steel.pdf

108 Chapter 2   Nature of Materials a. Using the diagram, briefly explain how spreading salt on ice causes the ice to melt. Show numerical examples in your discussion. b. At a salt composition of 10%, what is the temperature at which ice will start melting?...

108 Chapter 2   Nature of Materials a. Using the diagram, briefly explain how spreading salt on ice causes the ice to melt. Show numerical examples in your discussion. b. At a salt composition of 10%, what is the temperature at which ice will start melting? c. What is the eutectic temperature of the ice and salt combination? 2.34 What are the five classes of ceramic materials? 2.35 Name two common ceramic materials used in Civil Engineering structures. Are they organic solid or inorganic solids? 2.36 What are the four types of organic solids used in engineering applications? Define each one and give examples. 2.37 Using online search, prepare a table to show the following properties: crystal structures, atomic radius, atomic mass, and atomic packing factor for each of the following 15 metals: lead, zirconium, sodium, cadmium, iridium, magne- sium, iron, tungsten, titanium, aluminum, copper, nickel, beryllium, vana- dium, and lithium. 2.5 References Ashby M. F. and D. R. H. Jones. Engineering Materials 2, an Introduction to Microstructures, Processing and Design. International Series on Materials Science and Technology, Vol. 39. Oxford: Pergamon Press, 1986. Budinski, K. G. and M. K. Budinski. Engineering Materials, Properties and Selection. 9th ed. Upper Saddle River, NJ: Pearson, 2010. Callister, W. D., Jr. Materials Science and Engineering—an Introduction. 7th ed. New York: John Wiley and Sons, 2006. Derucher, K. N., G. P. Korfiatis, and A. S. Ezeldin. Materials for Civil & Highway Engineers. 4th ed. Upper Saddle River, NJ: Prentice Hall, 1998. Flinn, R. A., and P. K. Trojan. Engineering Materials and Their Applications. 4th ed. Boston, MA: Houghton Mifflin, 1995. Guy, A. G. and J. Hren. Elements of Physical Metallurgy. 3d ed. Reading, MA: Addison-Wesley, 1974. Jackson, N. and R. K. Dhir, eds. Structural Engineering Materials. 4th ed. New York: Hemisphere, 1989. Jastrzebski, Z. D. The Nature and Properties of Engineering Materials. 3d ed. New York: John Wiley & Sons, 1987. Shackelford, J. F. Introduction to Materials Science for Engineers. 8th ed. Upper Saddle River, NJ: Prentice Hall, 2014. Van Vlack, L. H. Elements of Materials Science. 2nd ed. Reading, MA: Addison- Wesley, 1964. Van Vlack, L. H. Elements of Materials Science and Engineering. 6th ed. Reading, MA: Addison-Wesley, 1989. worksaccounts.com C h a p t e r 3 Steel The use of iron dates back to about 1500 B.C., when primitive furnaces were used to heat the ore in a charcoal fire. Ferrous metals were produced on a relatively small scale until the blast furnace was developed in the 18th century. Iron products were widely used in the latter half of the 18th century and the early part of the 19th century. Steel production started in mid-1800s, when the Bessemer converter was invented. In the second half of the 19th century, steel technology advanced rapidly due to the development of the basic oxygen furnace and continuous casting methods. More recently, computer-controlled manufacturing has increased the efficiency and reduced the cost of steel production. There are many organizations throughout the world that provide standards and test methods for steel. In this chapter the American Society for Testing and Materials (ASTM) and the American Institute for Steel Construction (AISC) standards and test methods are presented. However, when possible, dimensional quantities have been converted to SI. The structural steel industry consists of four components (AISC, 2015): 1. producers of structural steel including hot-rolled structural shapes and hollow sections 2. service centers that function as warehouses and provide limited preprocessing of structural material 3. structural steel fabricators that prepare the steel for the building process 4. erectors that construct steel frames on the project site Civil and construction engineers will primarily interact with the service centers to determine the availability of products required for a project, the fabricators for detail design drawings and preparation of the product, and the erectors on the job site. Currently, steel and steel alloys are used widely in civil engineering applications. In addition, wrought iron is still used on a smaller scale for pipes, as well as for general blacksmith work. Cast iron is used for pipes, hardware, and machine parts not sub- jected to tensile or dynamic loading. Steel products used in construction can be classified as follows: 1. structural steel (Figure 3.1 vertical columns) produced by continuous casting and hot rolling for large structural shapes, plates, and sheet steel worksaccounts.com 110 Chapter 3   Steel Figure 3.1 Steel trusses and columns for the structural support of a building. 2. cold-formed steel (Figure 3.1 trusses and decking) produced by cold-forming of sheet steel into desired shapes 3. fastening products used for structural connections, including bolts, nuts and washers 4. reinforcing steel (rebars) for use in concrete reinforcement (Figure 3.2) 5. miscellaneous products for use in such applications as forms and pans Civil and construction engineers rarely have the opportunity to formulate steel with specific properties. Rather, they must select existing products from suppliers. Even the shapes for structural elements are generally restricted to those readily available from manu­ facturers. While specific shapes can be made to order, the cost to fabricate l­ow-volume members is generally prohibitive. Therefore, the majority of civil engineering projects, with the exception of bridges, are designed using standard steel types and structural shapes. Bridges are a special case, in that the majority of bridge structures are fabricated from plate steel rather than hot-rolled sections or hollow structural sections (AISC 2015). Even though civil and construction engineers are not responsible for formulating steel products, it is beneficial to understand how steel is manufactured and treated and how it responds to loads and environmental conditions. This chapter reviews steel pro- duction, the iron–carbon phase diagram, heat treatment, steel alloys, structural steel, steel fasteners, and reinforcing steel. This chapter also presents common tests used to characterize the mechanical properties of steel. The topics of welding, corrosion, and sustainability of steel are also introduced. worksaccounts.com  Section 3.1  Steel Production 111 Figure 3.2 Steel rebars used to reinforce portland cement concrete wall. 3.1 Steel Production The overall process of steel production is shown in Figure 3.3. This process consists of the following three phases: 1. reducing iron ore to pig iron 2. refining pig iron (and scrap steel from recycling) to steel 3. forming the steel into products The materials used to produce pig iron are coal, limestone, and iron ore. The coal, after transformation to coke, supplies carbon used to reduce iron oxides in the ore. Limestone is used to help remove impurities. Prior to reduction, the concentra- tion of iron in the ore is increased by crushing and soaking the ore. The iron is mag- netically extracted from the waste, and the extracted material is formed into pellets and fired. The processed ore contains about 65% iron. Reduction of the ore to pig iron is accomplished in a blast furnace. The ore is heated in the presence of carbon. Oxygen in the ore reacts with carbon to form gases. A flux is used to help remove impurities. The molten iron, with an excess of carbon in solution, collects at the bottom of the furnace. The impurities, slag, float on top of the molten pig iron. worksaccounts.com  Ironmaking