R.C.C. Structure design

R.C.C. Structure design :a combination of concrete and steel reinforcement that are joined into one piece and work together in a structure. The term “reinforced concrete” is frequently used as a collective name for reinforced-concrete structural members and products. The idea of combining in reinforced concrete two materials that are extremely different in properties is based on the fact that the tensile strength of concrete is significantly lower (by a factor of 10–20) than its compressive strength. Therefore, the concrete in a reinforced-concrete structure is intended to take compressive stresses, and the steel, which has high ultimate tensile strength and is introduced into the concrete as reinforcement rods, is used principally to take tensile stresses. The interaction of such different materials is extremely effective: when the concrete hardens, it adheres firmly to the steel reinforcement and protects it from corrosion, since an alkaline medium is produced during the process of hydration of the cement. The monolithic nature of the concrete and reinforcement also results from the relative closeness of their coefficients of linear expansion (7.5 × 10−6 to 12 × 10−6 for concrete and 12 × 10−6 for steel reinforcement). The basic physicomechanical properties of the concrete and steel reinforcement are virtually unchanged during temperature variations within a range of –40° to 60°C, which makes possible the use of reinforced concrete in all climatic zones.

In R.C.C. Structure design :The basis of the interaction between concrete and steel reinforcement is the presence of adhesion between them. The magnitude of adhesion or resistance to displacement of the reinforcement in concrete depends on the mechanical engagement in the concrete of special protuberances or uneven areas of the reinforcement, the frictional forces from compression of the reinforcement by the concrete as a result of its shrinkage (reduction in volume upon hardening in air), and the forces of molecular interaction (agglutination) of the reinforcement with the concrete. The factor of mechanical engagement is decisive. The use of indented bar reinforcement and welded frames and nets, as well as the arrangement of hooks and anchors, increases the adhesion of the reinforcement to the concrete and improves their joint operation.

R.C.C. Structure design : Structural damage and noticeable reduction of the strength of concrete occur at temperatures above 60°C. Short-term exposure to temperatures of 200°C reduces the strength of concrete by 30 percent, and long-term exposure reduces it by 40 percent. A temperature of 500°-600°C is the critical temperature for ordinary concrete, at which the concrete breaks up as a result of dehydration and the rupture of the cement stone skeleton. Therefore, the use of ordinary reinforced concrete at temperatures exceeding 200°C is not recommended. Heat-resistant concrete is used in thermal units operating at temperatures up to 1700°C. A protective layer of concrete 10–30 mm thick is provided in reinforced-concrete structures to protect the reinforcement from corrosion and rapid heating (for example, during a fire), as well as to ensure its reliable adhesion to the concrete. In an aggressive environment the thickness of the protective layer is increased.

R.C.C. Structure design :The shrinkage and creep of concrete are of great importance in reinforced concrete. As a result of adhesion, the reinforcement impedes the free shrinkage of concrete, leading to the emergence of initial tensile stresses in the concrete and compressive stresses in the reinforcement. Creep in concrete causes the redistribution offerees in statically indeterminate systems, an increase in sags in components that are being bent, and the redistribution of stresses between concrete and reinforcement in compressed components. These properties of concrete are taken into account in designing reinforced-concrete structures. The shrinkage and low limiting extensibility of concrete (0.15 mm/m) cause the inevitable appearance of cracks in the expanded area of structures under service loads. Experience shows that under normal operating conditions cracks up to 0.3 mm wide do not reduce the supporting capacity and durability of reinforced concrete. However, low cracking resistance limits the possibility of further improvement of reinforced concrete and, particularly, the use of more economical high-strength steels as reinforcement. The formation of cracks in reinforced concrete may be avoided through the method of prestressing, by means of which concrete in expanded areas of the structure undergoes artificial compression through mechanical or electrothermal prestressing of the reinforcement. Self-stressed reinforced-concrete structures, in which compression of the concrete and expansion of the reinforcement are achieved as a result of the expansion of the concrete (manufactured with so-called stretching cement) during specific temperature-moisture treatment, is a further development of prestressed reinforced concrete. Because of its high technical and economic indexes (profitable use of high-strength materials, absence of cracks, and reduction of reinforcement expenditures), prestressed reinforced concrete is successfully used in supporting structures of buildings and engineering structures. A basic shortcoming of reinforced concrete, high weight per volume, is eliminated to a considerable extent by the use of lightweight concrete (with artificial and natural porous fillers) and cellular concrete.

