practice is nevertheless considerable and they
are categorised here into the four basic types
of one-way-spanning flat-slab structures, two-
way-spanning flat-slab structures, beam-and-
slab structures and loadbearing-wall structures
(Fig. 4.45).
All four basic types are beam-and-post
arrangements and can be considered to consist
of a floor deck system supported on a vertical
structure of either columns or walls. Their
properties, span capabilities and sizing
requirements are summarised in Table 4.1.
In all cases, the design of the floor deck is
determined principally by the requirements of
gravitational load and this in turn dictates the
pattern of the column or wall grid which is
provided to support it. One-way-spanning floor
systems are best carried on rectangular grids
while two-way systems perform better on
column or wall grids which are square. In
either case the grid is normally kept as regular
as possible for reasons of economy but it need
not be perfectly regular.
The principal effect of lateral loads is upon
the design of bracing systems. In many cases
this will have no influence on the overall form
of the structure because the beam-column
arrangement will be self-bracing due to the
high level of structural continuity which is
possible with reinforced concrete. Some
reinforced concrete structures require bracing
walls for stability, however, and where these
are necessary the internal planning of the
building is affected.
Only the most basic, regular forms of each
type of structure are described here to give an
indication of the general arrangements and
span ranges for which they are suitable. Often
these basic forms are manipulated and
distorted to produce more complex structural
geometries (see Section 2.5).
4.4.2.2 One-way-spanning flat-slab structures
In this system the floor slab spans one way
between rows of columns (Fig. 4.46). The
arrangement is also known as ribbed-slab
because the slab is frequently given a ribbed
cross-section in order to improve its efficiency
by removing concrete from the tensile side of
Fig. 4.46 One-way-spanning flat-slab system. The ribbed
version depicted is used at the long span end of the span
range for improved efficiency. For short spans, one-way-
spanning flat-slabs have a simple rectangular cross-
section.the cross-section (Fig. 4.46). The ribs follow the
span direction of the slab. The voids between
the ribs are stopped short of the strip of slab
between the columns and this solid area acts
as a beam spanning between the columns. As
this is a one-way-spanning system the column
grid is rectangular with the slab spanning
parallel to the long side of the rectangle. The
normal span range is4mX6mto8mX llm
but the maximum span can be increased to
8 m X 14 m if pre-stressing is used. The
span/depth ratio is normally around 25 but can
be as high as 36 in pre-stressed versions.
Ribbed-slab structures are braced either by
rigid frame action or by walls acting as
diaphragm bracing in the vertical plane.
The particular advantages of the ribbed-slab
system are the relative simplicity of the
construction and high structural efficiency,
which allows relatively long spans to be
achieved with a small volume of concrete and
a small depth of structure.4.4.2.3 Two-way-spanning flat-slab structures
In two-way-spanning slab structures the floor
deck system consists of a two-way-spanning
flat plate of reinforced concrete which is
supported directly on a grid of columns (Fig.
4.47). The column grid is normally square to
maximise the two-way-spanning action. The 135Reinforced concrete structures