Making of Modern Steel
Conventionally, steel was manufactured through integrated mill process. This method is an aggregate of intermediate processes that enable conversion of iron ore into steel I- beams and other steel products. Raw materials are introduced in to the furnace through the top in alternate layers enabling the raw materials to mix effectively. Stove positioned at the bottom, feeds the furnace with hot air hence igniting the coke, the molten iron and slag streams down from the melting iron ore and limestone. Molten iron is taped to the BOF where oxygen is blown through it converting the carbon present in iron to carbon dioxide. The resultant molten steel undergoes vigorous casting that yields billets, slabs and blooms that later are reheated to yield different steel cross sections.
As the world adopts environmentally friendly industrial methods, manufacturing of steel from recycled steel based scrap has become common. Integrated mill method is the most preferred and used method. This method, allows electric arc furnace to be charged by scrape metals through the top opening. Scrape metals are converted to molten steel by three graphite electrodes emanating high temperatures. Calcium oxide and magnesium oxide are introduced into the furnace proportionately, leading to formation of a foamy slag that insulates the furnace lining from the high arc temperatures. Transportation of the molten steel to a ladle necessitates modification of the steel chemistry through addition of required chemical components. After this stage, the molten steel undergoes a continuous casting that yields blooms, slabs and billets of desired cross sections. Electric arc and blast furnace generate a waste product called the slag.
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Design and construction experts group steel classification into three; classification based on steel strength, classification based on steel metallurgy, and classification based on steel’s application.
Cold formed steel are manufactured by bending thin steel sheets to different corrugated profiles at room temperatures. Its products is applied in roofing and flooring decks. Additionally, cold formed steel has been used in partitioning walls of non-combustible type buildings. Metallurgical difference between the steel used in building construction is alloy steel and the carbon steel. Increasing the quantities of carbon in steel proportionately corresponds to the strength of the steel but lowers its formability and ductility.
Structural Steel-Hot-Rolled Sections
Due to the financial constrains depicted during production of structural steel sections, construction experts have standardized their sizes and shapes, such as M-shapes, W-shapes, HP-shapes, and S-shapes. In W-shape, the interior surface of the flanges is parallel whereas for the S-shaped the interior flanges are inclined at a slope. W-shaped are majorly used in beams and columns because their flanges are wider than those of S-shape. C-Shapes steel channels are of similar profile to S-shaped channels, in that, their inner flange surfaces are inclined at an angle of 2:12. The number that follow C entails the overall depth of the section while the second number gives the weight of a 1-ft length of the section. T-Shapes steel channel is a result of splitting a W-shape, M shape, S shape proportionately. Pipes, l-shapes, bars, tubes, and plates. L shapes can present with either equal or unequal leg angles. A tube is referred to as a hollow structural section (HSS) and is made by bending a steel plate and welding it seamlessly.
Steel Joists and Joist Girders
Prefabricated steel members are commonly used for roofing and flooring purposes. The two common trusslike members are called the joist girders and joists. Joist girder is heavier in comparison to joists since it spans from the end of both columns.
Steel Roof and Floor Decks
There are two categories of steel decks; floor and roof decks. The significant contrast between the two is that a roof deck is generally topped with rigid insulation and a roofing membrane, while a floor deck is topped with structural concrete fill. The thickness of a sheet steel is depicted by its gauge number. Roofing decks are majorly grouped according to depths, and gauges. Roofing decks are presented in various paint finish as simple prime paint or hot dip galvanized. Priming of the roofing decks offers seasonal protection. They are anchored by puddle welds to the wide flange beams used for support. Steel floor decks offer a working platform that doubles as a permanent formwork for concrete fill. Two types of floor decks used include; a composite deck, and a form deck
Corrosion Protection of Steel
Unlike aluminium, steel does not form a protective oxide coating, hence must be protected against corrosion. Several protective coatings are available for steel to suit different environmental conditions, aesthetic requirements, and budgets. These include zinc coating, acrylics, polyurethanes, epoxies.
Protecting the Steel against Fire
Generally, the incombustible property of steel offers unreliable sense of security since steel has a poor fire endurance. Thin or medium sized unprotected steel cannot withstand long extremes of fire. There are basically two ways to protect steel against fire: either insulate the steel component with a non-combustible thermal insulation.
CHAPTER 19: STRUCTURAL STEEL CONSTRUCTION PRELIMINARY LAYOUT OF FRAMING MEMBERS
Early preparation of the preliminary framing layout of steel building embodies detailed structural considerations with no limits to non-structural considerations like building envelope, fire resistance, interior finishes, aesthetics and the cost. All the structural design should have different framing plan for both floor and roof. Structural framing of a building must consider the lateral loads and gravitational pull. Collaboration between the team players should yield a better load resistance to the structure.
