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EN358 Ship Stuctures. Loads, Responses, Failure Modes. Naval Architecture and Ocean Engineering Department U.S. Naval Academy. Classifying Loads On Ships. Static loads Stillwater loads Hydrostatic, weight Other static Drydocking (grounding) loads Thermal loads Slowly-varying loads

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Ship Stuctures

Loads, Responses, Failure Modes

Naval Architecture and Ocean Engineering Department

U.S. Naval Academy

classifying loads on ships
Classifying Loads On Ships

Static loads

Stillwater loads

Hydrostatic, weight

Other static

Drydocking (grounding) loads

Thermal loads

Slowly-varying loads

Wave-induced dynamic pressure

Wave encounter + ship motions

Sloshing liquids

Shipping (green) water

Wave slap (sides/foredeck)

Launching, berthing

Rapidly-varying loads


Mechanical vibration

Propeller, machinery

Other dynamic loads

Combat loads, collision, grounding, ice-breaking

Loads to be combined

Basic loads

“Live” loads

“Dead” loads

Liquid loads

Equipment loads

Sea environment loads

Hydrostatic loads

Still water, waves (sagging, hogging)

Other sea loads

Heeling, transverse waves, shipping (green) water, slamming

Ship motion loads (DAF)

Individual loads

Operational environment loads

Flooding, aircraft landing, docking, ice loads, etc.

Combat loads

UNDEX, topside missile, airblast, gun-blast (self), etc.

Hughes / PNA


static loads
Static Loads
  • “Stillwater” loads
    • External pressures (hydrostatic/buoyancy – no waves)
    • Internal pressures (tanks)
    • All weights onboard
      • Fixed (“lightship”) weights
        • Structure (steel), machinery and piping (propulsion & non-propulsion including fluid in piping systems), fixed deck gear, outfitting/furnishing, fixed portions of weapons systems
      • Variable weights
        • Cargo (incl. “non-fixed” portion of weapons systems), fuel & lube, water (variable ballast & fresh water), holding & waste, provisions & stores, crew & effects, etc.
static loads4
Static Loads
  • Special static loads
    • Grounding
    • Drydocking
    • Lifting, parbuckling, etc.
    • Thermal loads
slowly varying loads
Slowly-Varying Loads
  • Wave-induced “dynamic” pressure distribution
      • Due to both wave motion and ship motion
    • Wave-induced buoyancy distribution
      • Longitudinal → longitudinal bending
      • Transverse → transverse “racking”
      • Oblique → torsion/twisting, bending, racking
slowly varying loads6
Slowly-Varying Loads
  • Wave-induced buoyancy distributions (longitudinal)
    • Hogging
      • Wave crest amidships
      • Main deck in tension
      • Keel & bottom plating in compression
    • Sagging
      • Wave trough amidships
      • Main deck in compression
      • Keel & bottom plating in tension
      • Most Navy combatants → worst case loads WHY?
slowly varying loads7
Slowly-Varying Loads
  • Other slowly-varying loads
    • Wave slap on sides and on foredecks
    • Sloshing of liquids in tanks
    • Shipping (green) water on deck
    • Localized inertial loads (masts, elongated structures, other heavy objects)
    • Launching & berthing loads
rapidly varying loads
Rapidly-Varying Loads
  • Slamming

→ whipping (2 or 3 node flexural vibration)

→ local buckling, shell plating damage

  • Springing
      • Flexural hull vibration due to increased frequency of encounter with waves in head seas
  • Mechanical Vibration
      • Propeller, machinery
rapidly varying loads9
Rapidly-Varying Loads
  • Other rapidly-varying dynamic loads
    • Combat loads
      • UNDEX
        • shock wave, bubble pulse → whipping
      • Above water weapons
        • nuclear air blast, missiles, etc.
      • Self-launched (missile blast, gun blast)
    • Impacts: collisions & groundings
response analyses
Response Analyses
  • Basic types of response analyses
    • Static only vs. static & dynamic vs. “quasi-static”
    • Probabilistic vs. deterministic
    • Linear vs. non-linear
    • Naval architects usually deal with “quasi-static”, deterministic, linear analyses
      • Consider slowly-varying wave loads as “quasi-static” pressure/buoyancy distribution (neglect ship motions)
      • Sometimes deal with dynamic analyses via dynamic amplification factor (DAF) → “quasi-static”
      • Probabilistic nature of loads and strength capacities are addressed via factor of safety (FOS)
      • Suitable FOS keep material behavior in linear range
        • But, geometric nonlinearities may occur !