ABSTRACT
Representative mechanical properties of reinforcing bars are shown in Figure 4.2 in reference to a tensile test. The plot shows the changes in unit strain as an axial tensile force is applied on the bar monotonically and slowly. The vertical axis indicates the tensile force divided by the nominal cross-sectional area of the bar. This quantity, which is a unit stress, is usually referred to as stress. It has units of force per unit area. The extension of the bar divided by its original length is referred to as unit strain, or simply as strain. It has no units. Sometimes it is also called engineering strain to distinguish it from true strain, a denition very seldom used in structural engineering practice. True strain is the unit strain obtained as the ratio of deformation increment to the actual length of the bar immediately before the occurrence of that increment. It is instructive to recognize that while stress has units of unit stress (pounds per square inch or newtons per square millimeter), unit strain has no units. It is important to repeat that unit stress has physical units, while unit strain has no units. Unit strain is strictly an abstraction unless it refers to its occurrence over a given length.
