Load Combinations and Design Methodology for the Design of Structural Beams
There are two major design methodologies for the structural design of beams. The major difference between Allowable Stress Design and Ultimate Strength Design is which side of the equation the safety factor is placed.
Design Methodologies
Allowable Stress Design (ASD)
Allowable Stress Design places one safety factor on the material stress side of the equation by using a factor less then 1.0. Multiplying the code value of the bending stress or shear stress by a number less than 1.0 and using that in the structural design equation lowers the stress value that is allowed to be used in the structural design of the beam, providing a factor of saftey. This works well for most situations as it is much easier to calculate because the loads use for the design are the working level actual loads the beam will experience.
Ultimate Strength Design (USD) / Load and Resistance Factor Design (LRFD)
Because some loads are more predictable than others, to be more precise in the structural design of a beam the Ultimate Strength Design methodology places the safety factor on the load side of the equation by multiplying each of the different load types by a different value that is greater than 1.0. For example, dead loads are much more predictable than snow loads are so the safety factor for dead loads is 1.2 and the safety factor for snow loads is 1.6 for most Ultimate Strength Design load combinations.
This can provide a more precise beam design because the safety factor that is being applied to each part of the load is a more accurate safety factor for each load type, instead of one safety factor that ends up on all loads.