AISC LRFD 1.pdf

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Sect. C3.] STABILITY BRACING 19all members in the vertical bracing system shall be included in the lateral stabilityanalysis.1a. Design by Plastic AnalysisIn braced frames designed on the basis of plastic analysis, as limited in SectionA5.1, the axial force in these members caused by factored gravity plus factored horizontalloads shall not exceed 0.85 c times A g F y .2. Unbraced FramesIn frames where lateral stability depends upon the bending stiffness of rigidly connectedbeams and columns, the effective length factor K of compression membersshall be determined by structural analysis. The destabilizing effects of gravityloaded columns whose simple connections to the frame do not provide resistance tolateral loads shall be included in the design of the moment-frame columns. Stiffnessreduction adjustment due to column inelasticity is permitted.Analysis of the required strength of unbraced multistory frames shall include theeffects of frame instability and column axial deformation under the factored loadcombinations stipulated in Section A4.2a. Design by Plastic AnalysisIn unbraced frames designed on the basis of plastic analysis, as limited in SectionA5.1, the axial force in the columns caused by factored gravity plus factored horizontalloads shall not exceed 0.75 c times A g F y .C3. STABILITY BRACING1. ScopeThese requirements address the minimum brace strength and stiffness necessary toensure member design strengths based on the unbraced length between braces withan effective length factor K equal to unity. Bracing is assumed to be perpendicularto the member(s) to be braced; for inclined or diagonal bracing, the brace strength(force or moment) and stiffness (force per unit displacement or moment per unitrotation) must be adjusted for the angle of inclination. The evaluation of the stiffnessfurnished by a brace shall include its member and geometric properties, as wellas the effects of connections and anchoring details.Two general types of bracing systems are considered, relative and nodal. A relativebrace controls the movement of the brace point with respect to adjacent bracedpoints. A nodal brace controls the movement at the braced point without directinteraction with adjacent braced points. The strength and stiffness furnished by thestability bracing shall not be less than the required limits. A second order analysisthat includes an initial out-of-plumbness of the structure or out-of-straightness ofthe member to obtain brace strength and stiffness can be used in lieu of the requirementsof this section.2. FramesIn braced frames where lateral stability is provided by diagonal bracing, shearwalls, or other equivalent means, the required story or panel bracing shear force is:P br = 0.004P u(C3-1)<strong>LRFD</strong> Specification for Structural Steel Buildings, December 27, 1999AMERICAN INSTITUTE OF STEEL CONSTRUCTION
20 STABILITY BRACING [Sect. C3.The required story or panel shear stiffness is:2ΣPuβ(C3-2)brLwhere = 0.75P u = summation of the factored column axial loads in the story or panel supportedby the bracing, kips (N)L = story height or panel spacing, in. (mm)These story stability requirements shall be combined with the lateral forces anddrift requirements from other sources, such as wind or seismic loading.3. ColumnsAn individual column can be braced at intermediate points along its length by relativeor nodal bracing systems. It is assumed that nodal braces are equally spacedalong the column.(a) Relative BracingThe required brace strength is:P br = 0.004P u(C3-3)The required brace stiffness is:Puβ2 br = (C3-4)φLb= φuwhere = 0.75P u = required compressive strength, kips (N)L b = distance between braces, in. (mm)(b) Nodal BracingThe required brace strength is:P br = 0.01P uThe required brace stiffness is:(C3-5)8Pβ(C3-6)br= φ Lbwhere = 0.75When the actual spacing of braced points is less than L q , where L q is the maximumunbraced length for the required column force with K equal to one, then L b in EquationsC3-4 and C3-6 is permitted to be taken equal to L q .<strong>LRFD</strong> Specification for Structural Steel Buildings, December 27, 1999AMERICAN INSTITUTE OF STEEL CONSTRUCTION
Page 1 and 2: LOAD AND RESISTANCEFACTOR DESIGNSPE
Page 3: iiCopyright © 2000byAmerican Insti
Page 6 and 7: vTABLE OF CONTENTSSYMBOLS . . . . .
Page 8 and 9: TABLE OF CONTENTSviiSPECIFICATION (
Page 10 and 11: TABLE OF CONTENTSixSPECIFICATION (C
Page 12 and 13: TABLE OF CONTENTSxiCOMMENTARY (Cont
Page 18 and 19: SYMBOLSxviistrength (for those stee
Page 20 and 21: SYMBOLSxixS eff Effective section m
Page 22 and 23: SYMBOLSxxit s Web stiffener thickne
Page 24 and 25: xxiiiGLOSSARYAlignment chart for co
Page 26 and 27: GLOSSARYxxvEffective moment of iner
Page 28 and 29: GLOSSARYxxviiLateral bracing member
Page 30 and 31: GLOSSARYxxixPost-buckling strength.
