May 25, 2001

Seattle’s heavyweight champ

  • Seattle Cancer Care Alliance clinic is made of some of the heaviest concrete in the world.
    KPFF Consulting Engineers

    Seattle Cancer Care Alliance clinic
    Photo courtesy of KPFF
    The clinic’s underground linear accelerator vault uses concrete that weighs 350 pounds per cubic foot — more than twice that of typical concrete. This allowed the vault’s walls to be just 4 feet thick, instead of the standard 8-9 feet.

    The Seattle Cancer Care Alliance roots come from three of the Pacific Northwest’s most prestigious oncology institutions: Fred Hutchinson Cancer Research Center, University of Washington and Children’s Hospital and Regional Medical Center. These institutions are regionally and world renowned for their state-of-the-art care, leading-edge treatments, improved outcomes, and strong cancer prevention programs.

    In addition to numerous contributions in the healthcare field, the alliance has contributed to a noteworthy innovation in the concrete industry. The design of the alliance clinic led to the development of what is probably the heaviest concrete in the world. This heavyweight concrete enabled the alliance to gain 2,000 square feet of valuable treatment space.

    A life-saving mission

    The alliance’s goal is to be one of the world’s finest clinical cancer-care programs. Located on Fred Hutchinson Cancer Research Center’s Robert W. Day Campus at South Lake Union, the seven-story facility provides more than 159,000 square feet of occupied space. That includes specialized outpatient clinical functions, radiation oncology services, service labs, offices, a pharmacy and a patient education center.

    Zimmer Gunsul Frasca Partnership designed the clinic to provide a warm, non-institutional feel for patients and their families. The building also provides an efficient, uplifting atmosphere for physicians and staff.

    ZGF’s design team included structural and civil engineers from KPFF Consulting Engineers, mechanical and electrical engineers from Affiliated Engineers Inc., and medical planners from Kaplan McLaughlin Diaz.

    Linear accelerator vaults

    As a facility dedicated to the elimination of cancer, the alliance building houses a variety of advanced medical equipment. One of the most powerful pieces of equipment is the linear accelerator that irradiates cancer cells with an 18-megavolt photon beam.

    To protect surrounding spaces from this radiation, linear accelerators are typically enclosed in massive concrete vaults. Conventional vault design relies on normal weight concrete, which weighs about 150 pounds per cubic foot (pcf). Using normal weight concrete to provide shielding usually calls for parts of the walls and ceiling to be 8 to 9 feet thick. That puts the top of the ceiling slab about 20 feet above the floor.

    At the alliance clinic, the linear accelerator is located with other radiation oncology services on the first floor. With a conventional design, the top of the vault would have been about 6 feet above the second floor, and about 2,000 square feet of valuable floor space would have been lost.

    To reduce the concrete thickness, some vaults are designed with steel plates or lead bricks embedded in the slabs and walls. If the alliance vaults used 12 inches of steel, the thickness of the ceiling slab might have been reduced to 5 or 6 feet. However, the top of the slab would still have been higher than the second floor. In addition, the steel plates would have raised the construction costs, draining money away from treatment and research.

    To align the top of the vault with the second floor required reducing the slab thickness to 4 feet. From our prior experience, KPFF knew that heavyweight concrete was a potential solution. However, the structural engineers estimated the concrete would need to weigh almost 330 pcf. That would probably be the heaviest concrete ever produced.

    To verify that this design would provide adequate shielding, the team called on Doug Jones, a nuclear physicist and director of the Northwest Medical Physics Center. Jones determined that 44 inches of heavyweight concrete plus 4 inches of normal weight concrete would provide shielding for the Alliance’s 18mV accelerator, as well as a 30mV model that may be used in the future.

    Innovative concrete design

    The alliance team turned to Stoneway Concrete to develop the heavyweight concrete. Several years ago, Stoneway began research of a concrete mix that weighed about 300 pcf. The company hoped to use the material as a counterweight on a movable bridge. That idea never materialized, but the possibility of gaining space at the alliance clinic revived the development.

    Typical concrete is made of cement, water, gravel, sand and admixtures. The heavyweight concrete at the alliance project called for steel rather than gravel or sand. Stoneway salvaged steel punchings from miscellaneous steel fabricators. Using salvaged materials dramatically reduced the cost of the shielding and added an element of sustainability to the design.

    The heavyweight concrete that Stoneway produced for the alliance averaged more than 350 pcf. “This is one of the heaviest, if not the heaviest, concrete mixes in the world,” says Greg McKinnon of Stoneway. “Prior to this, I have not heard of concrete that weighs more 300 pounds pcf.” An engineer from the American Concrete Institute agreed.

    Construction challenges

    The extraordinary weight of this concrete created several challenges during construction. Most of Stoneway’s trucks are designed to carry 10 to 12 cubic yards of normal weight concrete. The heavyweight mix exerted extra stress on the front of the trucks, limiting the loads to 3 cubic yards. That limit meant delivery required 3 to 4 times more trucks than normal. Therefore, Stoneway and Turner Construction started the heavyweight operation at 4 a.m. on a Saturday.

    Turner was the general contractor and performed the concrete work. Because the heavyweight concrete utilized a very high cement content, Turner had to ensure the mix was properly retarded. Otherwise, the concrete would set too quickly or not at all. Turner also needed to discharge the concrete slowly to prevent the steel aggregate and cement slurry from separating.

    A common goal

    The Seattle Cancer Care Alliance outpatient clinic opened its doors on Jan. 29. “What we did (developing the heavyweight concrete) is not really the best part of this project,” says McKinnon. “What is important is the tremendous value this brought to SCCA.”

    Creating the heavyweight concrete was a remarkable innovation. Creating 2,000 square feet of additional space in such a special facility was even more tangible and rewarding.

    Brian Pavlovec, PE and SE, is an associate with KPFF Consulting Engineers and was project manager for the Seattle Cancer Care Alliance outpatient clinic. Nate Schwab was KPFF’s project engineer on the job.


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