Page 184 - Structural Steel Designers Handbook AISC, AASHTO, AISI, ASTM, and ASCE-07 Design Standards

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BUILDING CODES, LOADS, AND FIRE PROTECTION*

4.12 CHAPTER FOUR

In their provisions for roof design, codes and standards also allow for the effect of roof slopes,
snow drifts, unbalanced snow loads, rain-on-snow surcharge, and ponding instability. The structural
members should be investigated for the maximum possible load effects that might be induced.

4.6 WIND LOADS

Wind loads are randomly applied dynamic loads. The intensity of the wind pressure on the surface
of a structure depends on wind velocity, air density, orientation of the structure, area of contact sur-
face, and shape of the structure. Because of the complexity involved in defining both the dynamic
wind load and the behavior of an indeterminate steel structure when subjected to wind loads, the
design criteria adopted by building codes and standards have been based on the application of an
2
equivalent static wind pressure. This simplified equivalent static design wind pressure p s (lb/ft ) is
defined by SEI/ASCE 7-02 as
(4.4)
p s =λI w p S30
where p s = simplified net design wind pressure for the main wind force-resisting system of low-rise,
2
simple, regular, enclosed, diaphragm buildings (lb/ft )
λ= adjustment factor for building height and exposure
I w = importance factor for wind loads
p S30 = net design wind pressure for Exposure B, at height of 30 ft, with I w = 1.0

For low-rise buildings with conditions discussed below that qualify for application of this sim-
plified method, SEI/ASCE 7-02 Method 1, the net wind design pressure, p net , on the components and
cladding can be similarly obtained from Eq. (4.4) by substitution of appropriate variables.
2
SEI/ASCE 7-02 specifies a minimum net wind pressure of 10 lb/ft for the main wind-force
resisting system as well as the components and cladding.
Velocity pressure is computed from
2
q z = 0.00256K z K zt K d V I w (4.5)
where K z = velocity exposure coefficient evaluated at height z
K zt = topographic factor
K d = wind directionality factor
I w = importance factor
V = basic wind speed (mph) corresponding to a 3-s gust speed at 33 ft above the ground in
Exposure C
Velocity pressures due to wind to be used in building design vary with type of terrain, distance
above ground level, importance of building, likelihood of hurricanes, and basic wind speed recorded
near the building site. The wind pressures are assumed to act horizontally on the building area pro-
jected on a vertical plane normal to the wind direction.
Unusual wind conditions often occur over rough terrain and around ocean promontories. Basic
wind speeds applicable to such regions should be selected with the aid of meteorologists and the
application of extreme-value statistical analysis to anemometer readings taken at or near the site of
the proposed building. Generally, however, minimum basic wind velocities are specified in local
building codes and in national model building codes but should be used with discretion, because
actual velocities at a specific site and on a specific building may be significantly larger. In the
absence of code specifications and reliable data, basic wind speed at a height of 10 m above grade
may be estimated from Fig. 4.1.
For design purposes, wind pressures should be determined in accordance with the degree to which
terrain surrounding the proposed building exposes it to the wind. Exposures are defined in Table 4.6.

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