Mortenson Construction's Bill Gregor is a big fan of off-site construction. Still, he is careful not to do prefab for prefab's sake. To nail down the contractor's component preassembly strategy for the $623-million Exempla Saint Joseph Hospital Heritage Project in Denver, Mortenson held a prefab charrette during the job's preconstruction and programming phase that included the owner, the contractor and the architect.

The exercise "is an analysis of what makes sense to drive cost, quality, safety and schedule," says Gregor, Mortenson's Exempla hospital construction executive.

For the 831,000-sq-ft project, a replacement for a hospital that faced a shutdown for code violations, schedule was the biggest pressure. Mortenson has to have the new hospital operational by Jan. 1, which left 29.5 months for a job that should take three years.

Over several months starting late in 2010, after a two-year hiatus prompted by the recession, the building team fine-tuned its off-site plan. Ultimately, the team elected to prefabricate 166 multitrade overhead utility racks, nearly a mile in total length; 440 of 562 bathrooms; 346 exterior panels and 376 patient-room head walls. Mortenson elected for site construction for the lower three floors of the seven-story building because of a lack of repetition of elements.

The only volumetric modules of the building are the bathroom pods for the 360 patient rooms and the administrative offices. Eggrock Modular Solutions, a division of Oldcastle, supplied them. The pods were shipped from Littleton, Mass. They contained all the finishes, down to towel bars, mirrors and toilet-paper holders.

Shipping the pods from the East Coast was not the greenest solution, but, "four years ago, there were only two experts that prefabbed bathrooms," says Maja Rosenquist, Mortenson's project director. "There are eight today, and they have multiple locations."

Workers mocked up a patient room on-site to work out all the design details. "The last thing you want to do is prefab [something] the wrong way 360 times," says Rob Davidson, a principal with architect H+L Architecture, part of hospital architect H+L/Davis, A Joint Venture, in association with design architect ZGF Architects LLP.

During the mock-up work, "we dropped the ceiling two inches to fit the largest number of pods in a standard-sized truck," says Davidson.

Mortenson rented a warehouse eight miles from the site to fabricate the exterior panels, typically 30 ft long by 15 ft tall. Crews erected 26 panels daily. After that, workers completed the detailing on the connections between the panels, installed the masonry on the clips set in the warehouse and added the 6-ft-by-8-ft windows.

The masonry was left off the prefab panel because of its weight. Glazing was left out because suppliers will not maintain warranties for off-site preassembly, says Gregor. Hoisting imposes different stresses on the panels. That could break the seals and shatter the glass.

The multitrade racks, typically 25 ft by 8 ft, contain hydronic piping, air ducts, cable trays, conduit, pneumatic tubing and drywall. Crews fabricated the racks at a 60,000-sq-ft warehouse four miles from the site. The work, which relied heavily on building information modeling, demanded an unusual amount of coordination. Mortenson, the owner, the design team, and the mechanical and electrical subcontractors were deeply involved, says Gregor.

Head walls were assembled at the same warehouse. The decision to prefabricate them was based on a goal of increasing quality and reducing congestion, not on increasing site productivity, says Gregor.

Mortenson was selected for preconstruction and construction services before Exempla put a hold on the project, in 2008, in the wake of the recession. In 2010, the project was reprogrammed and re-sited. Late in 2010, the architect was given the green light.