Mr A. N. Beal (M): It is surely a little optimistic to assume that restrictions on time of day for movement of unusual loads will have an ‘insignificant’ effect on their probability of combination with temperature differences (p179). Indeed, it is likely that correlations with time of day-and time of year-will have a dramatic effect on any
calculated probabilities and that the figures quoted in the paper can be regarded only as approximate in the extreme. However, in considering the problems of combined temperature and vehicle loading, there are non-probabilistic questions to ask-and these
may be more important. In particular, it should be remembered that temperature stresses arise from imposed deformations, whereas live and dead load stresses arise from imposed loads. The distinction is important because one is relieved by the onset of yielding, whereas, for the other, yielding leads to failure. Thus while temperature changes may lead to high calculated stresses, these are quite different in nature to, and much less worrying than, stresses due to applied loads-and this, rather than assumed probabilities of joint occurrence, may be the true reason for the satisfactory performance of bridges designed to pre-1973 rules.
Mr J. Wheeler (Main Roads Department, Perth, Western Australia):