Structural Loads: Ultimate Design vs. Allowable Stress Methods, Part 2
In our last post, we recapped the importance of structural loads and the most common types of structural loads, including dead loads, collateral loads, equipment loads, live loads and environmental loads. We also explained the two most common ways to determine structural loads: ultimate, or strength, design and allowable stress design. In this post, we will cover the ways these methods work and how to determine which method to use when calculating your building’s structural load.
Methods of Calculating the Load for Safety
Ultimate Design assigns each load a load factor (multiplying constant) and combines the resulting modified load values in various ways. The total load is then modified with a probability factor. The end result is compared to the ultimate capacity of the entire structure. This method provides a more consistent reliability figure than the Allowable Stress Design method but may not be as economical at higher failure rate probabilities (higher or more variable loads). This may be seen as a simpler method of determining structural loads but in turn it produces fairly conservative numbers.
Allowable Stress Design (ASD)
Instead of assigning a load factor to each structural load, ASD uses some fraction of the various loads that represents the probabilities that two or more loads will occur and/or fail simultaneously. These fractional loads are then combined in various ways to provide a failure probability or reliability number.
The total stress load for each load combination is compared to an “allowable stress” value that is derived from what is called the yield stress of the steel members (structural frame components such as columns and beams). There are also reductions and factors that come into play during certain circumstances such as earthquake or to modify the result of two or more loads that are added to the dead load.
ASD is a bit more complicated but may be more economical overall since more variables are taken into consideration.
Which to Use
As noted above, each building has its own unique use and environment. Engineering judgment is critical to making the appropriate decisions concerning the materials, composition, and construction of your building for the highest level of safety and reliability. Also, local building codes and national standards must be met.
Ultimate Design (Strength Design) provides a uniform reliability number for the building as a whole and in uses and environments where the safety factors are low. Ultimate Design will result in a more economical design for a building with fewer special needs for customized areas of reinforcement, such as that needed for heavy roofing or equipment or to withstand high winds or snow loads.
Allowable Strength Design better determines design for higher safety factor needs where a building is more prone to environmental pressures or must bear heavier loads that stress a specific portion of a building. The economy is found through building with materials that will keep the failure rate, and thus the repair and maintenance rate, to a minimum.
In all cases, it is important to communicate the need for any non-standard types of structural loads to the building manufacturer as early as possible, preferably during the bidding or negotiating stage, due to the changes they entail in materials, scheduling, and construction. Early consultation and clear communication with the building manufacturer will keep the project on track with fewer surprises.
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