Microsoft Word - staggered 8.03.010 Sep 12

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Microsoft Word - staggered 8.03.010 Sep 12











SEAOC Blue Book Seismic Design Recommendations
Staggered Truss Frames
Article 8.03.010
Page 1 of 4
September 2006
www.seaoc.org/bluebook
ASCE 7-02/05
reference section(s)
2001 CBC
reference section(s)
Other standard
reference section(s)
ASCE 7-02
ASCE 7-05
9.5.2.2
12.2.1
1629.9.2
BackgroundThe staggered truss system was developed at MIT in the 1960s (Scalzi 1971). Its arrangement of story-deep trussesin a staggered pattern allows large column-free areas and low floor-to-floor heights. With fewer columns than other
steel framing systems, staggered truss frames can also offer faster fabrication and erection schedules and reducedfoundation costs (Wexler and Lin 2003).
Most staggered truss systems are in areas of low seismic hazard. Because of the apparent benefits of the system,
AISC, structural steel contractors, and others have expressed interest in using the system in California. TheAmerican Institute of Steel Construction (AISC) has published a design guide with a chapter on seismic applications
(Wexler and Lin 2003). The following statement appears in its introduction:
One added benefit of the staggered-truss framing system is that it is highly efficient for resistanceto the lateral loading caused by wind and earthquake. The stiffness of the system provides the
desired drift control for wind and earthquake loadings. Moreover, the system can provide asignificant amount of energy absorption capacity and ductile deformation capability for high-
seismic applications.
In contrast to these assertions, the SEAOC Seismology Committee position is that the staggered truss system is notaddressed as a seismic force-resisting system in ASCE 7-02 Table 9.5.2.2 or in ASCE 7-05 Table 12.2-1 and that it
is an undefined structural system per 2001 CBC section 1629.9.2 and subsequent editions. Therefore, pendingreview of substantiating cyclic test data and analytical studies, the Committee recommends against use of the
staggered truss system as a seismic force-resisting system in ASCE 7-02/05 Seismic Design Categories (SDCs) Cthrough F and in 2001 CBC Seismic Zones 3 and 4. While SDCs C-F effectively cover all of California, the
substantiating test data requirements of ASCE 7 and the 1997 UBC apply to all SDCs and Seismic Zones.
Description of the SystemThe staggered truss system is contemplated for buildings from 6 to 25 stories tall (Wexler and Lin 2003). Its benefitsare most apparent in regular buildings with rectangular floor plans. The system consists of full story-deep trusses
spanning the transverse direction of the building; truss spans are typically 60 feet. From one story to the next, thetrusses are horizontally offset by one column bay (typically 20 to 30 feet) so that the truss locations are staggered up
the height of the building. See Figure 1. The stagger is typically of a uniform dimension and symmetric in plan.Floor diaphragms are typically precast planks spanning from the bottom chord of one