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.

 

What is Structural Engineering?

What is Structural Engineering? Structural Engineering deals with Structural Analysis and Structure design (A) Structural Analysis Deals With (B) Structure Design Deals with (i)Arrangement of structural Elements (i) Selection of material (ii)Determination of internal forces (ii) Preparation of the final layout of the structure (iii) Determination of state of stresses or critical combination of stresses at various points (iii) Design Drawing (iv) External Reaction (iv) Standard Specification Structural analysis deals with (i) Arrangements of Structural Elements Structural analysis deals  with the development of suitable arrangement  of structural elements for the structures to support the external loads or the various critical  combinations of loads which are likely to act on the structures. (ii) Determination of internal forces The analysis also deals with the  determination of internal forces in the various members Internal forces: ( a ) Axial forces ( b ) Bending moments ( c ) Shear forces (iii) State of…

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Design of singly reinforced rectangular section for flexure
Singly reinforced Beam

Design of singly reinforced rectangular section for flexure

Learn : Design of singly reinforced rectangular section for flexure, factored moment, ultimate moment of resistance, limiting moment of resistance factor, fixing dimension of the section, Area of tension steel.   Design of singly reinforced rectangular section for flexure The design problem is generally of determining dimensions (cross-sectional) of a beam (b X D) and the area of steel for a known moment or load. The basic requirement for safety at the limit of collapse (flexure) is that the factored moment Msbecause of loads should not exceed the ultimate moment of resistance Mulim of the section and the failure should be ductile. 2188ed6a96dad6de04b0dfb5853df1bee74e3d92 therefore \[M_{u}\leq M_{u lim}\] Taking equality \[M_{u}= M_{u lim}\] \[=0.36f_{ck}.\frac {x_{umax}}{d}\left ( 1-\frac {0.42 x_{u max}}{d} \right )bd^{2}\] \[M_{u}=R_{u}bd^{2}\] For the given material i.e., grade of concrete and type of steel, Ru is constant and is called as limiting moment of resistance factor. \[R_{u}=0.36f_{ck}\frac {x_{u max}}{d}\left (…

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Basic rules for design of beams
Beam Design

Basic rules for design of beams

Learn About design of beams, effective span, effective depth, reinforcement, nominal cover to reinforcement, curtailment of tension reinforcement  BASIC RULES FOR DESIGN OF BEAMS While designing R.C.C. beams, following important rules must be kept in mind: Effective Span (CI. 22.2, IS 456) The effective span of the beams are taken as follows : (a) Simply Supported Beam or Slab The effective span of a simply supported beam or slab is taken as least of the following: (i) Clear span plus the effective depth of beam or slab. (ii)Centre to centre distance between supports. (b) Continuous Beam or Slab In case of continuous beam or slab if the width of the supports is less than \[\frac {1}{12} \]of the clear span, the effective span is taken as in (a). If the width of the support is greater than \[\frac {1}{12}\] of the clear span or 600 mm whichever is less, the…

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Stress Strain relationship for concrete and Stress Strain relationship for steel
Stress Strain relationship for concrete, Stress Strain relationship for steel,Design Strength Values for Steel Design Stresses at Specified Strains.

Stress Strain relationship for concrete and Stress Strain relationship for steel

Learn : Stress Strain relationship for concrete, Stress Strain relationship for steel,Design Strength Values for Steel Design Stresses at Specified Strains. fe 415 and fe 500 Stress Strain relationship for concrete and Stress Strain relationship for steel Stress Strain relationship for concrete Stress Strain relationship for concrete : The experimental or actual stress strain curve for concrete is very difficult to use in design Therefore, IS code 456:2000 has simplified or idealized it as shown in Fig. 4.1. For design purpose, the compressive strength of concrete in the structure in taken as 0.67 times the characteristic strength. The 0.67 factor is introduced to account for the difference in the strength indicated by a cube test and the strength of concrete in actual structure. The partial safety factor (rmc), equal to 1.5 is applied in addition to this 0.67 factor. The initial portion of the curve is parabolic. After a strain of…

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Assumptions in limit state of collapse in flexure (Bending)
Assumptions in limit state of collapse in flexure (Bending)

Assumptions in limit state of collapse in flexure (Bending)

Learn Assumptions in limit state of collapse in flexure (Bending),relationship between the stress strain distribution in concrete,stresses in the reinforcement are taken from the stress-strain curve Assumptions in limit state of collapse in flexure (Bending) (REFER CL. 38, IS 456) The design of reinforced concrete sections for limit state of collapse in bending, is based on the following assumptions : (a)       Plane sections normal to the axis remain plane after bending. It means that the strain at any point in the cross-section is proportional to the distance from the neutral axis. (b)       The maximum strain in concrete at the outermost compression fibre is taken as 0.0035 in bending. (c)       The relationship between the stress-strain distribution in concrete is assumed to be parabolic, as shown in Fig. 4.1. For design purpose, the compressive strength of concrete is assumed to be parabolic, as shown in Fig. 4.1. For design purpose, the compressive…

