Ship Structural Response: Loads Ship Structures - EN358

Loads?

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

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

Basic Loads Live Loads Dead 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.

Typical Live loads Type of Compartment Live Loading 75 psf 100 psf Living and control spaces, offices and passages, main deck and above 75 psf Living spaces below the main deck 100 psf Office and control spaces below main deck 150 psf Shop Spaces 200 psf Storerooms and Magazines 300 psf Weather Decks (Main and 01 Levels) 250 psf

Basic Loads Liquid/Tank Loads Equipment 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.

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

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)

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

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

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

Hull Girder Load Effects Hughes 1988

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.

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

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.

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.

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.

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.

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.

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

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.

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

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.

Gun Blast Pressure Distribution Muzzle 5”/54