Brute Strength

October 16, 2008
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What does it take to hold up the 32 million pounds of concrete plus steel and machines in the radiation therapy rooms at Spectrum Health’s Lemmen-Holton Cancer Pavilion?

Why, more concrete, of course.

Underneath the Michigan Street cancer center — as well as the Michigan State University Medical School and RDV Corp.’s two planned medical office buildings —  is a 2,300-space parking garage. The Lemmen-Holton center opened last month.

Above the parking structure in the cancer pavilion are five rooms, known as vaults, where cancer patients receive radiation therapy. To keep radiation from escaping, the rooms have concrete walls 7 to 9 feet thick and 4-foot-thick ceilings, parts of them lined with steel. Linear accelerators, the machines which deliver high-dose radiation to tumors, can weigh as much as 20,000 pounds and require 100 tons of shielding.

Usually, radiation therapy rooms are located on the ground floor, where Mother Earth bears the massive weight and provides a natural shield. But with parking below, Lemmen-Holton architects and engineers, from Carl Walker Inc. of Kalamazoo and URS Corp. in Grand Rapids, worked with construction manager The Christman Co. of Lansing to figure out how to suspend the heavy rooms over thousands of cars. Company representatives said it was the first time they’d worked on above-grade vaults.

“The primary reason that the linear accelerator vaults are elevated and not on-grade, as they most commonly are, really has to do with the whole set-up of the parking spaces,” said Joe Greenam, URS principal-in-charge for the Lemmen-Holton project. “There is a base of 2,300 cars, and the cancer pavilion sits on top of a portion of that.”

He said the building’s designers at first considered putting the vaults on the ground. But results of a cost-benefit analysis showed it make more sense to retain the parking structure and locate the vaults on the cancer center’s first floor, he said.

“We went through an analysis based on good engineering principles, tested it with the construction manager for the facility, and then did a cost-benefit analysis to see if it was truly the best solution of all the options available,” Greenam said.

Four rooms were equipped for immediate use for megavoltage radiation therapy; a fifth was outfitted for high-dose radiation therapy, such as implanting a source of radioactivity at the cancer site; and a sixth was shelled in for future use.

“The vibration was one concern, and just supporting the weight of the concrete and steel plates that are used to contain the radiation,” Greenam said.

URS Project Manager David Byl said the ceilings in the radiation rooms are 4 feet of concrete, with 4 inches of steel plate, while the walls and floor range in thickness from 3 feet, 8 inches to 7 feet, with steel plates as thick as 8 inches.

“You’re talking probably 32 million pounds of concrete,” he said. “That would be like 10,000 passenger cars.”

Gail Vasonis, chief engineer for Carl Walker Inc. in Kalamazoo, a parking structure consultant, said planning was the key for execution of the job.

“We worked with a physicist to determine the thickness of the concrete (and) … to determine the locations of the accelerators,” Vasonis said. “The thickness of the materials was established that way.”

Unlike radiation vaults placed on terra firma, these vaults required floors that are as well shielded as the walls and ceiling, he said. Floors as thick as 7 feet required design planning to maintain a 20-foot ceiling-to-floor clearance to ensure headroom for emergency vehicles, he said.

“Where you have a 7-foot thick concrete floor slab, that’s over 1,000 pounds per square foot of weight for the floor and ceiling. That is pretty heavy,” Vasonis said. “The typical floor slabs for a parking area is typically 6 inches thick, so that’s 14 times that thickness.”

In the parking structure underneath the vaults, support columns, bolstered by rebar, are closer together and are 36-by 36-inches instead of 30-by-30 inches, he said.

The thick floors were poured in four layers. The thickness varied, being not as thick in areas where shielding requirements were diminished, to save on materials and design costs. The floors were poured in four separate layers, with a sandwich of eight 1-inch layers of steel plates after the second pour, Vasonis said.

Each concrete layer required a separate analysis of the formwork that supported the wet material, he said. Christman Project Manager Dave Schoonbeck said supports of wood and metal were specially built for each layer.

“There was a lot of design analysis that went into the form work. The walls are 20 feet tall. To place all that concrete at 20 feet, it wasn’t done in one layer. The weight of that wet concrete puts a tremendous amount of pressure on those wall forms,” Vasonis added.

“When the concrete hasn’t yet set or cured, it’s just adding weight itself. From a structural standpoint, you need a structural system just to support the form. The formwork itself becomes a complex system. We looked at sequencing, so the slab could be poured a portion at a time. The first layer was designed to support the next layer of concrete.”

Plus, the heat that would be generated by seven feet of curing concrete would be a major crack threat, Vasonis said.

“The construction sequencing aspect with respect to the overall weight of the concrete, and how that was sequenced with the formwork and shoring, was tightly coordinated between Christman and us to ensure everything was done safely and economically,” he said.

The metal was installed under a 16-by-20-foot area of each vault in 1-inch layers, because anything bigger would have been difficult to lift, Vasonis said.

For Schoonbeck, lifting the steel plates, which weighed several tons per piece, was a bigger construction challenge than pouring extra concrete.

“Partially through the process, they wanted us to add steel shielding,” Schoonbeck explained. “We had to figure out how to combine 8 inches thick of steel plate in the walls and ceiling. We ended up doing a quarter- or half-inch plates. We found a supply house in town that could cut it down to size and found a stabilization method for putting it vertically (in the walls) using pins.”

To place the plates, Schoonbeck’s team came up with a rigging system that used industrial magnets at each plate’s four corners, lifted it from an area on Michigan Street up and over the active construction area right next to the street, and then put it in place in the vault. He said concrete was poured during the day and the plate-placing occurred at night.

The ceilings required 24 to 32 stacked plates, and the walls, 16. “It was very time intensive,” he said.

The final concrete pour on the floor was a top layer incorporating electrical and mechanical systems.

“With this kind of brute strength, steel and concrete and everything involved, Chrisman has done a real nice job with those finishes so you don’t have a sense that you’re in this concrete vault,” Vasonis added. CQX

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