m11l28 - Module 11 Foundations - Theory and Design Version...

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Module 11 Foundations - Theory and Design Version 2 CE IIT, Kharagpur
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Lesson 28 Foundations - Theory Version 2 CE IIT, Kharagpur
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Instructional Objectives: At the end of this lesson, the student should be able to: explain the two major and other requirements of the design of foundation, identify five points indicating the differences between the design of foundation and the design of other elements of the superstructure, differentiate between footing and foundation, differentiate between shallow and deep foundations, identify the situations when a combined footing shall be used, explain the safe bearing capacity of soil mentioning the difference between gross and net safe bearing capacities, determine the minimum depth of foundation, determine the critical sections of bending moment and shear in isolated footings, draw the distributions of pressure of soil below the footing for concentric and eccentric loads with e L /6 and e > L/ 6, determine the soil pressure in a foundation which is unsymmetrical. 11.28.1 Introduction Till now we discussed the different structural elements viz. beams, slabs, staircases and columns, which are placed above the ground level and are known as superstructure. The superstructure is placed on the top of the foundation structure, designated as substructure as they are placed below the ground level. The elements of the superstructure transfer the loads and moments to its adjacent element below it and finally all loads and moments come to the foundation structure, which in turn, transfers them to the underlying soil or rock. Thus, the foundation structure effectively supports the superstructure. However, all types of soil get compressed significantly and cause the structure to settle. Accordingly, the major requirements of the design of foundation structures are the two as given below (see cl.34.1 of IS 456): Version 2 CE IIT, Kharagpur
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1. Foundation structures should be able to sustain the applied loads, moments, forces and induced reactions without exceeding the safe bearing capacity of the soil. 2. The settlement of the structure should be as uniform as possible and it should be within the tolerable limits. It is well known from the structural analysis that differential settlement of supports causes additional moments in statically indeterminate structures. Therefore, avoiding the differential settlement is considered as more important than maintaining uniform overall settlement of the structure. In addition to the two major requirements mentioned above, the foundation structure should provide adequate safety for maintaining the stability of structure due to either overturning and/or sliding (see cl.20 of IS 456). It is to be noted that this part of the structure is constructed at the first stage before other components (columns / beams etc.) are taken up. So, in a project, foundation design and details are completed before designs of other components are undertaken. However, it is worth mentioning that the design of foundation structures is
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m11l28 - Module 11 Foundations - Theory and Design Version...

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