LOAD COMBINATIONS

Because various loads may act on a structure simultaneously, load combinations should be evaluated to determine the most severe conditions for design. These load combinations vary from one document to another, depending upon the jurisdiction. There are a set of combinations for the allowable stress design and another set that incorporates load factors for strength design. Paragraphs below provide these load combinations.

The 2012 IBC has three sets of load combinations. Section 1605.2 provides the load combinations for strength design and Section 1605.3 provides two sets of load combinations for allowable stress design. The strength design load combinations and the first set of allowable stress design load combinations, found in Section 1605.3.1, are labeled as the “basic load combinations”. These basic load combinations align closely with the load combinations found in ASCE 7. The second set of allowable stress design load combinations, found in Section 1605.3.2, is labeled as the “alternative basic load combinations”. The alternative load combinations used to be important to masonry designers as using these load combinations allowed increases in allowable stresses for load combinations including wind or seismic loads. Since the 2011 MSJC Code no longer permits allowable stresses to be increased, the alternative combinations are less important.

The selection of the load combinations to be used for design is largely a matter of personal preference. With the recalibration of the allowable stress design provisions in the 2011 MSJC Code, the results obtained using those provisions are more comparable to those obtained using strength design. In addition, the design provisions themselves are becoming more similar over time. For example, the anchor bolt and shear design provisions are now the same between the two design methodologies, just with different factors for allowable stress design and strength design.

Lastly, it should be noted that the earthquake load, E, in the load cases discussed below includes both a vertical and horizontal component. The vertical seismic component is typically accounted for by adjusting the factor on the dead loads. The vertical seismic load is discussed in Section 3.9.4.1 of this chapter.