As a practical application of the method given in the preceding section, the author investigates the effective breadths in the following cases of plates with parallel sitiffeners, corresponding to the most usual ship structures, such as decks, shells and bulkheads etc., i.e.
1. with a concentrated load on every stiffener, both ends being supported or fixed.
2. with a uniform pressure over the plate, both ends being supported or fixed.
3. with a uniformly increasing pressure over the plate, both ends being supported or fixed.
It is found that the statically indeterminate bending moment along the stiffener due to the pressure over the plate, can be practically substituted by the bending moment along a “beam” carrying the whole pressure as a direct load, provided that the scantlings of the plate and stiffener are of the usual proportions. As a result of this, the applications as well as the calculations of the effective breadths are much facilitated.
The results of the calculations are given in curves, by which the properties of the effective breadths are deduced as follows; the effective breadths (a) are not constant along the lengths of the stiffeners, (b) but vary with the end conditions, (c) with the distribution of the loads, (d) and with the spacing of the stiffeners, (e) and are much affected by
s/l, (f) slightly by
d/l, (g) and scarecely by
t'/t, where
t is the thikness of the plate,
s, l and
t' are the spacing, the length and the thickness of the stiffener respectively, and
d is the depth of the flat bar stiffener or of any equivalent flat bar stiffener. For the purpose of calculating the deflections of the stiffeners, however, the effective breadths may safely be assumed to be constant throughout the lengths of the stiffeners and equal to their values at the point of application of a concentrated load (case 1), or at the centre of the stiffener carrying the uniform load (case 2 & 3).
As a standard for the practical use in shipbuilding, the author proposes to use thecurves of the effective breadths in the case of
d/l=1/20,
t'/t=1.
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