CHAPTER 3 Life cycle of SS frame and RC frame
3.2 Structural frames for buildings
3.2.1 RC frame
Reinforced concrete is one of the most widely used modern building materials.
The principle theory of reinforced concrete is extremely simple: Put the reinforcing steel where there are tensile forces in a structural member, and let the concrete resist the compression (Allen & Iano, 2009). Concrete is
―artificial stone‖ obtained by mixing cement and sand, which is then aggregated with water. Fresh concrete can be molded into almost any shape, which is an inherent advantage over other materials (Limbrunner & Aghayere, 2010). Concrete became very popular after the invention of Portland cement in the 19th century; however, its limited tension resistance prevented its wide use in building construction. To overcome this weakness, steel bars are embedded in concrete to form a composite material called reinforced concrete.
Developments in modern reinforced concrete design and construction practice were pioneered by European engineers in the late 19th century. At the present time, reinforced concrete is extensively used in a wide variety of engineering applications (e.g., buildings, bridges, dams).
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The extensive use of RC frames, especially in developing countries, is due to the following advantages (Fiona, 2009):
Low production cost. In most countries, the aggregates (sand and water) are produced locally. Thus, the cost of concrete is low because of low raw material costs and transportation costs.
Low labor costs. Skilled labor is not necessary because the production process and construction of RC frame are not complicated.
Ease of production. Concrete production does not require expensive manufacturing mills. In some cases, single-family houses or simple low-rise residential buildings are constructed without any engineering assistance.
Resistance to action of water. Concrete is used almost exclusively in water-retaining and underground structures such as bridges and piers, and so on.
Compressive loading applications.
3.2.2 Steel frame
Steel was first manufactured in the United States in 1856 (Aghayere & Vigil, 2009). Its first use in a bridge was a railroad bridge across the Mississippi River in St. Louis, in 1874. The first skyscraper to have steel beams incorporated in its frame is generally recognised as the Home Insurance Building in Chicago, which was built in 1885 and demolished in 1929 (Geschwindner, 2008). The first all-steel skyscraper was the Rand-McNally Building in Chicago, which was built in 1888-1890. It began a continuous evolution in steel building structures that continues today as new ideas continue to spring up in the minds of architects and engineers who continue to build with steel.
Using steel as a structural material has following advantages (McCormac, 2008):
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High strength. The high strength of steel per unit of weight means that the weight of structures will be smaller.
Uniformity. The properties of steel do not change appreciably with time, as do those of a reinforced-concrete structure.
Elasticity. Steel behaves more closely to design assumptions than most other materials because it follows Hook‘s law up to fairly high stresses.
The moments of inertia of a steel structure can be accurately calculated, while the values obtained for a RC structure are rather indefinite.
Permanence. Steel frames that are properly maintained will last indefinitely.
Ductility. In structural members under normal loads, high stress concentrations develop at various points. The ductile nature of normal structural steel enables it to yield locally at those points, thus preventing premature failure. The large deflection of a ductile structure will therefore give visible evidence of impending failure when overloaded or subjected to a sudden shake.
Toughness. Steel members can be subjected to large deformations during fabrication and erection without fracture—thus allowing them to be bent, hammered, and sheared, and have holes punched in them without visible damage.
Additions to existing structures. Steel structures are quite suited to having additions made to them, such as new bays or new wings.
Miscellaneous. This includes: 1) an ability to be fastened together by several simple connection devices, including welds and bolts; 2) adaption to prefabrication; 3) speed of erection; 4) ability to be rolled into a wide variety of sizes and shapes; 5) possible reuse after a structure is disassembled; and 6) scrap value since steel is the ultimate recyclable material.
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In spite having the above advantages, steel has following disadvantages (Salmon et al., 2009):
Corrosion. The fatigue strength of steel members can be appreciably reduced when the members are used in aggressive chemical environments and subjected to cyclical loads.
Fireproofing costs. The strength of steel structural members is tremendously reduced at temperatures commonly reached in fires when the other materials in a building burn. As a result, the steel frame of a building may have to be protected by materials with certain insulating characteristics.
Susceptibility to buckling.
Fatigue. Steel strength may be reduced if it is subjected to a large amount of stress reversal or even a large number of variations of tensile stress.
Brittle fracture. Under fatigue-type loadings, low temperatures, or triaxial stress conditions, steel may lose its ductility, and brittle fractures may occur.