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The fire resistance of composite deck slabs is presented in terms of their loadcarrying capacity, integrity, and thermal insulation. It is shown that 90 min fire resistance can be achieved with mesh reinforcement in the slab, provided the slab and reinforcement are continuous over a number of supports and imposed loads do not exceed 6.7 kN/m2. This was demonstrated by large-scale tests, and a summary of available UK test data is made. In other cases of design, the fire engineering method may be used. This takes account of the reduced strength of the elements in fire.
G.M.E. Cooke, R.M. Lawson and G.M. Newman
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
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