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m2l12

# m2l12 - Module 2 Analysis of Statically Indeterminate...

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Module 2 Analysis of Statically Indeterminate Structures by the Matrix Force Method Version 2 CE IIT, Kharagpur

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Lesson 12 The Three-Moment Equations-I Version 2 CE IIT, Kharagpur
Instructional Objectives After reading this chapter the student will be able to 1. Derive three-moment equations for a continuous beam with unyielding supports. 2. Write compatibility equations of a continuous beam in terms of three moments. 3. Compute reactions in statically indeterminate beams using three-moment equations. 4. Analyse continuous beams having different moments of inertia in different spans using three-moment equations. 12.1 Introduction Beams that have more than one span are defined as continuous beams. Continuous beams are very common in bridge and building structures. Hence, one needs to analyze continuous beams subjected to transverse loads and support settlements quite often in design. When beam is continuous over many supports and moment of inertia of different spans is different, the force method of analysis becomes quite cumbersome if vertical components of reactions are taken as redundant reactions. However, the force method of analysis could be further simplified for this particular case (continuous beam) by choosing the unknown bending moments at the supports as unknowns. One compatibility equation is written at each intermediate support of a continuous beam in terms of the loads on the adjacent span and bending moment at left, center (the support where the compatibility equation is written) and rigid supports. Two consecutive spans of the continuous beam are considered at one time. Since the compatibility equation is written in terms of three moments, it is known as the equation of three moments. In this manner, each span is treated individually as a simply supported beam with external loads and two end support moments. For each intermediate support, one compatibility equation is written in terms of three moments. Thus, we get as many equations as there are unknowns. Each equation will have only three unknowns. It may be noted that, Clapeyron first proposed this method in 1857. In this lesson, three moment equations are derived for unyielding supports and in the next lesson the three moment equations are modified to consider support moments. 12.2 Three-moment equation A continuous beam is shown in Fig.12.1a. Since, three moment equation relates moments at three successive supports to applied loading on adjacent spans, consider two adjacent spans of a continuous beam as shown in Fig.12.1b. , and respectively denote support moments at left, center and right supports. The moments are taken to be positive when they cause tension at L M C M R M Version 2 CE IIT, Kharagpur

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bottom fibers. The moment of inertia is taken to be different for different spans. In the present case and denote respectively moment of inertia of; left and
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m2l12 - Module 2 Analysis of Statically Indeterminate...

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