Advertising Supplement provided by
The American Institute of Steel Construction, Inc.
Use the following learning objectives
to focus your study while reading this months ARCHITECTURAL
RECORD / AIA Continuing Education article.
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 systemranging
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 erectedgenerally in one piece.
The Resulting System Achieves:
- Low floor-to-floor
heights (typically, an 8'-8" floor-to-floor height
- Large column-free
spaces (as much as 60 feet in each direction with
columns often only appearing on the perimeter of a
- Increased design
flexibility (especially for atrium placement and floor
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 projects scheduling
considerations. Steel framing, including spandrel beams and
precast floors, can generally be erected at a faster rate
than traditional systemsand 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
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
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.
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.