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Design Strength of Materials and Design Loads(Refer Clause 36, IS Code)
Design Strength of Materials,Partial Safety Factors for Materials,Design Loads,Values of Partial Safety Factor for Loads (Refer Clause 36,IS Code)

Design Strength of Materials and Design Loads(Refer Clause 36, IS Code)

Design Strength of Materials :  Partial Safety Factors for Materials (rm), and Design Loads : Values of Partial Safety Factor (rf) for Loads(Refer Clause 36, IS Code)  DESIGN VALUES (REFER CLAUSE 36, IS CODE) Design Strength of Materials The strength of any material obtained in a structure is always less than the characteristic strength of the material. It is because of the workmanship or quality control in the manufacture of materials. The reduced value of strength which is obtained by applying partial safety factors to the characteristic strength is called as design strength of the material. The design strength of the material fd is given by \[f_{d}=\frac{f}{r_{m}}\] where             f = Characteristic strength of the material rm = Partial safety factors appropriate to material and limit state being considered. Partial Safety Factors for Materials (rm) The values of partial safety factors for limit state of collapse, should be taken as : rmc…

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Types of shear reinforcement
Vertical Stirrups

Types of shear reinforcement

Types of shear reinforcement : Vertical stirrups, Bent up bars along with stirrups, Inclined stirrups, contribution of bent up bars TYPES OF SHEAR REINFORCEMENT The following three types of shear reinforcement are used : Vertical stirrups. Bent up bars along with stirrups. Inclined stirrups. Vertical Stirrups These are the steel bars vertically placed around the tensile reinforcement at suitable spacing along the length of the beam. Their diameter varies from 6 mm to 16 mm. The free ends of the stirrups are anchored in the compression zone of the beam to the anchor bars (hanger bar) or the compressive reinforcement. Depending upon the magnitude of the shear force to be resisted the vertical stirrups may be one legged, two legged, four legged and so on as shown in Fig. 5.5. It is desirable to use closely spaced stirrups for better prevention of the diagonal cracks. The spacing of stirrups near the supports is less…

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Effects of Shear Diagonal Tension
Effects of Shear  Diagonal Tension, maximum bending tensile stress, tensile stress (σ) as well as shear stress (τ),

Effects of Shear Diagonal Tension

Learn : Effects of Shear  Diagonal Tension, maximum bending tensile stress, tensile stress (σ) as well as shear stress (τ),crack pattern for a simply supported beam,  The maximum bending moment in this beam will be at midspan and the maximum shear force, at the supports. EFFECTS OF SHEAR  DIAGONAL TENSION Consider a beam AB subjected to transvers loads as shown in Fig. 5.3(a). The maximum bending moment in this beam will be at midspan and the maximum shear force, at the supports. The beam is subjected to bending and shear stresses across the cross-section. Let us consider a small element (1) from the tensile zone of the beam. It is subjected to bending tensile stress (σ) as well as shear stress (τ) as shown in Fig. 5.3. (b). At the midspan, the bending moment is maximum and the shear force is zero. So the element 2 is subjected to maximum…

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Shear Stresses in R.C.C. beams

Shear Stresses in R.C.C. beam (Reinforced cement concrete beam), Stress Based Approach (Elastic Theory), IS Code Approach SHEAR STRESSES IN R.C.C. BEAMS Stress Based Approach (Elastic Theory) R.C.C. is a composite materials so the exact shear distribution as per elastic theory is very complex. It is shown in Fig. 5.2(b) by the hatched portion of the curve.It is parabolic in the compression zone with zero at the top and maximum at the neutral axis. The value of shear-stress is constant in the tensile zone and is equal to the maximum shear-stress (q) because the concrete, below the neutral axis (tensile zone) is assumed to be cracked and neglected. The maximum value of shear stress (q) as per elastic theory is given by \[q=\frac{V}{bjd}\] where             V = shear force at the section b and d = dimensions of the section j = Lever arm depth factor IS Code Approach As per IS code…

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Characteristic Strength of Materials Characteristic Load or ultimate load
Characteristic Strength of Materials

Characteristic Strength of Materials Characteristic Load or ultimate load

Learn : Characteristic Strength of Materials :Characteristic Strength of Concrete, Characteristic Strength of Steel. and Characteristic Load or ultimate load CHARACTERISTIC VALUES  (REFER CLAUSE 36, IS CODE) Characteristic Strength of Materials The characteristic strength is based on the statistical analysis of the test results because there are variations in the strength of the material used. In order to simplify the analysis, it may be assumed that the variation in strength follows a normal distribution curve which is symmetric about the mean value as shown below in Fig. 3.1. Therefore, characteristic strength = Mean strength - k S where S is the standard deviation, k=1.64, corresponding to 5% probability \[\therefore                f_{ck}=f_{m} - 1.64 S\]  Characteristic Strength of Concrete The term characteristic strength means that value of strength of material below which not more than 5% of the test results are expected to fall. It is denoted by fck is N/mm2. The value…

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