Steelmaking Continuous casting Rolling Main products Rail M03_MAML5440_04_GE_C03.indd 112 Sheet pile Pellet Coke Shape Section mill Bar Wire rod Iron ore Sintered Limestone ore Wire rod mill Plate Hot direct Plate mill rolling Hot metal Billet Hot rolled coil (HDR) and sheet Hot strip mill Basic oxygen furnace (BOF) Bloom Cold rolled coil and sheet (also for plating) Slab Cold rolling tandem mill Blast furnace (BF) Welded pipe Butt welded pipe Welded pipe mill Scrap Electric arc furnace (EAF) Seamless pipe Seamless pipe mill Reheating furnace Steel castings Figure 3.3 Conversion of raw material into different steel shapes. worksaccounts.com 5/19/17 5:14 PM  Section 3.1  Steel Production 113 The excess carbon, along with other impurities, must be removed to produce high-quality steel. Using the same refining process, scrap steel can be recycled. Two types of furnaces are used for refining pig iron to steel: 1. basic oxygen 2. electric arc The basic oxygen furnaces remove excess carbon by reacting the carbon with oxygen to form gases. Lances circulate oxygen through the molten material. The process is continued until all impurities are removed and the desired carbon con- tent is achieved. The basic oxygen furnace process is used to produce construction products that require more drawability, such as hollow structural sections, studs, decking, and plate (Calkins, 2008). Electric furnaces use an electric arc between carbon electrodes to melt and refine the steel. These plants require a tremendous amount of energy and are used primar- ily to recycle scrap steel. Electric furnaces are frequently used in minimills, which produce a limited range of products. In this process, molten steel is transferred to the ladle. Alloying elements and additional agents can be added either in the furnace or the ladle. Structural members are primarily produced with the electric arc furnace process (Calkins, 2008). During the steel production process, oxygen may become dissolved in the liquid metal. As the steel solidifies, the oxygen can combine with carbon to form carbon monoxide bubbles that are trapped in the steel and can act as initiation points for failure. Deoxidizing agents, such as aluminum, ferrosilicon, and man- ganese, can eliminate the formation of the carbon monoxide bubbles. Completely deoxidized steels are known as killed steels. Steels that are generally killed include: Those with a carbon content greater than 0.25% All forging grades of steels Structural steels with carbon content between 0.15 and 0.25% Some special steel in the lower carbon ranges Regardless of the refining process, the molten steel, with the desired chemical composition, is then either cast into ingots (large blocks of steel) or cast continu- ously into a desired shape. Continuous casting with hot rolling is becoming the standard production method, since it is more energy efficient than casting ingots, as the ingots must be reheated prior to shaping the steel into the final product. Cold-formed steel is produced from sheets or coils of hot rolled steel which is formed into shape either through press-braking blanks sheared from sheets or coils, or more commonly, by roll- forming the steel through a series of dies. No heat is required to form the shapes (unlike hot-rolled steel) and thus the name cold- formed steel. Cold-formed steel members and other products are thinner, lighter, and easier to produce, and typically cost less than their hot-rolled counterparts (IBS Digest, 2005). worksaccounts.com M03_MAML5440_04_GE_C03.indd 113 5/19/17 5:14 PM 114 Chapter 3   Steel 3.2 Iron–Carbon Phase Diagram In refining steel from iron ore, the quantity of carbon used must be carefully con- trolled in order for the steel to have the desired properties. The reason for the strong relationship between steel properties and carbon content can be understood by examining the iron–carbon phase diagram. Figure 3.4 shows a commonly accepted iron–carbon phase diagram. One of the unique features of this diagram is that the abscissa extends only to 6.7%, rather than 100%. This is a matter of convention. In an iron-rich material, each carbon atom bonds with three iron atoms to form iron carbide, Fe 3 C, also called cementite. Iron carbide is 6.7% carbon by weight. Thus, on the phase diagram, a carbon weight of 6.7% corresponds to 100% iron carbide. A complete iron–carbon phase diagram should extend to 100% carbon. However, only the iron-rich portion, as shown in Figure 3.4, is of practical significance (­Callister, 2006). In fact, structural steels have a maximum carbon content of less than 0.3%, so only a very small portion of the phase diagram is significant for civil engineers. The left side of Figure 3.4 demonstrates that pure iron goes through two trans- formations as temperature increases. Pure iron below 912°C has a BCC crystalline structure called ferrite. At 912°C, the ferrite undergoes a polymorphic change to an FCC structure called austenite. At 1394°C, another polymorphic change occurs, returning the iron to a BCC structure. At 1539°C, the iron melts into a liquid. The high- and low-temperature ferrites are identified as d and a ferrite, respectively. 1600 1539 d 1 Liquid d 1400 Liquid 1394 g1d Liquid 1 g 1200 g, Austenite Liquid 1 Fe3C 2.11% 11488C 4.3% Temperature, 8C Upper critical temperature 1000 lines 912 g 1 Fe3C 800 a1g Lower critical temperature line 0.77% 7278C a, Ferrite 100% Fe3C 600 a 1 Fe3C 400 0 1 2 3 4 5 6 6.7 Percent weight of carbon Figure 3.4 The iron-carbon carbide phase diagram. worksaccounts.com  Section 3.2  Iron–Carbon Phase Diagram 115 Since d ferrite occurs only at very high temperatures, it does not have practical sig- nificance for this book. Carbon goes into solution with a ferrite at temperatures between 400°C and 912°C. However, the solubility limit is very low, with a maximum of 0.022% at 727°C. At temperatures below 727°C and to the right of the solubility limit line, a ferrite and iron carbide coexist as two phases. From 727°C to 1148°C, the solubility of carbon in the austenite increases from 0.77% to 2.11%. The solubility of carbon in austenite is greater than in a ferrite because of the crystalline structure of the austenite. At 0.77% carbon and 727°C, a eutectoid reaction occurs; that is, a solid phase change occurs when either the temperature or carbon content changes. At 0.77% car- bon, and above 727°C, the carbon is in solution as an interstitial element, within the FCC structure of the austenite. A temperature drop to below 727°C, which happens slowly enough to allow the atoms to reach an equilibrium condition, results in a two-phase material, a ferrite and iron carbide. The a ferrite will have 0.022% carbon in solution, and the iron carbide will have a carbon content of 6.7%. The ferrite and iron carbide will form as thin plates, a lamellae structure. This eutectoid material is called pearlite. At carbon contents less than the eutectoid composition, 0.77% carbon, hypoeu- tectoid alloys are formed. Consider a carbon content of 0.25%. Above approximately 860°C, solid austenite exists with carbon in solution. The austenite consists of grains of uniform material that were formed when the steel was cooled from a liquid to a solid. Under equilibrium temperature drop from 860°C to 727°C, a ferrite is formed and accumulates at the grain boundaries of the austenite. This is a proeutectoid fer- rite. At temperatures slightly above 727°C, the ferrite will have 0.022% carbon in solution and austenite will have 0.77% carbon. When the temperature drops below 727°C, the austenite will transform to pearlite. The resulting structure consists of grains of pearlite surrounded by a skeleton of a ferrite. When the carbon content is greater than the eutectoid composition, 0.77% ­carbon, hypereutectoid alloys are formed. Iron carbide forms at the grain boundaries of the