Summary
Highlights
Steel is the third most used construction material after concrete and asphalt. Its use dates back to 1500 BC. Large-scale production started in the 18th century with the development of the blast furnace, followed by the basic oxygen furnace and continuous casting method. Nowadays, computer-controlled manufacturing further enhances efficiency and reduces costs.
Unlike concrete and asphalt, engineers have limited direct influence over steel properties, which are primarily dictated by manufacturers. However, understanding production is crucial. Steel is used in various forms in construction, including structural steels (plates, bars, pipes, shapes), cold-formed steel (studs, trusses, roofing), fasteners (bolts, nuts, washers), and reinforcing steel (rebars).
Steel offers several advantages: high strength and stiffness (lighter structures compared to concrete), uniformity and permanence (properties don't change over time; indefinite lifespan with good maintenance), ductility and toughness (withstands large inelastic deformation, crucial for seismic regions), and sustainability (93.3% recycled content in US production, 98% recyclable at end of life without property loss).
Steel production involves three main steps. Step 1: Reduction of iron ore to pig iron using a blast furnace, combining iron ore, coke (to provide carbon), and limestone (to remove impurities). Step 2: Refining pig iron to steel by removing excess carbon and other impurities. This is done using either a basic oxygen furnace (reacting carbon with oxygen) or an electric arc furnace. This step can also process scrap steel. Step 3: Casting and forming the steel into desired shapes, either into ingots (less energy-efficient due to reheating) or directly into continuous casting for desired shapes (more energy-efficient).
The percentage of carbon significantly impacts steel's behavior. High carbon content (above 2% for cast iron, 0.8-2% for high carbon steel) leads to brittle steel. Structural steel, with 0.15-0.27% carbon, is ductile. Despite varying carbon content, the modulus of elasticity remains constant across all steel types. Alloying agents are added to steel to alter its properties, such as hardness, corrosion resistance, machinability, ductility, and strength.
Cold-formed steel is produced from hot-rolled steel sheets without additional heat, using machines to change its shape. This process results in thinner, lighter, and easier-to-produce products compared to hot-rolled counterparts, offering cost advantages. It's used for structural framing, walls, roofs, and partitions. The forming process causes plastic deformation and strain hardening, increasing strength but reducing ductility. Buckling and corrosion are critical considerations for cold-formed steel.
Hot-rolled structural shapes, plates, and bars are graded for use in columns, beams, and bridge girders. These grades specify mechanical properties like yield strength (Fy), ultimate tensile strength (Fu), and percentage elongation, as well as chemical composition (e.g., carbon content). Grades like A36 are common. Engineers need to understand these specifications when selecting steel.
Steel is used for various structural fasteners such as bolts, nuts, washers, anchor rods, threaded rods, and historically, rivets (now rarely used due to safety and efficiency concerns). For reinforcing concrete, which is weak in tension, steel rebars are essential. Reinforcing steel comes in plain bars (smooth, poor bond for tension), deformed bars (protrusions for good bond, acts as one unit with concrete), and wire fabrics (flat sheets for resisting temperature and shrinkage stresses in slabs and pavements).
Deformed rebars are available in various bar sizes (e.g., #3, #4, #5 up to #18), each with specific diameters and cross-sectional areas. There are special rules for calculating diameter and area for certain bar numbers. Steel is also crucial in prestressed concrete. In this technique, high-strength steel cables or wires are tensioned to introduce compressive stresses into the concrete, counteracting tensile stresses from applied loads. This requires high-carbon steels and high-strength alloy steels for their high strength and low relaxation properties.
Several tests are performed on structural steel to ensure quality and understand its properties. The tension test (ASTM E8) determines yield strength, ultimate tensile strength, elongation, and reduction of area under controlled temperatures. The stress-strain diagram generated from this test shows the linear elastic response, yielding, and non-linear behavior up to failure. Increasing carbon content increases yield and ultimate strength but reduces ductility. The modulus of elasticity remains constant. The torsion test (ASTM E143) determines the shear modulus, important for members subjected to twisting. Bend testing assesses the steel's ability to resist cracking during bending, crucial for rebars and other bent shapes often used on-site.