The Structural Systems oF Tall Buildings - Tall Buildings: Structural Systems and Aerodynamic Form - page 21

(a) (b)

(c)

Figure 3.6 Flat plate/slab systems: (a) without column capitals, (b) with column capitals,
(c) with gussets

thus attaining the maximum net floor height, is a major architectural advantage of this
system.
In resisting lateral loads, flat plate/slab systems may be insufficient, compared with
rigid frames. The reason for this is the shallow-wide-beam behaviour of the floor slab,
with low bending/flexural rigidity. Thus real frame behaviour that has beams having
sufficient depth cannot be achieved. The addition of shear walls to flat plate/slab
systems mitigates this problem and increases the resistance against lateral loads.
Flat plate/slab systems efficiently and economically provide sufficient stiffness
to resist wind and earthquake induced lateral loads in buildings of up to about 25
storeys.

3.4 core systems

Core systems are used in reinforced concrete buildings. This system consists of
a reinforced concrete core shear wall resisting all the vertical and lateral loads
( Figure 3.7 ). In general, a core wall is an open core that is converted into a partially
closed core by using floor beams and/or slabs so as to increase the lateral and torsional
stiffness of the building. Although the behaviour of closed cores is ideal against
building torsion under lateral loads, a partially closed core is used to approximate
this for architectural reasons. Thus, a partially closed core is produced by supporting
the open part of the core with beams and/or slabs having satisfactory strength against
shear and bending.
In core systems, floor slabs are cantilevered from the core shear wall independently
( Figure 3.8a ) , or else cantilevered modules of floor slabs are used ( Figure 3.8b ). In the
case of cantilevered modules, floor slabs, except the bottom slab of each module, are
cantilevered from the core shear wall and are supported by discontinuous perimeter
columns down through the height of the modules. The bottom slab of each module
is a strengthened cantilever floor slab which supports the perimeter columns of the
upper storeys in the module.