Structural steel refers to steel construction material formed with a given shape or cross-section as well as some standards of mechanical properties and chemical composition. The shape, size, composition, storage, strength etc. of structural steel is usually regulated in many industrialised nations. Structural steel material is used to construct many types of structures, such as bridges and buildings, across the globe. During manufacture, structural steel is fabricated to meet the applicable specifications of a project, such as strength, shape and chemical composition. The structural steel mainly consists of iron and carbon. Apart from carbon and iron, other additives include manganese, alloys and some chemicals which enhance its durability and strength. This article highlights the properties of structural steel as well as its benefits.
PROPERTIES OF STRUCTURAL STEEL
The structural steel properties heavily influence how it’s used in different applications. The properties, such as tensile strength, elasticity and yield strength, are highly valued by engineers as structural steel is majorly used in construction. Alongside general properties, such as density, these properties help to determine steel’s quality. Dependable and durable constructions can be achieved through the use of quality steel. In other words, these properties directly determine the performance of any structural steel material. Here are the major properties of structural steel.
Density is the mass per unit volume of a given material. The density of steel is between 7.85 g/cubic cm to 8.1 g/cubic cm.
The austenising temperature for steel, the temperature at which steel turns into an austenite crystal structure, starts at 900 C for pure iron but when more carbon is added, it drops to a minimum temperature of 724 degrees for eutectic steel (eutectic steel has only .83 percent by weight of carbon). When 2.1 percent carbon approaches, the austenizing temperature shoots back to 1,130 degrees. On the same note, the steel’s melting point changes according to the alloy.
The lowest temperature where plain carbon steel could begin to melt is 1,130 degrees. Steel does not turn into liquid below the given temperature. Pure Iron (which is basically 'Steel' with 0 percent Carbon) begins to melt at 1,492 degrees and completely turns liquid when it reaches 1,539 degrees. Steel with 2.1 percent Carbon by weight starts melting at 1,130 degrees and completely melts to liquid upon reaching 1,315 degrees. If 'Steel' has more than 2.1 percent Carbon, it’s no longer known as Steel, but as Cast iron.
Tensility of steel determines up to what limit steel can be stretched without fracturing. The breaking point is used to determine the tensility of steel. The breaking point refers to a point where steel breaks when subjected to stress. The structural steel has greater tensility as compared to other construction materials and hence is often preferred.
Yield strength refers to the capacity of structural steel to resist deformity. Yield strength is determined by measuring the minimum force which can cause deformation. Atomic and crystalline structure of steel will change due to deformation.
Elasticity is often confused with yield strength but they are completely different. Deformation of steel often occurs when it is subjected to stress. The deformation point is where the elasticity of a given material is measured. This property is measured using the Young's modulus of elasticity.
Steel usually loses strength when extremely heated. The critical temperature of any steel member refers to the temperature at which it is unable to support its load. According to structural engineering standards and building codes, critical temperatures are defined depending on the type, orientation, configuration and loading characteristics of the structural element. The critical temperature of structural steel is the temperature at which the yield stress of the steel has been minimized to 60 percent of the yield stress at room temperature.
To establish a steel member’s fire resistance rating, accepted calculations can be used. In Japan, the fire resistance rating is below 400C. In Europe, North America and China, this rating is about 530-810C (1000–1300F). The time a steel element being tested takes to reach the test standard’s temperature determines the fire-resistance rating duration. Fireproofing materials can be used to slow the heat transfer to steel, thereby limiting steel temperature. There are many fireproofing techniques for structural steel such as; intumescent, plaster and endothermic coatings; drywall, calcium silicate covering; and high temperature mineral wool insulation.
STRUCTURAL STEEL SHAPES
The shapes of structural steel used vary according to the construction project requirements. Some of the common shapes used to strengthen the structure being constructed are I-beam, L-shape, Z-shape and T-shape. But among these shapes, the most commonly used type is the I-beam. Skyscrapers, stadiums and ships all have some kind of I-beam construction. I-beam has higher second moment of area, allowing it to be stiffer with respect its cross-sectional area. When designing the I-beam, various properties or specifications are considered. To make steel structure more stable, you should focus on preventing unnecessary vibrations. The stiffness of steel helps to reduce these vibrations. Therefore, ensure you use structural steel with higher stiffness and mass levels to considerably reduce these vibrations in the steel structure. This will also prevent any future deformity in the structure.
BENEFITS OF STRUCTURAL STEEL
Structural steel design assures the structural stability and integrity of any steel structure, such as building, bridge, tower, etc. Structural steel designs have evolved from just concentrating on the rigidity benefits of steel to a structure, to the structural flexibility benefits of steel, which allow a structure to offer better resistance to the natural and human stresses. This evolution of the structural steel designs was majorly inspired by earthquakes. Multi-story steel structures can withstand any major earthquake.
Advantages of using structural steel for construction are that it’s cost effective to manufacture, does not require a lot of maintenance and is much cheaper to insure as compared to other building methods. Provided steel is protected from rust, it will indefinitely maintain its strength.
Some examples of popular structures made with structural steel are the Seagram Building, the Brooklyn Bridge and the Eiffel Tower. The Seagram Building is known as one of the best engineered buildings across the world. The Eiffel Tower is in Paris, France while the Brooklyn Bridge connects Brooklyn to Manhattan.