The Structural Systems oF Tall Buildings - Tall Buildings: Structural Systems and Aerodynamic Form - page 19

severe earthquakes (ductility is the ability to deform without a significant reduction in
strength). In rigid frame systems ductility is achieved by the formation of plastic hinges
in the columns and beams. In this way, when rigid frame systems under earthquake
loads are deformed beyond their elastic limits, a large part of the energy is dissipated
by the plastic hinges. Steel is a ductile material, while concrete is a brittle material,
however, the ductility of reinforced concrete depends on the design. Reinforced
concrete beams are designed to be under-reinforced to make them ductile, but this
is not required for columns. Thus, in the structural design of reinforced concrete rigid
frames, it is necessary to design the columns to be stronger than the beams so that
plastic hinges can be formed in the beams. In this way, a reinforced concrete rigid
frame is forced into ductile behaviour.
In the case of tall buildings, when designed for strength considerations only, the
biggest disadvantage in rigid frame systems is the magnitude of lateral drift, which
causes discomfort to occupants and damage to non-structural elements. There are
two causes of lateral drift: the first is the deformation due to cantilever bending of the
building (bending deformation), which is approximately 20 per cent of the total lateral
drift ( Figure 3.4a ). The second is that of the deformation due to bending of the beams
and columns (shear deformation), approximately 65 per cent is due to the bending of
the beams, and 15 per cent to the columns, totalling approximately 80 per cent of the
total lateral drift ( Figure 3.4b ) (Schueller, 1977).
Rigid frame systems efficiently and economically provide sufficient stiffness to
resist wind and earthquake induced lateral loads in buildings of up to about 25
storeys. Some examples of tall buildings using the rigid frame system with steel struc-
tural material include:

• the 12-storey, 55 m high Home Insurance Building (Chicago,1885) ( Figure 1.2 )
and
• the 21-storey, 94 m high Lever House (New York, 1952) ( Figure 3.5 )

(a) Deformation due to
cantilever bending

(b) Deformation due to the bending
of the columns and beams

Figure 3.4

Lateral drift in rigid frame systems