Consultation regarding ISO 12215-5

Discussion in 'Class Societies' started by TANSL, Nov 25, 2020.

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DCockeySenior Member

Below is a explaination of the concept from Comments on the Structural Design of High Speed Craft, Joseph G. Koelbel, Jr., Marine Technology, Vol. 32, No. 2, April 1995, pp. 77-100 This paper includes a history of methods of calculating bottom hydrodynamic loads and corresponding structural stresses for planning boats. Note that the maximum aspect ratio suggested by Koelbel is 2 rather than 2.5 as used by ISO 12215-5.

Consider a plating panel of uniform thickness and pressure loading, and with all four edges fixed as shown in Roark [20], Table 26, Case 8a, of the Sixth Edition. As the aspect ratio of the plate increases from unity, the maximum panel stress increases. As the aspect ratio approaches a value of two, the rate of increase of the stress drops off to zero. Thus the stress is constant, for aspect ratios greater than two, at a value equal to that calculated from beam theory. But, for a constant spacing of the longitudinals, an increase in the aspect ratio results in an increase in the plating area. Because of the pressure reduction, as shown in Fig. 6, a larger plating area sees a lower average pressure. The variation of stress in the plate with aspect ratio interacts with the variation of design pressure with design area (which is a direct function of aspect ratio) in such a way as to produce the maximum stress at an aspect ratio of two.

Therefore, the design area for plating is recommended to be taken as two times the square of the spacing of the longitudinals. Less than this produces too high a design pressure, thus increasing the weight of the plating, and greater than this produces too low a design pressure, which results in thin plating that could yield locally due to pressure peaks. The result, however, would not be catastrophic failure but permanent set (dishing) of the plating.

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TANSLSenior Member

Thanks a thousand DCockey, that's the answer I was looking for. In the end, as you might expect, everything is based on plate theory and on the values of the stress supported by a recessed edge plate, function, as you say, of the aspect ratio. It was clear but I have not been able to see that relationship.
I imagine that the case of reinforcements, which are not governed by plate theory but by embedded end beams, will have some similar justification.
What I still do not understand is why, and if that has been taken into account, this decision to limit the maximum value of the design area means that the design pressure for the reinforcements can be much higher than that of the attached panel.

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When getting into the nuances of the rules, this is also the value used by DNV-GL:

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DCockeySenior Member

Comments on the Structural Design of High Speed Craft, Joseph G. Koelbel, Jr., Marine Technology, Vol. 32, No. 2, April 1995, pp. 77-100 includes a comment about the DNV rule from circa 1994:

The general method, and some of the d e t a i l s , of the Det norske Veritas rules for high speed craft [4] are based, at least in part, on some of the above-cited source works. The American Bureau of Shipping (ABS) rules for high speed craft [15], which are nearing completion, are based on the above source works to a greater extent than are those of DnV. The U.S. Navy's small craft design department, currently known as the Naval Sea Combat Systems Engineering Station, Combatant Craft Engineering Department, Norfolk, has its own structural design guide [16] which is based largely on the Det norske Veritas rules.

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