m4l28

m4l28 - Module 4 Analysis of Statically Indeterminate...

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Module 4 Analysis of Statically Indeterminate Structures by the Direct Stiffness Method Version 2 CE IIT, Kharagpur
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Lesson 28 The Direct Stiffness Method: Beams (Continued) Version 2 CE IIT, Kharagpur
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Instructional Objectives After reading this chapter the student will be able to 1. Derive member stiffness matrix of a beam element. 2. Assemble member stiffness matrices to obtain the global stiffness matrix for a beam. 3. Write the global load-displacement relation for the beam. 4. Impose boundary conditions on the load-displacement relation of the beam. 5. Analyse continuous beams by the direct stiffness method. 28.1 Introduction In the last lesson, the procedure to analyse beams by direct stiffness method has been discussed. No numerical problems are given in that lesson. In this lesson, few continuous beam problems are solved numerically by direct stiffness method. Example 28.1 Analyse the continuous beam shown in Fig. 28.1a. Assume that the supports are unyielding. Also assume that EI is constant for all members. The numbering of joints and members are shown in Fig. 28.1b. The possible global degrees of freedom are shown in the figure. Numbers are put for the unconstrained degrees of freedom first and then that for constrained displacements. Version 2 CE IIT, Kharagpur
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The given continuous beam is divided into three beam elements Two degrees of freedom (one translation and one rotation) are considered at each end of the member. In the above figure, double headed arrows denote rotations and single headed arrow represents translations. In the given problem some displacements are zero, i.e., 0 8 7 6 5 4 3 = = = = = = u u u u u u from support conditions. In the case of beams, it is not required to transform member stiffness matrix from local co-ordinate system to global co-ordinate system, as the two co-ordinate system are parallel to each other. First construct the member stiffness matrix for each member. This may be done from the fundamentals. However, one could use directly the equation (27.1) given in the previous lesson and reproduced below for the sake convenience. Version 2 CE IIT, Kharagpur
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[] = L EI L EI L EI L EI L EI L EI L EI L EI L EI L EI L EI L EI L EI L EI L EI L EI k z z z z z z z z z z z z z z z z 4 6 2 6 6 12 6 12 2 6 4 6 6 12 6 12 2 2 2 3 2 3 2 2 2 3 2 3 (1) The degrees of freedom of a typical beam member are shown in Fig. 28.1c. Here equation (1) is used to generate element stiffness matrix. Member 1 : , node points 1-2. m L 4 = The member stiffness matrix for all the members are the same, as the length and flexural rigidity of all members is the same. [] 1
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m4l28 - Module 4 Analysis of Statically Indeterminate...

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