Going vertical
Vanderbilt's MRB 4 straddles two existing building

By Candy McCampbell

The Medical Research Building 4 going up at Vanderbilt University in Nashville is actually a joined pair of upward expansions that will add a total of 15 stories to two existing buildings, completely encapsulating one of them.

The $120 million project will provide 420,000 sq. ft. of laboratory, learning and office space.

An integral part of the support system is four post-tensioned, cast-in-place concrete trusses that will handle loads over the existing four-story Langford Auditorium, where 12 additional floors are being constructed.

From the trusses to the skin, the building - called MRB4 - will require 17,500 cu. yds. of concrete. Begun in May 2004, the building is scheduled for completion in early 2007, although some researchers will start occupancy in late 2006.

The structure itself is made of post-tensioned concrete, chosen for its continued strength over the life of the building, said Merrill Bowers, project manager for Turner Universal Construction of Nashville, the construction manager.

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"It's more fail-safe," he said.

The post-tensioned, cast-in-place trusses "are not common at all," he said. At about 24 ft. tall, 140 ft. long and 5 ft. thick, the trusses answer the need for a rigid building.

"Vanderbilt has restrictive criteria for deflection (or deviation)," said John Carpenter, partner in Carpenter Wright Engineers of Nashville, who designed the system. "The university wanted a very stiff building" that maintained deflection at about 1 in. over the 140-ft. span, he added.

"When you limit deflection this way it gets expensive," Carpenter said. The same type of concrete truss had been designed for an earlier Vanderbilt project, MRB3.

"They (the trusses) are one of the biggest challenges of the building" and each will require 500 tons of concrete to construct, Bowers said. The trusses will be erected and poured in a precise manner - so exact that Carpenter issued more than three pages of single-spaced, step-by-step instructions.

First, 12-in.-sq. tube steel and post-tensioning cables are installed to create the truss frames, followed by the installation of the form system.

Poured-in-place concrete then "wraps" the steel. Meters and sensors are placed in each truss, which will undergo five staged stress tests.

"There are 10 pours for each truss" over a two- to three-month period, Carpenter added. Some of the existing spread-footing foundations had to be strengthened, so the old base was removed and replaced.

Floors above the trusses are made of cast-in-place slab beams with post-tensioned cables to transfer some of the load back to the columns, Bowers said.

New 16-in.-sq. columns were joined to existing columns in the adjacent Light Hall to support an additional 12 floors, each 7 in. thick. The floors will be joined with the Langford expansion by hallways at the fifth floor and above.

Two of the building's 467 precast exterior panels will measure 61-ft.-long, the biggest panels ever poured at Gate Precast Co.'s plant in Ashland City, Tenn. One is 8 ft. 3 in. wide and the other is 5 ft. 6 in. wide.

"Our largest casting bed is 56 ft., so we will have to shut two beds down and bridge them," said Chris Winfield, Gate vice president.

The panels will be post-tensioned and have standard rebar enforcement "because of their size and the gravity forces that act on them," he added.

The largest panels weigh more than 50,000 lbs. and required that the contractor beef up the building's load capacity.

The total weight for all of the building panels is about 4.76 million lbs., he said.

Gate will bring in a 500-ton crane so workers from Precast Services of Columbus, Ohio, can lift the panels into place.

The concrete for the structure itself will come from Metro Ready Mix of Nashville, most of it pumped with a 60-meter pump set up on-site next to the building, Bowers said.

"The big slab pours will all be made at night, seven minutes apart, using 15 trucks," he added.

Sandwiched between a hospital and a medical library building, the jobsite is tight with only minimal laydown space.

"All the deliveries are just in time," Bowers said. "The superintendent schedules 24 hours for majors, 48 hours if it takes time. The tower crane and sub contractor coordinate every week."

Parking for the 280 workers is similarly tight. Some managers park in a nearby multi-level garage but most of the crew park at the local baseball stadium lots and ride shuttles the university runs in the morning and evening.