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.
|D||This is the base load case with only the dead load.|
|D + L||In addition to the dead load, there is an application of a live load.|
|D + (Lr or S or R)||The dead load combines with either rain, snow, or roof live loads. For example, during a storm.|
|D + 0.75L + 0.75(Lr or S or R)||Application of all loads from previous scenarios.|
|0.6W + D||Introduction of wind loads only.|
|D + 0.75L + 0.75(0.6W) + 0.75(Lr or S or R)||All ASD load combinations. Excluding seismic loads.|
|0.6D + 0.6W||Dead and wind loading.|
|D + 0.7Ev + 0.7Eh||Basic loading but includes vertical and horizontal seismic loading. For instance, an empty building during an earthquake.|
|D + 0.525Ev + 0.525Eh + 0.75L + 0.75S||An onerous loading scenario with a combination of seismic, live, and snow loads. For example, an earthquake occurring during a blizzard.|
|0.6D – 0.7Ev + 0.7Eh||Because seismic loads propagate via waves, the horizontal and vertical loads will act in both directions. So, this scenario is when the vertical load produces a lift force, which opposes gravity. Thus, the negative sign.|
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.
|Uses a constant safety factor, irrespective of the type of load.||Safety factors vary according to the variability of load type. So, load types with higher variance have higher factors and vice versa.|
|Generally, factors are applied to only loads to increase their capacity.||While factors are applied to both loads and resistance, to scale up loads and reduce resistance.|
|Results in a design with lower reliability because of lower values for the safety factor.||Produces higher reliability due to higher safety factor.|
|According to ASCE 7-05, this approach has ten load combination equations.||On the other hand, LRFD has just seven load combination equations.|
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.