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Note for Structural Design - SD by Saurabh Singh

  • Structural Design - SD
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Syllabus Module-I Properties of concrete and reinforcing steel, Philosophy, concept and methods of reinforced concrete design, Introduction to limit state method: Limit state of collapse and limit state of serviceability. Application of Limit state method to rectangular beams for flexure, shear, bond and torsion. Module-II Design of doubly reinforced beams. Design of T-and L-beams. Design of one way and two way slabs, Design of staircases. Module-III Design of short and long columns with axial and eccentric loading, design of isolated column footing. Module-IV Design principle of masonry structures: Brick and stone masonry. Design of masonry short and long walls, columns and retaining walls. Reference Books: 1. Limit state design of reinforced concrete by P.C. Verghese, PHI 2. Reinforced concrete: Limit state by A.K. Jain 3. Reinforced concrete by B.C. Punmia, A.K. Jain and A.K. Jain 4. SP-16 and SP-32.

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MODULE-1

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Introduction Reinforced concrete, as a composite material, has occupied a special place in the modern construction of different types of structures due to its several advantages. Due to its flexibilit y in form and superiority in performance, it has replaced, to a large extent, the earlier materials like stone, timber and steel. Further, architect's scope and imaginations have widened to a great extent due to its mouldability and monolithicity. Thus, it has helped the architects and engineers to build several attractive shell forms and other curved structures. However, its role in several straight line structural forms like multistoried frames, bridges, foundations etc. is enormous. Concrete Concrete is a product obtained artificially by hardening of the mixture of cement, sand, gravel and water in predetermined proportions. Depending on the quality and proportions of the ingredients used in the mix the properties of concrete vary almost as widely as different kinds of stones. Concrete has enough strength in compression, but has little strength in tension. Due to this, concrete is weak in bending, shear and torsion. Hence the use of plain concrete is limited applications where great compressive strength and weight are the principal requirements and where tensile stresses are either totally absent or are extremely low. Properties of Concrete The important properties of concrete, which govern the design of concrete mix are as follows (i) Weight The unit weights of plain concrete and reinforced concrete made with sand, gravel of crushed natural stone aggregate may be taken as 24 KN/m3 and 25 KN/m3 respectively. (ii) Compressive Strength With given properties of aggregate the compressive strength of concrete depends primarily on age, cement content and the water cement ratio are given Table 2 of IS 456:2000. Characteristic strength are based on the strength at 28 days. The strength at 7 days is about two-thirds of that at 28 days with ordinary portland cement and generally good indicator of strength likely to be obtained. (iii) Increase in strength with age

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There is normally gain of strength beyond 28 days. The quantum of increase depends upon the grade and type of cement curing and environmental conditions etc. (iv) Tensile strength of concrete The flexure and split tensile strengths of various concrete are given in IS 516:1959 and IS 5816:1970 respectively when the designer wishes to use an estimate of the tensile strength from compressive strength, the following formula can be used Flexural strength, fcr=0.7√fck N/mm2 (v) Elastic Deformation The modulus of elasticity is primarily influenced by the elastic properties of the aggregate and to lesser extent on the conditions of curing and age of the concrete, the mix proportions and the type of cement. The modulus of elasticity is normally related to the compressive characteristic strength of concrete Ec=5000√fck N/mm2 Where Ec= the short-term static modulus of elasticity in N/mm2 fck=characteristic cube strength of concrete in N/mm2 (vi) Shrinkage of concrete Shrinkage is the time dependent deformation, generally compressive in nature. The constituents of concrete, size of the member and environmental conditions are the factors on which the total shrinkage of concrete depends. However, the total shrinkage of concrete is most influenced by the total amount of water present in the concrete at the time of mixing for a given humidity and temperature. The cement content, however, influences the total shrinkage of concrete to a lesser extent. The approximate value of the total shrinkage strain for design is taken as 0.0003 in the absence of test data (cl. 6.2.4.1). (vii) Creep of concrete

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