STRUCTURAL STEEL STRESS - STRAIN BEHAVIOR BASIC INFORMATION AND TUTORIALS

Stress-Strain Behavior of Structural Steel

Structural steel is an important construction material. It possesses attributes such as strength, stiffness, toughness, and ductility that are very desirable in modern constructions.

Strength is the ability of a material to resist stresses. It is measured in terms of the material’s yield strength, Fy , and ultimate or tensile strength, Fu. For steel, the ranges of Fy and Fu ordinarily used in constructions are 36 to 50 ksi (248 to 345 MPa) and 58 to 70 ksi (400 to 483 MPa), respectively, although higher strength steels are becoming more common.

Stiffness is the ability of a material to resist deformation. It is measured as the slope of the material’s stress-strain curve.

With reference to figure below in which uniaxial engineering stress-strain curves obtained from coupon tests for various grades of steels are shown, it is seen that the modulus of elasticity, E, does not vary appreciably for the different steel grades.

Therefore, a value of 29,000 ksi (200 GPa) is often used for design. Toughness is the ability of a material to absorb energy before failure. It is measured as the area under the material’s stress-strain curve.

As shown in the figure, most (especially the lower grade) steels possess high toughness which is suitable for both static and seismic applications. Ductility is the ability of a material to undergo large inelastic, or plastic, deformation before failure.

It is measured in terms of percent elongation or percent reduction in area of the specimen tested in uniaxial tension. For steel, percent elongation ranges from around 10 to 40 for a 2-in. (5-cm) gage length specimen.

Ductility generally decreases with increasing steel strength. Ductility is a very important attribute of steel. The ability of structural steel to deform considerably before failure by fracture allows an indeterminate structure to undergo stress redistribution. Ductility also enhances the energy absorption characteristic of the structure, which is extremely important in seismic design.