Plastic Design to BS 5950





Introduction

1.1 Elastic and plastic properties Plastic design takes of structural steel

of mild steel, namely advantage its of an important and unique property

ductility. Figure 1.1(a) shows a typical strain curve obtained from a simple tensile test.

Unless a tensile test is carefully carried out using sophisticated equipment,

stress is difficult to determine and it has little practical significance. the upper yield Similarly, except

It is therefore ignored for one particular exception in

stress— design calculations. which will be considered in

Chapter 3, it is conservative and convenient to ignore strain hardening

and to consider the

The significant simplified stress—strain curve shown in

feature in both Figs 1.1(a) and (b) Fig. 1.1(b). is the

long yield plateau which allows the constant stress. Figure 1.1(a) possibility is

of considerable plastic of the carbon steels that have traditionally

been used in typical strain at plastic design. However, not all structural

steels have such a favourable stress—strain curve and the initial part

of the stress—strain curve for a range Fig. 1.21.1 shows of steels including some of the

It can be seen that the latter have a more gradually yielding higher grades. behaviour though

still exhibiting considerable ductility. Provided that adequate ductility

such steels can still be used for replaced by the 0.2% proof is available (see below),

plastic design with the stress in the conventional way. yield stress


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