Trans RINA, Vol 153, Part A4, Intl J Maritime Eng, Oct-Dec 2011
Figure 4: FE Model of the Midship Section for the Various Double Bottom Heights
7. SHEAR LOADING OF DOUBLE BOTTOM FLOORS AND LONGITUDINAL GIRDERS
The double bottom structure measures 22.7 m in breadth by 23.5 m in length, which forms an almost square grillage
type structure with an aspect ratio of
dDB/BDB=1.02, in each of the three holds analyzed. It consists of 8 transverse floors extending between the hopper tanks, equally spaced at
2580 mm and 5
longitudinal girders extending over the lower stools of the corrugated transverse bulkheads. These girders are spaced from the lower end of the hopper connection to the inner bottom at 4050 mm (Grd No 9), 4050 mm (Grd No 4) and 3250 mm (C.L. Grd) apart as show in Figure 2. (b). This compares well, in terms of the number of DB floors and girders, with MHI design shown in Figure 2 (a).
As shown in Table 2, the shear force distribution on the double bottom
grillage members is uneven. The
longitudinal girders are loaded heavier than the floors. This is due to the larger number of floors to girders (5 longitudinal girders as compared to 8 floors), although the double bottom grillage aspect ratio (dDB/BDB=1.02) is almost square. Evidently the 3 centrally located girders and 3 floors at the middle of the hold are the heavier load carrying members of the DB grillage. The 3 centrally located girders (center line and two adjacent girders) are
©2011: The Royal Institution of Naval Architects
carrying almost 150% of the load carried by the 3 mid- hold floors. However, A. Kawamura et al in their paper titled “Full scale measurements and strength analysis of 60,000 DWT bulk carrier – 1974” [14] (for a Panamax of 7 cargo holds bulk carrier – see Fig. 2a), have shown that at an aspect ratio of dDB/BDB=0.966 the 8 floors should take slightly more load than the 5 girders per hold. That means that the number of floors to girders of current designs of bulk carriers is not proportionally arranged for the anticipated DWT and maximum hull girder loading. In addition, the [15] paper entitled, “Structural strength of large bulk carriers” of Mitsubishi Heavy Industry, concludes that sufficient care should be paid for the balance
of Furthermore, the strength of the adjacent structure. the deformations in the hold will also
increase appreciably. In a companion paper to the present one we will show that if the aspect ratio of the DB grillage is increased to 1.02, this effect becomes appreciably larger (almost doubled).
The shear stresses on the double bottom girders for the basic ship, shown in Table 3, exceed the allowable limits, while the floors are stressed well below their allowable limits. Furthermore, the shear stresses decrease considerably as the DB height increases. This is due to the additional shear area available by the corresponding increased height
of the girders and floors. Current
shipyard practice to account for this overstressing is by fitting small thick inserts (commonly referred to as
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