STRUCTURAL DESIGN FOR ARCHITECTURE

Structural Design for Architecture

are required to produce a unit volume of fully

compacted fresh concrete and the principal

factors which affect the mix proportions are the

required strength and the required workability.

The required strength is determined during the

structural design calculations; the required

workability depends on the nature of the struc-

ture, specifically on the sizes of the elements,

the complexity of the pattern of reinforcement

and the type of equipment which will be avail-

able to assist with the compaction. Mix design

involves following a set of procedures in which

the various properties which are required of

the fresh and hardened concrete are used to

derive systematically the specification for the

concrete: a suitable water-cement ratio,

cement-aggregate ratio and aggregate grading.

The object is to obtain optimum mix propor-

tions for the requirements of a particular struc-

ture. The process is a fairly complicated one

and will not be described in detail here.

4.3.3.4 Reinforcement

The reinforcement which is used in concrete is

normally in the form of steel bars, either of

plain circular cross-section or with various

surface treatments which increase the bond

with the concrete (Fig. 4.30). The preferred

diameters are 6, 8, 10, 12, 16, 20, 25, 32 and

40 mm, and the normal maximum length is

12 m. Reinforcement for slabs is produced in

the form of square mesh, in individual pieces

or in rolls, in which the bars are welded

together at their crossing points. Meshes with

most combinations of bar sizes and spacings

can be readily obtained.

Reinforcement is produced in both mild

steel and high-yield steel. The latter allows

much higher tensile stress to be specified for

the reinforcement; its use is limited, however,

because the critical factor which determines

the amount of stress which can be permitted in

reinforcement is frequently the need to control

the amount of strain which occurs so as to

prevent cracking of the concrete. The full

tensile strength of high-yield steel cannot

therefore be used in the design of reinforced

concrete structures. The reinforcement which is

used in pre-stressed concrete is hard-drawn

Fig. 4.30 Reinforcing bars are produced with a number of

surface treatments to improve the bond with concrete.

steel wire; the preferred sizes are 4, 5 and

7 mm.

4.3.4 The behaviour of reinforced concrete

Reinforced concrete is a composite material

whose structural action is both subtle and

complex. The basic relationship between

concrete and reinforcement is, however, fairly

straightforward and is explained here in

relation to elements which are subjected to

bending-type loads.

The effect of bending on any structural

element is to place part of its cross-section in

compression and part of it in tension. In the

simplest types of reinforced concrete element,

the compression on one side of the cross-

section is resisted by the concrete and the

126 tension on the other side by the

Ribbed bars

Stretched and twisted ribbed bar

Ribbed and twisted bar

Square twisted bar