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Systems selection occupies much of a structural engineer's time during the concept phase of a project, and many factors affect the decision to use one type of construction over another. Desired floor-to-floor heights, winter construction schedules, local contractors' preferences, material costs, and aesthetics all influence the selection of structural components.
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The North Quadrangle Residential and Academic Complex at the University of Michigan in Ann Arbor exemplifies the program-driven results of structural systems selection for a $175-million building complex occupying 360,000 gross square feet. The multitude of systems and materials used in the final structure illustrates the thought process that occurs within the design team. North Quad's seven-story academic tower and 10-story residential tower connect in a series of classrooms, offices, study centers and television studios beneath the plaza between the towers. The dining hall on the building's academic side and the Media Gateway on the residential side add to the diverse programming of the complex.
The 450-student residence hall was designed and constructed to be 30 percent more energy efficient than required by the ASHRAE energy code. The building incorporates sustainability measures such as maximum insulation, energy efficient windows, occupancy sensors to reduce lighting levels and allow wider temperature swings when rooms are unoccupied, water conserving plumbing fixtures, and exhaust heat recovery.
Due to the complexities of the project and the large number of partners involved, the team embraced a decision-making process for this fast-track job, implementing many aspects of integrated project delivery, leveraging building information modeling software to great advantage.
Foundations
A detailed geotechnical investigation, including pressuremeter testing, found that site soils had an allowable bearing capacity of 12 kips per square foot, allowing the use of preferred spread footings with minor soil preparation.
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One of the primary concerns of the underground connecting structure that extended as much as 40 feet below the plaza was waterproofing, addressed by two key considerations. First, using a one-sided forming system allowed the concrete foundation walls to be placed directly against a waterproofing system layered on the perimeter soil retention structure. With no form ties penetrating the waterproofing, no leaks could form through breaches in the membrane. This system also maximized the buildable area up to the property line. Second, adopting a strategy of building a very rigid base beneath the plaza eliminated the need for expansion joints through the foundation walls and plaza slabs, as well as the potential for leaks from aging joint material located underground. Cast-in-place concrete walls support the masonry perimeter walls of the two towers, while substantially stiffening the base structure.
The base structure, itself, consists of several different types of concrete construction. The most prevalent plaza framing system consists of cast-in-place concrete beam and slab construction. This system provides capacity for heavily landscaped areas containing trees and large planters, substantial hanging loads over mechanical rooms, and heavy trucks at the loading dock. Furthermore, the beam and slab system adapts easily to the skewed framing conditions and multiple elevation changes located throughout the base structure.
Classroom space beneath the central plaza demanded higher overhead clearance, prompting the use of a two-way concrete flat slab. Although this system minimized the thickness of the plaza framing, the lower superimposed load capacity had to be coordinated with the landscape architect to scale back the density of planting in that area.
Perhaps the most unique type of plaza framing utilized is the sloped, multi-stepped, 27-inch-thick slab developed to frame sculpted entry terraces while maintaining long clear spans over television production studios. The stepped terrace incorporates a tree-lined ADA-compliant ramp spanning over column-free, acoustically-sensitive studios. The slab system incorporates the continuous reinforcing of a one-way slab as well as internal shear reinforcing of a concrete beam. The plaza steps and pavers are placed on top of insulation and waterproofing above the structural slab, resulting in an assembly with excellent resistance to sound transmission.
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Superstructure
Expansion joints separate each of the towers into three discrete structures. The joints occur at changes in mass and plan geometry, resulting in six structurally efficient rectangular building segments.
Braced frames represent one of the most cost-effective lateral force resisting systems for steel framed buildings. Concentrically braced frames were chosen for two of the three segments of the academic building, and for the transverse direction of the residential tower. With only a 10-foot floor-to-floor height for the residential floors, the braces were offset to allow proper window placement. Moment frames form the lateral force resisting system for the center segment of the academic building as well as the longitudinal direction of the residential tower segments because a regular bracing pattern would not accommodate windows or interior program demands.
In order to provide the desired number of beds within the footprint of the residential tower and under the desired overall building height, the team selected the girder-slab semi-proprietary steel framing system. Designed to work with precast hollow core plank, specially fabricated interior steel girders are embedded in the plank so that only the bottom beam flange projects below the bottom of the plank. Interior girders consist of castellated beams with a wide bottom flange used to support the plank, and a narrow bar forming the top flange. The narrow top flange allows unobstructed placement of the floor planks. The completed assembly is UL tested, carrying the required fire rating.
The structural system for the academic tower consists of conventional steel framing supporting concrete on metal deck. Floor slabs from 6-1/2 inches to 9-1/2 inches thick provide resistance to sound transmission at acoustically sensitive areas. A two-story Vierendeel truss supports building columns above a 40-foot-wide opening for an internal loading dock along the south wall of the academic building. This type of truss consists of rigid connections with no diagonal members, allowing unobstructed panels for window openings.
Design of the façade support structure created a challenging job of its own. Due to the fast-track nature of the project, the structural steel package was issued well before completion of the façade design. A separate package covering the façade steel included bent angles to support masonry arches, and tube subframes for bay windows, boundary elements between different envelope materials, and step-backs at different building heights. Slab edge support elements include extended slab cantilevers and articulated corner brackets that required a high level of detailing.
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Historic preservation
Incorporating a portion of the existing Carnegie Library into the new building design saved an important element of the region's historic fabric. Completed in 1907, the library on the Ann Arbor campus is one of over 2,500 libraries built with funds donated by Andrew Carnegie in the late 19th and early 20th centuries. Preserving one wall of this masonry structure required temporary and permanent supports coordinated to allow demolition of the remainder of the library, excavation below the existing footings, and unimpeded construction of the new building.
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The position of the existing front wall near a main street meant all bracing had to be located inside the building, in the same space to be occupied by the new structure. Permanent bracing, indicated by the silver color galvanized members, provides reinforcing for the three-story masonry wall to remain. Temporary bracing, indicated by the red prime-painted members, was designed to accommodate construction of the new foundations and steel framing with the least amount of interference. In its final configuration, the new building frame braces the existing wall laterally while allowing independent vertical movement.
Integrated project delivery
Fast-track projects differ from traditional linear design-bid-build projects in that the design team issues construction documents for portions of the building components as they are completed, rather than waiting to issue the entire construction document set when the design is finished. Early foundation and structural steel packages represent common fast-track design submittals. However, in order to expedite completion of the North Quad Project, the team issued 16 separate design packages, including structural submissions for bracing the Carnegie Library, foundations, slabs, structural steel, and façade support steel. Even within the submission packages, information was provided as early as possible for long lead items, such as heavy column sections that needed to be coordinated with mill rolling schedules.
The team embraced many elements of integrated project delivery (IPD) to improve communication and the dissemination of information. For example, AutoDesk's Revit BIM software was used to create a 3D representation of the structure. In addition to benefiting from automatic column schedule and braced frame generation provided by this software, the Revit model was also imported into CSI's ETABS software for analysis and design of the buildings by the structural team.
Leveraging the information-sharing capabilities of integrated modeling tools also helped the team during the contract administration phase. Requests for information came from the steel detailer not only in the form of written questions, but augmented by 3D PDF files that could be manipulated by the engineers – rotated and enlarged – to get a clear understanding of the virtual condition.
Franklin D. Lancaster, P.E., RA, LEED AP BD+C, F.ASCE, principal, is the structural engineering leader at EYP Architecture & Engineering. He can be reached at flancaster@eypae.com.
