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When the first wrought-iron suspension bridges were constructed in this country-the most famous were the Menai bridge by Telford and the Clifton bridge by Brunel-there was naturally some scientific interest in their design and properties. The considerable flexibility of these bridges, leading in some cases to excessive deflections under railway loads, and in some others to lateral and vertical oscillations under high winds, tended to concentrate scientific interest on the deflections and vibrations of suspension bridges, and this is reflected in the literature of the time. But the severe effects of these oscillations, coupled with
the increasing rolling loads on roads and railways, rapidly deterred responsible English engineers from the wi$espread use and further development of suspension bridges, and so we find that the early research work came to a premature end, and was not revived until some 90 years later, when proposals for bridging the Severn, the Humber and the Forth for road traffic brought long span suspension bridges to the fore
in this country. Meanwhile, as is well known, there had been very considerable developments in suspension bridge design in the U.S.A., culminating in the Golden Gate bridge with a maximum clear span of 4,500 feet. That the decision to proceed with the design of the Severn bridge should have roughly coincided in time with the oscillatory failure of the Tacoma Narrows bridge may well be looked upon by future historians as fortunate for any long span suspension bridges to be built in England; it has certainly reawakened scientific interest in the behaviour of suspension bridges, and is of course the indirect origin of this paper.
Professor A. L. L. Baker (Member) opened the discussion by saying that he agreed entirely with what the authors had said in their presentation but not with what they
had written in the paper. He agreed with the statistical, probabilistic approach. It was obviously the only logical way of dealing with the variations of strengths of materials and the variations and vagaries of loading. There were one or two points in the paper which Professor Baker found slightly misleading. First of all, the matter of basing a design entirely upon a probability of failure-10-7-and a minimum cost, and saying that one could not entirely guarantee a structure against failure, needed some qualification. He thought this was rather a dangerous statement.
Dr. D. Tordoff (BCSA): We are all conscious of the fact that changes are taking place in design specifications and I should like to ask Mr Needham to comment on the changing philosophy of connection design with the introduction of limit state principles. Will a different design approach be required, and is there any research needed, or being undertaken currently, for a new design approach?