austenite at temperatures above 727°C. The resulting microstructure consists of grains of pearlite surrounded by a skeleton of iron carbide. The lever rule for the analysis of phase diagrams can be used to determine the phases and constituents of steel. Sample Problem 3.1 Calculate the amounts and compositions of phases and constituents of steel ­composed of iron and 0.25% carbon just above and below the eutectoid isotherm. Solution At a temperature just higher than 727°C, all the austenite will have a carbon content of 0.77% and will transform to pearlite. The ferrite will remain as primary ferrite. The proportions can be determined by using the lever rule: Primary a: 0.022% C, 0.77 - 0.25 Percent primary A = c d * 100 = 69.5% 0.77 - 0.022 0.25 - 0.022 Percent pearlite = c d * 100 = 30.5% 0.77 - 0.022 worksaccounts.com M03_MAML5440_04_GE_C03.indd 115 5/19/17 5:14 PM 116 Chapter 3   Steel At a temperature just below 727°C, the phases are ferrite and iron carbide. The fer- rite will have 0.022% carbon, so we have 6.67 - 0.25 Percent ferrite, A: (0.022% C) = c d * 100 = 96.6% 6.67 - 0.022 0.25 - 0.022 Percent pearlite = c d * 100 = 3.4% 6.67 - 0.022  Figure 3.5 shows an optical 50 * photomicrograph of a hot-rolled mild steel plate with a carbon content of 0.18% by weight that was etched with 3% nitol. The light etching phase is proeutectoid ferrite and the dark constituent is pearlite. Note the banded structure resulting from the rolling processes. Figure 3.6 shows the same material as Figure 3.5, except that the magnification is 400 *. At this magnification, the alternating layers of ferrite and cementite in the pearlite can be seen. The significance of ferrite, pearlite, and iron carbide formation is that the prop- erties of the steel are highly dependent on the relative proportions of ferrite and iron carbide. Ferrite has relatively low strength but is very ductile. Iron carbide has high strength but has virtually no ductility. Combining these materials in different proportions alters the mechanical properties of the steel. Increasing the carbon con- tent increases strength and hardness but reduces ductility. However, the modulus of elasticity of steel does not change by altering the carbon content. All of the preceding reactions are for temperature reduction rates that allow the material to reach equilibrium. Cooling at more rapid rates greatly alters the F i g u r e 3. 5 Optical photomicrograph of hot rolled mild steel plate (magnification: 50 * ). worksaccounts.com  Section 3.3  Heat Treatment of Steel 117 F i g u r e 3. 6 Optical photomicrograph of hot rolled mild steel plate (magnification: 400 * ). microstructure. Moderate cooling rates produce bainite, a fine-structure pearlite without a proeutectoid phase. Rapid quenching produces martensite; the carbon is supersaturated in the iron, causing a body center tetragonal lattice structure. Time– temperature transformation diagrams are used to predict the structure and properties of steel subjected to heat treatment. Rather than going into the specifics, the different types of heat treatments are described. 3.3 Heat Treatment of Steel Properties of steel can be altered by applying a variety of heat treatments. For exam- ple, steel can be hardened or softened by using heat treatment; the response of steel to heat treatment depends upon its alloy composition. Common heat treatments employed for steel include annealing, normalizing, hardening, and tempering. The basic process is to heat the steel to a specific temperature, hold the temperature for a specified period of time, then cool the material at a specified rate. The temperatures used for each of the treatment types are shown in Figure 3.7. 3.3.1 Annealing The objectives of annealing are to refine the grain, soften the steel, remove internal stresses, remove gases, increase ductility and toughness, and change electrical and magnetic properties. Four types of annealing can be performed, depending on the desired results of the heat treatment: Full annealing requires heating the steel to about 50°C above the austenitic tem- perature line and holding the temperature until all the steel transforms into either worksaccounts.com 118 Chapter 3   Steel 1000 Normalizing 900 Full Anneal and Hardening Temperature, 8C 800 700 Process Anneal 600 500 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 Percent Carbon Figure 3.7 Heat treatment temperatures. austenite or austenite–cementite, depending on the carbon content. The steel is then cooled at a rate of about 20°C per hour in a furnace to a temperature of about 680°C, followed by natural convection cooling to room temperature. Due to the slow cooling rate, the grain structure is a coarse pearlite with ferrite or cementite, depending on the carbon content. The slow cooling rate ensures uniform properties of the treated steel. The steel is soft and ductile. Process annealing is used to treat work-hardened parts made with low carbon steel (i.e., less than 0.25% carbon). The material is heated to about 700°C and held long enough to allow recrystallization of the ferrite phase. By keeping the tempera- ture below 727°C, there is not a phase shift between ferrite and austenite, as occurs during full annealing. Hence, the only change that occurs is refinement of the size, shape, and distribution of the grain structure. Stress relief annealing is used to reduce residual stresses in cast, welded, and cold-worked parts and cold-formed parts. The material is heated to 600 to 650°C, held at temperature for about 1 hour, and then slowly cooled in still air. Spheroidization is an annealing process used to improve the ability of high carbon (i.e., more than 0.6% carbon) steel to be machined or cold worked. It also improves abrasion resistance. The cementite is formed into globules (spheroids) dis- persed throughout the ferrite matrix. 3.3.2 Normalizing Normalizing is similar to annealing, with a slight difference in the temperature and the rate of cooling. Steel is normalized by heating to about 60°C above the austenite line and then cooling under natural convection. The material is then air cooled. Nor- malizing produces a uniform, fine-grained microstructure. However, since the rate worksaccounts.com  Section 3.4  Steel Alloys 119 of cooling is faster than that used for full annealing, shapes with varying thicknesses results in the normalized parts having less uniformity than could be achieved with annealing. Since structural plate has a uniform thickness, normalizing is an effective process and results in high fracture toughness of the material. 3.3.3 Hardening Steel is hardened by heating it to a temperature above the transformation range and holding it until austenite is formed. The steel is then quenched (cooled rapidly) by plunging it into, or spraying it with, water, brine, or oil. The rapid cooling changes the grain structure forming martensite rather than allowing the transformation to the fer- rite BCC structure. Martensite has a very hard and brittle structure. Since the cooling occurs more rapidly at the surface of the material being hardened, the surface of the material is harder and more brittle than the interior of the element, creating nonhomo- geneous characteristics. Due to the rapid cooling, hardening puts the steel in a state of strain. This strain sometimes causes steel pieces with sharp angles or grooves to crack immediately after hardening. Thus, hardening must be followed by tempering. 