News

Structural Steel

• satyendra
• June 28, 2013
• 1 Comment
• corrosion, Ductility, elements, properties, steel, Steel sections, Steel shapes, strength, toughness,

Structural Steel

Steel construction is increasing in popularity all over the world, with every region benefitting from steel throughout the years. Many of the best architectural wonders have been constructed through the use of structural steel. More importantly, the usage of structural steel is beneficial in terms of eco friendliness than other modes of construction, and due to this factor alone, it is given more preference.

Structural steel comes in various different grades and shapes (sections). It is a category of steel which is used as a construction material. It is well known for providing structure the strength unlike any other material of construction, when it comes to construction. The durability and potency which structural steel provides is not matched by the likes of wood or concrete. More frequently than not, it is the case that structural steel is the material of choice for construction. People prefer it due to its various advantages.

Structural steel is produced in a variety of shapes. Many structural steel shapes take the form of an elongated beam having a profile of a specific cross section. It is designed to have a good strength / weight ratio (which is also called specific strength) and to be cost-effective in order to be benefited as a structural component in buildings, roads, bridges, etc. It is an economical construction material normally used in various steel structure applications. Most structural steel shapes have high second moments of area, which means they are very stiff in respect to their cross-sectional area and thus can support a high load without excessive sagging.

Structural steel exhibits desirable physical properties such as strength, uniformity of properties, light weight, and ease of use etc. It has the advantage of permitting long clear spans for horizontal members and requiring less floor space for columns than other common construction materials. This makes it one of the most versatile structural materials in use. Structural steel can also be used in combination with reinforced concrete to provide cost-effective building components. For large industrial buildings, where the structural frame can be exposed, it is frequently the material of choice.

The production process of structural steel is at the discretion of the steel producer. It is simply an alloy of iron which has a very low amount of carbon. There are also other alloying elements sometimes. Due to its high tensile strength and low price, it forms is a significant component in the materials being utilized in infrastructure. It consists of specific grades of steel and formed in a range of industry-standard cross-sectional shapes or sections. It normally consists of hot-rolled steel shapes with a thickness of 3.2 mm and above. Structural steel grades are designed with specific chemical compositions and mechanical properties formulated for particular applications. Majority of the structural steel sections are hot rolled products, with a cross section of special form like angles, channels and beams / joints.

The normal yield strength grades of the structural steel available are 195, 235, 275, 355, 420, and 460, although some grades are more commonly used than others. Higher grades are available in quenched and tempered material (500, 550, 620, 690, 890 and 960, although grades above 690 receive little if any use in construction at present). The unit of the yield strength grade indicated value is MPa.

While many sections of structural steels are made by hot rolling or cold rolling, others are made by welding together flat or bent plates (for example, the largest circular hollow sections are made from flat plate bent into a circle and seam-welded. There are also some special profiles (e.g. Z profile, elliptical section etc.) which are rolled from structural steels. In addition to rolling, structural steel sections are also fabricated. Most of the structural steel sections are hot rolled, while some of the thinner sections are sometimes cold rolled.

Hot-finished hollow sections are structural hollow sections of uniform cross-section and a thickness greater than or equal to 2 mm, either hot formed with or without subsequent heat treatment, or cold-formed with subsequent heat treatment.

The properties of structural steel vary widely, depending on its alloying elements. Structural steel grades have specific chemical compositions and mechanical properties needed as per their application. These steels are produced as per the specifications included in different national or international standards which are issued for structural steels. The normal structural steel grade is an alloy of iron with low carbon content and manganese, in addition with small amounts of sulphur, phosphorus, silicon, and oxygen and is one of the most significant industrial material. This grade of structural steels is for use at ambient or moderately elevated temperatures. In the construction sector, structural sections made from mild steel have maximum application. The other types of steel used for structural application are high strength low alloy steel, weather resistant steel, stainless steel, quenched and tempered steel (heat treated high tensile strength steel), constructional alloy steels, and high performance steels.

The common shapes in which structural steels are available consist of sections (beams, channels, Tees section and angles), squares and rounds, rectangular and hexagons, plates, pipes, hollow square sections, Z sections, and cold formed sections etc. The shape, size, tolerances, dimensions, cross-sectional measurements, and mechanical property etc. of common structural steel sections are regulated by standards in most countries.

Typical shapes of standard structural steel are shown in Fig 1. These shapes are normally 4 meters to 12 meters long and 100 millimeters to 600 millimeters wide. The most popular one is a universal beam (either H-beam or I-beam) with an ‘H’ or ‘I’ shaped cross-section. While many sections are formed by hot rolling or cold rolling, others are made by welding together flat or bent plates.

Fig 1 Structural steel shapes

There is not just one type of structural steel. There exist various different shapes and grades, depending on the needs for that specific application. Structural steels are classified by the shape of their cross-sections. Besides their shape, the grade of the steel directly affects the mechanical properties. Hence, different grades of structural steel are chosen according to different design requirements.  Structural steels are normally rolled in various shapes which include IPN sections, IPE sections, HEB sections (base), HEA sections (light), HEM sections (heavy), standard channel section (UPN), commercial channel sections (U), equal leg angles, unequal leg angles, rounds, squares, rectangular section, hexagonal section, plate section, and hollow section.

