A Ravenna fourth-grader said the new bridge in town looked “just like the San Francisco bridge.”
At first glance, the long arch and narrow rods conjure images of the Golden Gate Bridge. But the greatest similarity between the two bridges is that both are significant engineering feats, expected to be industry models for decades.
“This bridge is the icon of the Nebraska highway system right now and is receiving attention around the world,” said Lyman Freemon, director of the Nebraska Department of Roads Bridge Division. The $4.2 million bridge is a tied arch, a shallow structure that can span long distances. It is one of the best choices for navigational channels, said civil engineering professor Maher Tadros.
He and Freemon first thought of building a tied arch bridge during a 1997 trip to Japan, where they met with leaders of the Japanese construction industry. As a result of the meeting, the United States and Japan each agreed to build a tied arch bridge for demonstration purposes. (Japan has not built its bridge yet.)
For the next five years, the university and NDOR researched how to build such a bridge. The university also applied for federal funding.
In 2002, Tadros and Freemon found an ideal location for their experiment. Ravenna was already planning to replace a storm sewer and several culverts. The infrastructure lay near the viaduct that stretches from Utica Street to Highway 68. Built in the 1930s, the viaduct was corroding and obsolete, so NDOR determined that the bridge needed replacement.
Tadros said the site presented many challenges that ultimately led to engineers choosing a tied arch design. First, the bridge crosses Burlington Northern-Santa Fe railroad tracks, and the railroad requires a 25-foot clearance between the tracks and the bridge. Second, BNSF allows only three hours of train traffic interruption at any time during construction. Third, the project had a tight budget, and the bridge had to be built quickly.
A design team considered several options.
The Inverted Tee, a system Tadros developed for short-span bridges, would have easily met railroad clearance requirements. However, it could not span the length of the viaduct—174 feet—without pier supports. Tadros said building piers close to railroad tracks would have been difficult, dangerous and expensive.
The team also considered the NU I-Girder system, developed by Tadros and other university researchers. The I-Girder met the length requirement, but the deck would have been more than six feet deep, requiring NDOR to raise the road. In contrast, the tied arch bridge is just 32 inches deep.
Tadros said engineers determined that raising the road would be expensive, time-consuming and harmful to local businesses.
Therefore, the team embraced the challenge of building the United States’ first tied arch bridge that has post-tensioned steel tubes filled with concrete.
“The pieces of the puzzle fit together nicely,” Tadros said. “Nothing was forced.” The defining element of the bridge is the steel tubes used for the arch.
Two 12-inch tubes, separated by a 12-inch spacer, are filled with concrete and reinforced with steel tendons. The steel pipe prevents the concrete from crushing. “When you fill a steel tube with concrete, it is much greater than the sum of the capacities of its two parts,” Tadros said. The twin arches are as sturdy as one large arch, but streamlined. “It’s just like a string in the air,” Tadros said. “It’s very slender and attractive, yet it’s wide and wind resistant.” The feature Tadros is most proud of is the bottom UNL patented tie beam that supports the floor. The tie beam is composed of steel tubes filled with concrete and post-tensioned with high-strength steel strands.
“The trouble with conventional design is that the steel tube becomes fracture-critical, which means we have to over-design them so they won’t fracture under traffic. Post-tensioning essentially eliminates that problem by preloading the steel to put it in permanent compression,” Tadros said.
The slab resting on the floor beams also is post-tensioned, guaranteeing the bridge will not crack.
Kirk Weber, NDOR project manager, said the project was tricky because nothing like it had been attempted.
At first, designers thought bridge elements could be built offsite and installed in a few weeks. However, the department could not find a crane powerful enough to lift the arches.
“You always run into problems, but coming up with the solutions is part of the fun,” Weber said.
Construction on the four foundation shafts and abutment columns began in February 2005. Normally, Weber said, NDOR removes an old structure before starting construction on a new one. However, the contractor decided to keep the old viaduct in place to support pieces of the new bridge during construction. The arch was built in pieces at Capital Steel in Lincoln and transported to Ravenna for assembly.
Once the arches were assembled, temporary stilts braced them on the abutment columns. NDOR used the existing deck as a platform for placing the steel floor beams between the arches.
The viaduct was completed in November 2005. Tadros said it’s already being used as a template for a 240-foot bridge in Columbus and would be feasible for a proposed 410-foot Missouri River bridge.
Tadros said building “a bridge Ravenna could be proud of” was as important as the engineering itself. A passenger walkway connects southern Ravenna with the downtown business district. This spring NDOR will add the finishing touches when it paints the arches royal blue, Ravenna’s school color, and stains the retaining walls.
“This viaduct is certainly going to be a great addition to the community,” Mayor Gerald Reimers said.
It also continues a 17-year partnership between UNL and NDOR. The design team included Tadros and civil engineering professors Christopher Tuan and Amgad Girgis from the College of Engineering and Freemon, Weber, designer David Fritz and assistant bridge engineer Dan Sharp from NDOR.
During the Ravenna bridge’s dedication ceremony, Freemon joked that he and Tadros were “joined at the hip” after spending so much time together. That’s fine with Tadros, who said he considers the NDOR Bridge Division the best in the country.
Freemon offers equal praise.
“He [Tadros] is probably the most outstanding engineer of concrete structures not in the state, not in the country, but in the world,” he said.