Structural Steel
Structural steels are the variety of steel customarily used for construction such as buildings, bridges and as reinforcements to concrete. It is used because of its strength and ductility (ability to deform substantially prior to failure). The form, shapes, and size of structural steels are dependent on the specific function that it will serve. There are several standards that must be followed when producing structural steels; these standards also include the specifications for the steel’s mechanical properties, chemical composition, specific cross section and shape profile (American Institute of Steel Construction np).
Structural steels have various chemical elements which give its strength, ductility, fracture toughness and other mechanical properties. It is important that the constituents are understood in order to be able to manufacture structural steel that complies with existing standards. The following are the major elements used in manufacturing structural steel:
Carbon is considered the most important element contained in steel; it gives lower ductility and higher strength. Thus, structural steels must have carbon content of 0.15%-0.30% to achieve required strength. However, although carbon is an important component in achieving strength, it must be limited so as to not substantially decrease the ability of the material to sustain significant deformations before failure.
Manganese is commonly combined with carbon and is contained at 0.50%-1.70%; it is a necessary element as it conglomerates with sulfur and oxygen during the steel’s hot rolling process. Besides being a key alloying (glue-like) element in the manufacturing process, manganese levels are also associated with higher ductility.
Vanadium is generally contained at 0.02%-0.15% and gives the structural steel an increase in the fracture toughness; its percentage differs and is specific for various ASTM grades and function. Fracture toughness refers to the amount of energy that is required to break an element that is already cracked, whereas strength refers to the stress required to break an un-cracked element.
Aluminum on the other hand acts as a deoxidizing material and it is a component which gives a “fine-grained crystalline microstructure” to the steel; combined with silicon, fully-killed or semi-killed steel may be produced. Fully-killed (oxygen fully removed) structural steels contain between 0.01% and 0.02% aluminum, while Semi-killed (oxygen partially removed) steels contain no more than 0.005% aluminum.
Chromium increases the corrosion resistance of structural steel and is mostly present in considerably small amounts; it is usually combined with copper and nickel to reinforce the corrosion resistance. For stainless steel, which is known as the “18-8”, it means that the components are 18% nickel and 8% chromium.
Copper also gives the same properties as that of chromium which is contained at 0.2%.
Nickel which also gives corrosion resistance also develops the fracture toughness of structural steel. This in turn gives the steel its improved behavior in low temperatures. The percentage of nickel added in making the structural steels also varies with the grade; an A588 grade structural steel contains 0.25%-1.25% while ASTM A514 has nickel ranging from 0.30%-1.5%
Columbium is an element used to increase the strength of structural steels and is most commonly used for HSLA steels.
Silicon is also used as a deoxidizer like aluminum and usually contained at 0.4%.
Sulfur and phosphorus on the other note are chemicals which are considered to be undesirable components in structural steel production. They are impurities that represent residues of the manufacturing process. Both are being strictly monitored to be only contained at not more than 0.04%-0.05% since they reduce the ductility and stimulates segregation in the internal matrix of steel. However, they do have positive effects on the machinability of steel (easier to cut), and corrosion resistance, which decreases the incentive to completely remove them.
Lastly, there are other elements present in trace amounts such as titanium, nitrogen and boron which complement the aforementioned elements to increase their effects on structural steel production.
The process of producing structural steels comes in two different ways as of today. One way is using an Electric Arc Furnaces or EAF steel mills which generally produce wide-flange sections, channels as well as angles, all of which are under the hot-rolled category of structural steels. The other one is through a Basic Oxygen Furnace or BOF wherein Hollow Steel Sections or HSS are produces with numerous rolls of steel sheets. Plate steels which are very popular are created or fashioned from EAF or BOF.
For the Basic Oxygen Furnace, it uses coke (processed coal) and iron ores to produce rolled steel sheets and steel plates. The iron ore is melted inside a furnace fire blasted with the coke and then is transported into a ladle. Chemical pretreatment is used to process the molten iron and is then subjected to the basic oxygen furnace through a steel scrap. Inside the BOF, the entire mixture with the addition of oxygen is then melted together. The molten mixture is then cooled and poured into another ladle for it to be fashioned in plates or sheets. The process being efficient and effective results in an average of 25% recycled materials.
On another note, the Electric Arc Furnace employs the use of scrap metal or steel as its starting material. Usually, scrap steels coming from industrial waste, automobiles and appliances are scavenged and melted using gigantic electric furnace. To achieve a good metallurgical balance, impurities such as silicon, sulfur and phosphorous are removed from the molten steel through the introduction of calcium oxide (CaO) and magnesium oxide (MgO). At high temperatures, these chemical additives, known as ‘slag formers’, help break the bonds between the impurities and the base metals, creating a cementitious (paste-like) residue called ‘slag’, which floats on top of the melt and can be mechanically removed. The molten steel is then poured into “beam blanks” or molds which are specific for the desired shape. The blanks are then cooled then subjected again to heat then are passed on through numerous rollers to ensure that the product is of great form and specified precise shape. The steel beams being sturdy and of great quality go through cutting machines and are again cooled and furnished to be shipped. The process of EAF is more efficient and effective compared to the BOF with an average of 90% reprocessed content.
