Ingenuity and education will come together next spring when one of the nation's most unusual bridges is put in place across a creek in Ithaca. The pedestrian bridge deck, 8 feet wide and 42 feet long, will contain no heavy steel bars but instead will be constructed from concrete reinforced with plastic-and-carbon bars so light in weight they can be lifted with a finger and thumb.
Appropriately for a space-age bridge, the new structure will be called the Carl Sagan Bridge, for the late David Duncan Professor of Astronomy and Space Sciences at Cornell.
The bridge will be unusual in appearance as well as in construction. Because it is named for astronomer Sagan, the bridge's two longitudinal beams will have nine, 2-foot-diameter circular holes to be filled with the signs of the nine planets. The bridge will cross from Adams Street to a city park also named in Sagan's honor.
The project, and the lightweight bars, are the inspiration of Petru Petrina, a lecturer in Cornell's School of Civil and Environmental Engineering (CEE) who is leading a group of senior undergraduate members of the student chapter of the American Society of Civil Engineers (ASCE) -- Steven Caldwell, Erik Boe, Dave Pittman, Babak Farhad, Dan Mullins and Wei Kin Quek-- in giving his invention its first trial. The project is an outcome of a CEE course in materials that Petrina teaches.
This fall, Petrina and his students are making the composite bars, called fiber reinforced polymer (FRP), and working on the design for the bridge. In the new year they will turn to precasting sections, then managing the replacement of the decaying Adams Street Bridge across Cascadilla Creek in downtown Ithaca.
Petrina began developing his replacement for steel reinforcing bars, called rebar, six years ago as an answer to corrosion, an ever-present problem for the nation's infrastructure. The idea of composite building materials, made from a polymer, or plastic, mixed with carbon or glass fibers, as a replacement for steel goes back nearly three decades. In Europe and Japan there is an increasing emphasis on the use of various FRP rebar as a substitute for steel in reinforcing concrete bridge decks.
However, these lightweight bars, usually round in shape and made by a process called pultrusion, tend to be brittle and to have lower bonding qualities in reinforced concrete. Petrina's idea was to make rectangular sheets of FRP rebar built up from thin sheets of glass or carbon fibers, thickest in the regions of greatest beam bending. The sheets are "baked" in an industrial oven at a temperature of 121 degrees Celsius for about two hours then cut into bars.
"I have created a bar that, when it stretches, expands and enhances the bond to the concrete," says Petrina. "Carbon seems to be the best fiber for this because it has very high tensile strength and stiffness."
Clearly it was an idea whose time has come because last summer the Cornell Research Foundation licensed the patented rebar to the technology-transfer company Utek Corp., of Plant City Fla. Teamed with its subsidiary, Nucor Enterprises of Tulsa, Okla., Utek is sponsoring further research on the rebar. Utek also has made a gift of $40,000 to Cornell.
In researching the properties of the rebar, particularly length, Petrina needed to put the new building material to work. He knew that the nation's first all-FRP-rebar bridge had been built in West Virginia in 1996. When he heard the city of Ithaca was considering replacing the Adams Street bridge, he suggested his rebar be used to build the new structure. After discussions with city engineers, it was agreed that the city would pick up $13,800 of the cost of the $18,400 structure, with Cornell paying the balance.
The Petrina and the ASCE student chapter will design the bridge, and Richard White, Cornell emeritus professor of civil and environmental engineering, will review the design. The students will also make the construction drawings, make the FRP, cast the precast deck and manage the assembly at the site, hopefully next April.
After preparing several rebar sheets in their self-built lab oven, the students recently started to make a 2.5 feet by 8 feet deck section reinforced with Petrina's FRP and test it in the CEE's George Winter Structural Laboratory with the help of lab manager Tim Bond, who also is the ASCE chapter's faculty adviser. "You just don't get a chance like this to do something really new in engineering education very often," says Caldwell, a civil engineering major from Oyster Bay, N.Y. "This is state-of-the-art engineering," says Boe, also a civil engineering major, from Mawah, N.J. Pittman, a structural engineering major from Lansdale, Pa., simply comments, "Neat."