Page 99 - Marine Structural Design

Chapter 4 Scantling of Ship’s Hulk by Rules 75

The material behaves elastically.
Shear effects can be separated hm, and not influence bending stresses or strains.
The derivation of the equations for stress and deflection using the same assumptions as those
used for elementary beam theory may be found in textbooks on material strength.
This gives the following well-known formula:

Where SM, is the section modulus of the ship. The maximum stress obtained from Eq. (4.4) is
compared to the maximum allowable stress that is defined in the rules provided by
Classification Societies for ship design. If the maximum stress is larger than the maximum
allowable stress, the ship’s section modulus should be increased, and the drawing changed.
The maximum bending moment is usually found in the mid-section of the ship, and thus the
longitudinal strength at the mid-section of the ship is usually the most critical.
In general, the maximum shear stress is given by Eq. (4.5):

where FT, is the total shear force. t and I denote the web thickness of the hull girder, and the
moment of inertia of the hull. S is the first moment of effective longitudinal area above or
below the horizontal neutral axis, taken about this axis.
4.2.3 Corrosion Allowance

The strength requirements in ship design rules are based on a “net” ship approach. The
nominal design corrosion allowance is to be accounted for, because the scantlings correspond
to the minimum strength requirements acceptable for classification regardless of the vessel’s
design service life. Apart from coating protection for all ballast tanks, minimum corrosion
allowance for plating and structural members is to be applied, as shown in Figure 4.5.
For regions of structural members, where the corrosion rates might be higher, additional
design margins should be considered for primary and critical structural members. This may
minimize repairs and maintenance costs throughout vessel’s life cycle.