Structural steels can be classified in several ways and one of which is mainly based on their chemical composition:
High-strength quenched and tempered alloy steels have yield stress of 90 to 100 ksi, where ksi is the United States customary unit for stress (kilo-pounds per square inch). It is made to undergo heat treatment to increase the strength. There are only few of these types such as A514 that are only available in plate form.
High-strength, low alloy steels is one of the most advanced types of steel. With the addition of other chemicals, the strength of this type of structural steel ranges from 42ksi-65ksi.
Carbon-manganese steels have both manganese and carbon as its chief component aside from iron. It is widely used due to its satisfactory ductility as well as strength. It is usually known to be mild structural or carbon steels. Carbon steels are less expensive compared to the other two and has a yield stress of more than or equal to 36ksi.
With all the types of steel available, why do designers prefer to use structural steel over the others? One reason is that structural steels can help make the construction speed faster. Structural steels can be fabricated in shops or plants with a good construction tolerance making the productivity in construction higher. In addition, with the use of structural steels, projects are less costly, than with reinforced concrete or fiber-reinforced polymers. The use of structural steel renders faster construction times than reinforced concrete, and lower material costs than fiber-reinforced polymers.
Moreover, structural steel is aesthetically appealing given its natural finish; its strength and slenderness catches the interest of minimalists. It is also very advantageous for architects since they can exercise their creativity and come up with sleek designs. Structural steel entitles both its designer and user functionality and style. There are structural steels that can be rolled and event bent so as to meet the non-symmetric designs the client wants.
Structural steel also has an outstanding strength that can accommodate 50,000 psi of compressive or tensional stress in every square inch which is a great leap compared to standard steel which can only accommodate 12000 to 15000 psi of compressive stress and 3000 to 5000 psi of tensional stress. Likewise, it is a strong but light material making the building more cost effective and less extensive, as required foundations are smaller (One Steel Market Mills np).
Furthermore, structural steels are sustainable being the most recyclable construction material all over the globe. Structural steels are comprised, surprisingly, of 88% recycled materials; hence, it is easier for it to be recycled as well when time comes its functionality declines. Moreover, it is somewhat ecofriendly ever since 1990 with a carbon footprint reduced to 47% already. Fortunately, the manufacturing of structural steel continues to be leading a cleaner environment with a decrease of 30% energy consumption in the past thirty years. The manufacturing also uses make-up water conserving our potable resources of water. The make-up water used is looped to recycle and use it for other batches (Cengage Learning np).
When the life-cycle of the structure comes to an end, all of the structural steel (100%) can be recycled. Therefore, it can be concluded that structural steel produces zero waste. The director of AISC also stated that “Rather than utilizing land for quarrying operations to provide aggregates or as landfills for construction material waste, structural steel is emptying salvage yards allowing that land to be used for other purposes.”
Another of the numerous advantages of employing structural steels is that it is greatly modifiable. This means that whenever there is a need for change in the design or requirements, your ordered structural steel could be modified in a way that minimizes material waste and cost, as compared to reinforced concrete.
Structural steel buildings can be modified to add more levels or change the architecture in a way that is not possible with reinforced concrete. As long as the engineering requirements are met for any proposed changes, the construction can be carried out efficiently. Another good thing about structural steel is that it can be used as a standalone construction material (unlike concrete, which needs reinforcement). The mechanical properties of structural steel are standardized by the American Institute for Steel Construction (AISC) and comply with the standards of the American Society for Testing and Materials (ASTM) for a construction material (MIT Department of Civil and Environmental Engineering np).
Finally, it must be noted that structural steel is readily available. In the United States, the production of structural steel is very high. This industry gained popularity in the 2000’s and had a breakthrough of 8 million tons production in 2007. Most of the manufactured structural steel in 2007 was in hot-rolled form. There were also 6 million tons of structural steels manufactured in the form of wide flange sections and a great number of 800,000 tons were manufactured in excess to accommodate the increasing demand. Recently, the structural steel industry is greatly expanding in different parts of the world, particularly China, from which the US has been increasing imports because of lower costs. Structural steel continues to be popular for its properties; functionality, cost-effectiveness, aesthetically-appeal and flexibility. Indeed, structural steel is one of the most desired materials for construction of houses, bridges and commercial buildings (Fletcher Easy Steel np).
Works cited:
American Institute of Steel Construction : “Structural Steel Solutions” Web. <https://www.aisc.org/content.aspx?id=3792>
Cengage Learning “Properties of Structural-Steel Shapes” Web. <http://www.cengage.com/resource_uploads/downloads/1111136025_277030.pdf>
Fletcher EasySteel “Structural Steel Properties and Design Chartbook” Web. <http://www.easysteel.co.nz/site_files/8545/upload_files/EasysteelStructuralPropertiesbook(1).pdf?dl=1>
MIT Dpartment of Civil and Environmental Engineering “Chemical Composition of Structural Steels” Web. <http://web.mit.edu/1.51/www/pdf/chemical.pdf>
OneSteel Market Mills “Hot Rolled and Structural Steel Products”. Web. <http://www.cim.mcgill.ca/~paul/hotrolled.pdf>