 

Analysis of singly reinforced beam Working stress method

Learn : Analysis of singly reinforced beam Working stress method,Equivalent or Transformed Section,Strain Diagram,Stress Diagram,Neutral Axis,Stresses in Concrete and Steel , Dimensions of the Beam and Area of Steel,Percentage of Steel,Lever arm, moment of resistance Analysis of singly reinforced beam Working stress method Analysis of singly reinforced beam Working stress method : A singly reinforced beam section is shown in Fig. 2.3(a). To analyse this section, it is necessary to convert it into a transformed or equivalent section of concrete. Equivalent or Transformed Section As per the assumption (3), all the tensile stresses are taken by steel and none by concrete i.e., concrete in the tensile zone is cracked. So, the concrete area below the neutral axis is neglected and the effective area or the equivalent area of the section in terms of concrete is shown in Fig. 2.3(b). The equivalent area is equal to the area of concrete in…

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Concept of transformed or equivalent section of a RCC beam

Learn : Concept of transformed or equivalent section of a RCC beam,load carried by steel.load carried by concrete,section in terms of concrete CONCEPT OF TRANSFORMED OR EQUIVALENT SECTION Consider an R.C.C. section shown in Fig. 2.2(a) subjected to a compressive load P. Let      A         = Area of cross-section Ac        = Area of concrete Ast       = Area of steel m         = Modular ratio ss        = Stress in steel sc        = Stress in concrete es         = Strain in steel ec         = Strain in concrete Ps         = Load carried by steel Pc         = Load carried by concrete Aeqc     = Equivalent area of section in terms of concrete Es        = Yong's modulus of elasticity of steel Ec        = Young's modulus of elasticity of concrete P          = Ps + Pc \[ P          = \sigma _{s}A_{st}+\sigma _{c}A_{c}  \] The bond between steel and concrete is assumed to be perfect so the strains in steel and…

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PERMISSIBLE STRESSES (CLAUSE B-2, IS456:2000)

PERMISSIBLE STRESSES (CLAUSE B-2, IS456:2000),Permissible Stresses in Concrete ( IS 456),Permissible Stress in Steel Reinforcement ( IS 456) PERMISSIBLE STRESSES (CLAUSE B-2, IS456:2000) The working stress method is based upon the concept of permissible stresses. Permissible stresses are obtained by dividing the ultimate strength of concrete or yield strength of steel (0.2% proof stress) by appropriate factors of safety. The factors of safety used in working stress method are: (i)        For concrete (a) in bending compression - 3.0 (b) in direct compression - 4.0 (ii)       For steel - 1.78 There are greater chances of variation of strength of concrete due to improper compaction, inadequate curing and variation in the properties of concrete. The chances of variation in the properties of steel are less as it is fabricated in factories where good workmanship and better quality control is possible. So, lesser value of factor of safety is used for steel as…

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Types of R.C.C. beam, Fundamental assumptions of elastic theory of bending

Learn: Types of R.C.C. beam,Singly,doubly and T, Flanged Reinforced Beams,fundamental assumptions of elastic theory, Assumptions in the theory of simple bending of R.C.C. beams(Working stress method) Plain Cement Concrete has low tensile strength. A beam made up of plain cement concrete will have low load carrying capacity and will fail by cracking in the tension zone. It is therefore reinforced by placing steel bars in the tensile zone. These bars will take up the tensile stresses and thus increase the load carrying capacity or strength of the beam. The steel placed in the tensile zone, is called as longitudinal steel or main steel.  TYPES OF R.C.C. BEAM            Types of  R.C.C. beam are of following :             (i)        Singly Reinforced Beams : This is the one of Types of  R.C.C. beam in which steel reinforcement is placed in the tensile zone only are called as singly reinforced beams.             (ii)      …

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Working stress method load factor or ultimate load method limit state method

In this Article Learn : Various methods used for the design of R.C.C. structures,Working stress method,Load factor or ultimate load method,Limit state method, The main drawbacks of the working stress method of design,the advantages of Ultimate load method,The main limitations of the ultimate load method,The two important limit states to be considered in design:Limit State of Collapse,Limit State of Serviceability. DESIGN PHILOSOPHIES Design of any R.C.C. member comprises of the following : (i)        To decide the size (dimensions) of the member and the amount of reinforcement required. (ii)       To check whether the adopted section will perform safely and satisfactory during the life time of the structure.             Various methods used for the design of R.C.C. structures are as follows : (i)        Working stress method. (ii)       Load factor or ultimate load method. (iii)     Limit state method.  Working Stress method This method of design was the oldest one. It is based on the…

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Dead loads, Live load or imposed load, Wind load, Snow Load,Earthquake loads.

