# Load and Resistance factor design (LRFD) | Ultimate Design | Limit State design

Sunday, July 29, 2012 16:51**Load and Resistance factor design (LRFD), Ultimate Design, or Limit State design**

If the major part of factor of safety is applied on the service loads to increase loads called factored loads. The material strength is divided by the minor remaining part of factor of safety. The design method is called load and resistance factor design (LRFD), Ultimate design, or Limit State design.

**Overload factor**

The factor of safety by which any load is increased for load and resistance factor design is calledoverload factor.

**Resistance factor or Reduction factor (phi)**

The reciprocal of factor of safety or reduction factor by which the material strength is slightly adjusted is calledResistance factor.

**Limit state**

Limit state is defined as the limiting stage in the loading after which the structure cannot fulfill its function due to strength considerations.

Analysis of structures for loads is performed considering the structure to be within elastic range. However, inelastic behavior, ultimate failure modes and redistribution of forces after elastic range are considered in this method. This is more realistic design as compared with the old allowable stress design.

**Nominal strength (Rn)**

Nominal strength is defined as the strength of a structure or its component determined by using formulas given in specifications.

**Required strength (Ru)**

Any particular load effect increased by the load factors is called required strength (Ru).

**Design strength**

The nominal strength is reduced by the resistance factor is called design strength.

Hence, the deign equation in case of LRFD reduces to the following equation:

**Ru less than or equal to phi Rn**

Reference : Steel structures by Zahid Ahmad siddiqi and Muhammad ashraf