Structural Design,Ship Stresses
After having established the principal dimensions,form,and general arrangement of the ship,the designer undertakes the problem of providing a structure capable of withstanding the forces which may be imposed upon it,The hull of a steel merchant ship is a complex structure,unique in the field of engineering structures in that it is primarily a plate structure,depending for its major overall strength on the plating of the shell,decks,and in most cases,also on the inner bottom and longitudinal bulkheads,The framing members,each of which has its own function to perform,are designed primarily to maintain the plate membrances to the planned contours and their positions relative to each other when subjected to the external forces of water pressure and breaking seas,as well as to the internal forces caused by the services for which the ship is designed,Unlike most other large engineering structures,the forces supporting the ship’s hull as well as the loads which may be imposed upon it vary considerably,and in many cases,cannot be determined accurately,As a result,those responsible for the structural design of ships must be guided by established standards,
The problem of the development of a satisfactory structure generally involves the following considerations,
It is necessary to establish the sizes of,and to combine effectively,the various component parts so that the structure,with a proper margin of safety,can resist the major overall stresses resulting from longitudinal and transverse bending.
Each component part must be so designed that it will withstand the local loads imposed upon it from water pressure,breaking seas,the weight of cargo or passenger,and other superimpose loads such as deckhouses,heavy machinery,masts,and so on,including such additional margins as sometimes may be required to meet unusually severe conditions encountered in operation.
Rules of classification societies
The various classification societies have continued to modify and improve their rules to keep pace with the records of service experience,an increasing amount of research,and the constantly growing understanding of the scientific principles involved,In the modern rules of the societies,the designer has available to him formulas and tables of scantlings,dimensions of framing shapers,and thicknesses,These are directly applicable to practically all the ordinary types of sea-going merchant vessel being built today,and contain a flexibility of application to vessels of special types.
The design of structural features of a merchant ship is greatly influenced by the rules of classification societies; in fact,the principal scantlings of most merchant ships are taken directly from such rules.
Scantling are defined as the dimensions and material thicknesses of frames,shell plating,deck plating,and other structures,together with the suitability of the means for protecting openings and making them sufficiently watertight or weathertight,The classification society rules contain a great deal of useful information relating to the design and construction of the various component parts of a ship’s structure,Scantling can be determined directly from the tables given in these publications,In many cases,a good conception of the usual,good-practice” construction can also be gleaned from the sketches and descriptive matter available from the classification societies.
(From,McGraw-Hill Encyclopedia of Science and Technology”,Vol.12.1982)
The ship at sea or lying in still water is being constantly subjected to a wide variety of stresses and strains,which result from the action of forces from outside and within the ship,Forces within the ship result from structural weight,cargo,machinery weight and the effects of operating machinery,Exterior forces include the hydrostatic pressure of the water on the hull and the action of the wind and waves,The ship must at all times be able to resist and withstand these stresses and strains throughout its structure,It must therefore be constructed in a manner,and of such materials,that will provide the necessary strength,The ship must also be able to function efficiently as a cargo-carrying vessel.
The various forces acting on a ship are constantly varying as to their degree and frequency,For simplicity,however,they will be considered individually and the particular measures adopted to counter each type of force will be outlined,The forces may initially be classified as static and dynamic,Static forces are due to the
Fig,1 Ship movement------the six degrees of freedom
differences in weight and buoyancy which occur at various points along the length of the ship,Dynamic forces result from the ship’s motion in the action of the wind and waves,A ship is free to move with six degrees of freedom—three linear and three rotational,These motions are described by the terms shown in Figure,1.
These static and dynamic forces create longitudinal,transverse and local stresses in the ship’s structure,Longitudinal stresses are greatest in magnitude and result in bending of the ship along its length.
Fig,2 Static loading of a ship’s structure
Static loading If the ship is considered floating in still water,two different forces will be acting upon it along its length,The weight of the ship and its contents will be acting vertically downwards,The buoyancy or vertical component of hydrostatic pressure will be acting upwards,In total,the two forces exactly equal and balance one another such that the ship floats at some particular draught,The centre of the buoyancy force and the centre of the weight will be vertically in line,However,at particular points along the ship’s length the net effect may be an access of buoyancy or an excess of weight,This net effect produces a loading of the structure,as with a beam,This loading results in shearing forces and bending moments being set up in the ship’s structure which tend to bend it.
The static forces acting on a ship’s structure are shown in Figure 2(a),This distribution of weight and buoyancy will also result in a variation of load,shear forces and bending moments along the length of the ship,as shown in Figure 2(b)-(d),Depending upon the direction in which the bending moment acts,the ship will bend in a longitudinal vertical plane,The bending moment is known as the still water bending moment (SWBM),Special terms are used to describe the two extreme cases,where the buoyancy amidships exceeds the weight,the ship is said to,hog”,and this condition is shown in Figure 3,where the weight amidships exceeds the buoyancy,the ship is said to,sag”,and this condition is shown in Figure 4.
Excess of buoyancy
Fig,3 Hogging condition
Fig,4 Sagging condition
Dynamic loading If the ship is now considered to be moving among waves,the distribution of weight will be the same,The distribution of buoyancy,however,will vary as a result of the waves,The movement of ship will also introduce dynamic forces.