In this Article: Learn Types of loads on R.C.C. Structures, Dead loads, Live loads or imposed loads, Wind loads, Snow Loads,Earthquake loads. See also The loads coming on the foundations TYPES OF LOADS ON R.C.C. STRUCTURES Structures are designed to withstand various types of loads. The various types of loads expected on a structure are as follows : (i)        Dead loads (ii)       Live loads or imposed loads (iii)     Wind loads (iv)      Snow loads (v)       Earthquake loads   Dead Loads Dead loads are due to self weight of the structure. Dead loads are the permanent loads which are always present. Dead loads depends upon the unit weight of the material. Dead loads includes, the self weight of walls, floors beams, columns etc. and also the permanent fixtures present in the structure. The unit weight of commonly used building materials are given in the code IS 875 (part-I)-1987. The unit weights of…

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Reinforcing material types of steel reinforcement charectristic strength of steel

Content of this Article : Reinforcing material, Suitability of Steel as Reinforcing Material, charectristic strength of steel and types of steel reinforcement REINFORCING MATERIAL Concrete is weak in tension and it is to be reinforced properly with suitable material. The purpose of providing reinforcement in R.C.C. is : To take up all the tensile stresses developed in the structure. To increase the strength of concrete sections. To prevent the propagation of cracks developed due to temperature and shrinkage stresses. To make the sections thinner as compare to plain concrete section. To fulfill above criteria the reinforcing material should satisfy the following requirements : The reinforcing material should develop a perfect bond with concrete to transfer stresses from one material to other. It should have high tensile strength. It should be cheap, easily available and durable. The coefficient of thermal expansion of the reinforcing material should be nearly same as that…

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Concrete Mix Proportioning

 Concrete Mix Proportioning Concrete Mix Proportioning Concrete Mix Proportioning : The determine of the proportions of cement, aggregates and water to attain the desired strength and properties such as workability, durability etc., is called as concrete mix proportioning. The design of concrete mix is classified into following two types by IS 456:2000 (Clause 9) : (1)       Nominal mix concrete (2)       Design mix concrete.   Nominal Mix Concrete A concrete mix in which the proportions of cement, aggregate and water are adopted is called as nominal mix. It is not necessary that such a mix will give the desired strength and properties. For example, nominal mix proportions of M15 is 1:2:4. Nominal mix is not used for grades higher than M20. It is used for ordinary concrete works only. As per IS 456:2000 (Clause 9.3.1). The proportional of materials for nominal mix concrete shall be in accordance with the following Table…

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Properties of concrete

Conetnt of This Article  Properties of concrete : compressive strength,work-ability,Workability depends upon the following factors, The work ability can be measured by the following tests, Workability Requirements for Different Works,Durability Different Exposures with Normal Weight 20 mm Aggregates,Tensile Strength, Modulus of Elasticity, Poisson's Ratio, Creep, creep coefficient, Shrinkage, Freezing and Thawing, percentage of en trained air for nominal maximum size of aggregate, Sulfate, Attack Various Types of Environment Exposure Conditions See also: Requirements of good concrete mix and Grades of concrete Properties of concrete in plastic and hardened state Water cement ratio and slump test CEMENT CONCRETE AND ITS INGREDIENTS Properties of Concrete The properties of concrete depend upon the properties and proportions of its ingredients. The following are the important properties of concrete which are used in the design :   Compressive Strength The compressive strength of concrete is determined by the cube test. The characteristic compressive strength of…

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Concrete making materials Cement Aggregate and Water

Content of this article: Concrete making materials Cement: Ordinary portland cement, Rapid hardening Portland cement, Low heat Portland  cement,  Portland slag cement, Sulfate resisting cement, Portland pozzolana cement, white cement, Aggregate and Water Concrete Making Materials (1)       Cement Cement is the only chemically active ingredient of concrete which shows binding properties after reacting with water. It consist of silicates and aluminates of calcium which form a hardened mass after mixing with water. This type of cement is also known as hydraulic cement. There are many types of cements available in market, some of which are explained below : (i)        Ordinary Portland Cement Ordinary portland cement (OPC) is commonly used in construction. The Bureau of Indian Standard has classified OPC in three grades. This classification is based on the compressive strength of cement-sand mortar cubes. The face area of these cubes shall be 50 cm2. The cube is made of 1…

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