13.6 CLASSIFICATION AND PROPERTIES OF STRUCTURAL STEELS Steel is employed as the structural frame in various forms of construction (bridges, buildings, etc.), as well as in reinforced portland cement concrete structural members. The composition and engineering properties of the steels used in these diverse applications vary substantially. Although a detailed examination of all structural quality steels is beyond the scope of this text, the main grades, types, and classes of these steels will be briefly described. As noted previously, structural steels are classified on the basis of strength (class or grade) as well as composition (type). For example, the composition and selected properties of some (1) structural carbon steels (ASTM A-36M), (2) high-strength, low-alloy manganese vanadium steels (ASTM A-441/A-572M), and (3) high-yield strength, quenched and tempered alloy steels (ASTM A-514M) are provided in Table 13.2 and Fig. 13.15. The differences in strength and ductility among the various types can be significant, and the selection of an appropriate steel should be based on the end use of the structural element. Steels used as reinforcing elements in portland cement concrete structural members are available in several forms and with a variety of properties. Distinction must be made between (1) plain and deformed reinforcing bars (Fig. 13.16(a)), (2) wire for welded wire fabric (Fig. 13.16(b)), and (3) bar, wire, and strand (Fig. 13.16(c)), for prestressing. Gradings and selected properties are presented in Table 13.3, and typical stress-strain curves are shown in Fig. 13.17. The time-dependent behavior of steels is of interest in prestressed members, where relaxation of the steel reduces its stress level with time, as shown in Fig. 13.18. Note that the stress loss is greater for higher prestress levels (see Chapter 7).

2

TABLE 13.2 Properties and Composition of Selected Structural Steels Chemical Composition, % wt. Mechanical Properties Yield Tensile C P S Strengh Strengh Elongation Type (max) Mn (max) (max) Si (min)(MPa)(MPa) (%) Carbona 0.25-0.29d 0.60-1.20d 0.04 0.05 0.15-0.40 250 400-550 20-23d High strength 0.21-0.26d 1.35-1.65d 0.04 0.05 0.15-0.40d 290-450d 415-530d 15-24d low alloyb Quenched & 0.10-0.21d 0.40-1.50d 0.03 0.04 0.15-0.80d 620-690d 690-895d 18 tempered alloyc a ASTM A-36M. b ASTM A-572M. c ASTM A-514M. dRange of values, depending on shape and size of structural element.

TABLE 13.3 Properties of Selected Steels for Concrete Reinforcement Type of Grade Yield Strength (min) Tensile Strength (min) Reinforcement MPa MPa Barsa 300 300 500 400 400 600 500 500 700 Steel wire for Smoothb 485 550 welded wire fabric Deformedc 485 550 Prestressing barsd Plain 880 1035 Deformed 825 1035 Seven-wire 250 1725 strande 270 1860 a ASTM A-615M. b ASTM A-82. cASTM A-496. dASTM A-722. eASTM A-416.

3

Figure 13.15 Stress-strain curves of selected structural steels (from S. E. Cooper, Designing Steel Structures, Prentice Hall, 1985, p. 63, adapted from Steel Design Manual, 1981, U.S.Steel Co.).

4

Figure 13.16 Shape of selected steels used for concrete reinforcement: (a) deformed bars, (b) welded wire fabric (Courtesy of Tree Island Industries, New Westminster, B.C.) (c) wire strand for prestressing

5

Figure 13.17 Stress-strain curves for selected steels for concrete reinforcement (after A. E. Naaman, Prestressed Concrete Analysis and Design, McGraw-Hill, 1982, p. 34). Figure 13.18 Stress relaxation in pretensioned steel for prestressing (after A.E. Naaman, Prestressed Concrete Analysis and Design, McGraw-Hill, 1982, p. 34). Attention also should be given to the behavior of structural steels in service. Corrosion under general atmospheric conditions was discussed in Section 13.5, and in portland cement concrete in Section 12.5. In addition, performance at high temperatures with respect to fire safety is important. Above 300'C there is a tendency for some loss in strength (Fig. 13.19) and stiffness, which may become very significant above 600C.

6

Figure 13.19 Effect of temperature on the strength of various types of steels (after B. K. Bardhan-Roy, "Fire Resistance-Design and Detailing," Chapter 14 in Handbook of Structural Concrete, F. K. Kong, R. H. Evans, E. Cohen, and F. Roll, Eds., Pitman Advanced Publishing Program, 1983, pp. 14-16) Although the strength loss may be recovered (depending on the level of temperature) on cooling, one should consider the load-carrying capacity of the steel in the high-temperature condition with respect to its ability to support the loads imposed on the structure without failing while the conditions of high temperature exist. Excessive deformation in a steel structure exposed to fire may be aggravated by the reduction in modulus of elasticity at high temperature.

BIBLIOGRAPHY D. T. LLEWELLYN, Steels: Metallurgy and Applications. Butterworth-Heinemann Ltd., 1992. Manual of Steel Construction, (8th Edition), American Institute of Steel Construction, 1980. Metals Handbook Volume 1, Properties and Selection: Irons, Steels, and High-Performance Alloys, (10th Edition), ASM International, 1990.

7

PROBLEMS 1. What is a structural steel? What is the difference between a plain carbon and a low alloy structural steel? 2. What is the effect of an increase in carbon content from 0 % to 1 % on the (i) iron carbide (cementite, Fe3C) content, (ii) strength and (iii) toughness of an annealed, plain carbon steel? 3. Identify and describe the two microstructure] constituents which make up a typical structural steel. How is each of these microconstituents formed? How does each affect the mechanical properties of the steel? 4. What is a "killed" steel and how is it produced? 5. What is the mechanism by which carbon and nitrogen strengthen low alloy steels? 6. Briefly describe the strengthening mechanism resulting from work-hardening (strain-hardening). Why is this method of strengthening not commonly used for structural steels? 7. How are bainite and martensite formed? What is the difference in the properties of these two phases? Why are these phases not shown on the iron-iron carbide phase diagram? 8. What differences in microstructure and mechanical properties are produced by quenching, normalizing, or fully annealing a sample of structural steel? 9. Explain the differences between (i) elastic and plastic deformation, (ii) proportional limit and yield strength, and (iii) discontinuous yielding and strain-hardening. 10. How can the brittle fracture of structural steel members be prevented? 11. What is "weldability" and how is it affected by increasing carbon and alloy contents in a structural steel? 12. What conditions may result in the formation of a galvanic microcell on the surface of a structural steel? 13. Identify and briefly describe the most common form of steel corrosion. 14. What factors influence the usefulness of zinc coatings as a form of corrosion protection for structural steels?