advertisement

1 Reinforced Concrete Design-I By Dr. Attaullah Shah Swedish College of Engineering and Technology Wah Cantt. Course objectives: − To familiar the students with the behavior of RCC, various loads, building Code etc. − To understand the rationale of major design methods − To understand the safety and serviceability requirements of the structural components. − To enable the students to understand the design of various structural components like beams, columns, slabs, footings etc. − To understand the detailing of various designed elements for field implementation. Expected Class Behavior − Attendance in the class will be compulsory except genuine reasons for absence. − All students to bring scientific calculators. Students not having calculator will not be allowed in the class − Students participating the solution of problems will get extra sessional marks − Assignments will be given on group basis for discussion amongst each other. However individual submission will be required. − Assignment must be submitted in next class. Failing which one mark per week delay will be deducted − No late assignment beyond one month will be accepted − Surprise quizzes will be administered in addition to other sessional activities − Students absent for two consecutive classes may not be allowed without any cogent reason. Course outline − Mechanics of reinforced concrete − Behavior of reinforced concrete at working and ultimate loads. − Design of simple beams and slabs by working stress method. − Design of simple beams and slabs by ultimate strength design method. − Analysis and design of T beam, Doubly reinforced beam and continuous beams by Ultimate strength design method (USD). − Analysis and design of one-way and two-way simple and continuous slabs by Ultimate strength design method (USD). − Safety and serviceability provision − Shear strength in beams and design of shear reinforcement. − Bond in concrete and development length. − Design for axial compression and tension. What is concrete? - Mixture of gravel, crushed rock, or other aggregates held together held together in a rocklike mass with a paste of cement and water - Admixtures are added to change the physical and chemical characteristics of concrete such as …… - Concrete is poor in tensile strength. - Steel reinforcement is added to provide the tensile strength, but steel is poor in compressive strength itself. - Reinforced Cement Concrete is combination of concrete and steel. - In RCC, the compressive strength is provided by the concrete and tensile strength is provided by steel/ Advantages of Reinforced Cement Concrete − RCC has considerable compressive strength as compared to other material. − Great resistance to fire and water − RCC structures are rigid − Has long service life − Has low maintenance cost − Versatile construction material can be used for all structural components like footings, columns, beams and slabs etc. − Can be molded to the desired form and shape − Use locally available material in most of the areas. − Lower grade of skilled labor required for casting and steel fabrication. − Can you name some more advantages? Disadvantages of Reinforced Cement Concrete − Without sufficient tensile reinforcement, RCC is poor in tensile strength. − Forms are essential for the concrete works. Formwork constitute a big part of the concrete cost − For long span structures, the RCC sections may become very heavy as compared to steel members − The quality control and mix proportioning has dominant effect on the properties of concrete. − The shrinkage of concrete due to drying and temperature effects. − Creep effect: the Deformation under long term sustained loading Types of Portland Cement − Type I Cement: Commonly used all purpose cement. − Type II cement: Modified type I cement having lower heat of hydration and can withstand exposure to sulfate − Type III cement: High early strength concrete which can produce the compressive strength twice the type I cement in 24 hours. − Type IV cement: Low heat cement can dissipate cement slowly and used for mass concrete. − Type V cement: Used for concretes exposed to high concentrations of sulfate Admixtures: − Admixtures are used to change the properties of concrete. − They are to concrete as beauty aids are to populace. − Air entraining Agent: ASTM C260 and C618 − To improve the freeze and Thaw resistance of concrete. − Add bubbles into the concrete-foam concrete − On freezing the water enters the bubbles. − Accelerators: High early strength − Retarders: − To decelerate the setting and thereby the temperatures of concrete. Used for mass concrete. − Superplsticisers of High Range Water Reducers (HRWR): − Reduce water content, increase workability and strength. Properties of Reinforced Cement Concrete − Compressive Strength: − − − − − − − − − − fc‘ determined from testing 6in by 12 in cylinder at 28 days Concrete from 2500 psi to 10,000 and even 20,000 psi is avaialble For ordinary concrete 3000 to 4000 psi concrete is used. For