To view the figures and tables associated with this article, please refer to the flipbook above.
The Stavros Niarchos Foundation Agora Building was designed to create a communal space on the JHU campus. It is made up of three separate volumes, about 57,000 total square feet, comprised of classrooms, office and a large community space for exhibitions, conferences, or other events. The three volumes maintain interconnectivity to each other and the surrounding campus at large through unique staircases, bridges, and a transparent facade designed to make the two end volumes look like floating cubes surrounded by a dense landscape. The design team studied a number of different materials to achieve the desired aesthetic and feel, ultimately landing on structural steel and load bearing precast concrete.
When working on a long-term project, you see the design evolve from its most original thoughts and aspirations to what is eventually achieved in the field. Engineers don’t often take the time to appreciate how the first pass 2-D framing sketch grows into a well-coordinated, on budget, goal-meeting new building. The Stavros Niarchos Foundation (SNF) Agora Institute (SNFAI) was no different; the first sketch looked like the 2-D stick model in Figure 1 and then grew through multiple iterations into the institute that will serve the students at John Hopkins University for years to come.
SNF Agora Institute is composed of two separate volumes—the main building and auditorium—interconnected by bridges and a center multi-level stair affectionately known as the “spider.” The main building has four stories with a roof, and the auditorium building has two stories with a roof and an interior mezzanine. The spider provides the pathway between the two buildings with a continuous staircase connecting all floors as well as interior bridges providing access to the two buildings from the second and third floors with additional exterior egress stairs on either side of the main building and the auditorium building.
The design of the buildings followed the 2018 International Building Code (IBC) with the 2018 Maryland Building performance standards. Other standard additional design codes such as ASCE 7-16, (Minimum Design Loads for Buildings and Other Structures), ACI 318-14 (Building Code Requirements for Structural Concrete and Commentary), and the American Institute of Steel Construction 341-16 (AISC) Design Guides were referenced for loading requirements and material specific requirements.
The building structure at and below grade is all cast-in-place concrete supported by over 120 90-ton axial capacity auger pressure grouted piles. The 12-inch thick cast-in-place concrete slab at the ground level supports an exterior public plaza and landscaping.
Challenges
A multitude of factors contributed to the challenges faced when framing SNF Agora for support of the main gravity and live loads: What material could be expressed best to match the owners and architects’ design aspirations? What framing layout provided the best path for MEP routing? How is that achieved across two separate volumes and make it buildable?
Early iterations of the auditorium used steel framing with a story truss and tension tie rods (to stop the building from tipping over) with two pairs of columns 7 feet apart on either end, as it was intended to look like a floating glass cube. The trusses got bulky, and the building movement became too much for the facade performance and vibration requirements for user comfort. The main building tested a wood framed building within a steel frame with the rendering shown in Figure 2.
This iteration used steel trusses, mass timber, and a thin steel-framed facade support around the perimeter. When this didn’t work out because of the heavy steel and wood, the designers tried a cast-in-place concrete option (standard reinforced, post-tensioned and void formed were all put on the table). However, this all proved costly given the need to support the building on piles, which drove up foundation costs and seismic loads.
The final product for the gravity framing was an amalgamation of all the different structural options studied. Both buildings are steel-framed with concrete on metal deck, which was chosen for its flexibility, comparative (to concrete) lightweight, buildability, and ability to provide framing for cantilevers and limited number of columns while still meeting vibration criteria and other serviceability requirements related to acoustic performance and deflection. The typical slabs are framed with 3 ¼ inches of lightweight concrete on a 2-inch 18-gage metal deck spanning 10 feet between floor beams. The standard floor beams in the main building are 30 feet long W14x26s with thirty 3/4-inch diameter headed shear studs with a ¾ -inch camber to meet deflection criteria and allow space for MEP runs below them. W30 members serve as 66-feet span girders at the auditorium’s 2nd floor and roof to provide column-free spaces below. The W30 roof girders support a green roof and all the necessary lighting and equipment support for the performance space below.
The lateral framing for the buildings is a hybrid of moment frames, braced frames, and precast concrete shear walls. The inter-connected buildings needed to be designed to move independently but still provide public access across seismic joints. Figure 3 shows how the movement joints were laid out in each direction, at the structural interface of each building. The joint movement, subsequent joint size and lateral system layout were each precisely coordinated so they fit within the architecture at the seismic joint location—allowing the necessary physical building movement without compromising architectural intent.
Per the diagram shown in Figure 3, the auditorium was seismically separated from the spider and the main building while providing the lateral resistance for the exterior precast concrete stair (which was directly attached to the slabs at each level). A response modification factor of three (not specifically detailed for seismic resistance) was utilized for seismic design with 120 mph, 3-second gust for MWFRS loads. Tension only, double-steel plate braced frames (moment frames were too flexible and the exposed connections at the ground floor didn’t fit the architectural intent) are provided in three discreet locations at the ground floor and four more locations between the second floor and the roof. The braced frame connections and attachment points were carefully coordinated with the architect, as they are exposed, with each weld and connection designed with custom shaped gusset plates and steel forks and pins. The noted braced frames met the design wind drift criteria (H/400) and the necessary seismic drift required to establish the seismic joint.
Moment frames are provided throughout the main building and provide the bulk of the lateral resistance. However, the flexible moment frame system was not enough to meet the wind and seismic drift needed to create the desired maximum seismic joint at the interface with the spider. The interior precast concrete shear walls in the main building at the stair vestibules (originally assumed to be decorative “interior facade”) were redesigned to provide lateral resistance as shear walls. TYLin provided shear loads and overturning moments to the design assist contractor for the reinforcing design, and they worked together to design the connections to the superstructure such that the lateral performance needed for drift and strength (and seismic joint size) was provided by the combined lateral system.
The Spider
The spider structure is a focal point of SNF Agora Institute. It is not just the center, physically, of the buildings, but also the main artery through which users will traverse the structures and experience the space. Precast concrete was chosen as the main framing for the superstructure. TYLin worked with the design assist engineer to frame the spider. Each level of precast exterior ring beams and facade glass are hung from the precast/pretensioned roof structure with exterior steel tension rods that deliver loads back to two columns and interior shear walls. The slabs that support the stairs and landings span between shear walls and the noted columns. The shear walls provide lateral resistance in the north-south direction, while the main building provides the lateral resistance in the east-west direction. The spider is tied to the main building in the east-west direction at their interface through short pieces of precast concrete bridge that provide the connection between the buildings and a lateral load path between structures. The noted bridge pieces needed to slip in the north-south direction but still transfer load in the east-west direction. The bridges that span from the main staircase in the spider to the auditorium are connected with a vertical seated connection that provides seismic slip resistance in the east-west and north-south directions (Figure 4), creating an independent lateral system from the Auditorium building while still transferring vertical loads through bearing. These connections were carefully coordinated with the architectural team so they could slip in each direction, support load vertically and fit within the architectural enclosure without being seen. In addition, the connections had to be coordinated with the design assist contractor and engineer such that they understood the design intent.
Conclusion
SNF Agora is intended to provide students on the JHU Homewood campus with a place for conferences, speaker presentations, art exhibitions, labs and classrooms. The design team had the opportunity to study multiple different iterations of structure (both gravity and lateral) and material layout to meet architectural intent. The final product of steel framing, cast in place, and precast concrete served to provide open sight lines, column-free spaces, and an abundance of natural light. ■
About the Author
Thomas Reynolds, SE, PE, is a structural engineer with TYLin in New York City. His experience encompasses a range of building types including healthcare, higher education, primary and secondary education, institutional and residential.
