If you look at the new Heights school in Arlington, Va., from above, you might think you’re seeing a massive hand fan.
The new school, which opened in time for the 2019-20 academic year and enrolls 775 sixth- through twelfth-grade students, consists of five stacked steel-framed “bars” that fan around a pivot. This fanning gives the feel of a one-story school building while also creating large open volumes beneath the bars. Fanning the bars around a pivot led to the development of an innovative load-path concept using floating buttresses to support the corners of each bar. In addition, landscaped terraces live atop each bar, creating multiple outdoor learning and relaxation spaces.
The pivot was a natural location for vertical circulation and distribution of services, so a concrete core was designed to resist torsional, lateral, and gravity forces. The bars create floating corners on each side, and multiple structural concepts were evaluated to facilitate this design scheme, including cascading cantilevered beams with column transfers, cantilevered trusses parallel to each bar, and helical columns. Ultimately, the floating buttress design evolved from the helical column concept, where each column leans as the bar fans out. This created one helical load path at each corner that, while beautiful in structural elegance and simplicity, created sloped columns that occupied valuable interior space that couldn’t be lost. To preserve this space, the helical columns were pushed out to the perimeter walls, forming a truss and floating buttress system framed with W12 and W14 wide-flange sections. Each truss uses standard bolted gusset connections and bearing plates, and the buttresses use welded connections. The floating buttress resulted in additional out-of-plane forces, which are resisted by horizontal diaphragm framing that transfers diaphragm forces back to the core. To simplify erection, each truss was designed to be fully erected into place by putting an upper truss on the truss below it, using a few shoring posts for stability during erection. Where trusses intersected in plan, the chords simply passed over one another in elevation. Structural engineer Silman collaborated with steel fabricator Banker Steel to simplify load-path continuity through geometrically complex connections at critical locations. In addition, limited laydown area necessitated multiple crane lifts to get the heavy trusses and girders into place.
As is typical in school design, the classroom modules for the Heights are standardized for uniformity of program. Each bar includes classrooms on either side of a central corridor. Early studies were conducted to economize steel tonnage, considering two primary options: a double line of columns down the corridor and a single line down one side of the corridor. A double line of 8-in. to 14-in. wide-flange columns was selected, as this scheme required fewer materials and facilitated shallow corridor framing (the standard floor-to-floor height is 14 ft, 6 in.) in order to accommodate MEP routing. Continuing with this interdisciplinary collaboration between engineer and fabricator, framing was kept shallow at the exterior to allow greater light ingress as well as linear HVAC diffusers. Silman paid specific attention to a condition where one bar fanned out over another, with the two modules intersecting. To avoid multiple column transfers, the repetitive framing on the upper level was transferred out at the level below.
The landscaped terraces were designed to support a variety of intensive plants, trees, and landscaping features. To maintain economy, repetitive framing was maintained, with varying beam sizes and deck profiles being employed depending on loading requirements. The slabs were stepped down 19½ in. between classrooms and terraces, which created an opportunity to accommodate the approximately 70-ft-long transfer girders where the bars intersected. A two-level steel and precast concrete stair cascades down radially to connect the terraces and classrooms and create a sense of community while aligning orthogonally with the bar framing.
Spreading out the five bars created large volumes below that were particularly favorable for active school functions. The gym, assembly atrium, and theater were inserted in these volumes, and a library and music room were included between classrooms and the gym and theater. These spaces required clear spans from side to side of their respective bars, and steel framing depths were restricted for overall building height limitations.
The framing above the gym, library, and atrium are all standard or built-up sections, and the framing over the theater uses shallow trusses. Trusses were not feasible for the available space above the gym, so plate girders and heavy W36 sections were used to transfer the columns from above, supporting bar floor and terrace framing, and double-W24 sections ended up being the most economical solution over the atrium. A dramatic cantilever over the atrium reaches toward Wilson Boulevard to the south. To achieve the shallow floor depth, as well as the aesthetic desires of the project’s architects, Leo A Daly and Bjarke Ingels Group, a dapped-end 24-in.-deep built-up double-web plate girder was used for the soffit. Due to the large terrace load from above and the short back span of this cantilever, the plate girder was anchored with a tension column in bar five. Above the theater, trusses were the optimal solution to meet the needs of potential future expansion, MEP routing, column transfers for the crossing bar above, and allowable floor depths.
The egress requirements for this unique building layout led to stairs being placed at the opposite ends of bars one, three, and five. Silman took advantage of these stairs and implemented shear walls at each stair shaft, which created a tangential force-resisting path that coupled with the core to provide lateral force resistance. Diaphragms in bars two and four were tied with collectors to the shear walls in bars one, three, and five to provide a continual load path radially around the building bar ends. Another unique challenge resulting from the truss configuration at the pivot was the global diaphragm forces that were developed from gravity and lateral forces on the building. As the trusses fan out around the pivot, a global lean and twist was created and was resolved with steel struts and unique chord arrangements at and below-grade levels.
Silman carefully collaborated with the design and construction team during the early project phases to develop and track costs associated with critical and typical structural steel features, integrating analysis and 3D models throughout the process to track tonnage and costs associated with all structural elements. Throughout the design process and especially early on, meetings with Banker Steel and general contractor Gilbane were essential to ensuring economical solutions and constructability throughout design, as well as coordinating steel availability with the construction schedule, erection methods, preferred connection types, and site logistics. Some standard sections were changed to plate girders through this collaboration, while others remained heavy W36 beams spliced together in the field. Minor revisions were made to transfer girder connections, truss node stiffener configurations, and column splice locations.
The Heights is a testament to excellence through true collaboration of structural design, architecture, steel fabrication, and construction. At every twist, innovative approaches were developed to achieve a monumental vision and a demanding school program, leading to a true architectural jewel to serve the students of Arlington. N
Owner: Arlington Public Schools, Arlington, Va.
Construction Manager: Gilbane
Architects: Bjarke Ingels Group (BIG), New York
Leo A Daly, Washington, D.C.
Structural Engineer: Silman, Washington, D.C.
Steel Team
Fabricator: Banker Steel, Lynchburgh, Va.
Erector: Memco, LLC, Culpeper, Va.
Detailer: Sanria Engineering, San Jose, Calif.
Författare:
Suzy DeHoratiis (dehoratiis@silman.com) is a project engineer, Silman
Jason Myers (myers@silman.com) is an associate, Silman
Timon Hazell (hazell@silman.com) is a senior BIM engineer, Silman.
