4結構設計原理

1、4 Philosophy of structural Design結構設計原理A structural engineering project can be divided into three phases: planning, design, and construction 、 （一個結構工程項目可以分為三個階段:規(guī)劃、設計與施工 ）Structural design involves determining the most suitable proportions of a structure and dimensioning the structural elements and

2、details of which it is composed 、 （結構設計內容 包括要確定結構最優(yōu)的比例尺寸 ,并確定結構構件與細部的尺寸 ） This is the most highly technical and mathematical phase of a structural engineering project, but it cannot and certainly should not-be conducted without being fully coordinated with the planning and construction phases of the

3、 project 、 （這就是整個結構工程項目中技術性最高、數(shù)學嚴謹性最強的階段 ,但如果不能同規(guī)劃與施工階段完全協(xié)調、配合,就不能也不應該付諸實施 ） The successful designer is at all times fully conscious of the various considerations that were involved in the preliminary planning for the structure and, likewise, of the various problems that may later be encountered in

4、its construction 、 （一個成功的設計者總就是會 全面考慮結構在初步規(guī)劃中所涉及到的各種條件,以及以后的施工中會碰到的各種問題）Specially, the structural design of any structure first involves the establishment of the loading and other design conditions that must be resisted by the structure and therefore must be considered in its design 、 （在結構設計中 ,應首先確定

5、結構所必須承受的荷載與其它 設計條件 ,她們就是結構設計中的必要條件 ） Then comes the analysis （or computation） of the internal gross forces （thrust, shears, bending moments, and twisting moments）, stress intensities, strains, deflections, and reactions produced by the loads, temperature, shrinkage, creep, or other design conditions

6、 、 （ 然后分析在荷載、溫度、收縮、徐變及其她設計條 件下結構所產生的總內力 （軸力、剪力、玩具、扭矩 ） 、應力強度、應變、變形與反力 ） Finally comes proportioning and selection of materials of the members and connections so as to resist adequately the effects produced by the design conditions 、 （最后確定各構件及 連接的尺寸并選擇其材料 ,以抵抗設計條件所產生的作用） The criteria used to judgewhe

7、ther particular proportions will result in the desired behavior reflect accumulated knowledge （theory, field and model test, and practical experience）, intuition, and,就要用到一些經驗知識judgement 、 ( 當判斷某些特殊部位的設計就是否符合要求時（ 理論、現(xiàn)場及模型試驗、實際經驗）,與直觀判別的方法 ） For most common civilengineering structures such as bridges

8、 and buildings, the usual practice in the past has been to design on the basis of a comparison of allowable stress intensities with those produced by the service loadings and other design conditions 、 （ 對于橋梁與房屋這些最常見的土木工 程結構來說 ,過去最常用的就是將容許應力與使用荷載與其它設計條件下產生的應力 進行比較 ,然后在此基礎上進行設計 ） This traditional basi

9、s for design is called elastic design because the allowable stress intensities are chosen in accordance with the concept that the stress or strain corresponding to the yield point of the material should not be exceeded at the most highly stressed points of the structure 、 （由于容許應力在選擇時就是 依據(jù)了如下概念 :即結構的

10、最大應力不得超過材料屈服點所對應的應力或應變,因此這種傳統(tǒng)的設計基礎被稱為彈性設計 ） Of course, the selection of the allowable stresses may also be modified by a consideration of the possibility of failure due to fatigue, buckling, or brittle fracture or by consideration of the permissible deflections of the structure 、 （當然 , 考慮到結構也可能由于疲勞、

11、 壓曲、 脆斷或容許變形而引起失效 ,容許應力的選擇也可 隨之調整 ）Depending on the type of structure and the conditions involved, the stress intensities computed bin the analytical model of the actual structure for the assumed design conditions may or may not be in close agreement with the stress intensities produced in actual st

12、ructure by the actual conditions to which it is exposed 、 （根據(jù)結構類型與所涉及到的條件 ,采用 實際結構的分析模型 ,在假設的設計條件下計算出的應力強度,與實際結構在實際條件下所產生的應力強度 , 可能相符也可能不相符 ） The degree of correspondence is not important, provided that the computed stress intensities can be interpreted in terms of previous experience 、 （只要計算出的應力強度能

13、根據(jù)以往的經驗來解釋,吻合程度就不就是很重要 ） The selection of the service conditions and allowable stress intensities provides a margin of safety against failure 、 （ 使用條件與容許應力強度的選擇為防止結 構失效提供了安全儲備 ） The selection of the magnitude of this margin depends on the degree of uncertainty regarding loading, analysis, design, ma

