Page 161 - Introduction to Naval Architecture

STRENGTH 147

M is a distribution factor along the length. It is taken as unity between
0.4L and 0.65L from the stern; as 2.5x/L at x metres from the stern to
0.4L and as 1 ,0 — (x— 0.65L)/0.35L at x metres from the stern between
0.651 and /,
The IACS propose taking the wave induced shear force as:

Hogging SF = O.SFjC/JStq, + 0.7) kN
Sagging SF = -0.3F 2CLB(Q, + 0.7) kN

F l and F 2 vary along the length of the ship. If F = 190 Q,/ [110(0,+
0,7)], then moving from the stern forward in accordance with:

Distance from stern 0 0.2-0.3 0.4-0.6 0.7-0.85 1
Length
F l 0 0.92^ 0.7 1.0 0
F z 0 0.92 0.7 F 0
Between the values quoted the variation is linear.
The formulae apply to a wide range of ships but special steps are
needed when a new vessel falls outside this range or has unusual design
features that might affect longitudinal strength.
The situation is kept under constant review and as more advanced
computer analyses become available, as outlined later, they are adopted
by the classification societies. Because they co-operate through IACS
the classification societies' rules and their application are similar
although they do vary in detail and should be consulted for the latest
requirements when a design is being produced. The general result of
the progress made in the study of ship strength has been more efficient
and safer structures.

SHEAR STRESSES

So far attention has been focused on the longitudinal bending stress. It
is also important to consider the shear stresses generated in the hull.
The simple formula for shear stress in a beam at a point distant y from
the neutral axis is:

Shear stress = FAy/It

where:
F = shear force
A - cross sectional area above y from the NA of
bending