must consider the capacity of the galvanizing kettle. The average kettle length in North America is 40 feet, and there are many kettles between 50-60 feet. If larger-scale design elements are specified they can almost always be accommodated by fabricating modules geared to available kettle facilities—a scenario that often generates savings as smaller modules simplify handling and transportation, and can be connected on-site by field-welding or bolting. Alternatively, if an item is too large for total immersion in the kettle, but more than half of the item will fit into the kettle, the piece may be progressively dipped; consultation with the galvanizer is recommended during the design process. Weight is another consideration as hoists and overhead cranes move the steel elements through the kettle. While elements less than 30 inches long are often galvanized in perforated baskets, larger assemblies are usually supported by chains or other lifting fixtures. Any marks from these fixtures can usually be touched up if desired for aesthetic reasons. Best practice is to discuss the weight-handling capacity with the galvanizer to ensure lifting capacity and the best places for lifting points.

Sustainability

Many architects and owners are interested in pursuing accreditation from the U.S. Green Building Council's Leadership in Energy and Environmental Design (LEED), the premier green rating system for building design and performance. While LEED v4 was approved in 2013, architects can select either that system or LEED 2009 through June 2015.

In LEED 2009, galvanizing can contribute to points primarily through credits for recycled content. Specifically, two points can be earned in the Materials & Resources Credit 4: Recycled Content as more than 20 percent of the total value of the material comes from recycled sources. With their ultra high recycling rates, zinc and steel allow the galvanizing process to score an “extra credit” point under Innovations in Design Credit 1: Path 2 Exemplary Performance because the recycled content actually exceeds 30 percent. Other possible points can be earned under MR Credit 5: Regional Materials, MR Credit 3: Materials Reuse, and ID Credit 1: Path 1 Innovation in Design and ID Credit 1: Path 2 Exemplary Performance.

LEED v4 is different than its predecessors in that it has moved from a prescriptive checklist to a more holistic measure of environmental impact and greater transparency with regard to materials. There is an emphasis on Environmental Product Declarations (EPDs) which detail the materials used in a building and their economic impact (such as an LCA), the environmental impacts all along the supply chain, and material ingredient lists. Currently, EPDs, and material ingredient lists and supply chain information are in the works for the hot-dip galvanizing industry.