Yale University’s School of Medicine extension
Design & Construction Team
Owner: Yale University School of Medicine, New Haven, Conn.
Architect: Payette Associates Inc., Boston
Structural engineer: Simpson Gumpertz & Heger Inc., Waltham, Mass.
M/E/P/FP engineer: Vanderweil Engineers, Inc., Boston
Geotechnical engineer: Gibble Norden Champion Brown Consulting Engineers, Old Saybrook, Conn.
General contractor: Turner Special Projects Division, Milford, Conn.
Steel fabricator: Cives Steel Company, Gouverneur, N.Y.
Steel erector: Capco Steel Corporation, Providence, R.I.
Concrete subcontractor: Waterbury Concrete Foundations, Watertown, Conn.
The $12 million extension of the Yale University School of Medicine’s Sterling Hall of Medicine’s B-Wing, located between the existing B-Wing and the Laboratory of Epidemiology and Public Health (LEPH), filled a gap in a crescent-shaped frontage of Yale School of Medicine buildings that face a major traffic artery in New Haven, Conn.
The project’s architect, Payette Associates, Inc., was charged with placing a state-of-the art medical research facility in the garb of a 1920s building at this highly-visible location, and designing the building to look as if it had always been there.
The addition to the Georgian-revival B-Wing consists of a mechanical level and three floors of wet laboratories, lab support space, offices, conference rooms, and break rooms. The new structure provides laboratory space predominantly for the Department of Pharmacology and serves as swing space for other departments. Total gross square footage of this program, including the loading dock, is approximately 30,000.
The central challenge of the project was the building’s location over a busy, existing loading dock that serves critical receiving and waste management functions for the Sterling Hall of Medicine and LEPH buildings and, via these buildings, other buildings throughout the complex. The new building had to allow for these, as well as expanded dock operations, to continue through construction and in the future. The solution—reminiscent of an offshore oil drilling rig— was to support the entire building on story-deep trusses framing into four mega-columns placed to interfere as little as possible with loading dock operations.
The placement of the mega-columns had to satisfy four constraints: provide maximum maneuvering space for trucks, stay clear of the foundations of the existing buildings surrounding the site, minimize the need to relocate the complex network of utilities that crisscrossed the site, and provide enough open floor space for the building’s air handling units. The optimal solution was four mega-columns laid out in a 40-foot by 60-foot rectangular grid. The megacolumns support an orthogonal system of story-deep transfer trusses with cantilevers ranging from 10 to 20 feet that support the irregular floor plates of the building superstructure above.
The column locations at the laboratory levels were set to maximize the efficiency of the laboratory program. To minimize the extent of the transfer trusses, on the east side of the building a number of relatively lightly loaded exterior columns that did not interfere with the loading dock were carried to ground.
Other than roof-mounted exhaust fans, all of the building’s mechanical equipment is placed within the trussed service level. This level also provides a handicapped accessible entry to the building from the LEPH forecourt.
The 13-foot inter-story height of the new truss and laboratory levels matches those of the existing B-Wing. The contextual nature of the addition required that its exterior match the BWing’s Georgian-revival façade.
The lateral-load resisting system had to maintain structural separations at the movement joint where the addition abuts the existing building.
Structural steel was the material chosen for the superstructure and the transfer trusses based on the following parameters: weight, schedule, mechanical- electrical-plumbing (MEP) coordination, cantilevers, column size, and cost.
Foundation
The building bears on glacial outwash sands that are typical in the costal areas of Southern Connecticut. The subsoil conditions allowed for shallow spread footings with an allowable bearing pressure of 2.5 tons per square foot. The water table was approximately 5 feet below the typical footing elevations.
Four large, reinforced concrete footings at the bases of the mega-columns spread most of the building load to the subsoil. Each of these footings had to be proportioned differently to minimize the number of existing utility relocations.
Two of the four mega-column footings were tiered to clear existing service lines running over the tops of the footings.
Superstructure
The lightweight concrete slabs for the laboratory framing levels have a total thickness of 6-1/4 inches cast onto a 3- inch galvanized composite metal deck.
This design allows for an unshored deck span in excess of 10 feet, which minimizes the infill framing. It also provides the required fire rating without sprayapplied fireproofing on the underside of the deck. The use of lightweight concrete provided the lowest slab weight achievable for the deck span.
The roof uses the same composite slab design. Although this is not ideal from the standpoint of weight, the concrete slab is used primarily to minimize sound and vibration transmission from rooftop strobic fans to the occupied spaces below. The concrete slab also provides some flexibility in the installation of other minor roof-mounted equipment. The composite action between the concrete and the steel spandrel beams achieves an efficient design of the spandrel members supporting hung relieving angles for masonry and a heavy cornice element.The concrete slab also provides an easy and flexible anchoring medium for the coping stones at the top of the exterior wall.
