# ASD Load Combinations, Differences vs. LRFD

#### EngineeringFabrication & Quality

ASD (Allowable Stress Design) is a design procedure or philosophy, also referred to as service load design or working stress design (WSD). Moreover, it entails ensuring that the service load or working stress on a member remains less than its elastic limit. In this article, you will learn more about the ASD load combinations, and compare the basic differences between LRFD vs ASD.

## More on ASD Load Combinations

ASD load combinations consider a variety of loading conditions or scenarios that could affect the safety of a structure when in operation. Moreover, these loads include:

• Dead Loads (D): Which are the structure’s weight and other permanent loads such as equipment weight and hydrostatic forces.
• Live Loads (L): Generally, these are the loads that the structure is designed to support such as people, vehicles, and other operational loads.
• Roof Live Loads (Lr): Loads due to maintenance activities from equipment, personnel or materials.
• Snow Loads (S): As a result of snowstorms.
• Rain Loads (R): Loads because of rainstorms.
• Wind Loads (W).
• Earthquake Loads (Ev and Eh): These are the vertical and horizontal loads on a structure because of earthquakes. Moreover, this type of loading has a tremendous effect on buildings, especially those with re-entrant corners.

### ASD Load Equations

Generally, the ASD load combinations consider ten different scenarios as the equations below express.

## ASD vs LRFD

Generally, ASD and LRFD (Load and Resistance Factor Design) are the leading design methodologies for dealing with load combinations. This especially holds true in the steel construction industry. However, there is a gradual shift to LRFD. Their differences highlight some of the reasons for using either approach.

To elaborate more on the difference between the ASD and LRFD methods and load combinations, the figure below is the load-displacement (or stress-strain) curve of an arbitrary member. Also, this curve highlights the strength level estimation using the ASD and LRFD methods. For the strength level using the ASD approach (Rn/Ω), which is in red, the safety factor (Ω), reduces the nominal material strength (Rn) to below the yield point. Thus, ensuring a safe design as stress levels beyond this point are not allowed. On the other hand, for the LRFD, the material strength level (ϕRn), in yellow, is above yield. As a result, the resistance factors (ϕ) for all load combinations in LRFD are above 1.0 to keep design load levels below the yield load.