Ship Structures, Strength and Structural Design
What this domain tests
Hull girder bending and section modulus, IACS longitudinal strength standards, the Common Structural Rules, local scantling, buckling and fatigue, and your ability to judge whether a structural detail will survive a 25-year design life.
Hull girder strength
- Total bending moment = still water bending moment from the loading condition plus wave-induced bending moment; check hogging and sagging separately.
- Bending stress = M y / I, and section modulus Z = I / y, quoted at deck and keel.
- IACS UR S7 sets the minimum longitudinal strength standard; UR S11 defines the wave bending moment and the North Atlantic wave environment.
- The harmonised Common Structural Rules (CSR) apply to bulk carriers and double-hull oil tankers; net scantlings plus a rule corrosion addition give the as-built thickness.
Local strength and failure modes
Plate thickness against lateral pressure, stiffener section modulus, buckling of plates and columns, tripping of stiffeners, shear capacity, and torsion in open-deck container ships. Fatigue uses S-N curves, hot-spot stress at the detail, and Miner's cumulative damage rule: the sum of n_i/N_i must not exceed 1.0.
Worked example
A midship section has I = 60 m4 and the deck is 12 m above the neutral axis, so Z = 60/12 = 5.0 m3. With a total hogging moment of 1.5 x 10^6 kN.m, the deck stress is 1.5 x 10^6 / 5.0 = 300,000 kN/m2, i.e. 300 N/mm2. That exceeds a 175 N/mm2 allowable for mild steel, so the required Z is 1.5 x 10^6 / 175,000 = 8.57 m3; the section must be deepened or the deck plating increased by roughly 70%.
Traps
Using only the still water moment; mixing units between kN.m, N/mm2 and MPa; checking fatigue at the nominal rather than hot-spot stress; ignoring buckling of slender panels in compression; treating the corrosion addition as strength thickness.