Ship Structural Response: Loads

1. Ship Structural Response: Loads Ship Structures - EN358

2. Loads?

3. Ship Structural Loads • Loads to be Combined: • Basic Loads • Sea Environment Loads • Individual Loads • Operational Environment Loads • Combat Loads

4. Basic Loads • Loads which are assumed to act on the structure regardless of environmental influences and special operational conditions • Standard Live loads • Dead Loads • Liquid/Tank Loads • Equipment Loads

5. Basic Loads • Live Loads • Used primarily in designing decks. • Represent typical loads due to weight of minor equipment, personnel, etc. • Loads usually depend on function of space. • Dead Loads • Weight of the structure itself. • The load is generally minor, but can not be ignored.

6. Typical Live loads

7. Basic Loads • Liquid/Tank Loads • Hydrostatic pressure exerted on tank boundaries by the liquid. • Must look for worst case loading combination to determine design load. • i.e., adjacent tanks; one full, one empty. • Equipment Loads • Usually in addition to live loads and act in concentrated area. • Wheel loads, aircraft loads, storage racks, etc.

8. Sea Environment Loads • Loads which arise from the vessel being at sea. These loads are considered to the most significant design loads. • Hull Girder Loads • Sea Loads • Weather Loads • Ship Motion Loads

9. Hull Girder Loads • Model the hull as a Free-Free Box Beam. • Beam is experiencing bending due to the differences between the Weight and Buoyancy distributions. • Navy standard procedure is to look at three cases: • Still water. • Hogging wave. • Sagging wave. Quasi-Static Analysis (Load * g “factor” ie DAF)

10. Still Water Condition • Static Analysis - No Waves Present • Most Warships tend to Sag in this Condition • Putting Deck in Compression • Putting Bottom in Tension

11. Sagging Wave Excess Weight Amidships - Excess Buoyancy on the Ends Compression Tension

12. Hogging Wave Excess Buoyancy Amidships - Excess Weight on the Ends Tension Compression

13. Hull Girder Load Effects Hughes 1988

14. Sea Loads • Represent the effects of sea and wave action on: • Shell and weather deck • Deckhouse and superstructure • Intended to account for : • Passing waves and bow submergence • Wave slap and slam • Heeling • Wave slap loads depend on the angle of the surface and the height above the water.

15. Example Sea Loads 30° Heeling Angle: Generally 30° Passing Wave h w h = 0.55 ÖLBP w DWL Pitch & Green Seas 12' Head at FP Decreases to 4' Head Constant Aft DWL AP FP

16. Weather Loads • Effects of temperature, wind, precipitation, humidity, etc. • The most important structural weather loads are: • Ice & Snow – use 7.5 psf on weather decks. • Wind – use 30 psf on exposed vertical (or nearly vertical) surfaces.

17. Ship Motion Loads • Sea conditions generate ship motions, which produce dynamic loads. • Customary in early design stages to estimate loads based on earlier designs and treat as quasi-static. • U.S. Navy determines design factors for two conditions for dynamic loads: • Storm conditions. • Moderate (normal) conditions. • Design factors are based on accelerations experienced and are used to increase dead loads and cargo or equipment weights.

18. Operational Environment Loads • These are loads which are normally not combined with other loads for analysis. • Some of these are extreme loads which may happen only once in a vessels life, if at all. • Others are loads which occur due to special circumstances. • The effect of these loads need to be determine for each special case or circumstance, in addition to the Basic and Sea Loads.

19. Operation Environment Loads • Flooding Loads • These are the critical design loads for bulkheads and decks below the main deck. • Hydrostatic pressure distribution loads. • Aircraft Landing Loads • High intensity loads of short duration. • Apply only to specific portions of the decks in the landing zones.

20. Operation Environment Loads • Docking Loads • Specific locations along the hull need to be strengthened to carry loads from docking blocks or tug positions. • Usual block load is about 20 LT/in2 and occurs every two or three frames. • Ice Loads • Certain classes of ships need special additional structure to be able to operate in ice regions. • Typically use Classification Society Rule (ABS, DnV, etc.) to develop hull structure.

21. Combat Environment Loads • Ships which are expected to operate in a combat environment should have certain loads taken into account. The main combat loads taken into consideration are: • Underwater explosions/shock • Nuclear air blast loading • Own weapons effects

22. Combat Environment Loads • Underwater blast/shock loads • Underwater explosions can cause the ship to “whip” or vibrate near its fundamental two node frequency. • Large amplitude hog-sag cycle deflections happen in a second or less. • Large amplitude high frequency vibration can cause machinery to break off foundations, equipment to fail, and may cause damage to the hull. • Usually treated in design by strengthening foundations and providing shock isolation mountings and absorbing systems.

23. Combat Environment Loads • Nuclear Air Blast • After a nuclear explosion the expansion of hot gases causes a huge pressure wave. • The impact of the shock wave upon exposed structure can be critical in a ship design. • Superstructure and hull plating • Masts, antenna, radars, fire control systems • This is usually considered in a later stage of design by strengthening exposed structure and equipment foundations

24. Combat Environment Loads • The effect of gun blasts and missile launching must be considered when designing all structure in the vicinity. • Gun blasts can generate significant pressures for very short durations. • The structure of missile motor stowage areas must be able to contain accidental ignition.

25. Gun Blast Pressure Distribution Muzzle 5”/54