The Allowable Stress Design (ASD) technique is the conventional method accepted for the design of steel structures over many years. The method is based on pure elastic theory and demands that the stresses produced in a component by the applied loads must not exceed a stipulated allowable stress. In 1986, the American Institute of Steel Construction (AISC) presented an alternative method called the Load and Resistance Factor Design (LRFD) method. In this method, factored loads are applied to a steel component to determine the required absolute strength and this is evaluated with the component's assumed strength and a suitable reduction factor.

Historically, the Allowable Stress Method (ASD) has delivered safe and reliable steel and composite structures; however, the method does not comprehend inconsistency of various load effects (live load, dead load) and resistances (i.e. shear capacity, bending, cracks, etc.). For this reason, the Load and Resistance Factor Design (LRFD) is the preferred method of structural steel design. LRFD method has two principle benefits over the ASD method. First, during limit state analysis, engineer does not have to presume linearity between force and load, or stress and force. Second, different load factors can be utilized to suggest the degree of uncertainty for various loads (dead and live). Due to these benefits of LRFD, more consistent reliability is achieved during the structural steel design process and in many cases a more cost-effective steel structure results.

The principal advantage of the Load and Resistance Factor Design method is that, by applying a statistical analysis to the random values of component strengths and loads, a consistent factor of safety may be achieved for all types of steel structures. LRFD models the behavior of the structure at definitive loads and provides an accurate estimation of the strength of the steel or composite structure. In recent years, LRFD method has been successfully employed to the design of hot-rolled and cold-formed steel sections and components in United States, United Kingdom and other countries.

Also when you need to evaluate structure strength under seismic circumstances, a truly elastic design approach is difficult to correlate with estimated structural response. The existing alternative provisions for ASD are totally misleading because they utilize a conservative load factor of 1.7 on all live and dead loads and a set of customized ASD factors to determine permissible strengths. At some point, we are creating more confusion and work by trying to use ASD for the intrinsic inelasticity of seismic design.

A third area where LRFD offers tremendous advantage is in the design of frames with PR steel connections. Steel Designers will be able to discard the restraining assumptions of perfectly fixed connection behavior. Modeling connections using their authentic strength and stiffness may result in a more economic structural frame due to easy connection details.

AISC’s LRFD method is a practical, world-class and trusted design specification. What’s more, because a metric version is now available, it is assumed that most international jurisdictions will recognize it, either as an alternative to their own code or in the lack of an established code. Even if this is not the case, the LRFD method would form an economical and convenient base for the US engineers to get familiar with the internationally established limit states design philosophy. While ASD may not be beyond its usefulness today, there can be no doubt that LRFD will replace it gradually as innovative ideas become normal practice for steel structures.