To create the roof shapes, the arches for the two bilaterally symmetrical roofs lengthen from the ends toward the center. The longest clear span is 225 ft.
Adding complexity to the geometry, the cables spanning between the abutments are not pure catenaries, as in most suspension bridges. Safdie uses the roof arches to spread the cables apart toward the center, outlining an ellipse. Within each half of an armadillo roof, each arch has a different length and radius. The complexity prompted BH to use Gehry Technologies Digital Projects to create a structural building information model.
For the erector of the three cable-roof structures, the museum's masterpieces are not the paintings but the cable-roof-erection BIMs, created by construction engineers James M. Ronning and Michael Miller, both Minneapolis. “We had to go to the big guns in cable- bridge erection engineering to turn the designers' concepts into realities,” says Kepple. Without the BIMs, the cable roofs would have been virtually unconstructible, he adds.
Erecting the cable structures ranks as a first for Bosworth. Kepple calls the work his most challenging job in 30-plus years. The entire exercise hinged on initially installing the cables at an elevation that would allow them to end up at the final desired elevation when the roof was fully loaded. The idea was to avoid jacking the cables after the entire roof was finished.
The strategy called for creating an erection BIM from the 2D structural and architectural drawings that would not only provide an erection sequence but simulate the sequential loading of each roof. For this, Miller had to calculate the weight of all the materials, which added up to 40 psf. After using the BIM to virtually load the roof, the engineers reversed the process, removing each roof element until only the cables were left. In doing so, the flexible cables, which sag during loading, sprang back to their initial unloaded length. That length became the start elevation.
The goal was to place all the load without affecting the geometry and to hold the geometry while placing the load. “It is almost a catch-22 situation,” says Miller. “We were in somewhat uncharted territory” because the roof is also a shell structure—like a Pringle potato chip—not simply a cable structure, he adds.
During erection, Bosworth's ironworkers had to guard against undue deflections that would threaten the structure's shape and stability. Every element bearing on the cables changes the geometry significantly, says Ronning, especially at the start of the process.
Bosworth tackled the longer armadillo first. After installing the cables and their ball joints to the engineers' predetermined elevation, ironworkers installed hydraulic struts from cable to cable to push the cables outward to their elliptical shape until the arches took over. Crews then installed the arches, starting at the abutments and moving toward the center. Lateral bracing and cross bracing followed. “Proactive engineering made this a simple rigging operation,” says Kepple.
After crews finished erecting the roof framing, LNJV initiated the roof's architectural buildup phase, working from the center to the ends based on a symmetrical loading sequence developed by Geiger Gossen Hamilton Campbell Engineers, Suffern, N.Y.
Once the roof was fully loaded, crews tightened bolts and cross bracing rods. Bosworth then installed the sloped mullions. When the whole roof was done, the cable elevations were within the structural engineer's specified tolerance of plus 2 in., minus 2 in. at the cables' midpoint. But the sloped glazing didn't quite fit. “We discovered that the design tolerances weren't tight enough for the glazing,” says Kepple.
To get the glazing to fit, workers made a 1¼-in. slot in the concrete sill. To avoid the problem with the other two cable roofs, Miller redid the models to tighten tolerances. The glass for the second armadillo fit. Kepple anticipates the same for the crab, even though that roof is the toughest to erect due to its asymmetry.
Safdie acknowledges that erecting the cable roofs is like piecing together a Swiss watch. “When we went out for bids and the contractor was first on board, there was concern about the unfamiliar,” he says. “But it came together fast, smooth and without complexity.
“I can't wait to see it finished,” he adds. “No matter how many models, renderings and 3D simulations you see, at the end, it's never what you imagined.”