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Authors: Saran, Swami
Issue Date: 1969
Abstract: The criteria for satisfactory action of a footing subjected to vertical loads and moments are its ultimate bearing capacity and the permissible tilt and settlement. These requirements are analogous to the satisfactory action of an isolated centrally loaded footing. The bearing capacity of such footings has been deter mined analytically first. In this analysis, the footing has been considered rigid and the vertical load and moment have been replaced by an eccentric vertical load. It has been assumed that rupture surface is a log-spiral and failure occurs on the same side as the eccentricity with respect to the center of the footing. The resistance mobilized on this . x side is full passive and that on the other is partial. The footing has been assumed to lose contact with increase in eccentricity and bearing capacity factors Ny, Nq and Nc are obtained considering three cases separately i.e. (i) c = q = o, (ii) c = Y = o and (iii) q = Y = o. The total bearing capacity Q^ is then determined by superposing the limit stresses obtained by the three above cases. The error because of superposition is very small. For verification of analytical solutions, model tests and plate load tests were conducted. In two dimensional model tests, footings of 5 cm and 10 cm width were tested in a box 11 65 cm x 7.2 cm x 41 cm high containing sand. In three dimen sional tests, rectangular footings (10 cm x 20 cm, 10 cm x 30 cm, 10 cm x40 cm) and square footings (7.5 cm x7.5 cm, 10 cm x[o cm, 15 cm x15 cm) were tested in abox 114 cm x114 cm x 49 cm high. Both the two and three dimensional tests were per formed on dry Ranipur sand in dense (Dr - 84*) as well as loose (DR =46^) states. The footings were tested both at the surface and at a depth such that Df/E =1 and eccentricity of load ranged from 0.1B to 0.4B where Df and B are respectively depth and width of the footing. Plate load tests were carried out on 38 cm x 38 cm footing at a site in the University campus with eccentricities of 0, 0.1B, 0.2B and. 0.3B for the load. Failure load, settlement of the point of load applica tion, tilt of footing and the extent of failure surface at ground level were observed in each case. Failure surface could be observed in two dimensional tests only. The contact width and maximum settlement of the footing (i.e. settlement of the edge of footing) were computed from these observations. The pattern of contact width did not appear to influ ence the computed values o*f Ny, Nq and Mc. The extent of failure surface is affected by the pattern of variation of contact width (x]_). The experimental values tally closely with the theoretical values for the conventional variation. Ill Acomparison of the two dimensional model test results was used to compute shape factors. The field test results agree reasonably with the theoretical values considering shape factors obtained above. A comparison of the proposed theory with the test data of other investigators showed excellent agreement. A non-dimensional correlation has been established between settlement of the centrally loaded footing (S0) and settlements of the eccentrically loaded footing at its edge (Sm) and below the load (Se), from model tests. There exist unique relationships between Se/S0 and e/Bf and Sm/S0 and e/B at arbitrarily defined factors of safety of 1, 2 and 3. These are independent of the size and shape of the footing. Using these relationships, settlements of the footing in the field were predicted under eccentric loads. An excellent agreement was found between the predicted and computed values of settle ments and tilts. These correlations were also found to hold good for the maximum settlements of Transcona Grain Elevator and Leaning Tower of Pisa within reasonable limits. Thus settlements and tilts of footings subjected to moments can be predicted from plate load test under central load. The ultimate bearing capacity can be determined using bearing capacity factors developed in this investigation. In this manner, a complete solution of a footing subjected to moments has been obtained for the first time.
Other Identifiers: Ph.D
Appears in Collections:DOCTORAL THESES (Civil Engg)

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