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Benefits of Staggered Truss Systems in Multi-Story Residential and Other Applications
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Advertising Supplement provided by The American Institute of Steel Construction, Inc.

Case Study: Embassy Suites Hotel

Embassy Suites Hotel

Totaling approximately 617,000 sq.ft., the mixed-use Embassy Suites hotel development is among the largest of several newly built hotels in the business district adjoining New York’s bustling Wall Street area. The L-shaped hotel is situated on a site measuring 62,500 sq. ft. and has two wings connected by a 9,200 sq. ft. east-west atrium with skylights and glass walls on its west. The development showcases a 463-room hotel, a 16' screen movie theater, a sizable retail complex and a 13-story-high, glass-walled public atrium forming a centerpiece that offers sweeping views of the Hudson River.

The functional needs of the project’s components could not have been more diverse. Movie theaters require long clear spans of up to 60', with high floor-to-floor heights, typically 32' In contrast, hotels generally need short spans for rooms and low floor-to-floor heights, normally in the range of 8'-8" to 10' Retail spaces require somewhat more flexibility: they benefit from longer open spaces, however spans of 30' to 40' are generally acceptable.

Space usage also dictated the types of ceilings that had to be used in the project. As large-volume, high-occupancy spaces, movie theaters need a ceiling plenum for air and service distribution, as well as acoustic enhancement. Where as, hotel rooms are small, private spaces served by individual HVAC units. With bathrooms stacked vertically, piping is distributed inside the walls, while the bedroom ceiling is usually the soffit of the floor slab above with a directly applied surface finish. Retail spaces will frequently use a ceiling plenum for service distribution and the installation of special features.

Certain pieces of conventional wisdom are generally applied when choosing structural systems for a project, most of them dependent on the project’s location and uses. For a New York mixed-use project like Embassy Suites, such wisdom would point to a concrete flat-slab system for the hotel and a steel-framed structure for the movie theaters. However, it soon became clear that if the project was to survive the designers needed to throw a few traditions out the window.

During the concept development, the project’s budget estimate showed a very tight pro-forma. Unless the construction cost per square foot could be reduced, the project would not be financially feasible. The conceptual cost estimate, based on construction costs for similar projects built in the New York City market, called for a cast-in-place concrete system. The design team was charged with the task of finding significant cost savings—and fresh ideas were needed.

Thornton Tomasetti Engineers, the project’s structural engineers, proposed the use of a staggered truss system for the hotel combined with a conventional steel frame and composite slab-on-metal deck for the theater and retail components. Cost estimates by Bovis Lend Lease, the general contractor, confirmed the value of the cost savings to be in excess of $2 million. The team had found a viable and economic solution, but next needed to satisfy the client of the ability of the system to meet the program requirements while still providing the estimated cost savings.

Why is concrete flat slab so often the system of choice for hotel buildings? Flat-slab construction provides several important benefits, including shallow uniform thickness of the structure facilitates lower floor-to-floor heights, reducing the cost of the structure and the area of the exterior wall, elimination of the need for hung ceilings, and a satisfactory fire rating for the floor construction.

The staggered-truss system used for the Embassy Suites Hotel had many of the same advantages:

  • The Filigree precast plank floor is a uniform 8", providing low floor-to-floor heights.
  • The soffit of the planks can be finished in the same way as the concrete slabs, without hung ceilings.
  • The Filigree precast plank satisfies fire rating of the floor system.

According to Jonathan Stark, Perkins Eastman Architects, P.C., “Staggered truss actually reduces the flexibility of the building layout. But in a project such as a hotel that does not need the flexibility, it did allow for column-free space in each hotel room. Each hotel room had quite a different layout and detail due to the use of staggered truss with the addition of 6" to 10" of space.”

The decision to use the staggered truss system was, in the end, a financial judgment. For the specific project and site under evaluation, the steel system offered significant advantages:

  • The weight of the steel and plank structure is approximately 20% less than that of an equivalent concrete frame. Since the project site is located on land that was filled/reclaimed in the 1960s, deep end-bearing-pile foundations were required to support the structure. With depth-to-rock averaging 80', the reduction in foundation loads resulted in a savings of approximately $1 million in foundation costs.
  • Reduced dead load translates directly into reduced seismic load. Since the 14-story building’s lateral systems were governed by seismic design, the 25% reduction in lateral design forces resulted in further cost savings.

In addition, the mixed-use nature of the project strengthened the case for an all-steel-framed structure. Composite steel framing with slab on metal deck was a logical, cost-effective choice for the theater clear spans of 40 to 60'. For retail uses, steel framing provides maximum flexibility, permitting easy modification when tenants change. The steel weight averaged 7 psf for the hotel and 15 psf for the theaters. In a concrete structure, transfer girders would have been necessary to achieve the longer, more open spans needed at the lower public spaces.

Story-deep staggered trusses span transversely across the narrow dimension of each hotel wing. The truss spacing matches the suite partition locations, typically varying from 26 to 30' on center. Staggering occurs on alternate lines from one floor to the next. The 8" deep precast Filigree planks span from the top chord of one truss to the bottom chord of the adjacent truss. Since the trusses are completely encapsulated by the partition walls of the hotel, minimal width truss members are desirable. Rectangular HSS sections were determined to be the most efficient.

Provision was made within the footprint of each theater for the tenant fit-out of a full stadium seating theater. Partial mezzanine levels were provided, framed in structural steel, approximately 19' above the floor, to house projection booths and provide secondary means of egress. The stadium seating was supported on light gage steel joists and stud walls sitting on top of the theater slabs. This solution permitted the design and construction of the theater fit-out to lag behind the base building construction and provided an economical framing method for the stadium-seating component.

Meeting the objectives of a client—whether they are related to time, cost or function—is the primary goal of every project. In the case of the Embassy Suites Hotel development, the client’s cost requirements determined whether or not the project got built at all. In this restrictive circumstance, engineers decided to ignore everything that was known about typical residential construction in New York City in favor of the staggered steel-truss system. The result was exactly what they—and their client—were looking for.

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