STRUCTURAL DESIGN FOR ARCHITECTURE

Structural Design for Architecture

number of other chemical elements which act

as alloying agents and which have a critical

effect on its properties. The most important of

these is carbon; steel is defined as ferrous

metal with a carbon content in the range of

0.02% to 2%. Low-carbon steels are relatively

soft and ductile while those with a high carbon

content are hard and brittle. The range of

properties is fairly wide, depending on the

precise levels of carbon and of other trace

elements which are present. The term 'steel'

refers therefore not to a single metal but to a

range of alloys.

Structural steels are 'mild steels', which have

a carbon content of around 0.23%; the other

principal alloying agent is manganese which is

maintained at around 1.6%; sulphur and

phosphorus are also present. A range of struc-

tural steels is available in most countries.

Those which are used in the UK are specified

in BS 4360 'Weldable Structural Steels' which

is currently being superseded by a European

Standard EN 10 025. The latter incorporates

the provisions of BS 4360.

BS 4360 specifies four grades of structural

steel: 40, 43, 50 and 55. The grade numbers

refer to the tensile strength values (400, 430,

500 and 550 newtons per square millimetre).^6

Within each grade there are various sub-

grades: A, B, C, etc. determined by minor vari-

ations in chemical composition, principally

carbon content. The higher sub-grades (lower

carbon content) have slightly improved

mechanical properties and perform better in

respect of welding.

The properties of steel can be manipulated

by heat treatment. If the metal is cooled very

rapidly (quenched) the crystalline structure is

quite different from that which results from

gradual cooling. Quenched steel is extremely

hard and brittle and is not used in structural

engineering. Following re-heating, however,

the metal regains its ductility and the level of

6 It is intended to replace these designations with grades

based on yield stress values. Thus grade 43A will

become 275A as the yield stress for this type of steel is

275 N/mm^2.

7 Blanc, McEvoy and Plank, Architecture and Construction in
62 Steel, Chapter 3.

Fig. 3.12 The relationship between stress and strain in

typical structural steels. The short section of the graph

between the elastic and plastic ranges, in which the graph

is more-or-less horizontal, is of fundamental importance in

determining the excellent structural behaviour of steel.

brittleness/ductility (and therefore yield

strength) which is achieved can be accurately

controlled in this process (which is called

tempering). Heat treated steels are used in

structural engineering only for very specialised

applications, the most common of which is in

the manufacture of high strength friction-grip

bolts.^7

Steel is a high-strength material which has

equal strength in tension and compression;

the ultimate strength and design strength

values which are used in the UK are given in

Table 3.1, which is reproduced from BS 5950

The Structural Use of Steelwork in Building'.

The relationship between stress and strain of

a typical structural steel is shown in Fig. 3.12

and it will be seen that 'elastic' behaviour

(linear behaviour in which the graph of stress

against strain is a straight line) occurs in the

lower part of the load range. In the higher load

range the relationship is curved (inelastic, non-

linear behaviour) and a larger increase in strain

results for a given increase in stress. The

location in the graph at which the transition to

Stress

N/mm^2

600

450

30C

150

(^5101520) 25%
Strain