Stainless steel brings aesthetic appeal, low maintenance and long life to structural projects
January 2012 - Bridges transport pedestrians and vehicles over obstacles. They can be as simple as a log over a stream, but the most iconic structures have elements of grace and beauty, from moon bridges to the Golden Gate Bridge, Ponte Vecchio and London Bridge.
A new addition to San Diego’s already picturesque skyline is the Harbor Drive Pedestrian Bridge, which, according to the Centre City Development Corp., fulfills a California Public Utilities Commission requirement as well as “unites downtown’s urban core.”
The bridge links Balboa Park with the San Diego Bay, providing a safe pedestrian crossing over Harbor Drive and train and trolley tracks.
Safdie Rabines Architects and engineering services company T.Y. Lin International collaborated on the project. “The city of San Diego and CCDC had a longtime goal of completing the link between the San Diego Bay and the historic core of the city in the Gaslamp district and Balboa Park,” says Dan Fitzwilliam, P.E., senior bridge engineer at T.Y. Lin International. “T.Y. Lin International was asked to provide a landmark structure that would act as a final piece in that link as well as serve as the gateway to San Diego for vehicles entering from the East Village. Personally, our goal was to create a one-of-a-kind structure that would not only serve the public by crossing the busy downtown street and keeping pedestrians off the heavily trafficked trolley, commuter and freight tracks but also serve San Diego as an icon for the city’s revitalized downtown area.”
Major structural component
The Harbor Drive bridge is the second major pedestrian bridge in the United States to use a significant amount of stainless steel in its structure. The main cable uses 2205 duplex stainless steel from Outokumpu Inc., Itasca, Ill., to support the bridge deck with tension cables through the pipe and the heavy plate railing posts.
Typically, T.Y. Lin International works with concrete as the structural component for bridges. “We have not worked with stainless steel on a project of this magnitude,” Fitzwilliam says. “Traditionally, stainless steel has been used only for architectural finishes or non-structural items. We have used stainless steel for pedestrian railings or small architectural components of a structure but never as one of the main structural members.”
The Harbor Drive bridge needed to have a striking finish to attract pedestrians who would be interacting with the bridge as well as a 100-plus-year life span. “The selection of duplex stainless steel allowed us to provide the iconic structure with high-quality aesthetic finishes in an area that has an aggressive marine environment where we knew it would not get much maintenance,” Fitzwilliam says. “Traditional corrosion protection using painted carbon steel or lower-grade austenitic steel, such as 304L, would not have the life span that we needed, and the aesthetics would be rapidly compromised by the lower corrosion resistance.”
T.Y. Lin International determined three main criteria as integral parts of a successful design: High-strength materials above 50 kilograms per square inch yield strength, very high level of corrosion resistance and a high-quality aesthetic finish, and sustainability with minimum maintenance. The company, in conjunction with metallurgy consulting firm TMR Stainless, Pittsburgh, Pa., chose Grade 2205 duplex stainless steel. Plates for the project were produced in Outokumpu’s New Castle, Ind., mill and fabricated into pipe by Bristol Metals, Bristol, Tenn. Cleveland-based AMECO performed fabrication for the bridge.
“The bridge design engineers were advised to use 2205 for two reasons. No. 1, the primary structural load for the bridge deck is carried by the large pipes above the bridge railing and the plate sections that are welded to them. There is a high-tension cable going through those to accomplish this, and they needed the strength of a duplex,” says Catherine Houska, senior development manager at TMR Stainless. “No. 2, I was aware of Type 316 corrosion staining issues on railings in the harbor area. The micro-climate in that area is more corrosive than the typical coastal environment, and they wanted corrosion-free performance with essentially no maintenance. That made 2205 the logical choice.”
Stainless steel was used for several of the vital structural components of the bridge, which has a 354-foot-long main span over six railroad tracks and several lanes of traffic. It also features a 131-foot-tall concrete pylon and curved deck that is suspended along one side. Thirty-four individual suspenders are attached to the main cable and support the 20-foot-wide deck.
“The main cable pipe is an 8-inch nominal load-carrying element that carries a portion of the dead load from the stays down to the stairs and eventually into the concrete foundations,” Fitzwilliam says. “At the top of the pedestrian railing, we have incorporated another 8-inch nominal stainless steel pipe, which transfers the main longitudinal post-tensioning forces into the deck. The structural railing posts themselves are custom-fabricated stainless steel boxes, which the entire concrete deck is suspended from. Alternating with the suspender posts are secondary stainless steel posts, which transfer a radial correction force from the top of the railing to the deck and provide a torsional balancing force—which is a critical piece of the structural system.
“One of the main challenges we faced was in the form of the main cable system,” Fitzwilliam continues. “In a conventional suspension bridge, the suspenders or hangers are connected to the main cable directly through bulky cable clamps that wrap around the main cable and provide a fixed connection point. For our bridge, we wanted to have a smooth, uninterrupted line for the main cable from the deck all the way to the top of the pylon. We decided to utilize stainless steel guide pipe to create a corrosion protection barrier to the main tension cable. We took advantage of the structural capacity of the guide pipe to act as a secondary load-carrying element that would act in concert with the conventional post-tensioned main cable. In our solution, we were able to create a system that would carry the bridge loads using a combination of tension in the main cable and compression in the guide pipe to increase the efficiency of the system. At the same time, our solution allowed us to have an unbroken line of sight all the way up the cable, creating a continuity that allows the pedestrian’s eye to visualize the load path of the forces being carried up to the pylon in a natural way.”
Beauty and strength
Architectural structural applications, such as sculptural pedestrian bridges, “should be differentiated from vehicular bridges, which have different structural design requirements,” Houska says. “The firms involved in their design are often designing buildings and other architectural structures. The number of structural architectural applications for 2205 are still limited, but there is growing interest on the part of both architects and structural engineers because 2205 is a cost-effective means of obtaining improved performance in corrosive applications exposed to coastal or deicing salts or industrial pollutants. Of course, all duplexes offer a significant strength increase, which can be an advantage when there is a desire to reduce the size of structural components or where the higher strength is necessary to achieve the design requirements.”
Because of stainless steel’s aesthetic beauty, strength and long life, Outokumpu is working to educate engineers and architects around the world about the material’s potential in structural applications.
“In the U.S., we have been telling the story of pedestrian bridges made of stainless steel by referencing several European projects and Outokumpu’s experience with these bridges,” says Poul-Erik Arnvig, Outokumpu’s vice president of market development. The company’s 2205 duplex stainless is in use on the Helix pedestrian bridge in Singapore; Outokumpu’s 2304 duplex is the material featured both on the Padre Arrupe pedestrian bridge in Bilbao, Spain, and the Celtic Gateway bridge currently under construction in the United Kingdom.
“Bridges are a major first step, but there are many other areas in the architectural and construction industry that benefit from the advantages of stainless steel,” Arnvig says. “Just look at all the architectural uses of stainless steel for facades and columns on modern tall buildings. Also, sculptural uses have become more popular, just think of ‘the Bean’ [“Cloud Gate” by Anish Kapoor] in Chicago’s Millennium Park as well as the new ‘The Runners’ sculpture at O’Hare airport. But stainless steel can also be found in more hidden parts of construction, such as reinforcement. This is an application where stainless steel is now growing and where we believe a major breakthrough in usage of stainless rebar is imminent in North America.”
Outokumpu also is participating in the growing stainless roofing market, developing new matte surfaces “to avoid too much reflection from sunlight in different alloys, including 2205,” Arnvig notes. “The list is almost limitless. Once you realize what a wonderful and sustainable material stainless steel is, then there is no going back to good, old rusty constructions.” MM