The hotel is a combination of five separate building structures, utilizing a combination of shear walls and in-slab moment frames to resist lateral wind and seismic forces. One structural challenge for the hotel was the design and installation of the 220’ wide steel structure of the lower atrium roof. A second challenge was the placement of a 9,000 square foot ballroom directly under the high-rise portion of the hotel. This required the design of four 9’ deep x 5’ wide x 65’ long transfer girders each supporting two interior concrete columns.
Designing the spectacular 1.6 acre atrium that includes 120,000 square feet of glass walls and roofs was another hurdle. The atrium connects the wings of the central hotel tower and the two freestanding lower hotel towers. Structural design challenges of the atrium included design and construction of long-span bowstring roof trusses, purlins, girts and vertical trusses that are capable of resisting the imposed loads. This structural solution respects the architectural design intent and uses the least amount of framing to allow the maximum amount of light to enter through the skylight and curtain wall and maximizes the unobstructed views of the Potomac River, Alexandria, VA and Washington, D.C.
The Convention Center, supported on spread foundations, was configured using a 60’ x 60’ column grid to optimize the useable space on the exhibit hall floor. Due to the sheer size of the two Convention Center structures (400’ each), the structure was split up using a series of construction joints and intermediate stressing utilizing bonded post-tensioning. To stabilize the Grand Ballroom’s lateral system, a Mechanically Stabilized Earth (MSE) retaining wall system was utilized internal to the building to raise the soil grade in two separate steps. A system of slip/rocker slabs and slip connections were used to isolate the movement and lateral soil load of the MSE wall from the Convention Center’s lateral system.
The atrium required a separate set of considerations, including ease and speed of erection, erection tolerances and adjustability, and glass system deflection limitations to account for both the architectural intent and structural requirements. All told, the Atrium utilized 5,500 tons of structural steel. An elaborate trussing system along the atrium roof, the roofs of the central and lower hotel towers and vertical walls resulted in increased force protection while allowing slip and rotation where needed. Bow-string trusses used for the atrium roof include 42”diameter curved steel pipe top chord, center king post and double 3” and 4” diameter rod bottom chords. Intermediate posts were provided to vertically support the rods, while custom clevises were fabricated specifically for the heavily loaded trusses.
Other structural elements used in the design are HSS purlins and girts which span between roof trusses and vertical wall trusses to support the roof loads and curtain wall. The HSS purlins were designed to be modular such that erection time and cost was significantly reduced.