Discussion. Long Span Cable Roof Structures.

Abstract

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 ...