7/27/2019 C-GuestColumn-SEAOC-June08 Seismic Design of Steel Staggered Truss Systems

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Seismic Design of Steel Staggered Truss SystemsSeismology Committee, Structural Engineers Association of California

This is the first in a series of STRUCTUREarticles that have been condensedand adapted from the Structural Engineers Association of California(SEAOC) online series called the SEAOC Blue Book: Seismic DesignRecommendations. Beginning in 1959 and extending to 1996, the SEAOCSeismology Committee published updated printed editions of RecommendedLateral Force Requirements and Commentary, which was commonly called

the Blue Book. The Requirements portion of those publications was in largepart adopted verbatim by the International Council of Building Officials asthe seismic regulations of the Uniform Building Code. With the transition tothe unification of the three major model building code organizations into theInternational Code Council, and the nationwide use of the NEHRP seismicdesign provisions that are developed under the auspices of FEMA and theBuilding Seismic Safety Council, SEAOC decided the Requirements portionof the Blue Book no longer served a purpose, but that there was a still a need forstatements of position and guidance for practicing engineers and code officialsto help them to resolve ambiguities among various codes and standards and tointerpret research. A number of Blue Book articles are in development and arebeing published one-by-one, after each has passed through an extensive SEAOCSeismology Committee review process.

The staggered truss system was devel-oped at MIT in the 1960s (Scalzi 1971).Its arrangement of story-deep trusses ina staggered pattern allows large column-free areas and low floor-to-floor heights.

With fewer columns than other steelframing systems, staggered truss framescan also offer faster fabrication and erec-tion schedules, and reduced foundationcosts (Wexler and Lin 2003).Most staggered truss systems are in

areas of low seismic hazard. Becauseof the apparent benefits of the system,

AISC, structural steel contractors andothers have expressed interest in usingthe system in high-seismic regions of theUSA. The American Institute of SteelConstruction (AISC) has published adesign guide with a chapter on seismicapplications (Wexler and Lin 2003).The following statement appears inits introduction:One added benefit of the staggered-truss

framing system is that it is highly efficientfor resi stance to the lateral loading caused

by wind and earthquake. The stiffnessof the system provides the desired driftcontrol for wind and earthquake load-ings. Moreover, the system can provide asignificant amount of energy absorptioncapacity and ductile deformation capa-bility for high-seismic applications.The staggered truss system is not

a recognized seismic force-resistingsystem in ASCE 7-02 Table 9.5.2.2 orin ASCE 7-05 Table 12.2-1, equivalentto an undefined structural system per2001 CBC section 1629.9.2. Therefore,

pending review of substantiating cyclictest data and analytical studies, or projectspecific peer review in combination

with analytical studies, the Committeerecommends against use of the staggeredtruss system as a seismic force-resistingsystem per ASCE 7-02/05 SeismicDesign Categories (SDCs) C through Fand in 2001 CBC Seismic Zones 3 and4. While SDCs C-F effectively coversall of California, the substantiating testdata requirements of ASCE 7 and the1997 UBC are applicable to all SDCsand Seismic Zones.

Description of the SystemThe staggered truss system is contem-

plated for buildings from 6 to over 20stories tall (Wexler and Lin 2003). Itsbenefits are most apparent in regularbuildings with rectangular floor plans.The system consists of full story-deeptrusses spanning the transverse directionof the building; truss spans are typically60 feet. From one story to the next, thetrusses are horizontally offset by onecolumn bay (typically 20 to 30 feet) sothat the truss locations are staggered upthe height of the building (Figure 1).The stagger is typically of a uniform di-mension and symmetric in plan. Floordiaphragms are typically precast planksspanning from the bottom chord of onetruss to the top chord of the adjacenttruss. Exterior columns support the endsof the truss and provide frame columnsfor the lateral force-resisting system inthe longitudinal direction of the build-

ing. To maximize the architectural benefits of the system, there are frequentlno continuous interior columns.Each truss acts as a braced frame in th

transverse direction. A Vierendeel paneis often provided at the midspan of th

truss to accommodate passagewaysUnder transverse seismic loads, thVierendeel panel would be subject thigh deformations (much like the similapanel in a special truss moment frame)and would therefore have to be designeto dissipate energy through flexurayielding. The trusses resist transversshear, overturning forces, and interstordrift, and the floor diaphragm acts aa load path element between adjacentrusses. The longitudinal lateral forceresisting system is typically a perimete

moment frame or braced frame.

Response toEarthquake Loads

Acceptable earthquake performance oa staggered truss system will be limiteby the following attributes:

The lateral and gravity force-resisting systems of the building areone and the same. Every gravity-resisting truss and column is alsointegral to the transverse lateralforce resisting system.

Long transverse spans limit theability of the system to redistributegravity loads in the event of acolumn failure.

The ground story is usually muchmore flexible than the floors above.Customarily, moment framesreplace the staggered truss elementsat the ground story.

Diaphragms are critical to thelateral load path of this structuralsystem, transferring relatively highforces between vertical elements.

This is especially true at lowerstories, where the diaphragm anddiaphragm-to-truss connectionsmust transfer nearly the entire baseshear from one story to the next.

