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DiscussionMr A. J. Harris (Consulting Engineer) commented first on the question of safety.He had always hesitated t,o design on the basis of ultimate load and whilst he was becoming persuaded of the soundness of that principle for small structures, he was still very doubtful if the ultimate load was the right basis of design for heavy large-span bridges where the live load quite commonly represented little more than one-quarter of the tota load. Was it right to design on the ultimate limit in such structures? Considerations of the basis of design had led him to ask one or two questions. For example in 9 24 the Author had stated that it was necessary to have a very good bond between cable and concrete and went so far as to say "experience shows" that the rupture load might be reduced by 30% or even more for large spans, if the bond was not efficient. Was there in fact any such experience of large spans 4 There was much experience of testing very small spans, but could one call on experience of large spans to justify that view 9 221. The Author had further referred to the possibility of using cables of parabolic form which were completely free of the beam. How was that to be reconciled with the Author's remarks on bond, sincc with fiee cables there would be very little bond at all? On the ultimate basis of design such cables would be rejected, but many such had been successfilly used. Equally, on the question of factor of safety, the footnote to 5 121 Downloaded by [] on [11/09/16].

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