Page 32 and 33: GLOSSARYxxxiStrain-hardening strain
Page 34 and 35: 1CHAPTER AGENERAL PROVISIONSA1. SCO
Page 36 and 37: Sect. A3.] MATERIAL 3Cold-Formed We
Page 38 and 39: Sect. A3.] MATERIAL 5ASTM A449 bolt
Page 40 and 41: Sect. A6.] REFERENCED CODES AND STA
Page 42 and 43: Sect. A7.] DESIGN DOCUMENTS 9tion A
Page 44 and 45: Sect. B4.] STABILITY 11(a) When the
Page 46 and 47: Sect. B10.] PROPORTIONS OF BEAMS AN
Page 48 and 49: Sect. B10.] PROPORTIONS OF BEAMS AN
Page 50 and 51: 17Chap. CCHAPTER CFRAMES AND OTHER
Page 54 and 55: Sect. C3.] STABILITY BRACING 214. B
Page 56 and 57: Sect. C3.] STABILITY BRACING 231.0;
Page 58 and 59: Sect. D3.] PIN-CONNECTED MEMBERS AN
Page 60 and 61: 27Chap. ECHAPTER ECOLUMNS AND OTHER
Page 62 and 63: Sect. E4.] BUILT-UP MEMBERS 29(a) F
Page 64 and 65: 31Chap. FCHAPTER FBEAMS AND OTHER F
Page 66 and 67: Sect. F1.] DESIGN FOR FLEXURE 33Lp=
Page 68 and 69: Sect. F2.] DESIGN FOR SHEAR 35Lpd
Page 70 and 71: 37Chap. GCHAPTER GPLATE GIRDERSI-sh
Page 72 and 73: Sect. H3.] ALTERNATIVE INTERACTION
Page 74 and 75: Sect. I2.] COMPRESSION MEMBERS 41ti
Page 76 and 77: Sect. I3.] FLEXURAL MEMBERS 434. Lo
Page 78 and 79: Sect. I3.] FLEXURAL MEMBERS 45welde
Page 80 and 81: Sect. I5.] SHEAR CONNECTORS 47A c =
Page 82 and 83: 49Chap. JCHAPTER JCONNECTIONS, JOIN
Page 84 and 85: Sect. J1.] GENERAL PROVISIONS 518.
Page 86 and 87: Sect. J2.] WELDS 53Welding ProcessT
Page 88 and 89: Sect. J2.] WELDS 55For end-loaded f
Page 90 and 91: Sect. J2.] WELDS 57Types of Weld an
Page 92 and 93: Sect. J3.] BOLTS AND THREADED PARTS
Page 100 and 101: Sect. J4.] DESIGN RUPTURE STRENGTH
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Sect. J8.] BEARING STRENGTH 69J6. F
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71Chap. KCHAPTER KCONCENTRATED FORC
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Sect. K1.] FLANGES AND WEBS WITH CO
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For P u 0.4P yR v = 0.60F y d c t
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Sect. K3.] DESIGN FOR CYCLIC LOADIN
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79Chap. LCHAPTER LSERVICEABILITY DE
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81Chap. MCHAPTER MFABRICATION, EREC
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Sect. M4.] ERECTION 83(2) Bottom su
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Sect. M5.] QUALITY CONTROL 85The fa
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Sect. N3.] EVALUATION BY STRUCTURAL
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89App. BAPPENDIX BDESIGN REQUIREMEN
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App. B5.] LOCAL BUCKLING 914kc=h/tw
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App. B5.] LOCAL BUCKLING 93é0.877
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App. E3.] DESIGN COMPRESSIVE STRENG
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App. F1.] DESIGN FOR FLEXURE 97For
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App. F1.] DESIGN FOR FLEXURE 99TABL
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App. F1.] DESIGN FOR FLEXURE 101Cri
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App. F3.] WEB-TAPERED MEMBERS 103j
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App. F3.] WEB-TAPERED MEMBERS 105Fw
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107App. GAPPENDIX GPLATE GIRDERSThi
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App. G3.] DESIGN SHEAR STRENGTH 109
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App. G5.] FLEXURE-SHEAR INTERACTION
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App. H3.] ALTERNATIVE INTERACTION E
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115App. JAPPENDIX JCONNECTIONS, JOI
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App. J3.] BOLTS AND THREADED PARTS
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App. K2.] PONDING 119æçMetric:lC
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App. K3.] DESIGN FOR CYCLIC LOADING
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NUMERICAL VALUES 141TABLE 2ItemShap
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NUMERICAL VALUES 143Klr123456789101
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NUMERICAL VALUES 145TABLE 3-50Desig
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NUMERICAL VALUES 147TABLE 4Values o
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NUMERICAL VALUES 149TABLE 5 (cont
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NUMERICAL VALUES 151TABLE 7Values o
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NUMERICAL VALUES 153f vVAwnTABLE 8-
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NUMERICAL VALUES 157hf vVAwnTABLE 9
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NUMERICAL VALUES 161ht wf vVAwnTABL
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163COMMENTARYon the Load and Resist
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Comm. A2.] TYPES OF CONSTRUCTION 16
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Comm. A3.] MATERIAL 167and Chen, 19
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Comm. A3.] MATERIAL 169than the ant
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Comm. A4.] LOADS AND LOAD COMBINATI
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Comm. A5.] DESIGN BASIS 173where =
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Comm. A5.] DESIGN BASIS 175( )bsln(
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177Comm. BCHAPTER BDESIGN REQUIREME
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Comm. B5.] LOCAL BUCKLING 179(c) Al
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Comm. B5.] LOCAL BUCKLING 181load.
