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Partners) supported very strongly the Authors' conclusions that, for this type of central development, the cast in situ flat-plate slab was undoubtedly the cheapest, and the quickest to use. It also achieved the minimum depth of deck construction and allowed for the easiest distribution of mechanical, electrical and sanitary services. It had in addition a satisfactory natural fire resistance.73. His own firm was at present concerned with the design of the new town centre at Cumbernauld, near Glasgow, which was roughly 600 ft in one direction and would extend to about half a mile in the other direction. Here, too, flat-plate slabs were being used for the six lower deck-levels.74. The main difference between Cumbernauld and the Elephant and Castle was that whereas the spans at the latter were generally 24 ft, those at Cumbernauld were generally 35 ft and 44 ft. Many different forms of construction, including ordinary concrete beams and slabs, precast concrete, structural steelwork, composite construction, and prestressed concrete had been studied, but for the particular dimensions waffle flat-plate slabs gave the cheapest overall solution to the job.75. The greatest loads carried by the floors at Cumbernauld were those of the construction of the next floor up, and, indeed, as a result, the structure had already been pre-tested, so to speak, with a good load factor. 76. Fig. 21 showed the ingenious method of formwork which had been devised by the contractors, Messrs Duncan Logan. The rib sofits were formed by steel T-sections in pieces generally about 6 ft long. These were supported on special props arranged at 6 ft centres in the two directions. The indentations were then created by fibreglass pans pushed up from beneath. The advantage of inserting the pans from beneath was that after the concrete had been placed the pans were then free to be released first while the ribs were still supported by the T's. 77. Another feature of this formwork system was that the special props gave double support to the waffle ribs. At the end of each prop there was a bearing plate, which formed its own little piece of soffit shuttering, and the main lengths of the steel T s were then supported separately from the props with independent adjustment. This enabled the rib soffits to be struck earlier than the removal of the props, which were required to support the ribs in between.78. The sequence of construction therefore was to set up the props and T's; then to place the reinforcements; then to present the fibreglass pans from underneath; and then to pour the concrete. The pans were then removed by a brief shot of compressed air 48 hours later; the rib soffits were struck at seven days, but the props at 6 ft centres in both directions were left in position for 14 days.79. The Authors were correct in saying that the problem arising from the use of flat-plate slabs was the inflexibility which this could lead to in terms of changed requirements. At Cumbernauld they were trying to meet this point by providing large openings in the floors t...

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].

It is indicated in 5 19 that the end blocks are prestressed in three directions. I took this to mean the transverse stressing of the end beam on which the precast planks stand; nothing, however, appears on Fig. 8. Can the Authors clarify? 81. I was interested in the toothed surfaces between the units 'precast', as it were, on the falsework. I would like to know why they were toothed. Did the Authors carry out any tests? This cannot have been an inexpensive construction, and it seems to me that some justification should be given for having such detail. 82. The Authors say later that any jointed construction in the beams over the piers would be inadequate in strength. Can they give the grounds for this statement? 83. The Authors, in 5 36, state that the relaxation of the wire did not exceed 7%. I may be mistaken in my dates, but I would have thought that wire with 3% relaxation was available at that time. 84. The encasement of tendons was one of the points which for some time exercised the minds of those who sit on CP 11 5 as to whether it was necessary to have any further protection, as clearly the casing of the tendons must be unstressed. Although one Author has said that there were no visible cracks, this does not mean that there were no cracks. Have the Authors given any thought to this matter, and what is their view? There are not enough case histories of this type of construction to formulate the right sort of guidance in a code of practice. 85. I was most interested in the various analyses done for the prestressing of the end blocks vertically. It would be useful to know which of the values of the envelope, found from all the various work done, was used.