Iron and Steel in Construction

Steel Properties and Classification

• Each type of steel is designated by a group of numbers, where:
  • The first digit indicates the class of steel (e.g., carbon steel is No. 1, nickel steel is No. 2)
  • The next one or two digits indicate the approximate percentage of the major alloying element for alloy steels
  • The last two or three digits indicate the carbon content in hundredths of a percent
• American Iron and Steel Institute (AISI) has adopted the SAE system with some variations, adding letter prefixes to designate the steel-making process used and other letters to designate special conditions

Steel Production and Processing

• Ingots are cooled to a uniform temperature throughout in a soaking pit, which is a furnace where the steel temperature is allowed to decrease to about 2300°F (1260°C)
• Mechanical operations that change an ingot of steel into a useful shape include:
  • Rolling: compressing and shaping an ingot into a useful shape
  • Extrusion: forcing a billet of hot, plastic steel through a die of the desired shape
  • Drawing: pulling steel through a small die
  • Forging: deforming steel by pressure or blows into a desired shape
  • Normalizing: heating the steel to a temperature of about 1500°F (815°C) or higher
  • Annealing: heating the steel to a temperature slightly lower than for normalizing and cooling it several hundred degrees very slowly
  • Quenching: cooling steel very rapidly in oil, water, or brine from a temperature of about 1500°F (815°C)
  • Tempering: reheating the quenched steel to a temperature of 300 to 1200°F (150 to 650°C) and cooling in air to reduce the residual stresses and increase ductility

Steel Tensile Test

• Mechanical tests for steel include tension, bending, hardness, and impact
• For structural steel, the tension or tensile test is the most important
• Typical Tensile Test Specimen is a 0.500-in.-diameter cylinder machined to a smooth, accurate circular cross section
• Tensile stress is calculated by dividing the force by the original cross-sectional area
• Strain is calculated by dividing the increase in distance between the marks by the original 2-in distance

Steel Designations and Welding

• All designations begin with the letter E, followed by a four- or five-digit number
• The first two or three digits indicate the minimum tensile strength in Kips per square inch
• The next digit indicates the recommended welding positions (1: all positions, 2: flat and horizontal, 3: flat only)
• The next digit indicates current supply and recommended welding techniques
• Welding methods include:
  • Shielded Metal-Arc Method (manual method suited for field use)
  • Submerged-Arc Method (powdered flux is automatically spread ahead of the electrode)
  • Gas Metal-Arc Welding (a coil of electrode wire is constantly fed to a holder as the electrode melts)
  • Flux-Cored Arc Welding (used to facilitate continuous feeding of the electrode as welding takes place)
• Weld types include:
  • Fillet Weld (most frequently used type, triangular in cross section, placed at a right-angle joint)
  • Butt or Groove Weld (ends to be connected are butted together and welded, flat edges, Single-V, and Double-V are most common grooves)

Iron and Steel

• Iron in its various forms, including steel, is by far the most important of the metals used in the construction industry
• Ferrous metals include all forms of iron and steel
• Iron and steel appear to be smooth and uniform, yet they consist of particles called grains or crystals that can be distinguished under a microscope
• Grains are formed as the metal passes from the liquid to the solid state
• Constitution is an internal crystalline structure that determines to a great extent what mechanical properties the metal will have

Structural Steel

• Structural Steel encompasses rolled shapes, plates, connectors, and bracing used in structural frames
• It includes most steel directly attached to the frame, excluding certain items like grating, metal deck, and ornamental metal
• Yield Stress is the basis for designing steel structures, typically ranging from 24 to 100 Ksi
• Types of structural steels include:
  • Carbon Steel (contains a maximum of 0.29% carbon and is widely used for buildings and bridges)
  • High-Strength, Low-Alloy (HSLA) Steel (contains small amounts of alloying elements to enhance mechanical properties and resistance to rust)
  • Corrosion-Resistant, High-Strength, Low-Alloy (CR-HSLA) Steel (offers enhanced corrosion resistance and forms a protective coating over time when exposed to the atmosphere)
  • Quenched and Tempered Alloy Steel (available only as plates, with high yield stresses and specific heat treatment requirements)