Editor’s Note: Professor Jennings' contribution is, he says, "relatively non-technical, and is aimed primarily at laymen and engineers not directly engaged in structural engineering". Papers of this sort are so rare that we are delighted to offer this one from so respected an authority as Professor Jennings, not only for those of our members that the Professor intended should benefit, but also for structural engineers. All of us, deeply involved with technical detail, tend to lose something of our appreciation of the overall problem.