AISC LRFD 1.pdf

Comm. A5.] DESIGN BASIS 173where = summationi = type of load, i.e., dead load, live load, wind, etc.Q i = nominal load effect i = load factor corresponding to Q i i Q i = required strengthR n = nominal strength = resistance factor corresponding to R nR n = design strengthThe left side of Equation C-A5-1 represents the required resistance computed bystructural analysis based upon assumed loads, and the right side of EquationC-A5-1 represents a limiting structural capacity provided by the selected members.In LRFD, the designer compares the effect of factored loads to the strength actuallyprovided. The term design strength refers to the resistance or strength R n that mustbe provided by the selected member. The load factors and the resistance factors reflect the fact that loads, load effects (the computed forces and moments in thestructural elements), and the resistances can be determined only to imperfectdegrees of accuracy. The resistance factor is equal to or less than 1.0 because thereis always a chance for the actual resistance to be less than the nominal value R n computedby the equations given in Chapters D through K. Similarly, the load factors reflect the fact that the actual load effects may deviate from the nominal values of Q icomputed from the specified nominal loads. These factors account for unavoidableinaccuracies in the theory, variations in the material properties and dimensions, anduncertainties in the determination of loads. They provide a margin of reliability toaccount for unexpected loads. They do not account for gross error or negligence.The LRFD Specification is based on (1) probabilistic models of loads and resistance,(2) a calibration of the LRFD criteria to the 1978 edition of the AISC ASDSpecification for selected members, and (3) the evaluation of the resulting criteriaby judgment and past experience aided by comparative design office studies ofrepresentative structures.The following is a brief summary of the probabilistic basis for LRFD (Ravindra andGalambos, 1978, and Ellingwood, MacGregor, Galambos, and Cornell, 1982). Theload effects Q and the resistance factors R are assumed to be statistically independentrandom variables. In Figure C-A5.1, frequency distributions for Q and R areportrayed as separate curves on a common plot for a hypothetical case. As long asthe resistance R is greater than (to the right of) the effects of the loads Q, a margin ofsafety for the particular limit state exists. However, because Q and R are randomvariables, there is some small probability that R may be less than Q, (R