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Many low-rise steel frames are designed in practice on the basis of plastic theory. This factor is recognised by the purveyors of computer software, and programs for the plastic analysis and design of plane frames are proliferating. There are evident advantages in offering elastic-plastic analysis as an item on the menu, and some packages on the market now do this. However, there is a fundamental problem in elastic-plastic analysis of which the profession should be aware. The writer, and no doubt other readers of The Structural Engineer, would be interested to know whether any of the programs that are currently on commerical offer incorporate a solution. The problem in question concerns the ‘false mechanism’.
Professor J.M. Davies
In our June column, John Mayne, of the Building Research Station, argued against the validity of Mr L. Metter ’s thesis, contained in our March column, that the survival of structures designed to the I952 wind Code had shown the I972 Code to be ‘both onerous and incorrect’. Dr. Mayne quoted the ‘regular crop of damage to cladding and fixings occurring with every wind stronger than the most moderate’ as vindicating the pressures given in the 1972 Code. Mr Adrian Warburton now describes his experience in assessing the reasons for failure due to the storm of last October: After a night of lost sleep, and without communication until midday of Friday 16 October, it was with some trepidation that I returned to work. However, in the following (busy) weeks my surveys of damaged structures were interesting but, frankly, disappointing. While I was
sympathetic with owners, problems were invariably the result of mechanical damage or of some glaring construction weakness, caused by either faulty workmanship or lack of maintenance.
Results are reported of tests performed on reinforced concrete beams subject to thermal and short-term force loads. Simply supported beams were force loaded to collapse, and it was found that heated and unheated specimens did not exhibit significantly different ultimate moment and rotation capacities. Continuous beams were thermally loaded without force load. Beams which had been precracked by force loading produced thermal reactions which were 50-65 % of those measured on similar, initially uncracked beams. Although designed for moment redistribution, no significant reduction in ultimate loadcarrying capacity could be detected when the heated continuous beams were subsequently loaded to collapse. All ultimate loads were within 8 % of their equivalent unheated values. It is concluded that no significant reduction in the ultimate load capacity occurred as a result of coexistent thermal loading.
J.G. Church and L.A. Clark