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SIR,-The paper reported in the April issue on La Roche Guyon Bridge, which was presented at a joint meeting of the Institution of Structural Engineers and the British Section of the Societe des Inghieurs Clivils de France, appears to be in the nature of further propaganda on the use of long span bridges in ferroconcrete. At a similar joint meeting some time ago, this subject was dealt with in a more general way. At both meetings the authors of the papers were advocating the use of reinforced
concrete in preference to steel for long span bridges. Statements have been made that reinforced concrete is more economical than steel for spans of well over 1,000 ft. It is difficult to believe that such extravagant claims would bear impartial investigation. They are apparently arrived at by using very high stresses in the concrete-in fact far greater than have ever been dreamed of in this country. It would therefore seem that the comparison is made by using high stresses in the concrete and normal stresses in the steel. If a concrete structure can be over-stressed with safety, surely it would seem still safer to over-stress a steel one by a proportionate
amount, in which case the comparative economic span lengths would be very different from those claimed.
THE use of arc welding in multiple storey steel frame buildings is quite a recent
development, but already sufficient structures have been erected to enable the nature of the problem to be appreciated and the probable future progress to be estimated. It is certain that when efficient workshop methods are adopted and proficiency in erection acquired, welding will gradually become the normal method of construction.
A. Ramsay Moon
MR. A. S. GRUNSPAN, BSc., M.Inst.C.E., A.C.G.I. (Member), having voiced his great appreciation of the paper and congratulated the lnstitution upon it, said that welding had come to stay definitely. The results of numerous tests that had been carried out were such as to convince even the most conservative that welding afforded a very excellent and economic means of joining parts of metal together. But, be that as it may, we must not think that we knew very much about welding; we really knew very little. The designer would have to give very careful consideration to temperature stresses in the weld and in the parent metal arranging his joints in order to rcduce the temperature effects to the absolute minimum. Another problem, which was perhaps less understood, was that of contraction in the weld and the stress resulting therefrom: longitudinal contraction was perhaps not so important as transverse contraction. Some very excellent examples of plate girders had been illustrated by Mr. Helshy; in welding plate girders, it was very important to weld the stiffeners on first and the flanges afterwards, otherwise distortion would occur-which was further evidence of the great importance of studying contraction. Mr. Grunspan asked for Mr. Helsby’s comments and experiences in regard to the welding of stiffeners in Staggered fashion or back-to-back; he was surprised to hear the Germans considered that staggering was the better method, for he had always understood that staggering was not
advisable and that the back-to-back method gave rise to less distortion due to temperature effects.