The most unfavourable values for C are 0,34, found for Aster&Koch's tests Nr.11 and 16, with d = 500 and 750 mm and po = 0,46 and 0,42% respectively.
So, with some rounding off a value C = 0,35 would be appropriate for the simplified design equation. In prEN 1992-1-1:2001 a value 0,40 is used. An argument might be that the utmost part of the practical cases consists of slabs with smaller depths, subjected to uniform loading, where the maximum shear force does not coincide with the maximum moment, and the reinforcement ratio's are small enough to ensure failure by bending. The seldom case of a slab spanning in one direction, with a high cross-section, a critically low reinforcement ratio and a line load just at the most critical position from the support would then have a slightly lower safety. On the other hand formula's should always be safe enough to take account of any possible (not likely) case, which would be an argument in favour of the use of 0,35.
Some questions may be raised with regard to the definition of bw being "the smallest width of the cross-section in the tensile area". Tests on tapered cross-sections showed that there is certainly an influence of the definition of the web width, as shown in Fig. 6.14, left (tests by Leung, Chew and Regan, 1976). Fig. 6.14, right, shows that a definition of bw as the average width of the beam would be appropriate for this case.
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