3.3.4 Tempering The predominance of martensite in quench-hardened steel results in an undesirable brittleness. Tempering is performed to improve ductility and toughness. Martensite is a somewhat unstable structure. Heating causes carbon atoms to diffuse from mar- tensite to produce a carbide precipitate and formation of ferrite and cementite. After quenching, the steel is cooled to about 40°C then reheated by immersion in either oil or nitrate salts. The steel is maintained at the elevated temperature for about 2 hours and then cooled in still air. 3.3.5 Example of Heat Treatment In the quest to produce high-strength low-alloy steels economically, the industry has developed specifications for several new steel products, such as A913. This steel is available with yield stresses ranging from 345 to 510 MPa. The superior properties of A913 steel are obtained by a quench self-tempering process. Following the last hot rolling pass for shaping, for which the temperature is typically 850°C, an intense water-cooling spray is applied to the surface of the beam to quench (rapidly cool) the skin. Cooling is stopped before the core on the material is affected. The outer layers are then self-tempered as the internal heat of the beam flows to the surface. The self-tempering temperature is 600°C (Bouchard and Axmann, 2000). 3.4 Steel Alloys Alloy metals can be used to alter the characteristics of steel. By some counts, there are as many as 250000 different alloys of steel produced. Of these, as many as 200 worksaccounts.com 120 Chapter 3   Steel may be used for civil engineering applications. Rather than go into the specific char- acteristics of selected alloys, the general effect of different alloying agents will be presented. Alloy agents are added to improve one or more of the following properties: 1. hardenability 2. corrosion resistance 3. machinability 4. ductility 5. strength Common alloy agents, their typical percentage range, and their effects are sum- marized in Table 3.1. T a b l e 3. 1 Common Steel Alloying Agents (Budinski and Budinski, 2010) (© Pearson Education, Inc. Used by permission.) Typical Ranges in Alloy Steels (%) Principal Effects Aluminum 62 Aids nitriding Restricts grain froth Removes oxygen in steel melting Sulfur 60.05 Adds machinability Reduces weldability and ductility Chromium 0.3 to 4 Increases resistance to corrosion and oxidation Increases hardenability Increases high-temperature strength Can combine with carbon to form hard, ­   wear-resistant microconstituents Nickel 0.3 to 5 Promotes an austenitic structure Increases hardenability Increases toughness Copper 0.2 to 0.5 Promotes tenacious oxide film to aid atmospheric   corrosion resistance Manganese 0.3 to 2 Increases hardenability Promotes an austenitic structure Combines with sulfur to reduce its adverse effects Silicon 0.2 to 2.5 Removes oxygen in steel making Improves toughness Increases hardenability Molybdenum 0.1 to 0.5 Promotes grain refinement Increases hardenability Improves high-temperature strength Vanadium 0.1 to 0.3 Promotes grain refinement Increases hardenability Will combine with carbon to form wear-resistant   microconstituents worksaccounts.com  Section 3.5  Structural Steel 121 By altering the carbon and alloy content and by using different heat treatments, steel can be produced with a wide variety of characteristics. These are classified as follows: 1. Low alloy Low carbon Plain High strength–low alloy Medium carbon Plain Heat treatable High carbon Plain Tool 2. High Alloy Tool Stainless Steels used for construction projects are predominantly low- and medium-car- bon plain steels. Stainless steel has been used in some highly corrosive applications, such as dowel bars in concrete pavements and steel components in swimming pools and drainage lines. The Specialty Steel Industry of North America, SSINA, promotes the use of stainless steel for structural members where corrosion resistance is an important design consideration (SSINA, 1999). The use and control of alloying agents is one of the most significant factors in the development of steels with better performance characteristics. The earliest speci- fication for steel used in building and bridge construction, published in 1900, did not contain any chemical requirements. In 1991, ASTM published the specification which controls content of 10 alloying elements in addition to carbon (Hassett, 2003). 3.5 Structural Steel Structural steel is used in hot-rolled structural shapes, plates, and bars. Structural steel is used for various types of structural members, such as columns, beams, brac- ings, frames, trusses, bridge girders, and other structural applications (see Figure 3.8). 3.5.1 Structural Steel Grades Due to the widespread use of steel in many applications, there are a wide variety of systems for identifying or designating steel, based on grade, type and class. Virtually, every country with an industrial capacity has specifications for steel. In the United States, there are several associations that write specifications for steel, such as the Society of Automotive Engineers, SAE, the American Iron and Steel Institute, AISI, and the American Society for Testing and Materials, ASTM. The most widely used worksaccounts.com 122 Chapter 3   Steel F i g u r e 3. 8 Structural and cold-formed steel used to make columns, beams, and floors for the structural support of a building. designation system was developed cooperatively by SAE and AISI based on chemi- cal composition (www.key-to-steel.com). However, the materials and products used in building design and construction in the United States are almost exclusively des- ignated by ASTM specifications (Carter, 2004). ASTM specification names consist of a letter, generally an A for ferrous materials, followed by an arbitrary, serially assigned number. For example, ASTM A7 was a specification for structural steel written in 1900 and ASTM A992 was published in 1999 (Carter, 2004). The desig- nation or specification number does not contain any meaningful information other than to serve as a reference. Within ASTM specifications, the terms grade, type, and class are used in an inconsistent manner. In some ASTM steel specifications, the term grade identifies the yield strength, while in other specifications, the term grade can indicate requirements for both chemical compositions and mechanical proper- ties. ASTM and SAE have developed the Unified Numbering System, UNS (ASTM E527), based on chemical composition. This system uses a letter to identify the broad class of alloys, and a five-digit number to define specific alloys within the class. Several grades of structural steel are produced in the United States. Table 3.2 is a summary of selected information from various sources. The American Institute of Steel Construction, AISC, Manual for Steel Construction is an excellent reference on the types of steel used for structural applications. However, the best sources of information for structural steels are the various ASTM specifications. Of particular note is the fact that additional requirements are frequently included, dependent on the geometry of the product made with a particular steel. worksaccounts.com  Table 3.2 Designations, Properties, and Composition of ASTM Structural Steel Minimum Typical Chemical Composition3 (%) 1 1 Fy Fu Elonga- Steel Type ASTM Designation MPa MPa tion2 (%) C Cu5 Mn P S Ni Cr Si Mo V M03_MAML5440_04_GE_C03.indd 123 6 A36 250 400–550 23 0.26 0.2 0.8–1.2 0.04 0.05 A53 Gr. B 240 415 0.25 0.4 0.95 0.05 0.045 0.4 0.4 0.15 0.08 290 400 Gr. B 23 0.3 0.18 0.045 0.045 320 400 A500 Carbon 320 430 Gr. C 21 0.27 0.18 1.4 0.045 0.045 345 430 A501 250 400 23 0.3 0.18 0.045 0.045 Gr. 50 345 450–690 A529 19 0.27 0.2 1.35 0.04 0.05 Gr. 55 380 485–690 Gr. 42 290 415 24 0.21 - 1.35 0.04 0.05 0.15–0.4 Gr. 50 345 450 21 0.23 - 1.35 0.04 0.05 0.15–0.4 A572 Gr. 55 380 485 0.25 - 1.35 0.04 0.05 0.15–0.5 Gr. 60 415 520 18 0.26 - 1.35 0.04 0.05 0.4 High-strength Gr. 65 450 550 17 0.23 - 1.65 0.04 0.05 0.4 Low-alloy Gr. I&II 345 485 22 0.2 0.2 1.35 0.04 0.05 A618 Gr. III 320 460 22 0.23 - 1.35 0.04 0.05 0.3 50 345 450 21 0.12 0.45 1.6 0.04 0.03 0.25 0.25 0.4 0.07 0.06 A913 65 450 550 17 0.16 0.35 1.6 0.03 0.03 0.25 0.25 0.4 0.07 0.06 A9924 345–450 