In a project chock-full of tricky terrain, the contoured roof profile still stands out as the most convoluted, with the warped skylight over the piazza a close second. Crews erected roof steel in over 20 sequences, in careful coordination with concrete walls and curtain-wall operations, says Matt Rossie, a project director for Webcor Builders, the San Mateo, Calif.-based general contractor for the California Academy of Sciences.
For example, wall gang forms needed to be flown out after casting, which meant that only the primary roof steel could be installed prior to the third-floor pours.
Steel detailing, rolling, fabrication and matching connections of intersecting roof beams were difficult, he says. Each curved beam was designed as a spline, which had to be converted into a polyline before being imported into the detailing program of SME Steel Contractors, Salt Lake City.
To simplify the tricky roof steel-to-concrete wall interface, SME proposed resequencing third-level concrete work. Instead of casting walls first, workers erected roof steel on thin steel columns. Concrete was then cast around columns, embedding them.
Roof contours and 200 penetrations for windows and vents complicated the 5-in.-thick deck. Also, the slab could not be shored from below.
During mock-up work, Webcor had devised a wood-form system, and learned that shotcrete was necessary for steep surfaces. In the field, it became clear that the shore-free system would take too long. Instead, the team figured out a way to use brackets on the steel-beam flanges to support wood joists, spanning between beams. The joists, eventually cut out, then carried the deck form.
The glazed skylight over the 96 x 72-ft piazza presented more challenges. A strengthened perimeter truss connects the cable-net structure to the roof steel. The perimeter truss was complicated by the roof curving in multiple directions around it. Months were required to finalize geometry of the horizontal perimeter truss and resolve interface issues, says Webcor.
SME crews erected the truss on shores. Then, crews from Webcor cast the adjacent roof deck.
Workers from curtain-wall and skylight supplier Josef Gartner USA Inc., Schaumburg, Ill., then installed the piazza’s glass walls below the truss. Following that, SME welded gussets to the inner chord of the perimeter truss.
Gartner crews then site-assembled and lifted cable-net trusses and fastened them to gussets. Rods connecting net trusses came next. Initial stressing followed, then skylight glazing and more stressing. All told, the tension pulled roof steel toward the piazza’s center by up to 1 in., reports Webcor. Tension will vary depending on the temperature of the piazza and roof steel.
“It was an interesting roof to build,” says Rossie.