On Some Common Errors in Iron Bridge Design

5 There is a further qualification, however, and that is the allow- ance for the effect of friction of earth upon the sides of a bridge cylinder or caisson, and if this be taken into account, the pressure on the base is still further reduced. This friction is somewhat variable and has been stated as high as 800 and as low as 50 lbs. per square foot in different strata. Directing our attention to existing structures, great discrepan- cies appear in the size of cylinder foundations, not only between one structure and another, but between, different piers of the same structure. For example the Intercolonial Railway Bridge at Albury consists of two continuous spans of 160 feet each, carried on. three piers, each consisting of two cylinders of 10 feet diameter. The centre pair of these cylinders carry of the load, while the two end pairs together carry only Thus, while two cylinders carry a load represented by the number 10, four of equal size are provided to carry a load of 6 only, and these four are further surrounded by earth to a much greater height than the central ones, and therefore receive greater frictional support. It cannot, I think, be disputed that the bridge would have been both cheaper and safer had the end cylinders been reduced to 6 feet diameter, or even, less, for then any yielding would have been approxi- mately equal throughout, and the distribution, of bending moment in. the continuous girders consequently undisturbed. Similar remarks will apply to the Railway Bridges at Wagga, Bathurst, and Aberdeen, described in the Report of the Royal Commission on Railway Bridges, N.S.W., 1886. In all of these the terminal cylinders though carrying less than half the load, and more favourably circumstanced in other respects, ai’e just as large in diameter as their heavily loaded companions, see Fig. 1, which represents to scale the railway bridge at Aberdeen, N.S.W,. A reference to numerous successful cylinder and caisson bridge foundations leads to the conclusion that the subjoined are safe foundation pressures, the most unfavourable combination of load, wind and flood, being employed in. the calculation. Rock 10 tons per square foot at least. Fine compact sand at considerable depths, 6 tons per square foot. Very good clay 5 tons per square foot. Ordinary sand, clay, or loam 1 to 3 tons per square foot. Knowing then the superincumbent load and the nature of the