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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."
The AUTHOR introduced his paper by saying that in his view economies in material ought not to be achieved by a disproportionate expenditure of human energy or loss of amenities, and he hoped that if the paper had been studied in this light there would be
found much of interest which was not necessarily confined to a period of steel shortage.