North Carolina A&T University’s Samuel D. Proctor School of Education building was designed to make a statement. The intent of the university and architect was to create a building with a “wow factor” as the gateway into the revitalized main part of campus. This multi-award-winning building offers a unique visual experience and physically represents the university’s theme of creating a “catalyst for learning and leading.”
The 52-foot-long, 40-foot-wide, two-story-tall cantilever provides the visual impact. However, that was just one of the numerous challenges associated with this project, which required a finely tuned orchestra of team members to bring the entire design together. The building is located at the intersection of two main roads and is visible from all sides, so it had to be as aesthetically pleasing from the back door as it is from the front. The site slopes a full story height through the length of the building, causing a portion of the lowest level to be 17 feet below grade. The building has an intricate ribbon window wall panel system that parallels the long, lean shape of the building. The wide-open, three-story-tall, glass-encased entrance lobby, with its highly exposed ornamental stair, was a project within the project. All of these design challenges were met in the design process and carried out successfully during construction with everyone working together with an unparalleled team effort.
The building is a three-story, steel-framed structure housing classrooms, an auditorium, conference rooms, study rooms, and offices, including the Dean of the School of Education, who is proud to be located on the top floor at the end of the cantilever. The structural system consists of composite steel framing supported on steel columns with reinforced concrete spread footings. The lateral force resisting system of the main building is steel braced frames in one direction and ordinary steel moment frames in the other. The lateral system of the cantilevered portion of the building is built integrally with the gravity system to meet the challenge of the cantilever.
The architect’s design concept envisioned the building as a symbol of the university looking toward the future. Several design ideas were considered to create the focal point of the building. These included a single column centered at the end of the building, a single column off centered, and the less likely no column or cantilevered option. The architect was expecting the usual engineer approach of caution and expected a one- or even a two-column option. But as the design team sat around the conference room table, the architect posed the question: “Can we do it without any columns?” Willy Stewart’s response was, “Why not?” That attitude carried the team through this project and has become Stewart Engineering’s theme from that day forward. The engineers quickly began developing concepts to frame the 52-foot cantilever while keeping a close eye on construction costs and budgets. The final design concept incorporated a pair of three-story steel super trusses located in the exterior walls with composite steel beams framing perpendicular to them at each truss panel point. This option satisfied issues of cost, function, time of construction, and aesthetics.
The north half of the building, separated from the remainder of the building by an expansion joint, is 102 feet long. This results in a 52-foot cantilever with only a 50-foot back span to resist the overturning forces of the overhang. The super trusses are located in the eastern and western exterior walls of the building, where the back span is three stories tall and the cantilever is two stories tall. The trusses are displayed inside the building as well as from the exterior in select locations. The two-story cantilever supports the full gravity load of the two floors and roof, which results in an extremely large overturning force on the shorter back span. The trusses are supported by very few columns in the back span, in order to have long spans that develop the necessary resisting load. The remaining overturning forces are resisted by 12 rock anchors in groups of four at each of the three back span columns. The rock anchors consist of steel rods at a 5 degree batter angle, which are drilled through 12 feet of soil and then 6 feet into bedrock. Each anchor is then pretensioned to 75,000 pounds and grouted. The anchors are fastened to a pile cap located over a 6-foot-diameter, 12-foot-tall concrete pier founded on bedrock. The truss columns sit directly on the pile caps. At the onset of construction, the contractor and the design team met and more conventional anchoring methods were proposed; however, it was ultimately decided that the rock anchors were the best alternative. After construction, the contractor, owner, and design team were impressed with the system and even installed a glass plate in the lobby floor to display the top of one of the rock anchors.
The three columns between the two trusses were relied upon to resist the uplift forces. The initial design involved a fourth column, which was eliminated to accommodate a large sloped floor auditorium located in the first floor of the back span. The auditorium is as long as the back span area and 12 feet wider than the building above. This posed a difficult structural problem for the engineers. The super trusses support massive amounts of load and one of them had to be interrupted by an auditorium. The problem was solved by offsetting the truss to the east to align with the exterior wall. The engineers used the steel of the single story roof area as a horizontal truss to transfer the loads to the lowest level of the truss, 12 feet away. The design was accomplished by creating a comprehensive, three dimensional model using Risa-3D design software.
