Ship Structural Response: LoadsShip Structural Response: Loads

Ship Structures - EN358Ship Structures - EN358

Loads?Loads?

Ship Structural LoadsShip Structural Loads

Loads to be Combined: Basic Loads Sea Environment Loads

Individual Loads Operational Environment

Loads Combat Loads

Basic LoadsBasic 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 LoadsBasic 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.

Typical Live loadsTypical Live loads

Type of Compartment Live Loading

Living and control spaces, offices and passages, main deck and above

75 psf

Living spaces below the main deck 100 psfOffice and control spaces below main deck

150 psf

Shop Spaces 200 psfStorerooms and Magazines 300 psfWeather Decks (Main and 01 Levels) 250 psf

Basic LoadsBasic 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.

Sea Environment LoadsSea 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 Hull Girder LoadsLoads

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 AnalysisQuasi-Static Analysis

(Load * g “factor” ie DAF)(Load * g “factor” ie DAF)

Still Water Still Water ConditionCondition

Static Analysis - No Waves Present

Most Warships tend to Sag in this Condition

Putting Deck in Compression Putting Bottom in Tension

Sagging Sagging WaveWave

Excess Weight Amidships - Excess Buoyancy on the Ends

Tension

Compression

Hogging Hogging WaveWave

Excess Buoyancy Amidships - Excess Weight on the Ends

Tension

Compression

Hull Girder Load Hull Girder Load EffectsEffects

Hughes 1988

Sea LoadsSea 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 Example Sea LoadsLoads

30°

AP FP

DWL

Heeling

Angle: Generally 30°

Passing Wave

h = 0.55 LBPwDWL

Pitch & Green Seas

12' Head at FPDecreases to 4' HeadConstant Aft

h w

Weather LoadsWeather 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 LoadsShip 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 LoadsOperational 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 LoadsOperation 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 LoadsOperation 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 LoadsCombat 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 LoadsCombat 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 LoadsCombat 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 LoadsCombat 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 DistributionGun Blast Pressure Distribution

-4

0

4

8

12

16

20

-12 -8 -4 0 4 8 12

10 psi

5 psi

3 psi

5”/54

Muzzle