Building Design and Construction Handbook - Merritt - Ventech!

STRUCTURAL STEEL CONSTRUCTION 7.31
an arrangement of triangles possessing in their planes an inherent ideal rigidity both
individually and collectively.
3. There frequently is a need for bracing to resist erection loads and to align or
prevent overturning, in a direction normal to their planes, of trusses, bents, or frames
during erection. Such bracing may be temporary; however, usually bracing needed
for erection is also useful in supplying rigidity to the structure and therefore is
permanently incorporated into the building. For example, braces that tie together
adjoining trusses and prevent their overturning during erection are useful to prevent
sway—even though the swaying forces may not be calculable.
7.11 FRAME BRACING
Design of bracing to resist forces induced by wind, seismic disturbances, and moving
loads, such as those caused by cranes, is not unlike, in principle, design of
members that support vertical dead and live loads. These lateral forces are readily
calculable. They are collected at points of application and then distributed through
the structural system and delivered to the ground. Wind loads, for example, are
collected at each floor level and distributed to the columns that are selected to
participate in the system. Such loads are cumulative; that is, columns resisting wind
shears must support at any floor level all the wind loads on the floors above the
one in consideration.
7.11.1 Bracing Tall Buildings
FIGURE 7.16
buildings.
Wind bracing for multistory
If the steel frame of the multistory
building in Fig. 7.16a is subjected to lateral
wind load, it will distort as shown
in Fig. 7.16b, if the connections of columns
and beams are of the standard
type, for which rigidity (resistance to rotation)
is nil. One can visualize this
readily by assuming each joint is connected
with a single pin. Naturally, the
simplest method to prevent this distortion
is to insert diagonal members—
triangles being inherently rigid, even if
all the members forming the triangles
are pin-connected.
Braced Bents. Bracing of the type in Fig. 7.16c, called X bracing, is both efficient
and economical. Unfortunately, X bracing is usually impracticable because of interference
with doors, windows, and clearance between floor and ceiling. Usually,
for office buildings large column-free areas are required. This offers flexibility of
space use, with movable partitions. But about the only place for X bracing in this
type of building is in the elevator shaft, fire tower, or wherever a windowless wall
is required. As a result, additional bracing must be supplied by other methods. On
the other hand, X bracing is used extensively for bracing industrial buildings of the
shed or mill type.