450 18 0.23 0.6 0.5–1.5 0.04 0.05 0.4 0.15 0.11 Corrosion A242 50 345 485 21 0.15 0.2 1 0.15 0.05 resistant, High-strength 0.25– 0.4– 0.02– A588 345 485 21 0.19 0.8–1.25 0.04 0.05 0.4 low-alloy 0.4 0.65 0.1 1 Minimum values unless range or other control noted 2 Two-inch gauge length 3 Maximum values unless range or other control noted 4 A maximium yield to tensile strength ratio of 0.85 and carbon equivalent formula are included as mandatory in ASTM A992 5 Several steel specifications can include a minimum copper content to provide weather resistance 6 Range for plate given in table, bar range 0.6–0.9 worksaccounts.com 5/19/17 5:15 PM 124 Chapter 3   Steel Historically, dating back to 1900, only two types of structural steel were used in the United States: A7 for bridges and A9 for buildings. The specifications for these materials were very similar and in 1938, they were combined into a single specification, A7. The specification for A7 and A9 were limited to requirements for the tensile strength and yield point only; there were no chemical specifications. The chemical composition, particularly carbon content, became an issue during the 1950s, as welding gained favor for making structural connections. By 1964, AISC adopted five grades of steel for structural applications. The 2005 AISC Load and Resistance Factor Design Specification for Structural Steel Buildings (AISC 2005) identifies 28 different ASTM steel designations for structural applications. 3.5.2 Sectional Shapes Figure 3.9 illustrates structural cross-sectional shapes commonly used in structural applications. These shapes are produced in different sizes and are designated with the letters W, HP, M, S, C, MC, and L. W shapes are doubly symmetric wide-flange shapes whose flanges are substantially parallel. HP shapes are also wide-flange shapes whose flanges and webs are of the same nominal thickness and whose depth and width are essentially the same. The S shapes are doubly symmetric shapes whose inside flange surfaces have approximately 16.67% slope. The M shapes are doubly symmetric shapes that cannot be classified as W, S, or HP shapes. C shapes are channels with inside flange surfaces having a slope of approximately 16.67%. MC shapes are channels that cannot be classified as C shapes. L shapes are angle (a) (b) (c) (d) (e) (f) (g) (h) F i g u r e 3. 9 Shapes commonly used in structural ­applications: (a) wide-flange (W, HP, and M shapes), (b) I-beam (S shape), (c) channel (C and MC shapes), (d) equal-legs angle (L shape), (e) unequal-legs angle (L shape), (f) tee, (g) sheet ­piling, and (h) rail. worksaccounts.com  Section 3.5  Structural Steel 125 shapes with either equal or unequal legs. In addition to these shapes, other structural sections are available, such as tee, sheet piling, and rail, as shown in Figure 3.9. The W, M, S, HP, C, and MC shapes are designated by a letter, followed by two numbers separated by an *. The letter indicates the shape, while the two numbers indicate the nominal depth and the weight per linear unit length. For e­xample, W 1100 * 548 means W shape with a nominal depth of 1100 mm and a weight of 548 kg/m. An angle shape is designated with the letter L, followed by three numbers that indicate the leg dimensions and thickness in millimeters, such as L 102 * 102 * 12.7. Dimensions of these structural shapes are controlled by ASTM A6/A6M. W shapes are commonly used as beams and columns, HP shapes are used as bearing piles, and S shapes are used as beams or girders. Composite sections can also be formed by welding different shapes to use in various structural applications. Sheet piling sections are connected to each other and are used as retaining walls. Tables 3.3 and 3.4 summarize the applicable ASTM specifications/designations for structural steels, and plates and bars, respectively (Carter, 2004). These tables are guides only; specific information should be sought from the applicable specifica- tions for each material and application. In particular, the dimension and application of a member can affect some finer points about material selection, which are not cov- ered in these tables. In general, the materials identified as “preferred” in the tables are generally available in the market place. Those identified as “other applicable materials” may or may not be readily available. 3.5.3 Specialty Steels in Structural Applications As the ability to refine steels improves, it is possible to produce special products with sufficient economy to permit their use in construction projects. The Federal Highway Administration, US Navy, and AISI have taken a leading role in the development and application of high-performance steels. These are defined as materials that possess the optimum combination of properties required to build cost-effective structures that will be safe and durable throughout their service life (Lane et al., 1998). One of the products developed through this effort is high-performance steels, HPS. Cur- rently, two products are available: HPS 50W and HPS 70W. These are weathering steels that form a corrosion barrier on the surface of the steel when first exposed to the environment. This surface resists further corrosion, and hence reduces the need for maintenance. HPS 70W has stronger tensile properties than steel traditionally used for bridge construction, and hence bridges can be designed with a reduced quan- tity of material. These savings are somewhat offset by the cost of the material, but there is still a net reduction in construction costs. The tensile requirements of HPS 70W are yield strength 480 MPa, tensile strength of 580 to 750 MPa, and an elon- gation of 19% (50 mm gauge length). In addition, HPS must pass impact tests. HPS 70W is m­ anufactured to tight and extensive alloy content requirements, as shown in Table 3.5 (ISG Plate, 2003). Comparing the chemical requirements in Table 3.5 with those in Table 3.2 dem- onstrates that HPS 70W has more extensive chemical requirements, lower carbon content, and tighter controls on phosphorus and sulfur, which are detrimental alloy elements. The lower carbon content improves the weldability of the steel. worksaccounts.com  126 Chapter 3   Steel Table 3.3 Applicable ASTM Structural Shapes Applicable Structural Shapes HSS Pipe Steel Type ASTM Designation W M S HP C MC L Round Rect A36 A53 Gr. B Gr. B-42 Gr. B-46 A500 Carbon Gr. C-46 Gr. C-50 A501 Gr. 50 A529 Gr. 55 Gr. 42 Gr. 50 A572 Gr. 55 Gr. 60 High-strength Gr. 65 Low-alloy Gr. I&II A618 Gr. III 50 A913 65 A992 Corrosion Resist- A242 50 ant, High-strength A588 Low-alloy A847 = Preferred material specification = Other applicable material specification, the availability of which should be confirmed prior to specification = Material specification does not apply HSS = hollow structural shape worksaccounts.com M03_MAML5440_04_GE_C03.indd 126 5/19/17 5:15 PM  Table 3.4 Applicable ASTM Specifications for Plates and Bars Thickness Ranges (mm) ASTM Fy (MPa) M03_MAML5440_04_GE_C03.indd 127 Steel Types Designations or Grade 19 19 –32 32–38 38–51 51–64 64–102 102–127 127–152 152–203 >203 Carbon 220 A36 250 Gr. 50 b b b b A529 Gr. 55 b b High-strength Gr. 42 Low-alloy Gr. 50 A572 Gr. 55 Gr. 60 Gr. 65 Corrosion Resist- 290 ant, High-strength A242 320 Low-alloy 345 290 A588 320 345 Quenched and 620 tempered alloy A514a 690 Quenched 485 and tempered A852 low-alloy = Preferred material specification = Other applicable material specification, the availability of which should be confirmed prior to specification = Material specification does not apply a = Available as plates only b = Applicable to bars only above 1 in. thickness worksaccounts.com 5/19/17 5:15 PM 128 Chapter 3   Steel Table 3.5 Chemical Requirements of HPS 70W Element Composition (% by Weight) Carbon 0.11 max Manganese 1.10–1.35 Phosphorus 0.020 max Sulfur* 0.006 max Silicon 0.30–0.50 