On some bridge sites it may be advisable to adopt two or even more different types of piles. Conditions may demand the adoption of a combination of two forms of piling to obtain the best results. H.E. Brooke-Bradley
Sir,-In Dr. von Emperger’s lecture, and the ensuing discussion, considerable attention was paid to the significance of tension cracks in beams. I venture to suggest that they give no information regarding the actual strength of the beam. In the initial stages of loading, before the concrete is cracked, the tension is shared by the steel and concrete according to their modular ratio, and the principal stresses approximate to those in a homogeneous rectangular beam. As loading progresses, the concrete creeps in compression, and, as Dr. Faber showed, a parabolic or trapezoidal distribution of compressive stress results. This causes a lowering of the neutral axis, which is demonstrated by Dr. Probst’s experiment on the alternating loading of a non-reinforced beam, and raises the intensity of tensile stress in the concrete. In time the concrete will fail, and the failure will tend to follow the line of principal tensile stress, resulting in “diagonal tension” cracks. If the beam is not reinforced in shear, it is then in the condition of a Vierendeel truss, with each finger of concrete between adjacent cracks transmitting its increment of load through compression and bending. In time, the finger will fail in bending at one of its fixed ends, either at the steel or near the neutral axis. If shear reinforcement is present, the beam is in the condition of a lattice truss, and loading may be continued till it fails in tension or compression. Further opening of the cracks is due to elastic extension of the tension or shear reinforcement, and will disappear when the loading is removed. Thus the strength of the beam is entirely unaffected by tension cracks if proper shear reinforcement is provided.
Our review of a U.S. Government report under this title describing the various new forms of metal connectors that have developed for purposes of timber framework construction on the European continent appeared on page 362 of the Journal for August, 1933. We now reproduce a photograph of such connectors (Figure 1) and two examples of their use.
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
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.
The constructional side of large electricity generating stations is a subject which has perhaps not received the consideration it deserves. This appears somewhat surprising when it is realised that the cost of the building works only of a large modern power station, excluding coal and ash handling facilities, condensing water system, and external subsidiaries, may be a proportion of the order of some 20 per cent. to 25 per cent. of the total cost of the station completely equipped with plant. Arthur Creswell Dean
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