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m4l30

# m4l30 - 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 30 The Direct Stiffness Method: Plane Frames Version 2 CE IIT, Kharagpur
Instructional Objectives After reading this chapter the student will be able to 1. Derive plane frame member stiffness matrix in local co-ordinate system. 2. Transform plane frame member stiffness matrix from local to global co- ordinate system. 3. Assemble member stiffness matrices to obtain the global stiffness matrix of the plane frame. 4. Write the global load-displacement relation for the plane frame. 5. Impose boundary conditions on the load-displacement relation. 6. Analyse plane frames by the direct stiffness matrix method. 30.1 Introduction In the case of plane frame, all the members lie in the same plane and are interconnected by rigid joints. The internal stress resultants at a cross-section of a plane frame member consist of bending moment, shear force and an axial force. The significant deformations in the plane frame are only flexural and axial. In this lesson, the analysis of plane frame by direct stiffness matrix method is discussed. Initially, the stiffness matrix of the plane frame member is derived in its local co-ordinate axes and then it is transformed to global co-ordinate system. In the case of plane frames, members are oriented in different directions and hence before forming the global stiffness matrix it is necessary to refer all the member stiffness matrices to the same set of axes. This is achieved by transformation of forces and displacements to global co-ordinate system. 30.2 Member Stiffness Matrix Consider a member of a plane frame as shown in Fig. 30.1a in the member co- ordinate system ' . The global orthogonal set of axes ' ' z y x xyz is also shown in the figure. The frame lies in the xy plane. The member is assumed to have uniform flexural rigidity EI and uniform axial rigidity EA for sake of simplicity. The axial deformation of member will be considered in the analysis. The possible displacements at each node of the member are: translation in - and - direction and rotation about - axis. ' x ' y ' z Version 2 CE IIT, Kharagpur

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Thus the frame members have six (6) degrees of freedom and are shown in Fig.30.1a. The forces acting on the member at end j and are shown in Fig. 30.1b. The relation between axial displacement and axial forces is derived in chapter 24. Similarly the relation between shear force, bending moment with translation along axis and rotation about axis are given in lesson 27. Combining them, we could write the load-displacement relation in the local co- ordinate axes for the plane frame as shown in Fig 30.1a, b as, k ' y ' z Version 2 CE IIT, Kharagpur
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m4l30 - Module 4 Analysis of Statically Indeterminate...

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