Copper 0.25–0.40 Nickel 0.25–0.40 Chromium 0.45–0.70 Molybdenum 0.02–0.08 Vanadium 0.04–0.08 Aluminum 0.01–0.04 Nitrogen 0.015 max * All HPS 70 must be calcium treated for sulfide shape control The industry has recognized the advantages of designing with the high-perfor- mance steels. The first HPS bridge went into service in 1997. As of 2011, more than 250 bridges with HPS components have been constructed and 150 more are in the design or construction stage (fhwa.dot.gov/hfl/innovations/hps.cfm). The desire to improve the appearance and durability of steel structures has pro- duced an interest in designing structural members with stainless steel. The dura- bility of stainless steel has long been recognized, but the cost of the material was prohibitive. The ability of stainless steel to resist corrosion rests in the high chro- mium content. Whereas common structural steels have 0.3 to 0.4% chromium, stain- less steel has in excess of 10% chromium, by definition. Five AISI grades of stainless steel are used for structural applications (SSINA, 1999): 304: the most readily available stainless steel, containing 18% chromium and 8% nickel. Excellent corrosion resistance and formability. 316: similar to 304, but with the addition of 3–4% molybdenum for greater ­corrosion resistance. Generally specified for highly corrosive environments such as industrial, chemical, and seacoast atmospheres. 409: a straight chrome alloy, 11 to 12% chromium. Primarily used for interior applications. 410–3: a dual phase alloy with micro alloy element control that permits ­welding in up to 32 mm. 2205: a duplex structure with about equal parts of austenite and ferrite. ­Excellent corrosion resistance and about twice the yield strength of conven- tional grades. The chemical and tensile properties of these grades are summarized in Table 3.6. worksaccounts.com M03_MAML5440_04_GE_C03.indd 129 Table 3.6 Properties of Stainless Steels Used for Structural Applications 1 Components (Typically Maximum Percent by Weight) AISI Fy 1 Fu Percent Type (MPa) (MPa) Elong2 C Cr Mn Mo N Ni P S Si Ti 304 215 505 70 0.08 18–20 2 8–10.5 0.045 0.03 1 316 205 515 40 0.08 16–18 2 0.045 0.03 0.75 409 240 450 25 0.08 11.13 1 0.045 0.045 1 0.75 410 1230 1525 45 0.15 12.5 1 0.04 0.03 2205 515 760 35 0.02 22.4 0.7 3.3 0.16 5.8 0.25 0.001 0.4 1 Minimum values 2 Percent elongation is the percentage of plastic strain at fracture (2 ″ gauge length), minimum worksaccounts.com 5/19/17 5:15 PM 130 Chapter 3   Steel 3.6 Cold-Formed Steel Cold-formed steel is used for structural framing of floors, walls, and roofs as well as interior partitions and exterior curtain wall applications. The thickness of cold- formed steel framing members ranges from 0.455 mm to 3.000 mm. Cold-formed steel was formerly known as “light gauge” steel; however, the reference nomencla- ture “gauge” became obsolete with the ­adoption of a Universal Designator System for all generic cold-formed steel framing members in 2000. Cold-formed steel used for steel framing members is predominately manufac- tured from scrap steel using either electric arc or basic oxygen furnaces to cast slabs. The slabs are passed through a machine with a series of rollers that reduce the slab to sheets of the desired thicknesses, strengths, and other physical properties. The sheets are treated for corrosion resistance, usually hot-dipped galvanizing, and then rolled into coils that weigh approximately 900 kg. The primary method of manufacturing steel framing members is roll forming. At the roll former, the coils are slit into the required width and fed through a series of dies, to form the stud, joist, angle, or other cold-formed member, as shown in Figure 3.10. Steel framing members may be manufactured with holes in the member webs to facilitate utility runs during construction. The fabrication of all cold-formed steel construction materials is governed by industry standards including the Ameri- can Iron and Steel Institute’s Specification for the Design of Cold-Formed Steel Framing Members (NASPEC) and ASTM. 3.6.1 Cold-Formed Steel Grades Structural and non structural cold-formed steel members are manufactured from sheet steel in compliance with ASTM A1003/A1003M but limited to the material types and grades listed in Table 3.7. While multiple grades are acceptable for the Figure 3.10 Roll-forming of cold-formed steel framing member showing the stages of rolling. worksaccounts.com  Section 3.6  Cold-Formed Steel 131 Table 3.7 Structural Grades of Steel Used in Cold-Formed Products Type Structural Grade1 Designation US Designation Metric 230 ST33H ST230H H – high ductility 340 ST50H ST340H 230 ST33L ST230L L – low ductility 340 ST50L ST340L NS – nonstructural 230 NS33 NS230 1 Yield strength, MPa different steel types, the North American Standard for Cold-Formed Steel Framing recognizes two yield strengths 228 and 345 MPa (AISI S201-07). The large deformations caused by the cold-forming process results in local strain-hardening at the corners. The plastic deformation of the steel results in strain- hardening at the bends that increases the yield strength, tensile (ultimate) strength, and hardness but reduces ductility. Strain hardening can almost double the yield strength and increase the tensile strength by 40% (Karren and Winter, 1967). 3.6.2 Cold-Formed Steel Shapes A wide variety of shapes can be produced by cold-forming and manufacturers have developed a wide range of products to meet specific applications. Figure 3.11 shows the common shapes of typical cold-formed steel framing members. Figure 3.12 shows common shapes for profiled sheets and trays used for roofing and wall cladding and for load bearing deck panels. For common applications, such as structural studs, industry organizations, such as the Steel Framing Alliance (SFA) and the Steel Stud Manufacturers Association (SSMA) have developed standard shapes and nomenclature to promote uniformity of product availability across the industry. Figure 3.11 shows the generic shapes covered by the Universal Designator System. Note that the designator system is set up using inches. It is not reasonable to convert the designator system to a metric equivalent. The designator consists of four sequential codes. The first code is a three- or four-digit number indicating the member web depth in 1/100 in. The second is a single letter indicating the type of member, as follows: S = Stud or joist framing member with stiffening lips T = Track section U = Cold@rolled channel F = Furring channels L = Ang le or L-header worksaccounts.com  132 Chapter 3   Steel Flange Width Web Depth Flange Width Lip Depth Track (T) Web Depth Inside Bend Radius Flange Width Web Depth Stud or Joist (S) U-Channel (U) Flange Width "B" Flange Width Web Depth "A" Flange Width 12.7 mm Furring Channel (F) Angle (L) Figure 3.11 Generic cold-formed steel framing shapes. Figure 3.12 Common panel and deck shapes (TPU, 2009). The third is a three-digit numeral indication flange width in 1/100 in. followed by a dash. The fourth is a two- or three-digit numeral indicating the base steel thickness in 1/1000 in. (mils). As an example, the designator system for a 6 ″, C-shape with 1@5/ 8 ″ (1.62 ″) flanges and made with 0.054 ″ thick steel is 600S162-54. worksaccounts.com M03_MAML5440_04_GE_C03.indd 132 5/19/17 5:15 PM  Section 3.7  Fastening Products 133 3.6.3 Special Design Considerations for Cold-Formed Steel Structural design of cold-formed members is in many respects more challenging than the design of hot rolled, relatively thick, structural members. A primary differ- ence is cold-formed members are more susceptible to buckling due to their limited thickness. The fact that the yield strength of the steel is increased in the cold-form- ing process creates a dilemma for the designer. Ignoring the increased strength is conservative, but results in