The superstructure framing bays vary with the irregular floor plan, but the maximum bay plan dimensions are on the order of 20 feet by 28 feet. Infill framing members are typically W16 wide-flange shapes designed compositely with the slab.
Intense coordination by the design team provided an integrated architectural, structural, and MEP design that provided 10-foot ceiling heights in laboratories and offices and 8-foot, 1-inch ceiling heights in support spaces. All of this was possible with only 15 planned beam web penetrations on the laboratory floors. There were no field-coordinated beam web penetrations on the laboratory floors.
The lateral-load resisting system of the superstructure is a distributed moment frame utilizing essentially all of the building columns. Girders and beams on column lines are momentconnected to the columns with fieldwelded moment connections. The distribution of the moment frames throughout the floor plan keeps the depth of the lateral-load resisting beams and girders to 18 inches. The beamcolumn connection details conform to the latest (at the time of construction) available guidelines that were emerging from the post-Northridge SAC studies.
Transfer trusses
The superstructure column grid transfers to four mega-columns via a system of eight trusses. The trusses consist of W14 wide-flange chords and diagonals. The wide-flange chords carry a combination of axial loads from truss action and flexural loads from the floor system. Chords are typically continuous and are spliced only where necessary, such as at an intersection with another truss. W14 sections provide the best combination of strength, shallow depth for maximizing space available for mechanical equipment, and ample flanges and webs for arranging bolt patterns. Truss diagonals are bolted to gusset and flange plates shop welded to the chords. All truss connections are slip-critical on oversize bolt holes.
Bolted connections allowed two things: a maximum amount of flexibility and adjustment for erection tolerances, and minimized restraint stresses that would have been built into the trusses from the welding of heavy sections.
The trusses along the mega-columns, as well as the mega-columns the trusses frame to, constitute a lateral-load resisting moment frame. As such, it is important to have as direct a transfer of gravity and lateral forces as possible. This is accomplished by the use of a cruciform column built-up from a W36 section and two WT18 sections. The webs of the cruciform section align with the orthogonal planes of the trusses they support. The cruciform columns are encased in concrete for weather, impact, and fire protection. The truss chord connections to the cruciform columns are designed for the combination of gravity loads and the amplified seismic load cases of the 1997 American Institute of Steel Construction Seismic Provisions.
The grid of transfer trusses are thoroughly integrated with the mechanical equipment. The two main air handling units, installed during the steel truss erection, are now surrounded by trusses and, as such, will not be easily moved or accessed for major component replacement and upgrades. To provide future access, the floor framing in front of each unit has an 11-foot by 10-foot knockout panel that will provide access to the loading dock level below.
Stairs
In this contextual architectural design, the one external concession to modernity is a spiral rectangular stair on the north side of the building framed by a glazed aluminum curtain wall. The stair overhangs the structural edge of the building by approximately 8 feet without any support from the structure that supports the curtain wall. This is achieved by cantilevering the stair stringers from brackets connected to the building columns around the stairwell.
Our structural engineers worked closely with Payette Associates to define stringer layout, stringer sizes, and bracket designs required by this bold architectural gesture. The structural design of the cantilevered components was worked into the otherwise conventional stair design drawings and performance specifications.
Value engineering
The steel tonnage of the building’s lateral-load resisting system was largely driven by the drift limitations at the movement joint between the new and existing buildings. For the member sizes and layout of this project, the stiffer the moment-resisting frames, the more susceptible they are to flexural stresses induced by truss vertical deflections.
Initial pricing obtained from fabricators indicated that the original 3-inch-wide seismic joint width was something that the project could not afford. As a result, the architect modified all of the movement joint details to allow for a 6-inch-wide movement joint. The steel tonnage was reduced from the lighter and more flexible superstructure moment frames and transfer trusses.
Customary East Coast practice is to performance specify the truss connections that then are designed by the steel fabricator’s engineer to accommodate the fabricator’s shop set-up and fabricating preferences. However, in order to eliminate bidder uncertainties regarding the truss connections, our engineers designed all of the truss connections.
This provided full definition of the truss connection design and eliminated at least one cycle of fabricator calculation preparation, submittal, and review.
Conclusion
The project broke ground in summer and was completed one year later in the fall. Steel fabrication and erection proceeded without incident.
The erection was completed in nine weeks, including the casting of the mechanical level slab and the installation of the air handling units. The success of the contextual design is evident by the fact that the casual observer thinks that the B-Wing extension has always been there.
Pedro J. Sifre,P.E., is an associate principal and James C. Parker, P.E., is a senior principal at Simpson Gumpertz & Heger Inc. They can be reached at pjsifre@sgh.com and jcparker@sgh.com, respectively.
