Track lengths vary with petal width from 225 ft to 375 ft. Each petal will bear on eight, two-wheel bogies along the inner rail. The uplift on the outer rail will be resisted by six roller assemblies.

The maximum travel speed of a petal is 30 ft per minute. The system will open or close in 10 minutes.

Thanks to the long-span roof, there are "super-high, concentrated foundation loads" at the mega-column locations, says Rudolph P. Frizzi, managing principal of the job's geotechnical engineer, Langan Engineering and Environmental Services. Uneven subsurface conditions, with rock at 15 ft to 100 ft depths, prompted Langan to design a hybrid system of augur-cast piles and drilled piers.

Roof construction is driving the critical path for the entire project, says Wayne E. Wadsworth, principal in charge for the stadium's general contractor, the Holder-Hunt-Russell-Moody Joint Venture. "One of the engineering challenges is to build the roof within the tolerances for successful roof operation," he says.

A big question is whether the operable-roof steel can overlap construction of the flat-roof steel; otherwise, before starting the operable framing, the flat roof has to be completed in its entirety. Concurrent construction has the advantage of clearing the bowl's interior of falsework towers earlier, thus hastening the work on the precast seating risers and the halo scoreboard. But concurrent construction "may or may not be risky" with regard to meeting roof tolerances, says Wadsworth.

To help work out the roof erection sequence, which must be finalized before the guaranteed maximum price is set in early September, HHRM is using a 4D building information model, which links the schedule with the project's 3D BIM.

Though there are fewer than six weeks left to select a roof erection strategy, Wadsworth is not too worried about making a timely decision. "We've had lots of support from Buro Happold, which has been an integral part of making sure we understand the structure and the mechanization," he says. "We've kind of known where we've been going for a while."

This article originally appeared in the July 14 print edition of Engineering News-Record.