Building Design and Construction Handbook - Merritt - Ventech!

PROTECTION AGAINST HAZARDS 3.7
Consideration should also be given to the potential for wind blown debris impacting
a structure and damaging critical lateral force resisting elements.
3.2.4 Limitation of Drift
There are no generally accepted criteria for maximum permissible lateral deflections
of buildings. Some building codes limit drift of any story of a building to a maximum
of 0.25% of the story height for wind and 0.50% of the story height for
earthquake loads. Drift of buildings of unreinforced masonry may be restricted to
half of the preceding values. The severer limitation of drift caused by wind loads
is applied principally because they are likely to occur more frequently than earthquakes
and will produce motions that will last much longer.
Three basic methods are commonly used, separately or in combination with each
other, to prevent collapse of buildings under lateral loads, limit drift and transmit
the loads to the foundations. These methods are illustrated in Fig. 3.3. One method
is to incorporate shear walls in a building. A shear wall is a vertical cantilever with
high resistance to horizontal loads parallel to its length (Fig. 3.3a). A pair of perpendicular
walls can resist wind from any direction, because any wind load can be
resolved into components in the planes of the walls (Fig. 3.3b). Diaphragms developed
from wall, floor, and roof sheating can function similar to solid shear walls
when properly attached and laterally supported.
A second method of providing resistance to lateral loads is to incorporate diagonal
structural members to carry lateral forces to the ground (Fig. 3.3c). (The
diagonals in Fig. 3.3c are called X bracing. Other types of bracing are illustrated
in Fig. 3.6.) Under lateral loads, the braced bays of a building act like cantilever
vertical trusses. The arrows in Fig. 3.3c show the paths taken by wind forces from
points of application to the ground. Note that the lateral loads impose downward
axial forces on the leeward columns, causing compression, and uplift on the windward
columns, causing tension.
A third method of providing resistance to lateral loads is to integrate the beams,
or girders, and columns into rigid frames (Fig. 3.3d). In a rigid frame, connections
between horizontal and vertical components prevent any change of angle between
the members under loads. (Drift can occur only if beams and columns bend.) Such
joints are often referred to as rigid, moment, or wind connections. They prevent
the frame from collapsing in the manner shown in Fig. 3.2d until the loads are so
FIGURE 3.3 Some ways of restricting drift of a building: (a) shear wall; (b) pair of perpendicular
shear walls; (c) diagonal bracing; (d) rigid frames.