The building not only posed a challenge to support the gravity loads, it also had to cantilever to support the lateral loads. A stairwell located at the base of the cantilever created an opening in the two floors half as wide as the building. Originally, the design team feared that a large penetration located at the point of highest load would cause problems for the design. However, the design used the concrete shearwalls around the stairwell to support the cantilevering lateral load. The 20-foot-wide concrete core allowed for 10 feet of structure to pass on either side. Two, 10-foot-deep horizontal trusses were located in each of the floors and roof structures. This was done very economically by using the chords of the super trusses as the bottom chords of the horizontal trusses. The steel beams spanning 40 feet between the super trusses served as the web members of the horizontal truss. Additional steel was added to form the top chords and diagonal web members. The trusses spanned similarly to the super trusses with a 50-foot back span and then cantilevered off the concrete shear walls to the end of the cantilever. Steel-braced frames were located in the rear of the auditorium to resist overturning lateral loads.
An important architectural feature of the building is the black slate veneer located on the north face of the building wrapping around and running down the western face of the building. The black and grey pattern of the veneer is intended to resemble books in a bookshelf. The exterior veneer crosses through the glass curtain wall of the lobby space into the southern half of the building, visually tying the whole building together. This feature not only added significant weight that must be supported by the cantilevered structure, but also required strict deflection criteria to avoid cracking the slate.
Adjacent to the cantilevered portion of the building is a beautifully lit, glass-encased, three-story entrance lobby complete with tiered open space, an ornamental stair, and a cantilevered glass canopy mimicking the cantilevered building. The design of this lobby involved an intense coordination effort between architect, engineer, window wall supplier, contractor, and steel fabricator. Every detail was discussed and coordinated. Each floor had a clear story area, which is tiered back at each level, so that a person in that area can see down to the lowest lobby level.
The exterior glass curtain wall system spans from the third story roof down to the foundation. Exposed steel framing was added at each level to brace the system laterally. The team was very particular about the shape of each steel member, selecting wide flange beams and large pipe columns. The size of the members not only had to work structurally, but had to fit proportionally into the wide open lobby area. Every detail was precisely specified, including having the flanges of the beams rounded to follow the profile of the pipe column.
A solid wall panel feature, which is supported just outside of the window wall system, was added to the exterior of the building. The feature is located at the upper two floors and runs along the wall of the lobby. The system cantilevers 5 feet beyond the building and then bends 90 degrees and cantilevers an additional 7 feet along the north face of the lobby. This is supported by hollow structural steel members and moment connections within the wall panel.
The ornamental stair located in the lobby is visible from both inside the lobby and from the exterior through the glass curtain wall. The architects and engineers designed stringers composed of 21-inch-deep wide flange beams for the entire height of the stair. The same size stringer wrapped around each of the landings. The edge of the tiered open space in the lobby maintains the same 21-inch-deep wide flange beam resulting in a continuous line of steel which runs throughout the lobby and extends through all the floors and stairs.
The remaining facade of the building consists of an intricate wall panel system featuring ribbon windows running the length of the building. Typically, framing is provided above and below windows, which bears on either side of the openings. That is not possible with continuous ribbon windows.
The windows and the wall panels above and below the windows are supported by steel frames that cantilever up and down from the floor structures. Numerous hours of discussion and detailing were needed to detail and build this system. Every window and corner had to be specifically called out. With the help of the contractor and steel fabricator, the building went together seamlessly, resulting in the long, sleek exterior appearance — which was the goal of the design team.
This innovative project has received numerous design awards both for architecture and for engineering. The design team worked together to achieve numerous goals within one building while staying within the construction budget and schedule, resulting in a building that truly makes a statement.
Owner: North Carolina A&T University in Greensboro, N.C.
Structural and civil engineer: Stewart Engineering, Inc., Charlotte, N.C.
