AISC LRFD 1.pdf

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