# m15l38 - Module 15 Redistribution of Moments Version 2 CE...

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Module 15 Redistribution of Moments Version 2 CE IIT, Kharagpur

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Lesson 38 Redistribution of Moments – Theory and Numerical Problems Version 2 CE IIT, Kharagpur
Instructional Objectives: At the end of this lesson, the student should be able to: explain the concept of redistribution of moments in the design of statically indeterminate reinforced concrete structures, explain the behaviour of statically indeterminate reinforced concrete structures with increasing loads till the collapse of the structures, state the advantages of redistributing the moments in statically indeterminate reinforced concrete structures, identify structures when such redistributions are to be made based on the analysis of structures, mention the reasons why full redistribution is not allowed in reinforced concrete structures, specify the stipulations of IS Codes about the redistribution separately when the structure is designed by working stress method and by limit state method, apply the theories in solving numerical problems of statically indeterminate beams of one or more spans as per the stipulations of IS Code. 15.38.1 Introduction Statically indeterminate structures made of reinforced concrete like fixed ended one span beams, continuous beams and frames are designed considering internal forces like bending moment, shear force and axial thrust obtained from structural analysis. Either one or several sections of these structures may have peak values of the internal forces, which are designated as critical sections. These sections are dimensioned and reinforced accordingly. Flexural members, however, do not collapse immediately as soon as the loads at a particular section cause bending moment exceeding the maximum resisting moment capacity of that section. Instead, that section starts rotating at almost constant moment. This is known as formation of plastic hinge at that section reaching its maximum resisting moment capacity. The section then transfers loads to other sections if the applied loads are further increased. This process continues till the structures Version 2 CE IIT, Kharagpur

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have plastic binges at sufficient sections to form a failure mechanism when it actually collapses. However, significant transfer of loads has occurred before the collapse of the structure. This transfer of loads after the formation of first plastic hinge at section having the highest bending moment till the collapse of the structure is known as redistribution of moments. By this process, therefore, the structure continues to accommodate higher loads before it collapses. The elastic bending moment diagram prior to the formation of first plastic hinge and the final bending moment diagram just before the collapse are far different. The ratio of the negative to positive elastic bending moments is no more valid. The development of plastic hinges depends on the available plastic moment capacity at critical sections. It is worth mentioning that the redistribution
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