Ship Construction, Hull Structure and Strength: Load Paths, Scantlings and Structural Detail
Structural questions test whether you can follow a load from sea pressure through plating, stiffeners, girders and bulkheads into the hull girder, and whether you recognise details that create stress concentration. The exam favours primary hull girder strength, framing systems, special structures and the reason behind rule scantlings.
The hull girder
The hull bends as a beam. Still-water bending moment comes from distribution of weight and buoyancy; wave bending moment is added by rule. In a sagging condition the deck is in compression and the bottom in tension; hogging reverses both. Stress follows the beam formula, sigma = M y / I, with y measured from the neutral axis, and section modulus Z = I / y is what the classification rule prescribes.
Framing and primary members
- Transverse framing: closely spaced frames supported by longitudinals and web frames; efficient for local pressure, weaker for longitudinal bending.
- Longitudinal framing: closely spaced longitudinals supported by transverse floors and webs; standard in deck, bottom and, in tankers, the side shell.
- Primary members: keel, centre girder, side girders, transverse floors, web frames, stringers, hatch side coamings and longitudinal bulkheads.
- Continuity matters more than scantling: a bracket that ends abruptly, a pillar landing off a girder, or a small opening at a stress concentration is the origin of most cracks.
Special structures
The collision bulkhead sits between the minimum of 0.05L or 10 m and the maximum of 0.08L or 0.05L + 3 m from the forward perpendicular. Double bottoms, cofferdams, deep tanks, corrugated bulkheads and hatch coamings each have rule dimensions tied to length and position. The IACS Common Structural Rules apply to bulk carriers of 90 m and above and double-hull oil tankers of 150 m and above; the Enhanced Survey Programme controls how they are examined in service.
Worked example
A barge has its neutral axis 6.0 m below the strength deck and a second moment of area of 40 m^4. Combined still-water and wave bending moment is 900 MN m sagging. Deck stress = 900 x 10^6 x 6.0 / 40 = 135 N/mm^2, compressive. Against an allowable 175 N/mm^2, utilisation is 77 per cent.
Traps
Reversing sagging and hogging stresses; measuring the collision bulkhead from the aft perpendicular; assuming higher-tensile steel removes the need for fatigue checks; ignoring buckling of thin plating in compression.