Steel structures
3.3 The properties and composition of steel
Steel is a ferrous metal (its principal
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 Those which are used in the UK are specified in BS 4360 'Weldable Structural Steels' which is currently being superseded by a European Standard EN 10 025.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/mm2.
7 Blanc, McEvoy a n d Plank, Architecture and Construction in Steel, Chapter 3.
62
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/mm2 600 450 30C 150 5 10 15 20 2 5 % Strain
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.