14、terials, and construction and on the consequences of failure 、 （ 儲備量值的選擇依賴于相關荷載、分析、設計、施工與 失效后果的不確定性程度 ） For example, if an allowable tensile stress of 20000pis isselected for structural steel with a yield stress of 33000psi, the margin of safety (or factor of safety) provided against tensile yieldin

15、g is 33000/20000, or 1 、 65、 (比如 :對于結構鋼 ,屈 服應力為 33000,而容許應力為 20000,那么有屈服拉應力提供的安全儲備(安全系數(shù) )為33000/20000, 或 1、65)The allowable-stress approach has an important disadvantage in that it does not provide a uniform overload capacity for all parts and all types of structures 、 ( 容許應力法 有一個很大的缺點 ,就就是不能對各種類型的結

16、構及其各部分提供相同的超載能力) Asa result, there is today a rapidly growing tendency to base the design on the ultimate strength and serviceability of the structure, with older allowable-stress approach serving as an alternative basis for design 、 (因此 ,目前更趨向予以結構的極限強度與正常使用為基 礎的設計 , 這使得以往容許應力法為基礎的設計只能成為可選方案之一) The

17、newerapproach currently goes under the name of strength design in reinforced-concrete design literature and plastic design in steel-design literature 、 ( 最新的設計方法在鋼筋混凝土設 計規(guī)范中被稱為強度設計 , 在鋼結構設計規(guī)范中被稱為塑性設計) When proportioning isdone on the strength basis, the anticipated service loading is first multiplie

18、d by a suitable load factor (greater than 1), the magnitude of which depends upon the uncertainty of the loading, the possibility of its changing during the life of the structure, and, for a combination of loadings, the likelihood, frequency, and duration of the particular combination 、 (當結構依照強度設計的方

19、法確定尺寸之后,會首先將設計荷載乘以適當?shù)暮奢d系數(shù) (大于 1),該量值依賴于荷載的不確定性。在結構服役期內發(fā)生變化的可能 性 ,以及在荷載組合中各荷載的相似性、出現(xiàn)頻率與這種特殊組合的持續(xù)時間) In thisapproach for reinforced-concrete design, the theoretical capacity of a structural element is reduced by a capacity-reduction factor to provide for small adversevariations in materialstrengths, w

20、orkmanship, and dimensions 、 (在鋼筋混凝土強度設計法中 ,考慮到材料強 度、工藝與結構尺寸的不利變異性 ,將結構構件的理論承載能力乘上了一個小于1 的承載能力折減系數(shù) ) The structure is then proportioned so that, depending on the governing conditions, the increased load would (1) cause a fatigue or a buckling or a brittle fracture failure or (2) just produce yieldin

21、g at one internal section (or simultaneous yielding at several sections) or (3) cause elastic-plastic displacement of the structure or (4) cause the entire structure to be on the point of collapse 、 (隨后 ,考慮到增大后的荷載將會 :(1)引起疲 勞、壓曲或脆斷破壞 ;(2)在截面內部發(fā)生屈服 (或在幾個截面上同步屈服 );(3)產生結構 的彈塑性位移 ;(4)引起整個結構坍塌 ,所以,根據(jù)這些控

22、制條件 ,會重新調整結構尺寸 )Proponents of this latter approach argue that it results in a more realistic design with a more accurately provided margin of strength over the anticipated service conditions 、 （后一 種方法 （塑性設計 ）的倡導者提出 ,這種方法能夠在預期的使用條件下提供更明確的安全 儲備及更接近實際的設計 ） These improvements result from the fact that n

23、on-elastic and nonlinear effects that become significant in the vicinity of ultimate behavior of the structure can be accounted for 、 （這些進步的起因于這種事實 :那就就是當結構臨近極限狀態(tài)時 , 以能夠清楚地解釋非彈性與非線性效應 ）In recent decades, there has been a growing concern among many prominent engineers that not only is the term “ fact

24、orof safety ”improper and unrealistic, but worse still a structural design philosophy based on this concept leads in most cases to an unduly conservative and therefore uneconomical design, and in some cases to an unconservative design with too high a probability of failure 、 （最近幾十年 ,在許多著名的工程師當中 ,有 一個概念已經越來越明確 :那就就是不但“安全系數(shù)”這個名詞本身已不再合適,也不再符合實際 ,而且更糟糕的就是 ,以此概念為基礎的結構設計原理 ,在許多情況下會導致 一個過度保守的、不經濟的設計 ,或在某些情況下產生一個失效概率很高而毫無安全儲 備的設計 ） They argue that there is no such thing as certainty, either or failure or of safety of a s