LFD structural design and LRFD design are similar but has some important differences. Load Factor Design (LFD), also known as Ultimate Strength Design, requires a more detailed analysis than the popular design method used for many years by structural engineers, working stress analysis. The primary reason that many engineers move to LFD was the ability to design with more consistency in load carrying capacities. Load and Resistance Factor Design (LRFD) uses the same format of load factors and resistance factors as LFD, although these factors are based on a statistical determination instead of experience and judgment. The main concept of development of LRFD is to increase the standardization of factor of safety for different types of structures. On most federally funded projects in the United States, the design method must be LRFD. For many years, Allowable Working Stress (ASD), design was required. LRFD has been used in Canada since 1974. There, it is known as limit states design. LFRD is also the basis for the building code in many European countries.
Prior to Load Factors in design there was allowable stress design (ASD)
As early as the 1860’s, designers used a method of design based on allowable stress. When loads are applied to steel, it was observed to behave linearly up to a yield point. Designers reduced the amount of load on the structure creating a “factor of safety”.
Problems with allowable stress (ASD) method:
- ASD method assumes there is no residual stress from fabricating in the structural members
- ASD method assumes material strengths are always consistent.
- Bending and shear loading usually interact. This fact creates challenges with correctly applying allowable stress design.
- Reinforced concrete and timber are both non-linear materials. Strength varies with several factors including age, curing method, construction procedure and other factors. Designing structures with these materials benefit from strength factors.
The main differences between LFD and LRFD are:
The load and resistance factors used in LFD are based on experience and judgment. Load and resistance factors in LRFD are based on probability-based statistics. These factors take in account uncertainties in material properties, design theory, fabrication, and construction practices.
LFRD includes more refined lateral live load distribution factors.
LRFD incorporates four new limit states:
- Service Limit State - Normal service review to check stress deformation and crack width limits.
- Strength Limit State - Is the strength and stability of the structure adequate to resist expected normal loading?
- Fatigue and Fracture Limit State - Stress range limits for expected number of single truck loads to prevent fracture and limit crack growth.
- Extreme Event Limit State - Earthquake, flood, struck by a vehicle or ship, vehicle flow and ice flow checks.
A different live load is used in LRFD.