Lesson 35 Indeterminate Trusses and Industrial Frames Version 2 CE IIT, Kharagpur
Instructional Objectives: After reading this chapter the student will be able to 1. Make suitable approximations so that an indeterminate structure is reduced to a determinate structure. 2. Analyse indeterminate trusses by approximate methods. 3. Analyse industrial frames and portals by approximate methods. 35.1 Introduction In module 2, force method of analysis is applied to solve indeterminate beams, trusses and frames. In modules 3 and 4, displacement based methods are discussed for the analysis of indeterminate structures. These methods satisfy both equation of compatibility and equilibrium. Hence they are commonly referred as exact methods. It is observed that prior to analysis of indeterminate structures either by stiffness method or force method; one must have information regarding their relative stiffnesses and member material properties. This information is not available prior to preliminary design of structures. Hence in such cases, one can not perform indeterminate structural analysis by exact methods. Hence, usually in such cases, based on few approximations (which are justified on the structural behaviour under the applied loads) the indeterminate structures are reduced into determinate structures. The determinate structure is then solved by equations of statics. The above procedure of reducing indeterminate structures into determinate and solving them using equations of statics is known as approximate method of analysis as the results obtained from this procedure are approximate when compared to those obtained by exact methods. Also, approximate methods are used by design engineers to detect any gross error in the exact analysis of the complex structures. Depending upon the validity of assumptions, the results of approximate methods compare favourably with exact methods of structural analysis. In some way, all structural methods of analysis are approximate as the exact loading on the structure, geometry; the material behaviour and joint resistance at beam column joints and soil-structure interaction are never known exactly. However, this is not a good enough reason for using approximate methods of analysis for the final design. After preliminary design, it is important to analyse the indeterminate structure by exact method of analysis. Based on these results, final design must be done. In this module both indeterminate industrial frames and building frames are analysed by approximate methods for both vertical and wind loads. Version 2 CE IIT, Kharagpur
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