The traditional approach to solving this problem is to convert this dynamic situation into an equivalent static one,To do this,the ship is assumed to be balanced on a static wave of trochoidal form and length equal to the ship,The profile of a wave at sea is considered to be a trochoid,This gives waves where the crests are sharper than the throughts,The wave crest is considered initially at midships and then at the ends of the ship,The maximum hogging and sagging moments will thus occur in the structure for the particular loaded condition considered,as shown in Figure 5.
Still water Wave trough amidships Wave crest amidships
Bending moment curves
Fig.5 Dynamic loading of a ship’s structure
(a)----still water condition (b)---sagging condition (c)---hogging condition
The total shear force and bending moment are thus obtained and these will include the still water bending moment considered previously,If actual loading conditions for the ship are considered which will make the above conditions worse,e.g,heavy loads amidships when the wave through is amidships,then the maximum bending moments in normal operating service can be found.
The ship’s structure will thus be subjected to constantly fluctuating stresses resulting from these shear forces and bending moments as the waves move along the ship’s length.
Stressing of the structure The bending of a ship causes stresses to be set up in the bottom shell plating and compressive stresses are set up in the decks,When the ship hogs,tensile stresses occur in the decks and compressive stresses in the bottom shell,This stressing,whether compressive or tensile,reduces in magnitude towards a position known as the neutral axis,The neutral axis in a ship is somewhere below half the depth and is,in effect,a horizontal line drawn through ship’s section,
The fundamental bending equation for a beam is
Where M is the bending moment,I is the second moment of area of the section about its neutral axis,σis the stress at the outer fibres,and у is the distance from the neutral axis to the outer fibres,
This equation has been proved in full-scale tests to be applicable to the longitudinal bending of a ship,From the equation the expression
is obtained for the stress in the material at some distanceуfrom the neutral axis,The values M,I andуcan be determined for the ship,and the resulting stresses in the deck and bottom shell can be found,The ratio I/у is known as the section modulus,Z,whenуis measured to the extreme edge of the section,The Values are determined for the midship section,since the greatest moment will occur at or near midships (see Figure 2).
The structural material included in the calculation for the second moment I will be all the longitudinal material which extends for a considerable proportion of the ship’s length,This material will include side and bottom shell plating,inner bottom plating (where fitted),centre girders and decks,The material forms what is known as the hull girder,whose dimensions are very large compared to its thickness.
(Form,Merchant Ship Construction” by D.A,Taylor,1980)
framing members 骨架（构件）
plate membrance 板架
breaking sea 碎波
overall stress 总应力
superimposed load 叠加载荷
classification society 船级社
framing shape 骨架型材
shell plating 外板
deck plating 甲板板
operating machinery 转运机械
exterior force 外力
hydrostatic pressure 静水压力
cargo-carrying vessel 载运船舶
degree of freedom 自由度（d.o.f）
dynamic force 动力
shearing force 剪力
bending moment 弯矩
SWBM(still water bending moment) 静水弯矩
wave of trochoidal form 余波摆线
loaded/loading condition 装载状态
fluctuating stress 交变应力
tensile stress 拉伸应力
second moment 二次矩（惯性矩）
full-scale test 实船（实尺度）实验
section modulus 剖面模数
midship section 船中剖面
Additional terms and Expressions
longitudinal strength 总纵强度
transverse strength 横向强度
local strength 局部强度
docking strength 坐坞强度
strength criteria of ship 船舶强度标准
ultimate bending moment 极限弯矩
strength deck 强力甲板
bulkhead deck 舱壁甲板
longilutinal strength member 纵向强力结构
Notes to the Text
1,The hull of a steel merchant ship is a complex structure,unique in the field of engineering structures in that it is primarily a plate structure,depending for its major overall strength on the plating of the shell,decks,and in most cases,also on the inner bottom and longitudinal bulkheads,
句中unique…为后置形容词短句，修饰a complex structure,in that it is… structure中的in为unique所要求，表示独特性表现在那一方面，其后的that从句为介词(in)的宾语从句。
depending for…直到句末，为分词短语，用作状语。说明基本上是板结构的原因； (on) the plating…,also (on) the inner bottom…为 depending on所需求的两个介词宾语。
2,In the modern rules of the societies,the designer has available to him formulas and tables of scantlings,dimensions of framing shapes,and thicknesses.
句中available to him 为形容词短句，修饰formulas and tables，这类短句一般是后置的，但这里因句子结构平衡的需要，改变了语序，理解和翻译这类句子时希望读者加以注意。
3,It must therefore be constructed in a manner,and of such materials,that will provide the necessary strength,
句中的that…从句为in a manner和and of such materials公用，完整的全句为It must therefore be constructed in a manner that will provide the necessary strength,and of such materials that will provide the necessary strength.
4,This loading results in shearing forces and bending moments being set up in the ship’s structure which tend to bent it,
being set up…现在分词短句（被动态）修饰shearing forces and bending moments.
which tend to bend it定语从句，也修饰forces and moments.