STRUCTURAL DESIGN FOR ARCHITECTURE

non-linear behaviour occurs is termed the
'yield point'.
The portion of the stress-strain graph which
immediately follows the yield point is more-or-
less horizontal. This feature illustrates a very
important property of steel which is the
mechanism for the relief of stress concentrat-
ions, especially in the vicinity of connections.
If, for example, one part of a cross-section
tends to be highly stressed relative to other
parts (this might occur in the vicinity of a bolt),
the highly stressed material could reach its
yield point while the average stress was still
relatively small (Fig. 3.13). If more load were
applied the strain would increase equally in all
parts of the cross-section but the level of
stress in the most highly stressed area would
tend to remain constant because the horizon-
tal portion of the stress-strain graph would
have been entered by the material in that area.
In other parts of the cross-section, where the
stress was within the elastic range, the stress
level would continue to increase with increas-
ing strain, however, and the distribution of
stress would therefore tend to become more
even. The existence of the short horizontal
portion of the stress/strain graph is therefore a
stress-relieving feature. It is a very important
factor in determining the good structural
performance of steel.
Another significant aspect of the
stress-strain graph is the amount of deform-
ation which occurs before failure. A very large

Fig. 3.13 The stress-relieving mechanism. Each of the

four diagrams in the lower half of this illustration shows

the distribution of stress across the cross-section X-X. In

the first diagram all of the material is stressed within the

elastic range and the material which is closest to the bolt

hole is the most highly stressed. As the load rises the level

of stress also rises. Once the yield stress is passed at the

most highly stressed locations the horizontal portion of

the stress-strain graph is entered and it is possible for the

stress level to remain constant while stress levels in other

parts of the cross-section continue to rise. The distribution

of stress then becomes more even. This is an example of

the stress-relieving mechanism which is responsible for

the ability of steel to resist high levels of tensile load.

amount of deformation is in fact required

before steel fractures. This too is a safety

feature because it means that an overloaded

structure will suffer a large deflection which

gives warning of impending collapse.

3.4 Structural steel products

3.4.1 Introduction

Three basic techniques are used to form metal

components. These are casting, in which

molten metal is poured into a suitably shaped

mould, forging, where solid metal is beaten or

otherwise forced into a particular shape, and

machining, where material is cut away by

various means from a basic block of metal, to

form a component with a particular shape. All 63

Table 3.1 Basic design strengths for steel

BS 4360 Grade Thickness, Strength of sections,

less than or plates and hollow

equal to (mm) sections (N/mm^2 )

4 3 16 275
A, B and C 40 265
100 245
50 16 355
B and C 63 340
100 325
55 16 450
C 25 430
40 415

Steel structures

Yield stress

Increase in load