larger members, hence more costly, than is needed if the increased yield strength is considered. Corrosion creates a greater percent loss of cross section than is the case for thick members. All cold-formed steel members are coated to protect steel from corrosion during the storage and transportation phases of construction as well as for the life of the product. Because of its effectiveness, hot-dipped zinc galvanizing is most com- monly used. Structural and nonstructural framing members are required to have a minimum metallic coating that complies with ASTM A1003/A1003M, as follows: structural members – G60 and nonstructural members G40 or equivalent minimum. To prevent galvanic corrosion, special care is needed to isolate the cold-formed members from dissimilar metals, such as copper. The design, manufacture, and use of cold-formed steel framing is governed by standards that are developed and main- tained by the American Iron and Steel Institute along with organizations such as ASTM, and referenced in the building codes. Additional information is available at www.steelframing.org. 3.7 Fastening Products Fastening products include (Carter, 2004) Conventional bolts Twist-off-type tension control bolt assemblies Nuts Washers Compressible-washer-type direct tension indicators Anchor rods Threaded rods Forged steel structural hardware Table 3.8 summarizes the applicable ASTM specifications for each type of fas- tener (Carter, 2004). High-strength bolts have a tensile strength in excess of 690 MPa. Common bolts have a tensile strength of 410 MPa. The preferred material for anchor rods, F1554 Grade 36, has a yield stress of 250 MPa and an ultimate strength in the range of 400 to 550 MPa. A36, with a yield stress of 250 MPa, is preferred for threaded rods. Nuts, washers, and direct tension indicators are made with materials that do not have a minimum required strength. worksaccounts.com  Table 3.8 Applicable ASTM Specifications for Structural Fasteners Anchor Rods Threaded Rods Common Bolts Direct Tension High Strength Threaded & Indicators Fy Fu Washers Hooked Headed Yield ­Tensile Diameter Nutted Bolts Stressa Nuts ASTM Stress Range Designation (MPa) (MPa) (mm) - 725 725.4-38.1 A325 - 830 12.7–38.1 A490 - 1035 12.7–38.1 - 725 28.6 F1852 - 830 12.7–25.4 A194 Gr. 2H - - 6.4–101.6 A563 - - 6.4–101.6 F436b - - 12.7–38.1 F959 - - 6254 A36 250 400–550 7101.6-177.8 - 690 763.5 - 101.6 A193 Gr. B7 - 795 …63.5 - 860 6.4-101.6 Gr. A - 415 6.4–101.6 A307 Gr. C - 400–550 63.5–101.6 - 965 6.4–63.5 A354 Gr. BD 1034 38.1–76.2 - 620 38.6–38.1 c A449 725 6.4–25.4 c 830 6152.4 c Gr. 42 290 415 6101.6 Gr. 50 345 450 650.8 A572 Gr. 55 380 485 6 31.8 Gr. 60 415 520 631.8 Gr. 65 450 550 7127- 203 290 435 7101.6 - 127 A588 320 460 6101.6 345 485 15.9 - 76.2 A687 725 1034 max 6.4–101.6 Gr. 36 250 400–550 6.4–101.6 F1554 Gr. 55 380 520–655 6.4–76.2 Gr. 105 725 860–1034 = Preferred material specification = Other applicable material specification, availability should be confirmed before specifying = Material specification does not apply - Indicates that a value is not specified in the material specification a Minimum values unless range or max. is indicated b Special washer requirements apply for some steel to steel bolting, check design documents c LRFD has limitations on use of ASTM A449 bolts worksaccounts.com M03_MAML5440_04_GE_C03.indd 134 5/19/17 5:15 PM  Section 3.8  Reinforcing Steel 135 Structural connections are made by riveting, bolting, or welding. Rivet con- nections were used extensively in the past, but modern bolt technology has made riveting obsolete. Bolted connections may be snug tightened, pretensioned, or slip critical (Miller, 2001). Snug-tightened joints are accomplished by either a few impacts of an impact wrench or the full effort of an ironworker using an ordinary spud wrench to bring the members into firm contact. Pretensioned joints require tightening the bolt to a significant tensile stress with a corresponding compressive stress in the attached members. Four methods are used to ensure that the bolt is tightened to a sufficient stress level: turn-of-nut, calibrated wrench, twist-off-type tension-control bolts, and direct tension indicators. Bolts in slip-critical joints are also installed to pretensioned requirements, but these joints have “faying surfaces that have been prepared to provide a calculable resistance against slip.” When the joint is placed under load, the stresses may be transmitted through the joint by the friction between the members. However, if slip occurs, the bolts will be placed in shear, in addition to the tension stresses from the installation—hence the need for high-strength bolts. 3.8 Reinforcing Steel Since concrete has negligible tensile strength, structural concrete members sub- jected to tensile and flexural stresses must be reinforced. Either conventional or ­prestressed reinforcing can be used, depending on the design situation. In conven- tional reinforcing, the stresses fluctuate with loads on the structure. This does not place any special requirements on the steel. On the other hand, in prestressed rein- forcement, the steel is under continuous tension. Any stress relaxation will reduce the effectiveness of the reinforcement. Hence, special steels are required for pre- stress applications. 3.8.1 Conventional Reinforcing Reinforcing steel (rebar) is manufactured in three forms: plain bars, deformed bars, and plain and deformed wire fabrics. Plain bars are round, without surface defor- mations. Plain bars provide only limited bond with the concrete and therefore are not typically used in sections subjected to tension or bending. Deformed bars have protrusions (deformations) at the surface, as shown in Figure 3.13; thus, they ensure a good bond between the bar and the concrete. The deformed surface of the bar pre- vents slipping, allowing the concrete and steel to work as one unit. Wire fabrics are flat sheets in which wires pass each other at right angles, and one set of elements is parallel to the fabric axis. Plain wire fabrics develop the anchorage in concrete at the welded intersections, while deformed wire fabrics develop anchorage through deformations and at the welded intersections. Deformed bars are used in concrete beams, slabs, columns, walls, footings, pave- ments, and other concrete structures, as well as in masonry construction. Welded wire fabrics are used in some concrete slabs and pavements, mostly to resist temperature worksaccounts.com 136 Chapter 3   Steel Figure 3.13 Steel rebars used to reinforce PCC columns. and shrinkage stresses. Welded wire fabrics can be more economical to place and thus allow for closer spacing of bars than is practical with individual bars. Reinforcing steel is produced in the standard sizes shown in Table 3.9. Bars are made of four types of steel: A615/A615M (billet), A996/A996M, and A706/A706M (low-alloy), as shown in Table 3.10. Billet steel is the most widely used. A706 steel is often used when the rebar must be welded to structural steel. Reinforcing steel is produced in four grades. Note the conversion from the US grades to metric trades is a “soft” conversion as the conversion of the yield stress requirements is approximations. To permit ready identification of the different bar types in the field, marking symbols are rolled into the bars as they are being produced. As shown in Figure 3.14, there are four marking symbols: 1. Letter code for manufacturer 2. Numerical code for bar size, this code may be in either millimeters or “standard bar numbers,” which indicates the number of eighths of an inch of the nominal diameter of the bar 3. Letter code for type of steel (bars marked with both S and W have steel that meets all the requirements of types S and W steel) a. S for billet steel–A615/A615M b. I Rail steel–A996/A996M c. A Axle steel–A996/A996M d. W Low