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Mr. DWIGHT said that in the paper he had attempted to reduce the complication and to make life easier for the designer. Aluminium had two serious disadvantages which were rather like two millstones around the neck of the designer in aluminium-first the high cost of the material and secondly the low E. Aluminium cost about two and a half times as much as steel and it was three times as flexible. But there were two important advantages - first that it did not rust, and secondly we had the marvellously versatile extrusion process at our disposal. Because it did not rust, we had not to worry about using thin section; we might use 1/8 in. section, whereas in the case of steel the 1/8 in. might represent the rust allowance.
Simply-supported bridge-decks are frequently designed as grillages so that the interaction between longitudinal main members can assist the load distribution when the deck has to carry the abnormal heavy vehicle (the H.B. loading of B.S.153). The analysis of simply-supported single-span grillages is now well established by the use of the ‘ quasi ’ slab technique. In recent years the number of continuous or otherwise statically-indeterminate bridge decksparticularly in prestressed concrete-has been increasing and where continuous decks have been adopted they have often been constructed with beams of varying depth and section properties. A demand has, therefore, arisen for an analytical technique for such continuous grillages. A. Goldstein, E. Lightfoot and F. Sawko
Mr. F. T. Middleton (Member) asked what, in United States practice and taking 30 storeys as a maximum, was approximately in the author's estimation and experience the weight, cost and erection speed differential between a high strength bolted structure and a comparable site-welded partially bolted structure?