Bolts and Welds
Bolts and welding are connection of choice for structural steel members. Unlike in the past, riveting is currently not applicable. Steel bolts used is grouped into two types namely unfinished and high strength bolts. Unfinished bolts that are made from carbon and steel present with same stress strain as steel A36. High strength bolts are made from heat treated carbon steel and have stress strain of 85 ksi. In most structural steel connections bolts are tightened to snug tight condition. This connection is harnessed after members have been bolted together using a spud wrench. Slip critical bolted connection allows tightening of bolts to a high tensile stress in order to provide shear resistance through friction generated between connected surfaces. Comparing the applicability of welding and bolting, depicts a wide range of use of welding as opposed to bolting. Welded connections eliminates the use of bolts and the connection gusset plates which substantially saves a considerable amounts of steel. Continuity of structures is yielded through well secured welding. On the other hand, bolting requires less amount of skills, rapid and can be accomplished with less surface cleaning.
Connections between Framing Members
Common structural steel connections include column to beam connections, beam to beam connections, and column to column connections. Different variations accompanied by substituting bolting for welding and ease of erection are among the reasons governing the choice of connection. The American institute of steel construction has subdivided column beam connection into; rigid connection that is also called moment connection, simple connection that is also called shear connection, and lastly semi-rigid connection also called AISC type III connection.
Column-Beam Simple Connection is a simple connection that is flexible and allows the ends of the beam to rotate under the loads. The most frequently used column-beam simple connection uses two angles that are shop welded to a beam and field bolted to a column flange (or web), as required.
Column-Beam Rigid (AISC Type I) Connections. Since rigid connections transfer tensile and compressive stresses between the column and the beam, the flanges of the beam are connected to the column. Preferred way of connecting beam flanges to the column is through direct welding to the column using groove welds.
Beam-beam (or girder-beam) connections are most depicted in steel floors and roofs. These connections are similar to those of simple connection and considered simple.
Column Splices are ineluctable in multi-storey building due to limited length of the steel. In instances where column splices have been provided, it is positioned four feet above the lower floor so that the erector can use the floor as a platform to make other connections.
Steel Detailing and Fabrication
Unlike other forms of construction like masonry and site cast concrete construction, in which most components like columns and beams are constructed at the area of construction, structural steel components are shipped to the site of construction in the prefabricated and finished state that is ready for assembly and erection. For steel works that are ready to erect, their adjustments are done at the fabrication shop. Initially a fabricator composes sets of shop drawings based on the project’s contract documents.
Most of the steel detailing are done by the fabricator basing on the framing plans and other relevant information provided. Subsequently, fabricator orders structural steel sections from rolling mills after the shop drawing have been approved by the general contractor and the project architect. Application of modern technology has made fabrication process so prices and quick.
Steel Erection Principles in Practice: Fundamentals of Frame Construction
An erection company or an experienced fabricator can be trusted with erection of the structural steel frames at construction site. Steel erection is initiated with erection of the columns with the help of a crane. The height of the construction dictates the type of crane to be used. Anchor bolts are used to embed the steel columns to the foundations. A steel base plate is welded to the column bottom with predrilled holes to penetrate the anchored bolts. Levelling of the column is done to ensure they are plumb and can be accomplished in several ways.
The earliest predecessor of frame construction is the post and beam construction. This system entails columns and beams with little or no connection between the posts and the beams. This earliest construction system works effectively with gravity loads. The beam is subjected to bending under great loads of gravity. In early post and beam buildings, stability of building against overturning and sliding was achieved through large dead load of members and the friction between their surfaces.
The pin connected cantilevered frame depict connection that allow the members to rotate at the connection independently. It further allows the beam to bend and rotate under loads of gravity without transferring the bend to the columns.
Pin connected braced frame, is represented in the same way as a pin connection. A four pin frame can be stabilized by bracing the frame in its own plane. In rigid frame, the connections between the columns and beams are rigid, and retains a 90 degree between the connected members on deformation under the loads.
Rigid frame versus braced frame. A rigid frame does not need bracing for lateral stability, providing an unobstructed space. Braced frames are often inexpensive and allow faster erection. Rigid erections are used only when the braced frames are architecturally unacceptable.
For cases with multibay single story frames, all the fundamentals undertaken on single bay frames are applied. However, it is not important to brace all bay of a multibay frame. Enough frame stiffness can be obtained by bracing a few selected bays in a single story, multibay frame building.
