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It has appeared to the author for some considerable time that a reluctance on the part of designers to depart from the well implanted formula M/I = f/y, and the modular ratio concept of reinforced concrete design was leading them into considerable difficulties and perplexities when tackling prestressed concrete work. As a result one frequently hears the questions, " How do you find ' I ' for a prestressed beam ?"-"What is the modular ratio for a concrete stressed so highly as to have considerable plastic deformation ?"-"How can one deal with composite construction where part of the
work is prestressed and part cast in situ, the concretes being of different qualities ?"-etc. More and more elaborate formulae appear daily, which try to weigh the
imponderables involved, and "guestimation" plays a large part in much design work as a consequence, the errors being safelv masked by that ever to be cherished factor of "ignorance."
This investigation has shown that for narrow footings on inundated sand the ultimate bearing pressures are between 80 and 90 per cent. of those for footings on dry sand. According to the usually accepted theories, bearing pressures on inundated sand should only be about 50 per cent. of those on dry sand due to the reduction in the apparent density of the sand and also (possibly) to a slight reduction in the angle of internal
fnction. The reason for the discrepancy between the theoretical and experimental bearing values is that the actual mechanism of failure differs from that assumed in any of the several theories, and further is slightly modified when the footing becomes inundated. The locus of the sliding surface was determined experimentally for the dry sand and computed bearing pressures based on the inferred failure mechanism agree with the experimental loads.
Mr J. M. Mawditt (M): The paper is to be commended for simplifying the design procedure associated with the use of the material stress -strain curves as required by clause 18.104.22.168. of CP 110.