The load combinations in Table 3.1 are recommended for use with design specifications based on allowable stress design (ASD) and load and resistance factor design (LRFD). Load combinations provide the basic set of building load conditions that should be considered by the designer. They establish the proportioning of multiple transient loads that may assume point-in-time values when the load of interest attains its extreme design value. Load combinations are intended as a guide to the designer, who should exercise judgment in any particular application. The load combinations in Table 3.1 are appropriate for use with the design loads determined in accordance with this chapter.
The principle used to proportion loads is a recognition that when one load attains its maximum life-time value, the other loads assume arbitrary point-in-
time values associated with the structure’s normal or sustained loading conditions. The advent of LRFD has drawn greater attention to this principle (Ellingwood et al., 1982; Galambos et al., 1982). The proportioning of loads in this chapter for allowable stress design (ASD) is consistent with and normalized to the proportioning of loads used in newer LRFD load combinations. However, this manner of proportioning ASD loads has seen only limited use in current code-recognized documents (AF&PA, 1996) and has yet to be explicitly recognized in design load specifications such as ASCE 7. ASD load combinations found in building codes have typically included some degree of proportioning (i.e., D + W
+ 1/2S) and have usually made allowance for a special reduction for multiple transient loads. Some earlier codes have also permitted allowable material stress increases for load combinations involving wind and earthquake loads. None of these adjustments for ASD load combinations is recommended for use with Table 3.1 since the load proportioning is considered sufficient
It should also be noted that the wind load factor of 1.5 in Table 3.1 used for load and resistant factor design is consistent with traditional wind design practice (ASD and LRFD) and has proven adequate in hurricane-prone environments when buildings are properly designed and constructed. The 1.5 factor is equivalent to the earlier use of a 1.3 wind load factor in that the newer wind load provisions of ASCE 7-98 include separate consideration of wind directionality by adjusting wind loads by an explicit wind directionality factor, KD, of 0.85. Since the wind load factor of 1.3 included this effect, it must be adjusted to 1.5 in compensation for adjusting the design wind load instead (i.e., 1.5/1.3 = 0.85). The 1.5 factor may be considered conservative relative to traditional design practice in nonhurricane-prone wind regions as indicated in the calibration of the LRFD load factors to historic ASD design practice (Ellingwood et al., 1982; Galambos et al., 1982). In addition, newer design wind speeds for hurricane-prone areas account for variation in the extreme (i.e., long return period) wind probability that occurs in hurricane hazard areas. Thus, the return period of the design wind speeds along the hurricane-prone coast varies from roughly a 70- to 100-year return period on the wind map in the 1998 edition of ASCE 7 (i.e., not a traditional 50-year return period wind speed used for the remainder of the United States). The latest wind design provisions of ASCE 7 include many advances in the state of the art, but the ASCE commentary does not clearly describe the condition mentioned above in support of an increased wind load factor of 1.6 (ASCE, 1999). Given that the new standard will likely be referenced in future building codes, the designer may eventually be required to use a higher wind load factor for LRFD than that shown in Table 3.1. The above discussion is intended to help the designer understand the recent departure from past successful design experience and remain cognizant of its potential future impact to building design.
The load combinations in Table 3.1 are simplified and tailored to specific application in residential construction and the design of typical components and systems in a home. These or similar load combinations are often used in practice as short-cuts to those load combinations that govern the design result. This guide makes effective use of the short-cuts and demonstrates them in the examples provided later in the chapter. The short-cuts are intended only for the design of residential light-frame construction.
Note:
1The load combinations and factors are intended to apply to nominal design loads defined as follows: D = estimated mean dead weight of
the construction; H = design lateral pressure for soil condition/type; L = design floor live load; Lr = maximum roof live load anticipated
from construction/maintenance; W = design wind load; S = design roof snow load; and E = design earthquake load. The design or nominal
loads should be determined in accordance with this chapter.
2Attic loads may be included in the floor live load, but a 10 psf attic load is typically used only to size ceiling joists adequately for access
purposes. However, if the attic is intended for storage, the attic live load (or some portion) should also be considered for the design of
other elements in the load path.
3The transverse wind load for stud design is based on a localized component and cladding wind pressure; D + W provides an adequate and
simple design check representative of worst-case combined axial and transverse loading. Axial forces from snow loads and roof live loads
should usually not be considered simultaneously with an extreme wind load because they are mutually exclusive on residential sloped
roofs. Further, in most areas of the United States, design winds are produced by either hurricanes or thunderstorms; therefore, these wind
events and snow are mutually exclusive because they occur at different times of the year.
4For walls supporting heavy cladding loads (such as brick veneer), an analysis of earthquake lateral loads and combined axial loads should
be considered. However, this load combination rarely governs the design of light-frame construction.
5Wu is wind uplift load from negative (i.e., suction) pressures on the roof. Wind uplift loads must be resisted by continuous load path
connections to the foundation or until offset by 0.6D.
6The 0.6 reduction factor on D is intended to apply to the calculation of net overturning stresses and forces. For wind, the analysis of
overturning should also consider roof uplift forces unless a separate load path is designed to transfer those forces.
will provide reserve strength of l 6 percent and more.
