# Section 1605.3 Load Combinations Using Allowable Stress Design

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The purpose of this “Code Simple” is to shed some light on the load combinations used to check overturning and sliding for allowable stress design (ASD). First, a review of the two sets of load combinations used for ASD will be provided. The “basic” load combinations shown below set forth in International Code Council’s 2006 International Building Code (IBC) Section 1605.3.1 are taken directly from the American Society for Civil Engineers’ Minimum Design Loads for Buildings and Other Structures (ASCE 7-05), Section 2.4:

Equation 16-14: 0.6D + W + H
Equation 16-15: 0.6D + 0.7E + H

The code-prescribed earthquake load effect, E, is multiplied by 0.7 to align allowable stress design with earthquake effects set forth in the code, which are based on strength design.

The “alternative basic” load combinations set forth in 2006 IBC Section 1605.3.2 are taken from the 1997 Uniform Building Code. The following equations address the situations where the effects of lateral or uplift forces counteract the effects of gravity loads:

Equation 16-17: D + L + (ωW)
Equation 16-18: D + L + (ωW) + S/2
Equation 16-21: 0.9D + E/1.4

These two sets of ASD load combinations are based on different philosophies and are not specifically intended to be equivalent to each other. The “basic” set of ASD load combinations adopted from ASCE 7 is based on the premise that the design strength resulting from the allowable stress method should, in general, not be less than that resulting from the basic strength design method. The alternative basic set of ASD load combinations is based on the premise that the designs should be about the same as those resulting from the Uniform Building Code.

Q: Why does Equation 16-14 have a load factor of 0.6 on the dead load, D, but Equations 16-17 and 16-18 do not? Also, aren’t Equations 16-17 and 16-18 much less conservative than Equation 16-14 because they do not have a load factor on D and include live load, L, and snow load, S?