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CE 315: Design of Concrete Structures I Dr. Tahsin Reza Hossain Professor, Room No-649 Email: tahsin@ce.buet.ac.bd
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Syllabus• Fundamental behaviour of reinforced concrete• Introduction to WSD and USD methods• Analysis and design of singly reinforced, doubly reinforced and T-beams according to WSD and USD methods• Shear and Diagonal tension• Bond and anchorage according to WSD and USD methods• One-way slab
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Books• Design of Concrete Structures – Nilson, Darwin, Dolan 14th Ed• Structural Concrete- Theory and Design – Hassoun, Al-Manaseer 4th Ed• Reinforced Concrete- Mechanics & Design – Wight & McGregor 5th Ed Many more……..
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Concrete, Reinforced Concrete (RC), Prestressed Concrete (PC)• What is concrete? Constituents? – Stone like material, cement, coarse and fine aggregate, water, admixture• A bit of history• Advantages, disadvantages – Easy to make, relatively low-cost, formabilty, weather and fire resistant, good comp strength – Weak in tension• Reinforced concrete-mild steel• Where to place the reinforcement-examples• Prestressed concrete
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Roman Pantheon, unreinforced concrete dome, diameter 43.3m, 25BC, 125AD
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Serviceability, Strength and Structural Safety• To serve its purpose, a structure must be safe against collapse and serviceable in use• Strength of the structure be adequate for all loads• Serviceability – deflection small, hairline cracks, minimum vibration
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Strength and safety • If loads and moments, shears, axial force can be predicted accurately, safety can be ensured by providing a carrying capacity just barely in excess of the known demand. • Capacity= Demand
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Uncertainity• There are a number of sources of uncertainty in Analysis, Design and Construction• Read 7 points• Consideration given to consequence of failure• Nature of failure is also important
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Variability of Loads,Strength, safetyLoad can be considered asrandom variableForm of distribution curve(probability density function) canbe determined from large scaleload surveyProbability of occurrenceArea under curve is probability ofoccurrenceQd design loadSd Design strengthM is also a random variableBeta between 3 and 4corresponds to a probability offailure of 1:100,000
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Partial safety factor • Strength reduction factor X Nominal Strength > Load Factor X Design Load Why partial factors are different
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Design Basis• Strength Design • Service load design• Load factored- • Load unfactored hypothetical overload – Service load stage• Material stress level • Material stress level – Nonlinear inelastic – At allowable stresses – Concrete fc’ – Half of fc’ – Steel reaches fy – Half of fy – Both or one • WSD• USD – Working Stress Design – Ultimate Strength Design
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Design Codes and Specifications• International Building Code- consensus code• American Concrete Institute ACI Code- Building Code requirement for Structural Concrete -318- 2008• AASHTO- American Association of State Highway and Transportation Officials- for bridges• American Railway Engineering and Maintenance of Way Association –AREMA-Manual of Railway Engineering
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Bangladesh National Building Code• BNBC• First in 1993• Up-gradation is in progress
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Fundamental Assumption for RC Behavior1. Equilibrium2. Strain in steel=Strain in surrounding concrete3. Plane cross section remain plane4. Concrete does not resist any tension5. The theory is based on the actual stress- strain relationship of concrete and steel or some simplified equivalent.Read last para
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Behaviour of members subject to Axial Loads• Fundamental behaviour illustrated• Axial Compression – Economical to make concrete carry most loads – Steel reinforcement is always provided • Bending may exist • Cross section reduced
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RC Column Square, tied column Tie • Hold longitudinal bar during construction • Prevent bucking under load Circular spirally reinforced column Spiral • same • confinement to concrete
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Elastic behaviour• Up to fc’/2, concrete behave elastic• Also stress and strain proportional• Range extends to a strain of 0.0005• Steel is elastic nearly to yield 60 ksi, strain 0.002
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