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."