What are the advantages of LRFD over ASD design in Steel Structures?

The LRFD is certainly an improvement over the ASD. It takes better account of variability in both loading and resistance. LRFD is a reliability based design. Even before taking my reliability based design class, I always had my doubt in applying a single factor of safety for an output derived from inputs of varying reliability. The one disadvantage of LRFD is the lack of sufficient statistical data to select appropriate factors in certain specific cases, but still better than ASD.

LRFD design method is latest and results in more efficient design and thus sections of the members

The LRFD is certainly an improvement over the ASD as it takes better account of variability in both loading and resistance.

The advantages of LRFD the calculate more specific because that not only  gravity for observation, and calculate gravity and earthquake for observation calculate

The first and the most important difference between ASD and LRFD, has been that the old Allowable Stress Design compared actual and allowable stresses while LRFD compares required strength to actual strengths. The difference between looking at strengths vs. stresses does not present much of a problem since the difference is normally just multiplying or dividing both sides of the limit state inequalities by a section property, depending on which way you are going. In fact, the new AISC Allowable Strength Design (ASD), which replaces the old allowable stress design, has now switched the old stress based terminology to a strength based terminology, virtually eliminating this difference.

For me it's very simple : more lighter steel structures with LRFD. The criteria about of the "safety factors" must be discussed very careful between designer and client, accurate to the particular situation of each project and follow the code always. ASD is like design follow the "old school" but very valid in some enviroments, and in some countries, is the only way to get "the only constructibility"  to the project.

LRFD is more economical than the old ASD method. Actually, after the new modification on the AISC code both methods will yield a section with very close weight or cross sectional area.

ASD yields a design that is "somehow" safer, but actually ineconomical.

LRFD method considers the limits on the basis of limiting values such as deflection etc.while ASD method is considered on the basis of its factor of safety

he 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. In1986, 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. Inrecent years, LRFD method has been successfully employedto 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 of1.7 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.

LRFD should give a more economical design than ASD because it utilizes the material strength to the fullest possible.

1. When dead load is larger as compared to live load then LRFD becomes economical.
2. The behavior at the collapse including ductility, warning before failure are considered in LRFD. This is not directly possible in ASD method because here the structure is considered at service stage.
3. Plastic Design concept may be employed in LRFD method but cannot be applied in ASD method.
4. Every type of load may be given different factor of safety depending upon its probability of overload, number of occurences and changes in point of application, But in ASD same factor of safety is used for different loads.
5. Safer structures may result under LRFD method because of considering behavior atcollapse. But behavior at collapse is not considered in ASD method.