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The Structural Engineer, Volume 70, Issue 20, 1992
Dr J. B. Bellamy (F) (Marlow & Co. Ltd) The authors are ta be congratulated on a most interesting paper which shows that today, despite advances in technology, we are less willing to build large roofs in timber than our predecessors. Large numbers of Belfast truss roofs were built and appear to have given satisfactory service for many years; surely this provides us with a database of experience that could be used to assess the stress levels permitted in current Codes and the limit state factors proposed in EC5?
Mr A. C. G. Hayward (F) (Cass Hayward & Partners) The author’s interesting experiences in Papua New Guinea must be typical of those in other Far East countries where flooding and earthquakes occur, and I can echo similar conditions in Indonesia. Throughout history, bridge failures tend to occur by flood action, ship collision, earthquake or vehicle impact. Two of these are natural hazards, the others manmade which are the most frequent failure causes in more ‘stable’ environments. Only rarely does collapse occur from design overload which is the area on which most bridge designers spend their resources. There is a lesson here in that more attention is warranted in design to hazards such as vehicle impact and to the important matter of replacement, should this ever be necessary. Currently, this is a topical matter in the replacement of our bridges for motorway widening, where unit construction bridging may well have an increasing use and many existing bridges are of monolithic concrete construction, making their removal difficult.
Last words on the ‘genetic algorithm’ Last month (Verulam, 15 September) we attempted to summarise a long letter from Professor Jenkins in which he set out to expound, particularly in response to Stefan Tietz (21 July), the aims and potential achievements of his 'genetic algorith’ for structural optimisation. While we endeavoured to cover his main points, Professor Jenkins asks our indulgence in that he feeh we did not adequately present his answers to the ‘frank and uninhibited statements made by contributors’. His now proffered ‘closure’ is as follows: Mr Tietz has missed the point when he attributes functions to the GA that it does not possess. He questioned what he called ‘basic assumptions’ which did not appear in my paper. Nowhere do I talk about ‘rules’ for ‘perfection of design’ or for ‘construction costs’. I have been at pains to make the point that the GA is simply a tool in the designer’s toolkit. I am firm in my belief that structural design is an art practised by designers supported by scientific and engineering principles and methods, including computer-based methods. In this context I must urge Mr Tietz to take the opportunity to enlarge his definition of professional judgment. ‘The competent assessment of a huge number of variables’ is exactly what the GA does! Verulam