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163COMMENTARYon the Load and Resistance FactorDesign Specification for StructuralSteel BuildingsDecember 27, 1999INTRODUCTIONThe Specification is intended to be complete for normal design usage.The Commentary furnishes background information and references for the benefit of theengineer seeking further understanding of the basis, derivations and limits of the specification.The Specification and Commentary are intended for use by design professionals withdemonstrated engineering competence.LRFD Specification for Structural Steel Buildings, December 27, 1999AMERICAN INSTITUTE OF STEEL CONSTRUCTION
164Comm. ACHAPTER AGENERAL PROVISIONSA1. SCOPELoad and Resistance Factor Design (LRFD) is an improved approach to the designof structural steel for buildings. It involves explicit consideration of limit states,multiple load factors, and resistance factors, and implicit probabilistic determinationof reliability. The designation LRFD reflects the concept of factoring bothloads and resistance. This type of factoring differs from the AISC allowable stressdesign (ASD) Specification (AISC, 1989), where only the resistance is divided by afactor of safety (to obtain allowable stress) and from the plastic design portion ofthat Specification, where only the loads are multiplied by a common load factor.The LRFD method was devised to offer the designer greater flexibility, more rationality,and possible overall economy.The format of using resistance factors and multiple load factors is not new, as severalsuch design codes are in effect [the ACI-318M Building Code Requirementsfor Structural Concrete (ACI, 1999) and the AASHTO Load and Resistance FactorDesign for Bridges (AASHTO, 1996)]. Nor should the new LRFD method givedesigns radically different from the older methods, since it was tuned, or “calibrated,”to typical representative designs of the earlier methods. The principal newingredient is the use of a probabilistic mathematical model in the development ofthe load and resistance factors, which made it possible to give proper weight to theaccuracy with which the various loads and resistances can be determined. Also, itprovides a rational methodology for transference of test results into design provisions.A more rational design procedure leading to more uniform reliability is thepractical result.A2. TYPES OF CONSTRUCTIONConnection Classification. In the first edition of the LRFD Specification (AISC,1986), the types of construction were changed from Type 1 through Type 3 (AISC,1978) to the more general terms of fully-restrained (FR) and partially restrained(PR), to provide appropriate recognition of connection stiffness. The third editionof the LRFD Specification emphasizes the combined importance of stiffness,strength and ductility in connection design.Examples of connection classification schemes include those described byAckroyd and Gerstle (1982), Bjorhovde, Colson, and Brozzetti (1990), andEurocode 3 (1992) (Leon, 1994). The basic assumption made in classifying connectionsis that their most important behavioral characteristics can be modeled by amoment-rotation (M-) curve such as shown in Figure C-A2.1. Implicit in themoment-rotation curve is the definition of the connection as a region of the connectedmembers, along with the connecting elements. The connection is defined inLRFD Specification for Structural Steel Buildings, December 27, 1999AMERICAN INSTITUTE OF STEEL CONSTRUCTION
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LOAD AND RESISTANCEFACTOR DESIGNSPE
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iiCopyright © 2000byAmerican Insti
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vTABLE OF CONTENTSSYMBOLS . . . . .
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TABLE OF CONTENTSviiSPECIFICATION (
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TABLE OF CONTENTSixSPECIFICATION (C
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TABLE OF CONTENTSxiCOMMENTARY (Cont
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SYMBOLSxviistrength (for those stee
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SYMBOLSxixS eff Effective section m
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SYMBOLSxxit s Web stiffener thickne
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xxiiiGLOSSARYAlignment chart for co
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GLOSSARYxxvEffective moment of iner
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GLOSSARYxxviiLateral bracing member
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GLOSSARYxxixPost-buckling strength.
