Staggered solution

August 11, 2010

The staggered-truss structural system incorporating steel trusses was developed by the Massachusetts Institute of Technology in the late 1960s. However, designers' interest in the system has lagged largely because little documentation has been available for using it, according to Neil Wexler, president of the New York City-based structural engineering firm of Wexler & Associates.

Wexler has been instrumental in remedying that situation. He is the lead author of a new guide for incorporating staggered-truss systems into building designs (see 'New design resources,' page 36).

Wexler has designed staggered-truss systems mainly in response to clients' requests to use them. He says these clients are seeking the large, column-free interior spaces and the potential for substantial economies that staggered-truss systems can provide.

The most appropriate use for staggered-truss systems is for 'residence'-type facilities - including apartments, hotels, nursing homes and dormitories, according to Wexler. He says the system should be considered for any building of this type that has at least six stories.

Staggered-truss systems use geometry to reduce the weight of steel framing, compared with conventional post-and-beam construction.

Story-high trusses that extend the width of the building are located in the demising walls. They are staggered from floor to floor at alternate column lines so that the bottom chord of one truss aligns with the top chord of the next. This provides column-free areas as large as 60 by 70 feet. By comparison, typical column spacing for a concrete structural system is 18 feet; for conventional post-and-beam steel construction it is 25 feet to 30 feet.

Steel weight reduced

Because the number of required columns is greatly reduced, staggered-truss designs have achieved steel weights as low as 5 pounds per square foot, compared with 8 pounds or 9 pounds per square foot for typical steel-framing systems. Fewer columns translates to less extensive foundations, use of less concrete and faster construction completion.

A precast concrete floor system is usually used in conjunction with staggered-truss frames. Because they are prestressed in the factory, the units can span distances of as much as 30 feet without requiring the support of intermediate girders. The precast plank weighs as much as 30 percent less than cast-in-place concrete.

The decrease in the number of columns, compared with conventional steel framing, also reduces the time and cost of applying fireproofing to the steel. Additional savings typically accrue from the ability to use both precast concrete and prefabricated steel, providing an opportunity to increase competitive bidding among a larger pool of contractors.

Half Moon Harbor, a new 12-story apartment building in North Bergen, N.J., for which Wexler was the structural engineer, provides an example of the benefits of a staggered-truss design. The system allowed Edgewater, N.J.-based Cybul & Cybul Architects to design generous column-free spaces. Barrett Companies of Fort Lee, N.J., selected the staggered-truss system based on a cost/benefit analysis that also considered traditional systems, including poured-in-place concrete and post-and-beam steel. The lighter total weight of the staggered- truss system was particularly beneficial for reducing foundation requirements.

Wexler also was the structural engineer for the recently completed Mystic Marriott Hotel & Spa in Groton, Conn. An analysis by the project's design team determined that the system offered distinct advantages. It provided flexibility for New York City-based Brennan Beer Gorman/Architects in designing both the public spaces and the guest rooms. The staggered-truss system eliminated the need for second-floor transfer girders above the hotel's public spaces.

Fewer drawings required

Wexler says it is a misconception that design and construction with a staggered-truss system are more complicated than with more traditional systems. He notes that the typical staggered-truss project requires fewer drawings because fewer framing connections are needed, and most details are repetitious.

Nevertheless, he advises that designers should be mindful of issues associated with staggered-truss systems. For example, since the framing creates wall lines that do not align from floor to floor, shafts must be independent of the trusses. Mechanical, electrical and plumbing trades need to make appropriate notations on architectural drawings.

The first major use of a staggered-truss system in New York City was in 1986 for the Towers on the Park, a complex of 20-story and nine-story buildings containing 599 apartments.

A recent New York City staggered-truss project was the Embassy Suites Hotel in Battery Park City. The project's building team included Perkins Eastman Architects and structural engineer Thornton-Tomasetti Engineers.

The 2,600-room, 38-story Aladdin Hotel in Las Vegas was the first steel-framed, staggered-truss hotel to be built in Nevada's gambling capital, where concrete frames are the norm. The Aladdin's staggered trusses are 9 feet deep.

A concrete superstructure was originally contemplated for the Aladdin, according to New York City-based Tishman Realty & Construction Co., the owner's representative. But bids came in higher than anticipated, prompting an investigation of alternative designs. The cost of a staggered-truss plan proposed by SMI-Owen Steel Co., Columbia, S.C., was more than 10 percent less than that of the concrete-framed plan. Moreover, a shorter erection time allowed the hotel to be completed several months earlier than if it had a concrete frame.

Boston-based structural engineer McNamara/Salvia Inc. was a consultant to SMI-Owen. Principal Robert McNamara says the staggered-truss system helped to integrate the hotel tower with its function areas, which are located below the sixth level. Steel framing also facilitated the addition of structural reinforcement that became necessary when a swimming pool that was not part of the original design was added and placed on top of a low-rise section of the hotel.

 

         
 

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