STRUCTURAL DESIGN FOR ARCHITECTURE

Reinforced concrete structures

be adopted for columns is the need to avoid
high slenderness, which would make the
column susceptible to a buckling type of
failure. The slenderness ratio of a concrete
column is its effective length^12 divided by its
least width and the British Standard (BS 8110,
'Structural Use of Concrete') recommends a
maximum value of 60; 30 should be regarded
as a practical maximum for most forms of
construction, however, and to achieve reason-
able load-carrying capacities, slenderness
ratios in the range of 20-25 are normally
adopted. Reinforced concrete walls perform in
a similar way to columns and are made to
conform to the same slenderness ratio require-
ments. In the case of walls the slenderness
ratio is the effective height of the wall divided
by its thickness.

Slabs
Reinforced concrete slabs can be either one-
way- or two-way-spanning structures. The
behaviour of a one-way-spanning slab is
similar to that of a rectangular beam, and
these elements can in fact be regarded simply
as very wide beams. The primary reinforcement
consists of longitudinal bars placed parallel to
the direction of span on the tension side of the
cross-section (that is the lower part in the mid-
span position of slabs and the upper part near
the supports where slabs are continuous over a
number of supports).
Secondary reinforcement, in the form of
straight bars which run at right angles to the
direction of the span, is provided to control
shrinkage and to distribute the effects of
concentrated load (Fig. 4.39). The reinforce-
ment for a solid slab normally takes the form
of a mesh in which the bars of the primary and
secondary reinforcement are welded together
at the points where they cross. Shear stresses
in slabs are very low, except close to columns,
and primary reinforcement is not normally
provided to resist shear.

Fig. 4.39 Reinforcement in a slab. Slabs are provided

with mesh reinforcement close to the top and bottom

surfaces. If the bars are of equal thickness and spacing in

both directions the slab will be capable of spanning simul-

taneously in both directions (two-way-spanning slab). If

the degree of reinforcement in one of the directions is

greater than in the other (larger bars at closer spacing)

then the slab will span more effectively in that direction

(one-way-spanning slab). In the latter case reinforcement

is nevertheless provided in the non-span direction to

distribute concentrated load and to control shrinkage of

the concrete.

The depth of one-way-spanning slabs is

normally approximately one thirtieth of the

span for a simply supported slab and one

thirty-fifth of the span for a slab which is

continuous over a number of supports. The

economic span for a solid slab is in the range

4 m to 8 m but this can be extended by using a

ribbed form in which a proportion of the

concrete in the lower half of the cross-section

is removed. This type of slab is economic in

the range 6 m to 12 m. If pre-stressing is

applied the maximum spans are increased to

13 m for solid slabs and 18 m for ribbed slabs.

A two-way-spanning slab spans simultane-

ously in two directions and must be supported

on beams, walls or rows of columns around its

perimeter or at its corners. It should ideally

have a square plan and the primary reinforce-

ment is a square mesh of longitudinal bars; no

secondary reinforcement is required. Two-way

slabs are statically indeterminate structures

and allow a more efficient use of material than

one-way slabs. Similar span-to-depth ratios are 131

12 Effective length is based on the distance between

points at which the column is supported laterally by

other parts of the structure. Lateral support normally

occurs at each storey level.

Secondary reinforcement

to distribute the effect of

concentrated load

Main reinforcement in base of slab

resisting positive bending moment

at mid-span

main reinforcement in top

of slab over supports