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Comm. B7.] LIMITING SLENDERNESS RAT
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Comm. C1.] SECOND ORDER EFFECTS 185
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Comm. C1.] SECOND ORDER EFFECTS 187
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Comm. C2.] FRAME STABILITY 189Buckl
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Comm. C2.] FRAME STABILITY 191Notes
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Comm. C2.] FRAME STABILITY 193Where
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Comm. C3.] STABILITY BRACING 195age
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Comm. C3.] STABILITY BRACING 197pla
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Comm. C3.] STABILITY BRACING 199The
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Comm. C3.] STABILITY BRACING 201Par
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203Comm. ECHAPTER ECOLUMNS AND OTHE
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Comm. E4.] BUILT-UP MEMBERS 205TABL
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Comm. F1.] DESIGN FOR FLEXURE 207ha
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Comm. F1.] DESIGN FOR FLEXURE 2092b
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Comm. F4.] BEAMS AND GIRDERS WITH W
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Comm. H2.] UNSYMMETRIC MEMBERS AND
Page 248 and 249:
Comm. I2.] COMPRESSION MEMBERS 215d
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Comm. I3.] FLEXURAL MEMBERS 217may
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Comm. I3.] FLEXURAL MEMBERS 219The
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Comm. I3.] FLEXURAL MEMBERS 221wher
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Comm. I3.] FLEXURAL MEMBERS 223proc
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Comm. I4.] COMBINED COMPRESSION AND
Page 260 and 261:
Comm. I5.] SHEAR CONNECTORS 227tal
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229Comm. JCHAPTER JCONNECTIONS, JOI
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Comm. J1.] GENERAL PROVISIONS 231 P
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Comm. J2.] WELDS 2332b. Limitations
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Comm. J2.] WELDS 235When longitudin
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Comm. J2.] WELDS 237due to cyclic f
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Comm. J2.] WELDS 239(b) Plane 2-2,
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Comm. J3.] BOLTS AND THREADED PARTS
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Page 278 and 279:
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Comm. J4.] DESIGN RUPTURE STRENGTH
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249Comm. KCHAPTER KCONCENTRATED FOR
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Comm. K1.] FLANGES AND WEBS WITH CO
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Comm. K1.] FLANGES AND WEBS WITH CO
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Comm. K2.] PONDING 255Even on the b
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257Comm. LCHAPTER LSERVICEABILITY D
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Comm. L5.] CORROSION 259orthotropic
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Comm. M4.] ERECTION 261It is sugges
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Comm. N4.] EVALUATION BY LOAD TESTS
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Comm. N5.] EVALUATION REPORT 265N5.
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267Comm. A-EAPPENDIX ECOLUMNS AND O
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Comm. App. F3.] WEB-TAPERED MEMBERS
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271Comm. A-GAPPENDIX GPLATE GIRDERS
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273Comm. A-KAPPENDIX JCONNECTIONS,
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Comm. J2.] WELDS 275found to be con
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Comm. K3.] DESIGN FOR CYCLIC LOADIN
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279ReferencesAckroyd, M. H., and Ge
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REFERENCES 281Bjorhovde, R. (1972),
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REFERENCES 283Fielding, D. J., and
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REFERENCES 285Johnson, D. L. (1985)
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REFERENCES 287Ollgaard, J. G., Slut
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REFERENCES 289Zhou, S. P., and Chen
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SUPPLEMENTARY BIBLIOGRAPHY 291Kotec
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Metric Conversion Factors for Commo
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