ASD and LRFD Specifications

The American Institute of Steel Construction (AISC) has developed design specifications for structural steel with two different design approaches: ‘‘Specification for Structural Steel Buildings—Allowable Stress Design (ASD) and Plastic Design’’ and ‘‘Load and Resistance Factor Design (LRFD) Specification for Structural Steel Buildings.’’ Building codes either adopt by reference or incorporate both these approaches. It is the prerogative of the designer to select the approach to employ; this decision is generally based on economics. The approaches should not be mixed.
ASD. The AISC specification for ASD establishes allowable unit stresses that, under service loads on a structure, may not be exceeded in structural members or their connections.
Allowable stresses incorporate a safety factor to compensate for uncertainties in design and

construction. Common allowable unit stresses of the AISC ASD specification are summarized
in Table 6.18.
LRFD. The AISC specification for LRFD requires that factors be applied to both service loads and the nominal resistance (strength) of members and connections. To account for uncertainties in estimating the service loads, load factors generally greater than unity are applied to them (Art. 6.11). To reflect the variability inherent in predictions of the strength of a member or connection, the nominal resistance Rn is multiplied by a resistance factor  less than unity. To ensure that a member or connection has sufficient strength to support the service loads, the service loads multiplied by the appropriate load factors (factored loads) should not exceed the design strength Rn . Table 6.18 summarizes the equations for design strength specified in the AISC LRFD specification.
Other. The AISC also publishes separate specific-purpose specifications. Included are the following: ‘‘Specification for Allowable Stress Design of Single Angle Members,’’ ‘‘Specification for Load and Resistance Factor Design of Single Angle Members’’; ‘‘Specification for the Design of Steel Hollow Structural Sections,’’ which is in LRFD format (see Art. 6.30); and ‘‘Seismic provisions for Structural Steel Buildings,’’ which includes both LRFD and ASD formats (see Sec. 9). These separate specifications are intended to be compatible with, and provide a useful supplement to, the primary AISC ASD and LRFD specifications.