The Constructional Aspect of Large Electricity Generating Stations

WATER ratio rules appear to afford a simple method of controlling the strength of concrete. It is true that the application of such rules in practice is complicated by the necessity for allowances for the absorptions of the aggregates or for the free water which may be present in them, but this may be regarded as a legitimate problem for the engineer in charge. The real difficulty lies in the fact that strength is also affected by other factors relating to the composition of the concrete, and, in the brief introductory account which follows, methods of making allowance for the effects of these factors are mentioned. W. Hunter

Mr. M. N. RIDLEY, M.Inst.C.E. (Member of Council), proposing a vote of thanks to the author, welcomed the paper, particularly because it indicated the extent to which continuity in reinforced concrete work was being adopted. He had noticed that engineers were inclined to make pin joints in their bridges. They would put a pin joint in the centre of the arch and pin joints in the abutments, and he believed that in one or two cases there were as many as five. He had always maintained, however, that. a bridge of ordinary span was far safer and better, and sometimes more economically designed, if all pin joints were excluded. It was true that calculation was then more difficult, but he held that when that had been done one had the best type of work. Where there were very big spans, of course, one must consider pin joints or the equivalent, especially for expansion and contraction; but for shorter spans, at any rate up to 100 ft. and sometimes more, in reinforced concrete and steel work pin joints were quite unnecessary.

AS a Structural Engineer getting on in years I can look farther back than most of you. I have seen many changes in constructional engineering. I remember the change from wrought iron to steel. The change in working stress from 5 or 4 tons per square inch to 7 1/2 tons per square inch meant that the quantity of metal used was less. The smaller sections used presented problems which took us a little time to get used to. Nowadays no one sees any problem in it. There is one disadvantage in the use of steel: that is its greater liability to corrosion. I have had great experience in Promenade Pier work, and in that direction have noticed the excessive corrosion in steel as compared with wrought iron. For instance, in the Brighton West Pier there are rolled iron joists and girders still in good condition. This pier was opened in 1866; that gives a life of over 67 years to the wrought iron, while some steel lattice girders and joists have had to be taken out after a life of only 20 to 30 years. Rolled steel joists corrode very quickly, due usually to the thinness of their webs ; 3/8 in. metal for lattice girders also corrodes very quickly. In my practice I consider that 1/2 in. metal is the minimum thickness for pier work. The tendency at the present time, to reduce the thickness of webs of rolled steel joists, is, from the pier point of view, unsatisfactory. M. Noel Ridley