What differentiates the (LRFD) Load and Resistance Factor Design from the ASD Allowable Stress Design?

ASD vs LRFD

What differentiates the (LRFD) Load and Resistance Factor Design from the ASD Allowable Stress Design?

Allowable Stress Design (ASD)

The Allowable Stress Design (ASD) method, often referred to as Working Stress Design (WSD), is founded on the idea that stresses created in structural parts shouldn't go beyond a specific percentage of the elastic limit. This is an outdated approach of design that solely takes into account the material's elastic strength, which restricts the permissible stresses to a small portion of this limit (e.g., 40-50 percent). No factor is used to raise the service loads; all loads are treated as service loads. This method's primary flaw is that it ignores the Plastic and Strain Hardening stages of the material, which causes it to become excessively conservative in some circumstances while producing unsafe results in others because it only takes into consideration loads at service load values. Additionally, the ASD technique does not take serviceability restrictions into account, which could lead to structures that, while safe, fail to serve their original function.

Load and Resistance Factor Design (LRFD)

The Load and Resistance Factor Design (LRFD) method is based on the idea that the applied loads are scaled up while the strength (resistance) of different materials is scaled down by particular factors. As a result, structural parts are designed utilizing lower strength and increased loads. The final strength of the materials taken into consideration for design dictates the use of the elastic, plastic, and strain hardening stages of the material, yielding consistently affordable and secure designs. The factors that affect strength rely on how confident you are in the material's ability to anticipate its strength. Because steel's strength can be predicted and guaranteed with greater accuracy than concretes, for instance, the strength reduction factor for steel is lower than that for concrete. Similar to this, load factors are higher for extremely unexpected loads than for loads that can be forecast more precisely. Due to the fact that dead loads do not vary as much as live or wind loads, their load factors are lower than those of live or wind loads. In addition to strength design, the LRFD approach also takes into account serviceability restrictions such as maximum permitted deflection, cracking, etc.

Because LRFD offers a more logical approach than ASD does, it has essentially supplanted ASD in the design of RCC and steel structures.

Combinations of LRFD and ASD loads according to ASCE/SEI 7-10

ASD vs LRFD