m7l17 - Module 7 Limit State of Serviceability Version 2 CE...

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Module 7 Limit State of Serviceability Version 2 CE IIT, Kharagpur
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Lesson 17 Limit State of Serviceability Version 2 CE IIT, Kharagpur
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Instruction Objectives: At the end of this lesson, the student should be able to: explain the need to check for the limit state of serviceability after designing the structures by limit state of collapse, differentiate between short- and long-term deflections, state the influencing factors to both short- and long-term deflections, select the preliminary dimensions of structures to satisfy the requirements as per IS 456, calculate the short- and long-term deflections of designed beams. 7.17.1 Introduction Structures designed by limit state of collapse are of comparatively smaller sections than those designed employing working stress method. They, therefore, must be checked for deflection and width of cracks. Excessive deflection of a structure or part thereof adversely affects the appearance and efficiency of the structure, finishes or partitions. Excessive cracking of concrete also seriously affects the appearance and durability of the structure. Accordingly, cl. 35.1.1 of IS 456 stipulates that the designer should consider all relevant limit states to ensure an adequate degree of safety and serviceability. Clause 35.3 of IS 456 refers to the limit state of serviceability comprising deflection in cl. 35.3.1 and cracking in cl. 35.3.2. Concrete is said to be durable when it performs satisfactorily in the working environment during its anticipated exposure conditions during service. Clause 8 of IS 456 refers to the durability aspects of concrete. Stability of the structure against overturning and sliding (cl. 20 of IS 456), and fire resistance (cl. 21 of IS 456) are some of the other importance issues to be kept in mind while designing reinforced concrete structures. This lesson discusses about the different aspects of deflection of beams and the requirements as per IS 456. In addition, lateral stability of beams is also taken up while selecting the preliminary dimensions of beams. Other requirements, however, are beyond the scope of this lesson. 7.17.2 Short- and Long-term Deflections As evident from the names, short-term deflection refers to the immediate deflection after casting and application of partial or full service loads, while the long-term deflection occurs over a long period of time largely due to shrinkage Version 2 CE IIT, Kharagpur
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and creep of the materials. The following factors influence the short-term deflection of structures: (a) magnitude and distribution of live loads, (b) span and type of end supports, (c) cross-sectional area of the members, (d) amount of steel reinforcement and the stress developed in the reinforcement, (e) characteristic strengths of concrete and steel, and (f) amount and extent of cracking.
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This note was uploaded on 03/17/2012 for the course CENG 3012 taught by Professor Prof.j.n.bandopadhyay during the Summer '01 term at Indian Institute of Technology, Kharagpur.

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m7l17 - Module 7 Limit State of Serviceability Version 2 CE...

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