Edmund Fitzgerald

Date 9-8-06

Types of Loads and Forces

Ships, like other physical object, conform to certain physical laws. Many of these physical laws have been translated into mathematical formulas. Because of this we are able to determine certain characteristics of an item.

Loads & Definitions

Force is one of the basic elements of nature. It is an action on a body tending to make it move. A force can be defined as a vector, shown in the drawing below. The magnitude of a force is the number assigned to its strength and is expressed in units such as pounds. A force acts at a single point.

Pressure, on the other hand, is effectively an infinite number of tiny forces distributed over an area. Pressure is expressed as the force magnitude per unit of area to which it is applied.

Mass is another fundamental component of nature, in the same league as force. Mass is basically the characteristic of body which causes it to resist motion, or, more specifically, acceleration.

Bending loads occur throughout the ship frame and hull. If the bend is in the direction of the drawing below, the top layers are squashed in compression while the bottom layers are strected in tension.

Shear loads essentially shift one block of metal down or up from an adjacent block. One side of the shear's sharp edge pushes down while the other side pushes up. Shear may be an importance part of the design process for the flat part of the ship's deck.

Weight like a ship in the water on the earth is attracted by the action of gravity and thus exerts a force on whatever is supporting it.

Load means almost the same as force, but is more general in that a single load can mean some combination of multiple forces. Each type of force goes by its own unique name.

Couple is a pair of forces, equal in amplitude but opposite in direction and offset from one another so that they tend to cause a body to rotate. A couple is itself a vector, the amplitude of which is commonly expressed in units such as inch-pounds or foot-pounds. Depending on how a couple is applied to a body, it can show up as a bending moment or as torsion.

Torsion loads twist the beam or object as if squeezing a rag as to ring water out of it. With a ship, twisting of the frame or hull may occur when one side of the ship goes over a large wave and the other side does not at the same time. Designing a resistance to torsion is not easy, especilly if the result must be light-in-weigh and the deck flat with many hatch openings. L/D must also be ment using the standards for ship's design.

Motion & non-Motion Wheather it be bending, compression, tension, shear or torsion, any of these loads can appear statically or dynamically. Static loads result from fixed or stationary loading conditions. Loads resulting from moving conditions can fall into the category of dynamic, shock or fatigue accorging to Omer Blodgett's Basic Principles of Shock Loading. All types of these loads can affect the ship's structures.

Steady loads also called static, acting on a ship usually refer to the loads from cargo or the weight of the ship. These loads are described and visualized while the ship is standing still- before the ship moves in the water.

Shock loads, also called impact loads, are high intensity loads applied over a short time period. A shorter time increases the shock factor.

Fatigue is the mode of many structural failures and is generally the result of repeated loads. A fatigued structure first exhibits a crack which gradually increases in size to result in an eventual break.

Dynamic loads refer to the combination of both shock and fatique loads. Wind, vibration from wave condition, unballance loading and unloading, impact from walls and grounding occurrences can be responsible for these loads applications. Safety and design factors adjust the load and/or stresses to account for the possible increased magnitude of these loadings.

Service Factor Knowing how the ship will be used in service is essential and provides guidelines as to the applied loads and the multipliers necessary for calculations. Predicting and calculation all possible used is not an easy job, though, and in many cases is impossible. If only a basic knowledge of a load is know, a safety factor can be added, providing a cushion for a large majority of unknown possible conditions.

Material Properties Parent materials are more likely to be definable and consistent but manufacturing inconsistencies can exist, expecially with recycled steels. Properties may not always be perfit throughout a specimen.

Fabrication Processes Fabrication processes include bending and forming, welding and such basic processes as shearing, drilling and tapping. Of these, welding is probably the most apt to create descontinuity and material changes worthy of analysis. The physical location of holes is a close second. Factors based on shape are used as a measure of the level of internal stress in the material.

Environment Predicted environment is a topic worth considering. Cold water climates can cause certain materials to become brittle and high temperature can cause certain materials to become more pliable. Fortunately for steel ships, most operating temperatures are within a reasonable range and calculation adjustments are not necessary.

Rivets Rivets are basically an extension of theories used for bolts. Solid rivets, used where percision and non-variability is important are often closed on their ends with a ship. Rivets are designed for specialized loading situations which require resistance to primarily shear types of loads.

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