pre-stressed concrete of 5000 to 6000 psi is used. At two Unions Square USA, concrete up to 19000 psi was used. In many countries 6in cube is used. The cylinders given almost 80% strength. The stress strain diagram shows: The curve is straight from zero to half or one third of ultimate strength. Beyond this point it is non linear The concrete reaches the ultimate strength at 0.003 The stress strain curves for conc. are similar to the behavior of compression side of beams - The weaker grades of concrete are less brittle than the stronger one Modulus of Elasticity − There are several definitions for concrete − It is the slope of initial stress strain curve at the origin. − The slope MOE is the slope to the tangent of stress strain curve i.e. 50% of the stress strain curve. − The slope of the line drawn from the origin to the 25-50% of the ultimate strength is referred as secant modulus − Apparent modulus is the long term modulus determined from stress strain curve under sustained load. − According to ACI ( American Concrete Institute) section 8.5.1, the modulus of elasticity for concrete of density of 90-150 lbs/ft3, the − For normal weight concrete: − Concrete having compressive strength of 6000 psi and above is called high strength concrete. − For 6000-9000 psi − Poisson’s ratio: − For weaker grade concrete: 0.11 − For higher grade concrete: 0.21 − Average value: 0.61 − Shrinkage − Concrete shrinks and cracks after drying − The amount of shrinkage depends on type of exposure: − In concretes exposed to windy conditions-more shrinkage − Humid atmosphere has less shrinkage − Hot and dry atmosphere has more shrinkage − To reduce the shrinkage: − Keep the water amount minimum − Cure the concrete well − Place the concrete in small sections − Use construction joints to control position of cracks − Use shrinkage reinforcement − Use dense and non porous aggregates Creep or plastic flow: − The long term deformation under sustained loads − Long term loads also adversely affect the strength of concrete − The amount of creep is affected by the following factors: − Longer the curing time less will be the creep. Steam curing reduces the creep. − High strength concrete have smaller creep than normal strength concrete − Creep increases with higher temperatures − The higher the humidity, the less will be the pore water which can escape from concrete and less creep − Concrete with high content of cement paste has more creep. − The addition of reinforcing bars in the compression area will reduce the creep − Large members ( large volume to surface area) have less tendency to creep as compared to small members. Tensile Strength − Normally 8-15% of the compressive strength of concrete − The tensile strength has important role in crack development − The tensile strength varies with the square root of fc’ − Tensile strength is determined with the help of modulus of rapture ( flexural tensile strength) and split cylinder tests. − ACI proposes modulus of rapture= 7.5(fc’)1/2 − In Split cylinder test, the cylinder is placed under two plates on end to end and split along the length. The tensile strength is given as - Reinforcing steel − Mostly deformed bars having ribbed projections are used − The steel bars are identified as # which represents it as multiple of 1/8 i.e. #4= 4/8 in − Grade of the bars represents its yield strength i.e. 40 grade bars means 40,000 psi − The various types of steel bars specified by ASTM are: − ASTM A615: Deformed and plain billet steel bars ( S type) − ASTM A706: Low alloy deformed and plain bars: used where controlled tensile strength is required or used for welding ( denoted with W) − ASTM A996: Deformed rail steel or axle steel bars ( R) − Due to small difference in prices of grade 40 and 60 steel bars, the later is preferred. − The SI numbers of bars are now more commonly used also called metric bars or soft metric bars. Comparison of Standard in lb bars and SI units or soft metric bars Loads − Dead loads: − Constant magnitude mostly remain in one position − Frames, walls, beams, ceiling, stairways etc. − Live loads: Loads changing magnitude and positions − Traffic loads for bridges ,impact loads due to vibration, people loads etc. Loads − Environmental loads: Due to environment in which the building is placed. − − − − Snow and ice loads: Rain loads Wind Seismic or earthquake loads − To select the appropriate level of design load, the combinations of these loads are made as per guidelines given by various codes. − For Railway bridges: American Railway Engineering Association (AREA) − For highway bridges: AASHTO − For buildings International Building Code (IBC)/ACI Code Assignments 1 ( Target date: 07.02.12) Group 1 ( Even No) : Describe various structural loads used for design of reinforced concrete structures Group 2 ( Odd Nos) : Name several admixtures used in concrete mixes and explain the purpose of each. Late submission will lose two marks per week of delay. No assignment will be accepted after one month of the submission data.