Rigid Frame Bridges in Reinforced Concrete. Discussion on Mr. A.P. Mason's Paper

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.

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

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