Building Design and Construction Handbook - Merritt - Ventech!

3.4 SECTION THREE
it is known for a specific locality that extreme winds may come only from one
direction. As a consequence of this assumption, each wall of a rectangular building
should be considered in design to be subject to the maximum wind load.
Winds generally strike a building in gusts. Consequently, the building is subjected
to dynamic loading. Nevertheless, except for unusually tall or narrow buildings,
it is common practice to treat wind as a static loading, even though wind
pressures are not constant. High velocity winds can cause considerable vibrations,
particularly in lighter more flexible structures. Therefore, connections that tend to
loosen under heavy vibration should be avoided.
Estimation of design wind pressures is complicated by several factors. One factor
is the effect of natural and man-made obstructions along the ground. Another factor
is the variation of wind velocity with height above ground. Still another factor
complicating wind-pressure calculation is the effect of building or building component
shape or geometry (relationship of height or width to length) on pressures.
For important buildings, it is advisable to base design wind pressures on the results
of wind tunnel tests of a model of a building, neighboring buildings, and nearby
terrain.
3.2.2 Wind Pressures and Suctions
Pressures are considered positive when they tend to push a building component
toward the building interior. They are treated as negative for suctions or uplifts,
which tend to pull components outward.
Figure 3.1a illustrates wind flow over the sloping roof of a low building. For
roofs with inclines up to about 30�, the wind may create an uplift over the entire
roof (Fig. 3.1b). Also, as shown in Fig. 3.1b and c, the pressure on the external
face of the windward wall is positive and on the leeward wall, negative (suction).
If there are openings in the walls, the wind will impose internal pressures on the
walls, floors, and roof. The net pressure on any building component, therefore, is
the vector sum of the pressures acting on opposite faces of the component.
Because of the wind characteristics described in Art. 3.2.1 and the dependence
of wind pressures on building geometry, considerable uncertainty exists as to the
magnitude, direction, and duration of the maximum wind loads that may be imposed
on any portion of a specific building. Consequently, numerous assumptions, based
to some extent on statistical evidence, generally are made to determine design wind
loads for buildings. Minimum requirements for wind loads are presented in local
and model building codes.
Codes usually permit design wind loads to be determined either by mathematical
calculations in accordance with an analytical procedure specified in the code or by
wind-tunnel tests. Such tests are advisable for structures with unusual shapes, unusual
response to lateral loading, or location where channeling effects or buffeting
in the wake of upwind obstructions are likely to occur. Tests also are desirable
where wind records are not available or when more accurate information is needed.
Codes often require that the following conditions be met in execution of windtunnel
tests:
1. Air motion should be modeled to account for variation of wind speed with
elevation and the intensity of the longitudinal component of turbulence.
2. The geometric scale of the model should not be greater than 3 times that of the
longitudinal component of turbulence.