This last point about the diaphragmmight be unique to the staggered trussystem. The floor diaphragms are requireto participate in the lateral system afully as the trusses and columns. Modeearthquake design codes, however, assigseismic design factors (i.e. R, Cd, an0) without thorough consideration o

7/27/2019 C-GuestColumn-SEAOC-June08 Seismic Design of Steel Staggered Truss Systems

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Figure 1: Typical Staggered Truss Frame Layout.

diaphragm ductility and modes of inelasticitywithin the diaphragm. Use of seismic designfactors from moment-resisting frames, bracedframes, or special truss moment frames for astaggered truss system would be inappropriate.The assertions by Wexler and Lin quoted

above are apparently based on elastic analysisresults and theoretical response estimatesby Goel et al. (1973) and perhaps by aninappropriate extrapolation of inelasticbehavior modes expected in special truss

moment frames (Basha and Goel 1994).Additional information may be found inScalzi (1971), Goel et al. (1973), Gupta andGoel (1972), and Hanson and Berg (1974).

Recommended Research

The Seismology Committee is not aware ofany recent testing of the staggered truss systemfor use as a seismic force-resisting system. AISCand others are investigating the feasibility ofthe staggered truss system for areas of highseismicity. Testing and analysis are expected

to focus on sources of inelasticity, diaphragms,and diaphragm-to-truss connections.The development of eccentrically braced

frames and special moment-resisting framesperhaps offers examples for proponents of thestaggered truss to follow. Specifically, ASCE7-02 section 9.5.2.2 and ASCE 7-05 section12.2.1 give requirements for qualifying anundefined seismic-force-resisting system.The Seismology Committee expects thatadequate testing and/or analysis will berequired to adequately address at least thefollowing design and performance issues:

Identification of predictable inelasticmechanisms

Design forces and deformations inyielding Vierendeel panels and adjacenttruss members

Design forces related to diaphragm-truss interaction, considering expectedstrength, stiffness, and ductility

Force distribution and inelasticity inprecast diaphragms and topping slabsunder high in-plane forces

ReferencesASCE (American Society of Civil Engineers) (2002). ASCE 7-02,Minimum Design Loads

Buildings and Other Structures. ASCE, Reston, VA.ASCE (American Society of Civil Engineers) (2006). ASCE 7-05,Minimum Design Loads

Buildings and Other Structures, Including Supplement No. 1, ASCE, Reston, VA.Basha, H.S., and Goel, S. C. (1994). Seismic Resistant Truss Moment Frames with Duc

Vierendeel Segment, Report #UMCEE94-29, University of Michigan Department of Cand Environmental Engineering, Ann Arbor, MI.

California Building Standards Commission (2001).2001 California Building Code(California Cof Regulations, Title 24). California Building Standards Commission, Sacramento, CA.

Goel, S. C., Berg, G. V., and Hanson, R. D. (1973). Seismic Behavior of Staggered Truss FramSystem - Design Procedure for Earthquake Loading, Report UMEE 73R2 for AISI Pro175, University of Michigan, Ann Arbor, MI.

Gupta, R. P., and. Goel, S. C. (1972). Dynamic analysis of staggered truss framing systemProc. Paper 9038 in Journal of the Structural Division, ASCE, v. 98 n. ST7, July, 141492.

Hanson, R. D., and Berg, G. V. (1974). A seismic design of staggered truss building, PrPaper 10289 in Journal of the Structural Division, ASCE, v. 100, n. ST1, January, 1193.

Scalzi, J. B. 1971. The staggered truss system Structural considerations,AISC EngineerJournal, October.

Wexler, N. and Lin, F.-B. 2003. Staggered Truss Framing Systems(AISC Steel Design GuSeries No. 14), American Institute of Steel Construction, Chicago, IL.

STORY HIGH TRUSSES

ARE STAGGERED

VIERENDEEL TRUSS PANEL

USUALLY USED TO

ACCOMMODATE CORRIDOR

COLUMN FREE GROUND STORY

PRECAST CONCRETE PLANKS SPAN

FROM BOTTOM CHORD OF TRUSS TO

TOP CHORD OF ADJACENT TRUSS

TRANSVERSE MOMENT OR BRACED FRAMES

ARE REQUIRED AT THE GROUND STORY

MOMENT FRAMES ON EXTERIOR ARE OFTEN

USED IN LONGITUDINAL DIRECTION

Force distribution and inelasticityin diaphragms under verticaldisplacements related to trussdeflections and link deformation

Design of diaphragm-to-trussconnections, considering cyclic loadinand diaphragm or truss overstrength

Column design forces and ductilitydemands, considering dynamic trusscolumn interaction and sharing of

columns by lateral andtransverse systems

Vulnerability of the gravity system tofailure of seismic-force-resisting memb

Effects of openings and discontinuitiein highly loaded diaphragms

Disproportionate effects of atypical anirregular building configuration

Axial and flexural interaction in trusschords, diagonals, and connectors

In addition, because the system load pinvolves an out-of-plane offset at every fllevel, testing must consider the interaction

yielding (and possibly degrading) diaphragtrusses, and connections, as opposed to the behavior of individual components. Ethe testing of an entire truss frame would capture the essential aspect of shear tranbetween adjacent frames.

The complete SEAOC Blue Bookseries of on-line Seismic Design

Recommendations articles is available awww.seaoc.org/bluebook/index.htm