BEHAVIOUR OF ECCENTRICALLY LOADED RING FOOTINGS ON SAND

Abstract

Ring footings are generally used, for supporting tall circular structures such as water towers, chimney stacks, cooling towers and vertical silos. Besides being economical, ring foundation is often the only solution when dual condition of full utilization of soil capacity and no tension under foundation is to be satisfied. These foundations are subjected to eccentric loads due to bending moments caused by wind pressure or seismic force in addition of vertical loading. At present no theory exists to compute the bearing capacity of eccentrically loaded ring footings. The present model study has been carried out to study the failure load, settlement at the point of load application, tilt of footing and extent of failure surface. Three dimensional test has been carried out on nine model footings with three different external diameter viz 10 cm, 20 cm and 30 cm. Ratio of internal diameter to external diameter of footing in each case has been kept as 0.0, 0.4 and 0.8. Tests were carried out on poorly graded air dried Ranipur sand at medium dense state of packing. The footings were tested both at the surface and at shallow depth keeping Df/B = 0 .5 and eccentricity of load ranged from 0 .1B to 0 .3B, where D f and B are respectively depth and external diameter of the footing. Non-dimensional correlations have been established between settlement of centrally loaded footing (So) and settle-ment of the eccentrically loaded ring footing at its edge (Sin) and below the point of load application (Se), from the model tests. Equations showing relationship between Se/So and e/B and Sm/So and e/B for different openings of ring footings have been worked out by least square method of curve fitting. General correlations encompassing different parameters have also been obtained. Interpreting the results, an empirical formula for the ultimate bearing capacity of eccentr ca y.loaded ring footings has been developed based on di nsioval analysis.