//SYS21///INTEGRA/BST/VOL1/REVISES 21-7-2001/BSTC06.3D ± 197 ± [177±236/60] 26.7.2001 4:18PM
The ship girder 197
Fig. 6.19
means obvious. Moreover, it is not obvious in the early stages, whether the superstructure should be included in the strength section or whether the construction should be such as to discourage a contribution from the superstructure. Some measure of the e ect of adding material to the strength section is needed if we are to avoid calculating a new modulus for each trial addition. In order to provide such a measure, consider (Fig. 6.20) the addition of an area a at a height z above the neutral axis of a structural section whose second moment of area has been calculated to be Ak2 , area A, radius of gyration k and levers y1 to the keel and y2 to the deck. Now stress
M I=y
Fig. 6.20
By considering changes to I and y it can be shown that for a given bending moment, M, stress will obviously be reduced at the deck and will be reduced at the keel if z > k2 =y1 , when material is added within the section, z < y2 .