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Staggered But Steady

Contemplating the design of its Staybridge Suites Hotel, Chicago developer Miglin Properties, LLC joint owner with Atlanta's Dellisart Lodging, LLC eyed

Contemplating the design of its Staybridge Suites Hotel, Chicago developer Miglin Properties, LLC Û joint owner with Atlanta's Dellisart Lodging, LLC Û eyed spacious rooms in a building worthy of the city's architectural legacy. An exhaustive design process led to a June 2007 groundbreaking for what would be Chicago's first staggered steel truss and prestressed hollow core project.

The 17-story luxury hotel will comprise a structural steel system of 14 floors built over four cast-in-place concrete levels, including a parking garage and sky lobby. Since the use of a staggered-truss framing system allowed designers to arrange building masses creatively by incorporating cantilevered floors, a mid-section of the 186-ft.-tall structure will be slightly offset to produce the hotel's novel configuration. Adding further distinction, project principals note, are metal cladding on exterior lengthwise walls and a glazed-glass finish on outside surfaces spanning the building's width.

A STAGGERING PLAN

According to the American Institute of Steel Construction, Chicago, the staggered truss system is a new concept in structural steel framing for mid- or high-rise buildings. It consists of a series of story-high trusses staggered from floor to floor and spanning the entire building width between two rows of exterior columns. With columns only on exterior walls, the usual interior columns are omitted, providing a full-width, column-free area. Staybridge Suites, therefore, will feature 60-ft.-square spaces with no columns, while some open areas range to 60 _ 80 ft.

Implemented with precast plank, the system incorporates a floor system that spans from the top chord of one truss to the bottom chord of the adjacent member. The floor thereby becomes a major framing component, serving as a diaphragm that transfers lateral shears from one column line to another; thus, though the trusses lie in two parallel planes, the structure performs as a single braced frame. Overall, benefits of the composite system include minimal bending moments (from gravity and wind loads) in the columns; more open areas for design freedom; semi-finished floor and ceiling in one operation; reduced foundational loads due to fewer structural members and, in this case, hollowcore plank; plus, faster erection with all-weather construction.

SECURING STAYBRIDGE

As economy in labor and materials is a prime consideration for any commercial venture Û even a luxury hotel Û staggered truss steel framing with hollowcore plank affords the advantage of mass production of uniform components. Only one type of truss was required for the project, among a total of 815 tons of structural steel supplied by K&K Iron Works, McCook, Ill. The fabricator delivered 115 trusses, 100 columns, and 600 braces, spandrel beams and diaphragms. ATMI Precast of Aurora, Ill., shipped over 1,000 prestressed hollowcore plank, typically 4-ft. _ 30-in. or 4-ft.-square.

Ready mixed for the cast-in-place parking garage and hotel entrance was supplied by Bridgeview, Ill.-based Prairie Material Sales, Inc. Jeff Rodgers of general contractor Walsh Construction Co., Chicago, reports that approximately 4,000 yd. were required for the building's first four levels, plus columns and shear walls. Three Saturdays and one Sunday were devoted to site preparation before mid-week pours could take place. Coordinating installation during the cast-in-place phase, he adds, involved placement of anchor bolts and setting plates, timing erection with formwork, establishing temporary bracing, and determining rebar/post-tensioning/formwork/steel interactions. Aligned composite columns, i.e., steel columns encased in concrete, were deemed best suited to the project's structural demands.

With the progression to higher levels, crane selection and traffic circulation affecting steel and precast plank delivery became crucial. Accordingly, a single Liebherr 316 crane for both plank and steel placement, as well as a staging area along the site's northern perimeter, facilitated truck unloading and erection of structural components. Coordination of cycles for delivery and setting of steel and precast members enabled builders to erect steel on one side, east or west, as precast was set on the opposite side at a lower level. Grouting was performed only after installation of the floor above; and, a three-floor safety zone was established for all follow-up trades.

At the 10th to 13th stories, where cantilevered floors are offset by 7 ft. 6 in., the plank arrangement provided space at the edge of a unit for inset trusses. There, diaphragms had to be fully grouted to ensure stability. Maximizing economies of scale, offset floors used the same trusses, merely designed in reverse. Trusses ranged in length from seven to 44 feet and weighed from 1,500 to 9,300 lb. each.

Setting hollowcore plank on a staggered-truss frame provided flooring without the need for a structural topping or finish. ATMI's fabrication process using extruded, zero-slump concrete yielded precast plank sufficiently smooth to require only a thin latex veneer before laying of carpet in most areas.