D1.C.2 LRFD Fundamentals
The primary objective of the LRFD Specification is to provide a uniform reliability for all steel structures under various loading conditions. This uniformity can not be obtained with the allowable stress design (ASD) format.
The ASD method can be represented by the inequality
ΣQi < Rn/F.S.
The left side is the required strength, which is the summation of the load effects, Qi (forces and moments). The right side, the design strength, is the nominal strength or resistance, Rn, divided by a factor of safety. When divided by the appropriate section property (area or section modulus), the two sides of the inequality become the actual stress and allowable stress respectively. ASD, then, is characterized by the use of unfactored "working" loads in conjunction with a single factor of safety applied to the resistance. Because of the greater variability and, hence, unpredictability of the live load and other loads in comparison with the dead load, a uniform reliability is not possible.
LRFD, as its name implies, uses separate factors for each load and resistance. Because the different factors reflect the degree of uncertainty of different loads and combinations of loads and of the accuracy of predicted strength, a more uniform reliability is possible. The LRFD method may be summarized by the inequality
yiQi < RnØ
On the left side of the inequality, the required strength is the summation of the various load effects, Qi, multiplied by their respective load factors, yi. The design strength, on the right side, is the nominal strength or resistance, Rn, multiplied by a resistance factor, Ø.
In the STAAD implementation of LRFD, it is assumed that the user will use appropriate load factors and create the load combinations necessary for analysis. The design portion of the program will take into consideration the load effects (forces and moments) obtained from analysis. In calculation of resistances of various elements (beams, columns etc.), resistance (nominal strength) and applicable resistance factor will be automatically considered.