The letters ‘W’, ‘HP’, ‘M’, ‘S’, ‘C’, ‘MC’, and ‘L’ are frequently used for designating the structural steel sections (Fig 2). ‘W’ denotes a doubly symmetric wide flange with parallel flanges. These sections are normally known as beam. ‘HP’ is like the shape ‘W’ but the flanges and webs have the same thickness and the depth and the width are the same. These sections are known as H beams and are normally used as bearing piles. ‘S’ is doubly symmetrical wide flange with an inside flange slope of 16.67 %. These sections are also called tapered flanged beams and are used as beams or girders. ‘M’ is a doubly symmetrical section which do not fit in ‘W’, ‘HP’ and ‘S’ sections. ‘C’ section is with inside flange slope of 16.67 %. ‘MC’ section is a channel section which cannot be classified as a ‘C’ section. ‘L’ section is an angle section with equal or unequal legs.

Fig 2 section designation

The weldability of structural steels depends on its carbon equivalent (CE) which is defined by the equation CE = C + Mn/6 + (Cr+Mo+V)/5 + (Ni+Cu)/15. Structural steels with carbon equivalent below 0.45 are readily weldable with appropriate procedures. CE of structural steels greater than 0.45 indicates caution is to be observed during welding of such steels.

Structural steel differs from concrete in its attributed compressive strength as well as in tensile strength. Since it has high strength, stiffness, toughness, and ductile properties, it is one of the most commonly used materials in commercial and industrial building construction. Further, structural steel can be developed into nearly any shape, which are either bolted or welded together in construction. It provides a number of different advantages in relation to other building materials like concrete and wood as given below.

• A relatively large region of plastic deformation exists in the stress-strain diagram. Practically this means that structural steel is sufficiently flexible to withstand loads beyond its yield point until final failure. Elasticity of structural steel follows Hooke’s law accurately
• It has high strength which means that the weight of the structure made of structural steel is less.
• It has good ductility due to which structural steel can withstand extensive deformation without failure under high tensile stresses.
• It has good toughness which means that structural steel has got both strength and ductility.
• It has got flexibility which allows extension of existing structures made of structural steels can be carried out easily.
• It is the most environmentally friendly material, since it is 100 % recyclable.
• It has uniform properties which do not change as opposed to concrete. Compared to concrete, it is already at its full load capacity and does not require time to ‘cure’.
• Compared to wood, it does not deform, neither distorts and it is in fact more resistant to temperature changes and normally to extreme environmental conditions.
• It has exceptional and essentially constant mechanical properties, as well as higher strength to weight ratio, which ensures the design of lighter constructions.

Structural steels are non combustible materials but they lose strength when heated sufficiently. When heated to temperatures normally associated with fires, the strength and stiffness of the structural steel are significantly reduced. Structural steels have a critical temperature. This critical temperature is the temperature at which the structural steel cannot safely support its load. Critical temperature is normally the temperature at which its yield stress has been reduced to 60 % of the room temperature yield stress.

Structural steel has susceptibility to buckling. As the length and slenderness of a compression member is increased, its danger of buckling increases. For avoiding this, some additional steel is needed to stiffen the structure so that it does not buckle. Another undesirable property of structural steel is that its strength can reduce if it is subjected to a large number of stress reversals or even to a large number of variations of tensile stress. Further, under certain conditions, structural steel can lose its ductility, and brittle fracture can take place at the places of stress concentration. Fatigue type loadings and very low temperatures aggravate the situation. Tri-axial stress conditions can also lead to brittle fracture.

Structural steels are susceptible to corrosion when freely exposed to air or water. However, structural steels can be coated or painted periodically to provide it corrosion and fire resistance. The use of weathering steel, however, in suitable applications tends to eliminate this cost. Though use of weathering steels can be quite effective in certain situations, there are some cases where their use is not feasible. For example, corrosion fatigue failures can occur, where steel members are subjected cyclic stresses and corrosive environments. The fatigue strength of structural steel can be appreciably reduced when the structural member is used in aggressive chemical environments and subjected to the cyclic loads.

Structural steels are used in several ways and their application can be diverse. They are particularly useful because they offer the unique combination of good welding properties with guaranteed strengths. Structural steel is an extremely adaptable product and is frequently favoured by engineers trying to maximize strength of the structure while minimizing its weight.

The structural steel all over the world pre-dominates the construction scenario. This material has been exhaustively used in various constructions because of its various specific characteristics which are very much ideally suited for construction. Structural steel is durable and can be well molded to give the desired shape to give an ultimate look to the structure.

The construction industry is the biggest consumer of structural steel, where it is used for a number of purposes. Whether a small box lintel is used to carry the load of a structural wall in a residential building or a vast I-beam is bolted in place to hold the road surface on a bridge, structural steel can be specified, designed, and fabricated for any type of job.

With structural steel, structures can be created like no other material when it comes to construction. No other material has the potency and durability which structural steel does. Structural steel has undoubtedly become the preferred choice for construction due to the various benefits it has. The buildings which are made from structural steel need different structural frames. Also, constructing with structural steel needs much less time which makes it the most sought after way of constructing tall steel structures.

Major applications for these steels are in high rise and tall multi-storey buildings, industrial buildings, towers, tunnels, bridges, road barriers and industrial structures etc. Structural steel buildings tend to be built using various frames too, for example clear span, modular and single slope. Due to the fact that structural steel is much easier and less time-consuming when it comes to building, along with its numerous other factors which have proved advantageous. Construction with structural steel, these days, has become the most sought after way of constructing buildings.