Architect: The Freelon Group in Durham, N.C.
Contractor: New Atlantic Contracting, Inc., Winston-Salem, N.C.
MEP engineer: RMF Engineering, Durham, N.C.
Lighting designer: Light Defines Form, Greensboro, N.C.
Q&A with the structural engineer
Structural Engineer of Record Christopher R. Herron, P.E. (CH), of Stewart Engineering, Inc., discussed the Samuel D. Proctor Hall School of Education with Structural Engineering & Design Editor Jennifer Goupil, P.E. (JG).
JG: How did you select the final structural system?
CH: We considered a concrete building; however, economy quickly ruled it out. We chose a steel-framed system with the two super trusses based on the floor plan, the demands of the building on the structural system, and the costs. The final configuration and details of the trusses took form over several months of calculations and coordination. The remainder of the building was selected to be composite steel to be consistent with the cantilever and it was the most economical system. It also met the architectural requirements inside and outside of the building.
JG: How was the most challenging aspect of the structural design solved?
CH: We spent a considerable amount of time looking at the connections of the trusses and how best to frame the truss. We used continuous wide flange top and bottom chords. The vertical web members were wide flange columns which were interrupted at each of the chords forming a stacked column condition. This allowed the largest forces located within the chord members to be supported through a continuous member versus a beam-to-column connection.
JG: What was the most unique problem to solve on the project?
CH: One of my favorites was the wall panel around the glass lobby. The architect wanted a two-story floating wall panel feature just outside of the glass curtain wall. The feature cantilevered through the glass wall to the outside of the building. The wall panel was parallel to the window wall and then cantilevered out 5 feet past the building. The panel then bent 90 degrees and cantilevered an additional 8 feet. The double cantilever induced high torsional and bending forces throughout its structure. We used hollow structural steel members within the wall panels to support the feature. Welded connections that were capable of supporting the moment, torsion, and shear were used throughout.
JG: What construction considerations affected the structural design?
CH: We considered if the super trusses should be built in the shop and shipped in large pieces or if it was more economical to build the trusses completely in the field. The floor-to-floor height was too tall to ship in significant pieces so the trusses were built piece by piece in the field.
JG: What engineering ideas did you implement to save project costs?
CH: The lateral system of the cantilevered building was done very economically by using steel that was already in the building to form horizontal trusses within each level that cantilevered off the stair well wall. We designed the chord members of the super truss and the floor beams that spanned between them as the top chords and web members of lateral trusses.
President and CEO Willy E. Stewart, P.E., directs the 90 employees of Stewart Engineering, Inc. (www.stewart-whynot.com), which is headquartered in Raleigh, N.C. Established in 1994, the firm serves the following markets: education, health care, commercial, multi-family residential, municipal, sports, NCDOT, and military. In addition to structural and civil engineering, the firm provides the following services: transportation and geotechnical engineering, special inspections, materials testing, landscape architecture, geomatics, planning, water/wastewater, and transportation planning. Honored in many ways, the firm has been included on ZweigWhite’s Hot Firm ranking and has received multiple engineering excellence awards for many projects.
Samuel D. Proctor Hall School of Education
|Size, shape, and type|
|Number of square feet:||65,700|
|Number of stories:||3|
|Structural system types:|
|North building — Composite steel beams spanning between a pair of steel three-story super trusses located in each exterior wall|
|South building — Steel deck and beam roof structure, composite concrete and steel floor structure, combined braced frame and ordinary steel moment frame lateral system|
|A combination of reinforced concrete spread footings and 6-foot-diameter piers on bedrock anchored with rock anchors|
|Tons of structural steel:||548|
|Tons of rebar:||81|
|Cubic yards of concrete:||2,600|
|Square feet of deck:||65,700|
|Number of footings/piers:||55|
Christopher R. Herron, P.E., is a principal, vice president, and the structural engineering department manager in the Charlotte, N.C., office of Stewart Engineering, Inc. He can be reached at 704-334-7925 or firstname.lastname@example.org.