Bridge of Curves
Designers of the James D. Pfluger Bridge in Austin, Texas used conventional materials in unique ways to create a safe passage for pedestrians and bicyclists as well as a pleasing sight
By Tom Gibson
Driving into Austin on I-35 from the north, we encounter a maze of highways and a bustling downtown with modern tall buildings made of steel, glass, and concrete filling the skyline. In the spaces between existing structures, tall cranes toiled erecting new ones. After getting off on Riverside and heading west, we come to Lamar Boulevard and have to wait several minutes at the stoplight for the Friday lunchtime traffic to clear.
The growing cityscape and heavy traffic underscored why we had come to the capital of Texas: to see the new James D. Pfluger Pedestrian and Bicycle Bridge that runs next to an old bridge on Lamar Boulevard. Austin's population and subsequent traffic had increased so much in recent years that it made crossing the Colorado River on the Lamar Boulevard Bridge dangerous. In a town full of athletic runners and bicyclists, this was no small matter. Using a model program of citizen input, the city built a bridge that gave townspeople not only an avenue for crossing the river safely but also an architectural centerpiece. Full of curves rather than straight lines, the structure uses conventional materials in unconventional fashion, a feat that involved some novel and inspired engineering.
Domenic Coletti, a design engineer for HDR, the lead engineering firm on the project, describes the experience: "It was out of the ordinary, and you had to stop and think about what you were doing more than you would on a typical bridge. It wasn't the routine type of calculations you can do over and over again. That plus the fact it was intended to be a signature bridge for the city, something they wanted to look nice. This was one of the rare opportunities that bridge engineers get to work on a project where more than functionality is important."
Actually, the Pfluger Bridge spans Town Lake, formed along the southern edge of downtown Austin by damming the Colorado River. This created one of Austin's most popular locations for outdoor activity and one of the most heavily-traveled commuter corridors. Recreationists frequent Town Lake Hike and Bike Trail, which runs 10 miles along the north and south shores of Town Lake under the bridge and connecting to it. Other trails run off it northward through downtown Austin. Kalpana Sutaria, an architect by profession and project manager of the bridge for the Public Works Department of the City of Austin, describes Town Lake as "a jewel in Austin's downtown. It has become a very important part of Austin's lifestyle."
The Lamar Bridge comprises one of the major crossings over Town Lake. A six-span concrete deck arch bridge built in 1940, it features art deco detailing and has become a historically significant landmark. However, it has also become functionally obsolete with its 10-foot traffic lanes and narrow sidewalks. Austin's growth, combined with the bridge's key location near the center of the hike-and-bike trails, has resulted in heavy pedestrian and bicycle traffic in addition to vehicle traffic. Highlighting the problem with this mix, a cyclist was struck and killed by a drunk driver on the bridge in 1991, and in 2000, a jogger was struck and killed by an errant car on the bridge. The city desperately needed an improved facility for crossing Town Lake.
In the beginning, the citizens of Austin approved a bond package that included $8 million to widen Lamar Bridge, and in the early 1990s, the city secured $950,000 of matching federal funding through the Intermodal Surface Transportation Efficiency Act (ISTEA). In 1995, the city retained a consultant team led by HDR to study project alternatives. The team evaluated six options, all variations of widening Lamar Bridge, and held regular meetings with the city of Austin, the Texas Department of Transportation, the Texas Historical Commission, and other key stakeholders.
Some people wanted to widen the bridge and strongly opposed building a separate one. But others pointed out that Lamar Bridge is already cantilevered on each side, and adding lanes would compound that. The Texas Historical Commission indicated its preference to avoid any alterations to Lamar Bridge, to preserve the historical integrity of this landmark -- the commission is a state agency, and the state owns the bridge. In addition, attendees at a 1996 public meeting indicated they didn't want more traffic lanes added to the existing bridge.
In a transportation work session in 1998, Austin's city council directed the project team to seek public input in the design of a separate pedestrian/bicycle bridge. Council members passionately debated the idea of a separate bridge, but they ultimately decided to proceed with a design process. To develop concepts, the city held a public workshop for key stakeholders. This generated 15 proposed concepts, including conventional cable-stayed and arch bridges, relocation of an existing historical truss bridge, and several variations of beam bridges.
The city selected four of the 15 proposed concepts, including the Double Curve, for further development by the consultant team's architect, Kinney & Associates/Carter Design Associates of Austin. They set a criteria of no straight lines and conceived a structure with a double-hourglass-shaped deck plan resulting from the curved horizontal alignments and featuring helical ramps and curved connector spans at each end. The four developed concepts were presented to city council, and the Double Curve concept emerged as the winner.
