Northwestern Memorial’s Prentice Women’s Hospital

Design & Construction Team

Project Name: Northwestern Memorial’s Prentice Women’s Hospital
Owner: Northwestern Memorial Hospital, Chicago
Structural Engineer: Thornton Tomasetti, Chicago
Architects: VOA + OWP/P Design Collaborative, Chicago
Contractor: Power/Jacobs, Chicago
M/P/E engineer: Environmental Systems Design, Chicago

Opening in 2007, Northwestern Memorial’s Prentice Women’s Hospital is a 950,000-squarefoot, 17-story hospital dedicated to providing health services for women at all phases of life.

The state-of-the-art facility will be among the top three birthing centers in the nation, with the capacity for more than 13,000 births per year, and one of the largest centers for special-care infants.

Located on the campus of Northwestern Memorial Hospital and the Northwestern University Medical School in downtown Chicago, the new facility occupies a full city block. The urban setting presented challenges because of site constraints, existing utilities, and adjacent structures. Because it is a replacement hospital constructed on the site of a former building, the location of existing foundations was also an important consideration.

The new Prentice Women’s Hospital houses a variety of program elements that are vertically stacked because of the site’s relatively small footprint. Hospital functions include patient rooms, labor and delivery rooms, and a neonatal intensive care nursery. Diagnostic suites and a breast cancer center required the accommodation of MRI equipment and linear accelerators. Also, the facility includes column-free public spaces such as an extensive conference center, auditorium, and cafeteria.

The overall site area is 66,665 square feet, with a maximum building footprint of approximately 59,000 square feet after setbacks. There are 17 levels above grade, one level below grade, and two mechanical floors. With floor-to-floor heights ranging from 14 to 27 feet, the overall height of the building is 305 feet, which is comparable to a 30-story residential building. Twenty-five elevators are distributed between two elevator cores.

The structural system is comprised of concrete shear walls at the two elevator cores to resist wind loads and a structural steel frame to support gravity loads. The foundation system consists of belled caissons bearing on stiff clay at 90 feet below grade. The building skin has precast concrete and glass curtainwall elements.

With Northwestern Memorial Hospital as the owner and Power/Jacobs as construction manager, the Chicagobased project team was led by the VOA + OWP/P Design Collaborative.

Consultants included Thornton Tomasetti as the structural engineer and Environmental Systems Design as the mechanical/electrical/plumbing/fire protection engineer.

Site constraints

Building in an urban environment offered challenges resulting from adjacency to busy city streets and existing utilities, and the potential for damage to existing structures due to settlements caused by excavation. In addition, the proximity of the hospital site to Lake Michigan makes for a high groundwater table that fluctuates between 11 feet and 14 feet below grade, well above the basement slab, which is 17 feet below grade.

A 2-foot-thick concrete slurry wall along three sides of the site was a costeffective way to address these issues.The slurry wall was used as a temporary earth-retention system during construction.

By extending it into the soft clay, it served as a permanent water cutoff, eliminating the need to design the basement slab for water uplift. In the final condition, the slurry wall also functions as a basement wall.

A different approach was required along the west side of the site. A 100- year-old brick sewer was located in the alley immediately west of the property line. Steel sheet piling embedded in multiple layers of existing concrete basement walls followed the property line, and existing grade beams, piles, and concrete caissons were located directly against the existing concrete walls.

Therefore, the existing obstructions prevented feasible construction of a slurry wall along that edge. The solution was to vibrate a steel sheet pile system into the ground, working around the obstructions, to provide temporary earth retention until a basement wall could be constructed.Wide flange steel members welded to the steel sheet pile and cast into the slurry wall provided closure at the corners where the sheet piles and the slurry walls intersected, and prevented water infiltration into the site.

Getting out of the ground

To maximize the building footprint while meeting setback requirements along the north, east, and south sides of the site, the building was extended to the property line on the west side. However, existing conditions required the caissons to be pulled into the site by approximately 5 feet along the entire west side to allow sufficient clearance for the caisson drilling equipment. A series of cantilevered grade beams was designed to reach out over the top of the caissons to the property line to support the perimeter columns and foundation wall.

A 6-foot, 6-inch-thick mat foundation supporting 300-foot-tall concrete core walls along the west property line also was cantilevered over in-board caissons.

The challenges posed by existing conditions did not stop at the perimeter.

The site was occupied previously by the Wesley Memorial Hospital, constructed in 1937, and an addition built in 1956.

Re-using the existing timber pile foundations of the Wesley structure and the concrete caissons of its addition was not viable because their capacity was significantly less than the foundation loads of the new hospital. In general, where new caisson locations conflicted with existing timber piles, the piles were pulled out of the ground and the voids filled with grout.