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GLOSSARYxxxiStrain-hardening strain
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1CHAPTER AGENERAL PROVISIONSA1. SCO
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Sect. A3.] MATERIAL 3Cold-Formed We
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Sect. A3.] MATERIAL 5ASTM A449 bolt
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Sect. A6.] REFERENCED CODES AND STA
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Sect. A7.] DESIGN DOCUMENTS 9tion A
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Sect. B4.] STABILITY 11(a) When the
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Sect. B10.] PROPORTIONS OF BEAMS AN
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Sect. B10.] PROPORTIONS OF BEAMS AN
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17Chap. CCHAPTER CFRAMES AND OTHER
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Sect. C3.] STABILITY BRACING 19all
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Sect. C3.] STABILITY BRACING 214. B
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Sect. C3.] STABILITY BRACING 231.0;
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Sect. D3.] PIN-CONNECTED MEMBERS AN
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27Chap. ECHAPTER ECOLUMNS AND OTHER
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Sect. E4.] BUILT-UP MEMBERS 29(a) F
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31Chap. FCHAPTER FBEAMS AND OTHER F
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Sect. F1.] DESIGN FOR FLEXURE 33Lp=
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Sect. F2.] DESIGN FOR SHEAR 35Lpd
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37Chap. GCHAPTER GPLATE GIRDERSI-sh
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Sect. H3.] ALTERNATIVE INTERACTION
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Sect. I2.] COMPRESSION MEMBERS 41ti
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Sect. I3.] FLEXURAL MEMBERS 434. Lo
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Sect. I3.] FLEXURAL MEMBERS 45welde
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Sect. I5.] SHEAR CONNECTORS 47A c =
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49Chap. JCHAPTER JCONNECTIONS, JOIN
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Sect. J1.] GENERAL PROVISIONS 518.
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Sect. J2.] WELDS 53Welding ProcessT
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Sect. J2.] WELDS 55For end-loaded f
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Sect. J2.] WELDS 57Types of Weld an
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Sect. J3.] BOLTS AND THREADED PARTS
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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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Page 148 and 149: 115App. JAPPENDIX JCONNECTIONS, JOI
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Page 152 and 153: App. K2.] PONDING 119æçMetric:lC
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Page 174 and 175: NUMERICAL VALUES 141TABLE 2ItemShap
Page 176 and 177: NUMERICAL VALUES 143Klr123456789101
Page 178 and 179: NUMERICAL VALUES 145TABLE 3-50Desig
Page 180 and 181: NUMERICAL VALUES 147TABLE 4Values o
Page 182 and 183: NUMERICAL VALUES 149TABLE 5 (cont
Page 184 and 185: NUMERICAL VALUES 151TABLE 7Values o
Page 186 and 187: NUMERICAL VALUES 153f vVAwnTABLE 8-
Page 190 and 191: NUMERICAL VALUES 157hf vVAwnTABLE 9
Page 194 and 195: NUMERICAL VALUES 161ht wf vVAwnTABL
Page 198 and 199: Comm. A2.] TYPES OF CONSTRUCTION 16
Page 200 and 201: Comm. A3.] MATERIAL 167and Chen, 19
Page 202 and 203: Comm. A3.] MATERIAL 169than the ant
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Page 206 and 207: Comm. A5.] DESIGN BASIS 173where =
Page 208 and 209: Comm. A5.] DESIGN BASIS 175( )bsln(
Page 210 and 211: 177Comm. BCHAPTER BDESIGN REQUIREME
Page 212 and 213: Comm. B5.] LOCAL BUCKLING 179(c) Al
Page 214 and 215: Comm. B5.] LOCAL BUCKLING 181load.
Page 216 and 217: Comm. B7.] LIMITING SLENDERNESS RAT
Page 218 and 219: Comm. C1.] SECOND ORDER EFFECTS 185
Page 220 and 221: Comm. C1.] SECOND ORDER EFFECTS 187
Page 222 and 223: Comm. C2.] FRAME STABILITY 189Buckl
Page 224 and 225: Comm. C2.] FRAME STABILITY 191Notes
Page 226 and 227: Comm. C2.] FRAME STABILITY 193Where
Page 228 and 229: Comm. C3.] STABILITY BRACING 195age
Page 230 and 231: Comm. C3.] STABILITY BRACING 197pla
Page 232 and 233: Comm. C3.] STABILITY BRACING 199The
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Page 238 and 239: Comm. E4.] BUILT-UP MEMBERS 205TABL
Page 240 and 241: Comm. F1.] DESIGN FOR FLEXURE 207ha
Page 242 and 243: Comm. F1.] DESIGN FOR FLEXURE 2092b
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Comm. H2.] UNSYMMETRIC MEMBERS AND
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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
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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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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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