The steel bridge employs a Vierendeel truss system, with rectangular panels and rigid joints, but no diagonal braces. This system allows for an open and transparent environment. FKP studied about 30 bridges in the Houston area before selecting the Vierendeel truss, which is unique to Texas Children's Hospital, says Daniel Gaitan, the firm's design coordinator.
“It's not done often, because of a slightly additional cost,” he says, adding that the design allows for longer spans and better views from the bridge.
Adding to the project's overall challenges, the bridge's steel fabricator, Fort Worth-based North Texas Steel, went out of business in March 2010.
“We were fortunate enough to be able to make a deal enabling them to finish the fabrication prior to shutting their doors,” Kubin says. “They did a good job, and we had to keep tabs on them and sent extra resources to the fabrication plant in Fort Worth to supervise and inspect the fabrication before shipping.”
The fabricator shipped sections as it completed them, requiring Bellows to accept the steel early and store it in its yard until construction progressed enough to begin installation. Materials were also kept at the yard of the erector, Houston-based Peterson Bechner Industries.
Bellows assembled the bridge structure on site in two-story modules and set them into place with temporary shoring until the team made it across the street. They added a suspended work and debris protection platform from the bottom of bridge for the entire length, says Joe Baldridge, a principal with Walter P Moore.
Bechner Industries lifted 35-ft sections at a time during road and rail closures over six weekends. The steel framing weighed about 2,800 lb per linear ft, with each lift by two cranes ranging from 78,000 lb to 105,000 lb. “The erection of the bridge went better than people thought it might, and we credit that to careful planning,” Baldridge says.
Pearsall adds that the BIM modeling assisted with construction logistics and how the cranes would be able to lift the sections of the bridge around the constraints. The 800-ton bridge is supported at four points with pairs of 102-ft-long drilled piers and a concrete cap beam. Two are located at the new building and one each at the two other facilities.
“We had to shoehorn in the foundations for that bridge,” says Charlie Penland, a civil engineer with Walter P Moore, who used BIM to identify existing utilities, rail lines and foundations and map out where to place the drilled piers. “We were able to get down and analyze and understand how to make this work.”
None of the floor elevations aligned, so FKP slightly sloped the bridge. Each unit required renovations to receive the connections.
With completion of the bridge, the maternity center made both a literal and symbolic connection to Texas Children's Hospital. Outpatient services are set to open in November and inpatient beds in 2012. Despite numerous challenges, W.S. Bellows delivered the project on schedule and within budget. “There is nothing typical about this job,” Kubin says.