D r P. Broughton, Lloyd's Register of Shipping Long span cable roof structures remain a rather novel form of construction in the UK. Indeed, for reasons beyond my imagination, lightweight roof structures of both the tension and compression type remain rather few in number.110. I should like to make one or two comments on the analytical aspects of such structural forms. In 5 12 it is suggested that double-curvature nets of the single layer type suffer from the disadvantage that downward loading results in a load increase in the sagging cables together with a simultaneous load decrease in the hogging cables. I believe this statement to be true for most forms of cable roof structures without differentiating between single and double layered systems. 111.Referring to 5 25, I should have thought that the main disadvantage of the radial system as shown in Fig. 2 is that, unlike the rectilinear cable layout shown in Fig. 1, there appears to be insufficient interconnexion between any one radial truss and the next, that is, apart from the central cage. Thus the series of radial trusses will behave much more as a series of plain frameworks as opposed to a complex space frame. 112.In 8 52 it is suggested that the design process may be lengthy and costly because structures of this type behave in a non-linear manner and for complex systems may possess many degrees of freedom. However, current methods of analysis allow for the solution of large problems in an economic manner. I feel that such advances are generally attributable to the wit of the electronic engineer and his computer rather than the civil structural engineer.113. I do not agree with the statement in 5 54 that physical models of structures are inexpensive in relation to mathematical models, particularly for structures of this type. The complete non-linear analysis, allowing for gross changes in geometry, of structural forms outlined in the Paper, is by today's methods considered to be straightf0rward.l 114. Further, structural mathematical models also possess the advantage that they are more readily adaptable during the design process. However, I agree that the testing of physical models is desirable for structures of this type, particularly regarding the dynamic aspect, and in this respect I would suggest that even the testing of fullscale structures may be necessary for further correlation. 115.With reference to the design examples referred to in $5 57-64, it is known that the effect of a flexible ring beam is ro alter the behaviour of a structure of this type considerably, particularly if unsymmetrical loading is to be a main consideration.In this respect, the statement made in 5 64 somewhat contradicts that made in 0 18. 116.It is noticed in the first example that the upper cables suffer a reduction in tensile force from 92 tons force to 50 tons force when the framework is subjected to symmetrical loading. It is suggested that greater changes in cable forces will occur under even moderate unsymmetrical loading, say due to agglomerated snow loading, and with ...

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...

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Roof spans of 100 to 150 m are now common in American sports stadia and to accommodate audiences of up to 100 000 it is likely that, both in the US and Europe, spans of 250 or 300 m will be required within the next decade. The Paper discusses the various factors governing the design and construction of such roofs and gives information regarding existing structures. Detailed cost analyses of several types of cable roof are given, and on the basis of these a prediction of the approximate costs of very long span roofs is made. Structural form General principlesA cable roof structure, of whatever type, carries vertical loads by virtue of its shape in the same way as a suspension bridge. The load-carrying part of the structure always consists of sagging cables stressed in tension, any other cables being provided for ancillary purposes and not for carrying downward vertical loading.2. This arrangement results in large out-of-balance horizontal forces at the perimeter of the roof, which must be resisted by an anchorage or ring beam.3. The use of high-tensile steel cables stressed only in tension as the principal structural members results in a uniquely cheap method of achieving really long spans, providing the anchorage problem can be solved economically.4. If the loading were invariably symmetrical and in the downward direction, a single layer of sagging cables would provide the necessary structural basis for any roof. However, this arrangement would not be adequate to cater for asymmetrical live load conditions, wind uplift, or dynamic instability in the form of wind-induced oscillations. 5.It is therefore necessary to incorporate either a secondary inverted cable system or some type of rigid double-curvature cladding, such as a concrete shell, to deal with these ancillary loading conditions. Two-layer systems 6. In order to achieve the necessary stability the cables in two-layer systems are prestressed. This results in increased horizontal peripheral forces, together with an increased downward loading on the sagging cables in the form of a reaction transmitted from the hogging cables.7. The two-layer cable systems fall into two principal categories: (a) lens-shaped systems which are circular or elliptical in plan (b) systems which are rectangular in plan. In lens-shaped systems the peripheral horizontal forces are usually resisted by a ring beam which is stressed primarily in compression. The cable layout can be either rectilinear in the form of a square grid (Fig. 1) or radial as in the case of a 'bicycle wheel' system ( Fig. 2). In either case the average resultant peripheral forces under uniform loading are primarily radial.