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Note for Design of Concrete Structures - DCS By kruthika sekar

  • Design of Concrete Structures - DCS
  • Note
  • MVJ College of Engineering - mvjce
  • Civil Engineering
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1 DESIGN OF RC STRUCTURAL ELEMENTS SUBJECT CODE:15CV51 MODULE 1 INTRODUCTION TO LIMIT STATE DESIGN AND SERVICEABILITY INTRODUCTION TO WORKING STRESS METHOD: OUTLINE: • This theory in practise was started in 1990’s • This is traditional working stress design approach, which assumes that structural elements behave linearly in elastic manner • In this approach, the safety is assured by limiting the stress in the member, the limiting stress is obtained from stress-strain curve which is far below the yield stress, this limiting stress is called as permissible stress. • The permissible values for compressive stress and tensile stress is obtained from IS 456 200 table no- • The Factor of safety which is the ratio of strength of material /permissible stress is taken as for concrete-3 & for steel-1.78 • Strain in steel is linearly related to concrete by a factor called modular ratio. BASIC ASSUMPTIONS: • The behaviour of the structural element is linear in nature • Concrete and steel behave as elastic • There are no slip between concrete and steel • Section is designed by limiting the stress DRAWBACKS: • In this design procedure only safety is assured but serviceability of element is not utilised • The basic assumption that the material behaves linearly does not hold good since there will be increase in permissible stress, due to secondary effects. • Degree of safety against different end condition is not given. • Calculated moments are more due to influence in cracking • The calculated loads do not consider load factor LIMIT STATE METHOD (LSM): Philosophy: • Advancement over traditional deign of WSM and ultimate load method. MVJ COLLEGE OF ENGINEERING DEPARTMENT OF CIVIL ENGINEERING CKS

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2 • WSM is based on service load calculation considering safety only and ultimate load method is based on ultimate load calculation considering serviceability. • But LSM aims for comprehensive and rational solution to the design problem by considering safety at ultimate load and serviceability at working load. Limit states: it is a state which depicts impending failure, beyond which the structure ceases to perform its function satisfactorily. Two types of limit states: 1. Limit state of collapse/failure: Each states of limit may be attained due to different types of load configuration, i. Failure due to impact, earthquake, fire, frost ii. Failure due to fatigue iii. Failure of one or more critical sections in flexure, shear, torsion or due to combinations. iv. Failure due to elastic instability of structure v. Failure due to bond and anchorage failure of reinforcement vi. Failure due to destructive effects of chemicals, corrosion of reinforcements 2. Limit state of serviceability: The structure may be rendered unfit due to various Limit state of serviceability, i. Excessive deflection or displacement severely affecting the finishes and causing discomfort to the uses of the structure. ii. Excessive load damage leading to cracking and spalling of concrete impairing the efficiency or appearance of the structure. DIFFERENCES BETWEEN WORKING STRESS METHOD AND LIMIT STATE METHOD: The design of the components of the RCC structure can be done in the following two ways: 1.Working Stress Method 2.Limit State Method LSM WSM The Stresses in an element is obtained from The stresses are obtained from design loads and the working loads and compared with compared with design strength. permissible stresses The method follows linear behaviour of both the materials MVJ COLLEGE OF ENGINEERING stress-strain In this method, it follows linear strain relationship but not linear stress relationship DEPARTMENT OF CIVIL ENGINEERING CKS

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3 (one of the major difference between the two methods of design). Material capabilities are under estimated to The ultimate stresses of materials itself are used large extent. Factor of safety are used in working as allowable stresses. Partial safety factors are stress method. used in LSM Ultimate load carrying capacity cannot be The predicted accurately. material capabilities are not underestimated as much as they are in working stress method. Partial safety factors are used in The main drawback of this method is that it limit state method. results in an uneconomical section. MODULAR RATIO: Modular Ratio is defined as the Ratio between Modulus of Elasticity of Steel and Modulus of Elasticity of Concrete, Modular ration significance is to finding losses in prestress. This is because, a Reinforced Concrete is made up of Both Steel and Concrete. In this case, Steel is a Tension member and Concrete is a Compression Member. When Load is applied, a RCC member seem to carry same load but strain produced by both (concrete and steel) are different. FACTOR OF SAFETY: Factors of safety (FoS), also known as (and used interchangeably with) safety factor (SF), is a term describing the load carrying capacity of a system beyond the expected or actual loads. Essentially, the factor of safety is how much stronger the system is than it usually needs to be for an intended load. PARTIAL SAFETY FACTORS: Partial safety factors are expressed in terms of probability value that the structure will not become unfit for its intended function during its useful life. This probability value is applied for different limit states in the limit state method. For Example: A bridge is made and it is required to carry weight of vehicles upto 50KN. But when the bridge is made it is made in such a way that it can carry vehicles of weight upto 100KN, so we will say that the bridge has a safety factor of 2 (100KN/50KN). Since it can carry 2 time the weight it is intended to.. PSF=Maximum Load/Intended Load MVJ COLLEGE OF ENGINEERING DEPARTMENT OF CIVIL ENGINEERING CKS

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4 Partial safety factor for materials: It is necessary to reduce the characteristic strength values by suitable factors to obtain the design strength values, called as Partial safety factor for material represented as Ƴm (gamma m) Design strength is divided by Ƴm to obtain design strength, Example: design of concrete Fck is cube strength, but this strength in actual structure will be taken as 0.67Fck Partial safety factor for loads: The multiplying factor used to modify (increase) the characteristic load F to obtain Fd is termed as Partial safety factor of loads ƳS The Indian standard code IS-456 2000 recommends the use of Partial safety Factors for loads and strength as shown in table below, Partial safety Factors for loads, Table 18, IS 456-2000 CHARACTERISTIC LOAD: For dead loads, a characteristic load is defined as the value which has a 95%probability of not being exceeded during the life of the structure. This concept assumes a normal distribution of the values of a particular dead load. In the following figure, the shaded area above the characteristic value represents 5% probability of exceedance of the load in the design life of the structure. MVJ COLLEGE OF ENGINEERING DEPARTMENT OF CIVIL ENGINEERING CKS

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