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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.

Continuing
Education

Use the following learning objectives to focus your study while reading this month’s ARCHITECTURAL RECORD / AIA Continuing Education article.

Learning Objective:
After reading this article, you will be able to:

1. Understand the design of the staggered truss structural system.

2. Know the benefits of the staggered truss system design over other traditional concrete and steel structural designs.

3. Understand how the staggered truss system provides an efficient structural system with floor-to-floor heights of as little as 8'-8".

4. Review how the staggered truss system was used as an effective design solution in a hotel and a university case study.

First developed in the 1960s, the staggered truss system is now experiencing a renaissance, with a growing number of hotels, apartment buildings, dormitories, office buildings and even hospitals using this innovative and flexible system—ranging from an Embassy Suites hotel in New York City designed by Perkins Eastman Architects to the Baruch College Academic Center designed by Kohn Pedersen Fox Associates.

The staggered truss structural system consists of story-high steel trusses placed on alternating column lines on each floor so that the long axis of one truss is always between the trusses on the floor below. Typically, a precast concrete hollow core plank floor system is utilized, with the planks spanning from the bottom chord of one truss to the top chord of the adjacent truss (see schematic). With precast plank floors, economy is achieved by “stretching” the plank to the greatest possible span. Eight-inch plank can span 30 feet, while 10-inch plank generally can be used to span up to 36 feet, depending on the loading condition.

While earlier staggered truss systems utilized channels for web diagonals and verticals, today most of the trusses are designed with hollow structural sections (HSS) for vertical and diagonal members because they are more structurally efficient and easier to fabricate. The trusses are fabricated with camber to compensate for dead load and are transported to the site, stored and then erected—generally in one piece.

The Resulting System Achieves:

  • Low floor-to-floor heights (typically, an 8'-8" floor-to-floor height is achieved)
  • Large column-free spaces (as much as 60 feet in each direction with columns often only appearing on the perimeter of a structure)
  • Increased design flexibility (especially for atrium placement and floor plans)

Experience Reveals That Staggered Truss Systems Are:

  • Faster to erect than comparable concrete structures; and
  • Reduce the weight of the superstructure, which results in substantial savings in foundation work

In addition, because the vertical loads are concentrated at a few column points, less foundation formwork is required. And as with precast concrete floors, a good quality concrete plank floor structure yields a dry, semi-finished ceiling that requires little or no finishing.

The staggered truss system offers one of the most cost-efficient framing options in part due to the project’s scheduling considerations. Steel framing, including spandrel beams and precast floors, can generally be erected at a faster rate than traditional systems—and once two floors are erected, window installation can start and stay right behind the steel and floor erection. No time is lost in waiting for other trades, such as bricklayers, to start work. Except for foundations, topping slab, and grouting, all “wet” trades are eliminated.

The system works best for buildings in the five to 20-story range, there are several notable examples in the 30-40 story range, including the Trump Taj-Mahal in Atlantic City, the Renaissance Hotel in Nashville, and The Aladdin in Las Vegas. However, while the stiffness of the system provides the desired drift control for wind and earthquake loadings, staggered trusses systems may not be economical for projects above 20 stories in high-seismic regions due to the issue of diaphragm action of the plank system, though staggered truss designs may be used if other diaphragm systems are utilized. In some cases, cast-in-place topping slabs over pre-cast concrete planks that have been designed to act as the diaphragm have cracked along the edges of the precast elements and the welded wire fabric have fractured during earthquakes. However, untopped hollow-core plank diaphragms with grouted joints and chordes have performed satisfactorily in earthquakes and laboratory tests.

Fire resistance is another advantage, for several reasons. First, the steel is localized to the trusses, which only occur at every 58-to-70-feet on a floor, so the fireproofing operation can be completed efficiently. Furthermore, the trusses are typically placed within demising walls and it is possible that the necessary fire rating can be entirely by enclosing the trusses with gypsum wallboard. Finally, if spray-on protection is desired, the applied thickness can be kept to a minimum due to the compact nature of the truss elements.

In addition to saving their clients time and money, the staggered truss system provides architects with increased design flexibility. This flexibility translates into first-floor ballrooms, health clubs, restaurants and beautiful lobbies. And this flexibility extends to the layout of the back-of-house operations, a key concern for hotel developers.

Design Considerations

In many respects, designing a staggered truss system is simpler than a conventional steel-framed building. There are fewer structural drawings since there are fewer connections and most of the details are repeated. Detail drawings also are simplified. The main design consideration is the accommodation of shafts within the building that are independent of the trusses, since the framing system creates wall lines that differ from floor to floor. Also, it is important that the Mechanical/Electrical/Pluming (MEP) trades receive adequate information on the architectural drawings and that the lines of communication are kept open.

The key to obtaining maximum cost efficiency from a staggered truss project is careful attention to planning and coordination. Early steel fabricator and concrete plank manufacturer involvement is critically important. Coordination between the two contractors is particularly important for such details as plank cut-outs, plank camber, plank bearing supports, and clearances for stud welding. It is often advantageous for the steel fabricator and plank supplier to be in direct contact.

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