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The railway loading in this Paper is based on an equivalent loading which was evolved by London Transport in about 1973, and which has now been incorporated, slightly amended, into the loading section of the revised British Standard 153. This bridge is the first railway bridge in the world of this type of span to be designed to this reduced railway loading, applicable to rapid transit systems.67. For impact, the allowance appears to be exactly the same as in the revised BS 153 and the London Transport loading. This impact is less than is allowed on railway bridges elsewhere throughout the world and was based on tests which were carried out on certain London Transport bridges. We reduced our impact loading because our stock is somewhat different from main line stock in that it does not have locomotives generally so far as passenger stock is concerned. In the case of Tyne and Wear (Fig. 3), the axle loading for works vehicles is only about 25% higher than for passenger vehicles. I would like to ask what the operating speeds are and whether any separate calculation was made when determining the impact allowance on static loading.68. From Fig. 7, which shows the general cross-section of the bridge, the deck units appear to be separate troughs carried on the longitudinal stringers. I would like to ask what precautions were made regarding derailment, because it seems to me that if the diesel locomotive became derailed there could be local instability of the deck units, and that is contrary to what is laid down in the new British Standard. Furthermore, I wonder whether there has been as yet any loading test on the bridge or whether there is any plan for a test loading to prove the actual forces in the members.69. Although the works loading is more than the passenger loading, I am surprised that none of the elements is subjected to fatigue loading (4 12) in that fatigue decides the sizes. I wonder if the Authors could say how many passenger trains are expected to cross the bridge each day. In the light of the sort of frequency that we have in London, I would expect that certainly the stringers and some of the shorter loaded members would be determined by fatigue considerations.

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