Author: Cowan, H J
First published: N/A
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Cowan, H J
Dr R. D. Henshell (PAFEC Ltd)
I should like to congratulate Dale Carlton and his team, and the other teams he works with, on holding a number of records in respect of the largest problems of different categories that have ever been tackled in the history of finite elements.
On Saturday 13 June 1992 two of three spans of a masonry viaduct at St John’s Vale Station, Deptford, collapsed during a pause in demolition work. Two men were killed, buried under the fallen spans, and several others were injured as they descended on top of the debris. The collapse was of the ‘domino’ type and was described in New Civil Engineer as unexpected. Both the BBC and Grampian Television reported an apparently similar collapse in Aberfoyle, near Balmoral, in 1989. In both cases the structure collapsed when the crown of one span had been partly cut away. This feature, while prompted by these two failures, is not intended to present a detailed explanation of them. In the case of the Deptford failure particularly, only very limited information is available.
This paper is intended to show how wind excitation is a major effect in the design of steel chimneys. The static behaviour is well covered by publications and Codes, but the same is not true of the dynamic behaviour. Accordingly, this is presented in a form that closely parallels static methods and combines with the static calculations to obtain overall values of amplitudes, stresses or defections. Structurally sound approximations are given for values of the various parameters, which are within a few percent of ‘exact’. The calculations obtain the aerodynamic forces acting on a chimney, the resulting amplitude of vibration, the dynamic stress, and an assessment of whether metal fatigue might cause dificulty. An example is included of a 80m-high chimney of 3.25 m diameter to show that the dynamic procedure is quite feasible for hand calculations in a standard design office.
Professor A. Bolton