Building Design and Construction Handbook - Merritt - Ventech!

PROTECTION AGAINST HAZARDS 3.9
While shear walls, diagonal bracing, and rigid frames can be used even for very
tall buildings, simple framing arrangements, such as planar systems, are not so
efficient in high structures as more sophisticated framing. For example, shear walls
or rigid frames in planes parallel to the lateral forces (Fig. 3.5a) may sway considerably
at the top if the building is tall (more than 30 stories) and slender. Resistance
to drift may be improved, however, if the shear walls are arranged in the form of
a tube within the building (Fig. 3.5b). (The space within the tube can be utilized
for stairs, elevators, and other services. This space is often referred to as the service
core.) The cantilevered tube is much more efficient in resisting lateral forces than
a series of planar, parallel shear walls containing the same amount of material.
Similarly, rigid frames and diagonal bracing may be arranged in the form of an
internal tube to improve resistance to lateral forces.
The larger the size of the cantilevered tube for a given height, the greater will
be its resistance to drift. For maximum efficiency of a simple tube, it can be arranged
to enclose the entire building (Fig. 3.5c) For the purpose, bracing or a rigid
frame may be erected behind or in the exterior wall, or the exterior wall itself may
be designed to act as a perforated tube. Floors act as horizontal diaphragms to brace
the tube and distribute the lateral forces to it.
For very tall buildings, when greater strength and drift resistance are needed
than can be provided by a simple tube, the tube around the exterior may be augmented
by an internal tube (Fig. 3.5d) or by other arrangements of interior bracing,
such as shear walls attached and perpendicular to the exterior tube. As an alternative,
a very tall building may be composed of several interconnected small tubes,
which act together in resisting lateral forces (Fig. 3.5e). Known as bundled tubes,
this type of framing offers greater flexibility in floor-area reduction at various levels
than a tube-within-tube type, because the tubes in a bundle can differ in height.
Diagonal bracing is more efficient in resisting drift than the other methods,
because the structural members carry the loads to the foundations as axial forces,
as shown in Fig. 3.3c, rather than as a combination of bending, shear, and axial
FIGURE 3.5 Bracing of tall buildings: (a) diagonal bracing, rigid frames, or shear walls
placed in planes (bents) parallel to the lateral forces; (b) interior tube enclosing service core;
(c) perforated tube enclosing the building; (d) tube within a tube; (e) bundled tubes.