alloy steel–A706/A706M worksaccounts.com M03_MAML5440_04_GE_C03.indd 137 Table 3.9 Standard-Size Reinforcing Bars According to ASTM A615* Nominal Dimensions*** Deformation Requirements (mm)**** Bar Nominal Maximum Minimum ­Designation Mass Diameter Cross-Sectional Perimeter Average ­Average Maximum Number** (kg/m) (mm) Area (mm2) (mm) Spacing Height Gap***** 10 0.560 9.5 71 29.9 6.7 0.38 3.6 13 0.994 12.7 129 39.9 8.9 0.51 4.9 16 1.552 15.9 199 49.9 11.1 0.71 6.1 19 2.235 19.1 284 59.8 13.3 0.97 7.3 22 3.042 22.2 387 69.8 15.5 1.12 8.5 25 3.973 25.4 510 79.8 17.8 1.27 9.7 29 5.059 28.7 645 90.0 20.1 1.42 10.9 32 6.404 32.3 819 101.3 22.6 1.63 12.4 36 7.907 35.8 1006 112.5 25.1 1.80 13.7 43 11.38 43.0 1452 135.1 30.1 2.16 16.5 57 20.24 57.3 2581 180.1 40.1 2.59 21.9 * Copyright ASTM, Printed with Permission. ** Bar numbers approximate the number of millimeters of the nominal diameter of the bars. *** The nominal dimensions of a deformed bar are equivalent to those of a plain round bar having the same weight per meter as the deformed bar. **** Requirements for protrusions on the surface of the bar. ***** Chord 12.5% of Nominal Perimeter worksaccounts.com 5/19/17 5:15 PM  138 Chapter 3   Steel T a b l e 3. 1 0 Types and Properties of Reinforcing Bars According to ASTM (Somayaji, 2001) (© Pearson Education, Inc. Used by permission.) Tensile Yield Size ASTM* Strength Strength** ­Availability Steel Type Grade Min., MPa Min., MPa (US No.) Metric A615 Billet steel bars 280 483 276 3–6 (plain and 420 620 414 3–18 deformed) 520 689 517 11–18 A616 Rail steel (plain and deformed) 420 620 474 3–11 A617 Axle steel 280 483 276 3–11 (plain and 420 620 414 3–11 deformed) A706 Low-alloy steel 420 552 414–538 3–18  (deformed Bars) * ASTM types are per the cited reference. These types have been replaced: (1) A615 is now A615/A615M, A616 and A617 are now A996/A996M, and A706 is now A706/A706M. ** When the steel does not have a well-defined yield point, yield strength is the stress corresponding to a strain of 0.005 m/m (0.5% extension) for grades 40, 50, and 60, and a strain of 0.0035 m/m (0.35% extension) for grade 75 of A615, A616, and A617 steels. For A706 steel, grade point is determined at a strain of 0.0035 m/m. MAIN RIBS MAIN RIBS LETTER or SYMBOL LETTER or SYMBOL H for PRODUCING MILL H for PRODUCING MILL H BAR SIZE BAR SIZE 19 19 19 TYPE of STEEL TYPE of STEEL S S S GRADE GRADE (Grade Line or Number) (No Grade Marking Required) 4 GRADE 40 (300) GRADE 60 (420) Figure 3.14 ASTM reinforcing bar identification codes. worksaccounts.com M03_MAML5440_04_GE_C03.indd 138 5/19/17 5:15 PM  Section 3.8  Reinforcing Steel 139 4. Grade of steel designated by either grade lines or numerical code (a grade line is smaller and is located between the two main ribs which are on opposite sides of all bars made in the United States) a. Grade 40 or 300—no designation b. Grade 60 or 420—one grade line between the main ribs or the number 4 c. Grade 75 or 520—two grade lines between the main ribs or the number 5 3.8.2 Steel for Prestressed Concrete Prestressed concrete requires special wires, strands, cables, and bars. Steel for pre- stressed concrete reinforcement must have high strength and low relaxation prop- erties. High-carbon steels and high-strength alloy steels are used for this purpose. Properties of prestressed concrete reinforcement are presented in ASTM specifica- tion A416/A416M and AASHTO specification M203. These specifications define the requirements for a seven-wire uncoated steel strand. The specifications allow two types of steel: stress-­relieved (normal-relaxation) and low relaxation. Relaxation refers to the percent of stress reduction that occurs when a constant amount of strain is applied over an extended time period. Both stress-relieved and low-­relaxation steels can be specified as Grade 250 or Grade 270 , with ultimate strengths of 1725 MPa and 1860 MPa, respectively. The specifications for this appli- cation are based on mechanical properties only; the chemistry of wires is not per- tinent to this application. After stranding, low-relaxation strands are subjected to a continuous thermal–mechanical treatment to produce the required mechanical properties. Table 3.11 shows the required properties for seven-wire strand. Table 3.11 Required Properties for Seven-Wire Strand Stress-Relieved Low-Relaxation Property Grade 250 Grade 270 Grade 250 Grade 270 Breaking strength,* MPa 1725 1860 1725 1860 Yield strength (1% extension) 85% of breaking strength 90% of breaking strength Elongation (min. percent) 3.5 3.5 Relaxation** (max. percent) Load = 70, min. breaking — 2.5    strength Load = 80, min. breaking — 3.5    strength * Breaking strength is the maximum stress required to break one or more wires. ** Relaxation is the reduction in stress that occurs when a constant strain is applied over an extended time period. The specification is for a load duration of 1000 hours at a test temperature of 20 { 2°C. worksaccounts.com 140 Chapter 3   Steel 3.9 Mechanical Testing of Steel Many tests are available to evaluate the mechanical properties of steel. This section summarizes some laboratory tests commonly used to determine properties required in product specifications. Test specimens can take several shapes, such as bar, tube, wire, flat section, and notched bar, depending on the test purpose and the application. Certain methods of fabrication, such as bending, forming, and welding, or opera- tions involving heating, may affect the properties of the material being tested. There- fore, the product specifications cover the stage of manufacture at which mechanical testing is performed. The properties shown by testing before the material is fabricated may not necessarily be representative of the product after it has been completely fab- ricated. In addition, flaws in the specimen or improper machining or preparation of the test specimen will give erroneous results (ASTM A370). 3.9.1 Tension Test The tension test (ASTM E8/E8M) on steel is performed to determine the yield strength, yield point, ultimate (tensile) strength, elongation, and reduction of area. Typically, the test is performed at temperatures between 10°C and 35°C. The test specimen can be either full sized or machined into a shape, as pre- scribed in the product specifications for the material being tested. It is desirable to use a small cross-sectional area at the center portion of the specimen to ensure fracture within the gauge length. Several cross-sectional shapes are permitted, such as round and rectangular, as shown in Figure 3.15. Plate, sheet, round rod, wire, and tube specimens may be used. A 12.5 mm diameter round specimen is used in many cases. The gauge length over which the elongation is measured typically is four times the diameter for most round-rod specimens. Various types of gripping devices may be used to hold the specimen, depend- ing on its shape. In all cases, the axis of the test specimen should be placed at the center of the testing machine head to ensure axial tensile stresses within the gauge length without bending. An extensometer with a dial gauge (Figure 1.27) or an F i g u r e 3. 1 5 Tension test specimens with round and ­rectangular cross sections. worksaccounts.com  Section 3.9  Mechanical Testing of Steel 141 LVDT (Figure 1.31) is used to measure the deformation of the entire gauge length. The test is performed by applying an axial load to the specimen at a specified rate. ­Figure 3.16 shows a tensile test being performed on a round steel specimen using an LVDT extensometer to measure the deformation. As discussed in Chapter 1, mild steel has a unique stress–strain relationship (Figure 3.17). Here, a linear elastic response is displayed up to the proportion limit. As the stress is increased beyond the proportion limit, the steel will yield, at which time the strain will