Page 226 - Forensic Structural Engineering Handbook

7.10 CAUSES OF FAILURES

WIND LOADS

Nature and Consequences of Wind Loads
In general, wind pressures are derived from the effects of a moving stream of air passing
an object. Wind pressures generally originate either from the inertial effects caused by the
impediment that an object in the stream creates to the flow of air or as Bernoulli pressures
caused by the flow of a fluid over a surface. Depending on the orientation of the loaded sur-
face to the direction of the wind, the pressures are characterized as either drag forces that
cause loads parallel to the direction of the wind or lift forces that cause loads transverse to
the direction of the wind. In all cases, wind loads act essentially perpendicular to the sur-
face on which pressures are exerted.
Lift forces can be either vertical or horizontal, depending on whether the surface
exposed to wind pressures is horizontal or vertical. In general, lift forces on the exterior
surface are suction, acting outward, due to reductions in pressure that are associated with
the moving mass of air. As such, the general field of walls and other surfaces that face
the oncoming wind is subjected to inward, positive pressures due to inertial forces,
whereas walls that are oriented parallel to the direction of wind, low-sloped roofs, and
the leeward walls of buildings experience outward, suction pressures. With increasing
roof slope, and in walls that are oblique to the direction of the wind, positive, inward
pressures occur as well.
The total load due to wind on an object is the net of the forces due to pressures on all
surfaces. Therefore, a component of a building’s exterior cladding will respond to pressure
differentials between its inside surface and its outside surface, whereas a building in its
entirety will respond to pressure differentials that exist between windward and leeward
faces, and between opposite wall surfaces that are oriented parallel to the direction of wind.
The most common form of wind damage in buildings is failure of components of the
building envelope and collateral damage that results from intrusion of rain that often
accompanies wind storms. 5–7 Envelope damage often takes the form of glazing or roofing
failures (Fig. 7.6) or failure or damage to cladding systems. Roofing failures are common
when ballast or mechanical fasteners that restrain roofing materials are insufficient to resist
suction loads caused by wind moving over horizontal or inclined roofs. It is most common
for building envelope components to fail first near building and roof edges (Fig. 7.7).
Turbulence and vortices caused by building edges increase pressures locally. In addition,
the wind speed at edges usually exceeds that of the approaching wind because air acceler-
ates as it takes a longer path to move around the building.
Except in extreme cases, glass usually has sufficient strength to resist the direct pres-
sures that are caused by wind. Glazing failures are most common near grade level where
airborne debris lifted by severe winds can become missiles (Fig. 7.8) that impact brittle and
relatively weak glass.
Another form of collateral damage due to failure of glazing or other components of the
building envelope occur as wind pressure differentials change. During a wind storm, the
interior of an enclosed building will be pressurized at a level that is dependent on the rela-
tive air permeability of the various surfaces exposed to wind pressures. For instance, if
there are relatively few openings and other incidental paths for air exchange through the
windward face compared to the other surfaces of the building, then the interior of a build-
ing will be at negative pressure relative to the ambient pressure. This occurs because suc-
tion caused by air moving past openings—large and small—on sidewalls and leeward walls
will reduce the pressure inside the building.
Since the net effect on a building component is the difference between pressures on
opposite surfaces, negative pressure inside a building will increase the total demand on
windward surfaces. On these surfaces, interior and exterior pressures are additive. Interior