Caissons were then drilled through the grout. Within a 30-foot zone near the slurry wall, pulling existing timber piles may have caused potential movement of the slurry wall. Instead, new caissons were drilled through the existing timber piles within that zone. In cases where new caissons conflicted with existing concrete caissons, transfer grade beams straddling the existing caisson were designed to transfer the column loads to two adjacent caissons.

The thickness of the mat foundation and of the transfer grade beams was such that the heat of hydration and temperature differentials between the center of these concrete elements and the exposed surface became a concern.To prevent the potential for significant cracking, mass concrete provisions were specified for these members.These provisions included limits on the maximum temperature at pour and maximum permissible thermal gradient between the center of the thick concrete element and its exposed surfaces.

Low heat-generating mix designs were specified and mathematical simulations of heat generation and thermal gradients, using the properties of the mix design and ambient conditions, were performed in advance of placing the concrete. The mathematical simulations were verified in the field using thermal sensors located in the mass concrete elements.

To provide insight into the complexity of the below-grade construction, the site excavation and foundation construction process took 18 months—more than two-thirds of the overall structural construction schedule—while the superstructure was erected in just seven months.

Challenges of vertically stacked programs

The program called for a 60-foot by 90-foot, column-free conference center to be located on the third floor. Since the structural grid was 29 feet by 29 feet, at least two columns supporting 14 floors would have to be transferred.

Furthermore, the transfer system could not be located on the floor directly above the conference center because of program constraints.

The solution was to locate story-deep transfer trusses spanning 58 feet within the seventh floor mechanical space. The trusses were designed to carry column supporting the floors above the seventh floor, as well as hangers from which the structure of the fourth, fifth, and sixth floors were suspended to enable columnfree spaces on the third floor. An opentruss configuration minimizes the impact on the functionality of the mechanical space, allowing access between truss members for people, equipment, ductwork, and piping. A similar scheme was used for the column-free spaces in the third floor auditorium, with the truss located on the fifth floor in this case.The trusses were detailed to simplify assembly in the field, see photo at top right.

A grand entrance

A clearly defined entry point to the facility was created by delineating a threestory- tall space along the entire south elevation of the building, above a driveway located on the ground level, see rendering at middle right. This resulted in two rows of three-story-tall columns to support the floor structure above the driveway. The 60-foot unbraced length of the columns, carrying up to 15 floors, required large and heavy built-up steel shapes. The structural solution was twofold.Horizontal moment frames consisting of cruciform steel shapes were extended out from the building to brace the columns along one row of the tall columns.Where bracing was not possible, cruciform-shaped columns constructed out of standard steel shapes were used to optimize material quantities by increasing the stiffness about the weak axes of the columns.

The bracing system was required to satisfy both strength and stiffness requirements of the American Institute of Steel Construction’s Load and Resistance Factor Design (AISCLRFD) Steel Specifications Section C3.

The horizontal moment frame was designed to resist loads in the order of 800 kips, while allowing a horizontal movement of only about half an inch.

Special details were developed at the moment connections between cruciform- shaped horizontal frame members and steel columns to ensure the proper transfer of forces, see photo bottom right.

Linear accelerator rooms Linear accelerator rooms were located below grade to minimize the impact on the steel floor framing. Various options were considered for providing radiation shielding. Using 4-inch steel plates embedded in reinforced concrete walls would have required elaborate temporary stabilization systems to support the steel plates while the concrete wall was constructed around it; this option was eliminated to simplify construction.

Normal weight concrete walls would have resulted in significant loss of program space. Hence, the optimum solution was using heavyweight concrete walls with a density of 250 pounds per cubic foot. To take advantage of the shielding properties of the soil on two sides and achieve further savings, the linear accelerator was located at a corner of the lower level.

Future MRI rooms

The project team considered not only the weight of the MRI units and shielding, but also the path along which future replacement units would be transported into the facility. Removable sections of floor slabs were detailed in the fourth and fifth floors directly above the driveway below to allow equipment to be lifted on to the floors without removing the cladding, as is customary in many facilities. A clear load path with reinforced structural framing and slabs was defined on structural drawings that could be used to safely transport units to their final locations.

The structure at the current MRI rooms was depressed 12 inches to accommodate a secondary, shielded encasement. Locations of future MRI rooms were defined and a removable layer of structure was placed over a depressed area. The space can then be converted easily to MRI rooms in the future with minimal disruption of hospital operations.

A state-of-the-art facility

While creative solutions to structural design challenges may have contributed to the success of the overall project, the guidance provided by Northwestern Memorial Hospital, the direction provided by the construction manager, and the leadership of the architect in creating a design that responds to the needs of the hospital and the community resulted in a successful project.

Faz Ehsan, Ph.D., P.E., is a vice president at Thornton Tomasetti in Chicago and served as project manager of the Prentice Women’s Hospital. He can be reached at fehsan@thorntontomasetti.com. David Weihing, P.E., S.E., is an associate at Thornton Tomasetti in Chicago and served as project engineer.He can be reached at dweihing@thorntontomasetti.com.