Discussion on Structural Paper No. 53: Masonry Arch Bridges: A Study.

Abstract

DISCUSSION ON MASONRY ABCHBRIDQES: A STUDY used almost as extensively on these as for its original purpose. It has been operated from Hampshire to the County of Durham, travelling over some of the most severe routea used by abnormal loads without serious hitch and always to a strict timetable, while conforming scrupulously to all regulations governing such movements.on the positions required even when moving the machine in reverse, and in most unsuitable 137. The operating crew acquired an extraordinary skill in placing the bogies precisely circumstances.The Chairman, in moving a vote of thanks to the Authors, said that the Paper dealt in a very interesting manner with the assessment of the strength of masonry arches by investigation of their movements and deflexions under load. It reminded him how an arch, by virtue of its shape, was never a t rest. The weight of the crown and any live load on the arch tended to thrust the abutments outwards, although, as the Authors had shown, one could get quite unexpected movements from live loads at various places.141. He hoped that the Authors would be able to add the dates of construction and a note on the shapes of the arches of the bridges that had been tested.142. Had any significant differences been found in the results of the tests on arches of varying shapes? The three main shapes were the semi-circular, the pointed or ogival, and the elliptical or basket-handled arch. In Fig. 25, of the Yarm Bridge, t.he far arch appeared to be more or less of circular shape and the nearer arch was pointed. Had the Authors tested both, and, if so, had there been any significant differences? 143. Brunel's railway bridge at Maidenhead was perhaps one of the most famous of basket-handled arches. He wondered how it aould respond to deflexion tests, and thought it would deflect considerably because of its high span/rise ratio. But it was a wonderful bridge and had given extraordinary service.144. There was one further point he wished to ask. Who had first enunciated t.he rule of the middle third, and when? Professor A. J. S. Pippard (Emeritus Professor of Civil Engineering and Senior Research Fellow, Imperial College of Science and Technology) confined his attention to three points only.146. The first was the suggestion that elastic theory should be used for the design of the arch bridge. That had first been suggested by Castigliano, and his book contained a worked example of strain energy analysis applied to an actual voussoir arch bridge. Any parts found to be in tension wexe assumed to be cut out and a new analysis made on the resulting arch. If one carried that process sufficiently far, one would probably finish with an effective pin joint at the point where tension was first discovered. However that might be, thc work done by himself, Miss Chitty, and other collahorators had led to the conclusion that the elastic approach w w proper to thc arch structurc.147. His second point was that of elastic bohaviour, and the straight lines obtained by the Authors in their tests indicated that'...