Page 423 - Practical Design Ships and Floating Structures

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information of ship hull plates was collected to enrich, improve and correct the regression models.

2 CALCULATION METHOD OF ESSENTIAL LOCAL CONTRACTION VALUE FOR
COMPLEX CURVED PLATE FORMING ’’

2.1 Calculation method of the local contraction value for symmetric sail-shaped plate and symmetric
saddle-shaped plate

Symmetric sail-shaped plates and saddle-shaped plates being considered as a part of a circle, the
equations for the maximum contract length AL and contraction area AA of curved plates are written
as
8 Hh
Sailed-shaped plate: AL=-
L
8 HhB
AA=- 3L (2)
8 Hh
Saddle-shaped plate: AL=- (3)
L
1 6 HhB
AA=- 3L (4)
where L, R and H are the length, radius and the height of the longitudinal arc respectively. B, r and h
are the length, radius and height of the transverse arc across the contraction area respectively.

2.2 Expansion method ofjinite grid for sail-shaped plate
Hull plates are divided into finite grids. If the size of each grid is small enough, then the part of the
curved surface in the grid can be considered as a plane panel. All these small planes make up the
whole curved surface. When we imaginably unfold the bounded curved surface into a plane plate and
allow the crannies among adjoining panels, several crevice lines lying on the unfolded plate would be
observed. It is these crevice lines that indicate the essential local contraction values for the curved
surface forming.

1) Determination of the reference line of the curved plate. The imaginary crevice lines are defined as
the “reference lines”. Obviously different layouts of the reference lines will result in different
contraction values along the lines. After examination of different methods, geodesic line which links
the central point of the short edge lying on the surface is chosen as the reference line.

2) Grid division of the hull plate. For the convenience of calculation, the curved surface between two
ribs is regarded as a curved strip. Each strip is divided into small equal grids along the reference
1ine.By substituting space planes for those small grids, the whole curved plate is divided into a certain
number of small planes.

3) Development of the surface strip. The plan view of each surface strip is obtained by developing
each surface strip from the reference line to its two edges with the help of the geodesic method. The
developed form of the surface strip of the sail-shaped plate is a drum-shaped strip.