Children's Hospital bridge presents challenges for engineering, construction
When the new Helen DeVos Children’s Hospital opens for business on 1-11-11, visitors will be walking on air.
That’s because the steel and concrete, glass-clad bridge spanning Michigan Street NE from the Tower 35 medical office building and parking ramp across the street is mostly made of air, according to its structural engineer.
The 85-ton first segment of the bridge was installed earlier this month, and the rest is expected to be put into place in April. The completed bridge will weigh 99 tons.
“The bridge really and truly does not weigh a whole lot relative to the volume that it takes up. Most of it’s air,” said Richard Zinser, structural engineer for Zinser/Grossman Structural of Dallas. “The pipes are fairly light, compared to concrete sections of the same geometry. But the bridge itself is a fairly light structure.”
Design architect is Jonathan Bailey and Associates, and the project architect is URS. Wolverine Construction Management and Turner Construction are overseeing the project.
“We didn’t want just a standard square or rectangular bridge,” said Glenn Masty, Spectrum Health’s project manager. “We wanted an iconic feature for the project. We sat down and discussed many opportunities and proposals to come up with an iconic feature.
“The bridge is very unique. When you are on the bridge and walking from Tower 35, coming into the Children’s Hospital, it starts as a one-story bridge. It kind of looks like it is rotating, and it goes up to a four-story peak into the Children’s Hospital.”
The sweeping design features a myriad of unique angles, making the bridge an engineering challenge. Skinned by 628 pieces of glass, only two pieces are the same. The bridge is a truss system, said Aaron Davis, assistant project manager for Wolverine.
“They were looking for a unique bridge,” Zinser said. “There’s not very many sides of this bridge that are going to be straight or square to anything. It’s got slants and a sloped top that emulates a curve. It is very unique. It wasn’t your standard, walk-across-the-street bridge. Along with it came quite a few challenges regarding the design of the bridge.”
The hospital design was altered somewhat when Spectrum Health decided to abandon a plan to attach a needle-like spire and cables to the outside of the hospital. In that design, the cables also carried the bridge. With the spire scuttled, the bridge now relies on its connections to the two buildings flanking Michigan Street as well as an intermediate support on the north side of Michigan.
“We went through a number of gyrations with it,” Zinser added. “The cost of the different components and elements, the actual physical ability to design and construct some of these components and elements — we went through a rigorous evaluation, and when the hospital determined what their budget was, the bridge was the thing that they focused on as the piece that we keep.”
Zinser said they even ordered a wind tunnel study that looked at how the blustery side of Mother Nature might impact not only the bridge but the surrounding area.
“We ended up commissioning a wind tunnel study of the entire building, including the bridge,” Zinser said.
Zinser said the shape of the bridge presented a challenge in designing it to work with the wind that whistles from the west, over the Grand River and up the hill of the Medical Mile.
Masty said the study considered not only the wind, but air intakes, exhaust and odors. He said it looked at a four- to five-block area. The company created a scale model of the area for the wind tunnel test, Zinser explained. “It’s just a giant fan that blows wind on this assembly, this mini-city,” he said. Pressure taps were placed on the model to determine the scaled pressures of the wind on different parts of the building.
“The idea is that all of the surrounding buildings and whatever happens to occur on the site, that all has an impact in how the wind responds to the building. It more or less defined what pressures to use on the design of the building, both the structural frame as well as the glazing and any of the other skin components of the building.”
Key was coming up with a frame that fit the building and provided stability for the structure, but the wind tunnel study went a long way toward determining lateral loads that meet code requirements.
“The majority of the bridge is made out of what’s called structural pipe. They are round segments, and they are fully welded. The floor system that the bridge has got is just your standard steel composite concrete floor. We put a standard system inside this box truss that we developed,” Zinser said.
“Every single piece of glass, the glazing on the outside, because of how the structure of the bridge is, is different,” Masty said. “It’s not like we do a mock-up connection and we say, OK, that connection we do 100 times in the bridge. Every connection detail is specific to that particular corner.”
Wolverine found a company in St. Louis that works with these types of bridges frequently, Hillsdale Fabricators, which took Zinser’s 2-D drawings and turned them into a 3-D model that was used to build the bridge components. The pieces then were shipped to Grand Rapids and assembled on Bostwick Avenue.
To move the bridge off of Bostwick, a large flat-bed with multiple axles was loaded with beams, said Wolverine’s Tim Gray, project superintendent. The bridge was loaded onto the beams by a crane, then strapped to the beams for the moving process. A crane outfitted with a custom bumper pushed the engine-less flatbed onto Michigan Street.
“Basically, it comes down to we could not have built this bridge 10 years ago,” Masty said. “Without the CAD (Computer Aided Design) systems and the technology of the equipment and the laser cutters and things, we all believe from our side, it could not have been built 10 years ago.”