With only six months remaining before the matching ISTEA funding would expire, final design work on the new bridge proceeded on an accelerated schedule. The engineers and architects worked in parallel to refine aesthetic concepts while simultaneously developing construction plans. In stating the importance of aesthetics, Sutaria says, "Every structural element that was exposed was looked at as an architectural element."
Headquartered in Omaha, Nebraska, HDR covers many disciplines of engineering through offices around the country. Crews in its Dallas, Tampa, and Omaha offices worked on different parts of the bridge, while engineers in the Austin office handled program management and other non-structural parts. Jose I. Guerra, Inc. Consulting Engineers, a multi-discipline engineering firm in Austin, designed the helix ramp for the bridge and did all on-site construction inspection work.
Engineers initially considered several structural systems, including cast-in-place structures and concrete box girders, for the bridge's superstructure. But the need for quick, simple, low-cost design along with ease of constructability over the lake and the ability to conform to geometric complexities led HDR to use standard steel bridge elements. They selected steel plate girders, and the architect requested weathering steel, which takes on a dark layer of rust, for a natural appearance to fit with the wooded shorelines of Town Lake and to eliminate having to paint the bridge.
With a tight construction budget, the engineering team set out to produce plans resembling those for a major steel plate girder highway bridge, with the exception of the basic geometry and aesthetic treatments. This way, the plans would look familiar to typical highway bridge contractors bidding on the project and yield competitive bids.
Several sections of the bridge superstructure used framing design indistinguishable from typical highway bridge construction. The main girders on the bridge are plate girders, which have top and bottom flange plates welded to a vertical web plate. Sections abutting the southern end use three concentric, horizontally-curved composite plate girders and rolled beams -- similar to plate girders, rolled beams are rolled to their I-beam shape at the mill in one piece. These are connected by diaphragms, girders that run cross-wise to the main girders.
Real differences came in the center section of the bridge. Engineers struggled with the deck design because if parallel straight beams were used in conventional fashion, the deck would overhang up to 12 feet at its widest point due to the bulbous shape of the center section. They studied options such as cast-in-place decks and precast deck panels, but cast-in-place would have involved difficult construction over the lake, while the precast option meant complicated detailing for the interfaces between panels and connections.
Coletti explains the intricacies of engineering the design: "The hour-glass shape, when you do the superstructure with steel plate girders, becomes in some ways more complicated than a typical curved bridge, but in other ways it's actually simpler to design." In a typical curved girder bridge, the girders all curve the same way, and they're all concentric to each other, simpifying the geometry and some aspects of the structural analysis. On this bridge, the two outside girders curve opposite to each other. "That's unusual for any bridge structure. It was more challenging from a geometric standpoint and also more complicated to model in the finite element program we used. But at the same time, the opposing curvature negated global overturning effects found in typical curved girder bridges, allowing a much simpler design by conventional hand analysis techniques"
The substructure system, or piers, for the bridge are conventional in material but unconventional in form. They use reinforced, cast-in-place concrete shapes know as bents following the curved theme for the structure. The curved columns and a haunched bent cap at the top allow views of scenic Town Lake through the bents.
At the northern end of the bridge lies the Helix Ramp, a conventionally-reinforced helical ramp that rotates through 540 degrees to link with the hike-and-bike trail below. A second link comes via a stairway to the trail below. The helical ramp originally proposed at the southern end was eventually removed from the design. An aerial view of the bridge shows that the paths of travel to points north of the bridge have been blocked off for now; they will be continued as future expansion takes place.
Anyone involved in the Pfluger Bridge can tell you the number of stakeholders involved often made the project arduous. Coletti says, "One of the biggest challenges was finding a way to make sure everybody had a voice in the design, because all their opinions had value, and yet do it within the budget and the time constraints we had. That was the greatest success of the project." He adds, "Everybody was very excited about it. We had to work hard and fast, and sometimes we had to work long hours on it, but nobody ever complained because it was a fun project."
Since the bridge's opening in 2001, Sutaria says, "People have been extremely happy with the project, and I still come across people who tell me this is the best thing the city has done. It has created a new public place that people really enjoy. Many different people use this bridge." As an example, she cites a club of mothers with strollers that has formed and takes to the bridge several times week.
We parked near the bridge and walked across it to experience it first hand. Indeed, a host of people were using it including walkers, joggers, dog walkers, bicyclists, and parents with strollers. A kayak passed underneath the bridge, and ducks floated on the water, adding to the ambience. To compare the old with the new, we strolled over to Lamar Bridge and found it full of traffic. Signs on the entrance to the sidewalks on it said "For emergency use only." But most signs indicate the new Pfluger bridge has proven successful and filled a need. Austin residents can safely cross Town Lake on foot or bicycle and escape the city's burgeoning traffic.
For more information on the James D. Pfluger Bridge, visit www.ci.austin.tx.us/publicworks/images/pb_hdr.pdf