ssc-367 - Ship Structure Committee

An allowable stress method, also considered a screening process,
relies on both theory and experience. The procedure developed
should be calibrated against available fatigue failure data and
typically incorporatesthe following steps:
1. Computation of wave-induced loads
2. Determinationof applied stress levels
3. Determinationof allowable stress levels
4. Adjustment of allowable stress levels
5. Assessmentofvariouscomponents/detailsforsusceptibi lityto
fatigue failure.
The wave-inducedloads are computedusing simplifiedformulae,where
the long-storm distribution of fatigue loading is represented by a
single characteristic value. The vertical bending moment is
computed as a function of the vessel length, breadth and block
coefficient along the longitudinal axis. The applied (nominal)
cyclic stress amplitude is determined by using beam theory and
dividing the vertical bending moment at any point along the
longitudinal axis with hull girder section modulus.
The allowable stresses depend on many variables. For a simplified
method an allowablestress may be defined as a function of location
(deck, side shell, etc.) and detai1 geometry (1ocal stress factor).
Typically, such a method is based on a 20-year service life,
standard corrosioneffects and a nominal geographic area. Thus if
specific service life or routing information is available, the
allowable stress levels are adjusted. Two of the of the simplified
analysis methods are:
1. ABS’ Al1owable Stress Method
This allowablestressmethod by Thayamballi (Reference4.2) is
primarily intended for use in fatigue screening of tankers.
The simplifiedformulae presentedallow calculationof several
types of loadingon atanker due to wave-inducedmotions. The
loading types and their relevancy are:
4“3