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'...
(Divisional Road Engineer, Ministry of Transport, Birmingham), after congratulating the Authors on producing an interesting and informative Paper about a structure which was by far the biggest and most important bridge on the M50 Motorway, said it was of interest to note that the line of the Ross Spur had been finally established for the greater part of its length on a line originally conceived in 1945. As a result of the public inquiry held in 1953, three relatively small variations had been made at the Herefordshire end, but they had been very localized and there had been no major departure from the original.70. In the section of the Paper dealing with basic requirements it was interesting to note the difficulties mentioned in 0 4 concerning extreme flood conditions. During the course of construction of the work, flooding in full measure had been the unhappy lot of the contractor throughout the work but particularly in the early stages. In addition to the flooding difficulty, the clear headroom of 44 ft below the summer level of the River Severn would have entailed a very considerable embankment across the flood plain of the river and there had been considerable doubt about the ground being capable of sustaining an embankment of such great height. On that ground also the long viaduct was fully justified. 71. The structure as a whole was a most elegant work and it was therefore all the more unfortunate that very few people would see it in elevation, because it was tucked away in a corner between Worcestershire and Gloucestershire access to which, while possible by public roads, was nevertheless not known to many people. The original proposal for the main river bridge, a very long time ago, had been for a lattice girder type of structure. He was very glad that that had been dropped in favour of the plate girder design finally built.72. Turning to the construction of the work, the photographs accompanying the Paper showed very well how the work had been done. It was probably true to say that having regard to the size of the job and the site difficulties due to flooding, weather and so on, the work had been carried out with no more than the normal number of headaches to contractors and consulting engineers. Mr Farr, who would speak later, would bear out that statement.73. Mr Jones directed attention especially to the very excellent pier shutter shown in Figs 10 and 11, which must have proved an excellent investment for the contractor, who with very little trouble had been able to use it to construct every one of the 44 piers of the viaduct, apart from the anchor piers at the ends of the main steel span. The facility of handling the shutter must have proved of the greatest value in *
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