Design Loads for Residential Buildings .

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.

What is Structural Systems

Over many years, engineers have observed that some structural systems perform
better in earthquakes than others. Based on these observations, the Provisions
design criteria for building structures are based on the structural system used.
Structural systems are categorized based on the material of construction (e.g.,
concrete, masonry, steel, or wood), by the way in which lateral forces induced by
earthquake shaking are resisted by the structure (e.g., by walls or frames), and by
the relative quality of seismic-resistant design and detailing provided.
The Provisions recognizes six broad categories of structural system:

• Bearing wall systems,
• Building frame systems,
• Moment-resisting frame systems,
• Dual systems,
• Cantilever column systems, and
• Systems not specifically designed for seismic resistance.

In bearing wall systems, structural walls located throughout the structure provide
the primary vertical support for the building’s weight and that of its contents as
well as the building’s lateral resistance. Bearing wall buildings are commonly
used for residential construction, warehouses, and low-rise commercial buildings
of concrete, masonry, and wood construction. Figures 21, 22, and 23 show typical
bearing wall buildings.

Building frames are a common structural system for buildings constructed of
structural steel and concrete. In building frame structures, the building’s weight
is typically carried by vertical elements called columns and horizontal elements
called beams. Lateral resistance is provided either by diagonal steel members
(termed braces) that extend between the beams and columns to provide horizontal
rigidity or by concrete, masonry, or timber shear walls that provide lateral
resistance but do not carry the structure’s weight. In some building frame
structures, the diagonal braces or walls form an inherent and evident part of the
building design as is the case for the high-rise building in San Francisco shown in
Figure 24. In most buildings, the braces or walls may be hidden behind exterior
cladding or interior partitions.
Moment-resisting frame systems are commonly used for both structural steel and
reinforced concrete construction. In this form of construction, the horizontal
beams and vertical columns provide both support for the structure’s weight and
the strength and stiffness needed to resist lateral forces. Stiffness and strength are
achieved through the use of rigid connections between the beams and columns
that prevent these elements from rotating relative to one other. Although somewhat
more expensive to construct than bearing wall and braced frame structural
systems, moment-resisting frame systems are popular because they do not
require braced frames or structural walls, therefore permitting large open spaces
and facades with many unobstructed window openings. Figure 25 shows a steel
moment-resisting frame building under construction.
Dual systems, an economical alternative to moment-resisting frames, are commonly
used for tall buildings. Dual system structures feature a combination of
moment-resisting frames and concrete, masonry, or steel walls or steel braced

frames. The moment-resisting frames provide vertical support for the structure’s
weight and a portion of the structure’s lateral resistance while most of the lateral
resistance is provided either by concrete, masonry, or steel walls or by steel braced
frames. Some dual systems are also called frame-shear wall interactive systems.
Cantilever column systems are sometimes used for single-story structures or in
the top story of multistory structures. In these structures, the columns cantilever
upward from their base where they are restrained from rotation. The columns
provide both vertical support of the building’s weight and lateral resistance to
earthquake forces. Structures using this system have performed poorly in past
earthquakes and severe restrictions are placed on its use in zones of high seismic
activity.

In regions of relatively low seismic risk, the NEHRP Recommended Seismic
Provisions permits the design and construction of structural steel buildings that
do not specifically conform to any of the above system types. These buildings are
referred to as “structures not specifically detailed for seismic resistance.”

In addition to these basic structural systems and the primary materials of construction,
the Provisions also categorizes structural systems based on the quality
and extent of seismic-resistant detailing used in a structure’s design. Systems that
employ extensive measures to provide for superior seismic resistance are termed
“special” systems while systems that do not have such extensive design features
are typically called “ordinary” systems. The Provisions also includes design rules
for structural systems intended to provide seismic resistance that is superior to
that of “ordinary” systems but not as good as that of “special” systems; these systems
are called “intermediate” systems.