The principal scope of specifications is to provide general principles and computational methods in order to verify safety of structures. The 'safety factor', which according to modern trends is independent of the nature and combination of the materials used, can usually be defined as the ratio between the conditions which yield failure of the construction and the worst predictable working conditions.?This ratio is also proportional to the inverse of the?probability (risk) of failure of the structure.

規(guī)范的主要目的是提供一般性的設(shè)計原理和計算方法，以便驗算結(jié)構(gòu)的安全性。就日前的趨勢而言，安全系數(shù)是與所使用的材料性質(zhì)及其組合情況無關(guān)，通常把它定義為結(jié)構(gòu)發(fā)生破壞的條件與結(jié)構(gòu)可預(yù)料的最不利的工作條件之比值。這個比值還與結(jié)構(gòu)的破壞概率（危險率）成反比。

Failure has to be considered not only as overall collapse of the structure but also as unserviceability or, according to a more precise, common definition, as the reaching of a 'limit state' which causes the construction not to accomplish the task it was designed for. There are two categories of limit state:

(a) Ultimate limit state, which corresponds to the highest value of the load-bearing capacity. Examples include local buckling or global instability of the structure; failure of some sections and subsequent transformation of the structure into a mechanism; failure by fatigue; elastic or plastic deformation or creep that cause a substantial change of the geometry of the structure; and sensitivity of the structure to alternating loads, to fire and to explosions.

(b) Service limit states, which are functions of the use and durability of the structure. Examples include excessive deformations and displacements without instability; early or excessive cracks; large vibrations; and corrosion.

破壞不僅僅指結(jié)構(gòu)的整體破壞，而且還指結(jié)構(gòu)不能正常地使用，或者，用更為確切的話來說，把破壞看成是結(jié)構(gòu)已經(jīng)達到不能繼續(xù)承擔(dān)其設(shè)計荷載的“極限狀態(tài)”。通常有兩類極限狀態(tài)，即：

(1)強度極限狀態(tài)，它相當于結(jié)構(gòu)能夠達到的最大承載能力。其例子包括結(jié)構(gòu)的局部屈曲和整體不穩(wěn)定性；某些截面失效，隨后結(jié)構(gòu)轉(zhuǎn)變?yōu)闄C構(gòu)；疲勞破壞；引｜起結(jié)構(gòu)幾何形狀顯著變化的彈性變形或塑性變形或徐變；結(jié)構(gòu)對交變荷載、火災(zāi)和爆炸的敏感性。

(2)使用極限狀態(tài)，它對應(yīng)著結(jié)構(gòu)的使用功能和耐久性。其例子包括結(jié)構(gòu)失穩(wěn)之前的過大變形和位移；早期開裂或過大的裂縫；較大的振動和腐蝕。

Computational methods used to verify structures with respect to the different safety conditions can be separated into:

(a) Deterministic methods, in which the main parameters are considered as nonrandom parameters.

(b) Probabilistic methods, in which the main parameters are considered as random parameters.

根據(jù)不同的安全度條件，可以把結(jié)構(gòu)驗算所采用的計算方法分成：

(1)確定性的方法，在這種方法中，把主要參數(shù)看作非隨機參數(shù)。

(2)概率方法，在這種方法中，主要參數(shù)被認為是隨機參數(shù)。

Alternatively, with respect to the different use of factors of safety, computational methods can be separated into:

(a) Allowable stress method, in which the stresses computed under maximum loads are compared with the strength of the material reduced by given safety factors.

(b) Limit states method, in which the structure may be proportioned on the basis of its maximum strength. This strength, as determined by rational analysis, shall not be less than that required to support a factored load equal to the sum of the factored live load and dead load (ultimate state).

此外，根據(jù)安全系數(shù)的不同用途，可以把結(jié)構(gòu)的計算方法分為：

(1)容許應(yīng)力法，在這種方法中，把結(jié)構(gòu)承受最大荷載時計算得到的應(yīng)力與經(jīng)過特定安全系數(shù)折減后的材料強度作比較。

(2)極限狀態(tài)法，在這種方法中，結(jié)構(gòu)的工作狀態(tài)是以其最大強度為依據(jù)來衡量的。由理論分析確定的這一最大強度應(yīng)不小于結(jié)構(gòu)承受計算荷載所算的強度（極限狀態(tài)）。計算荷載等于分別乘以荷載系數(shù)的活載與恒載之和。

The stresses corresponding to working (service) conditions with unfactored live and dead loads are compared with prescribed values (service limit state). From the four possible combinations of the first two and second two methods, we can obtain some useful computational methods. Generally, two combinations prevail:

(a) Deterministic methods, which make use of allowable?stresses.

(b) Probabilistic methods, which make use of limit states.

把對應(yīng)于不乘以荷載系數(shù)的活載和恒載的工作（使用）條件的應(yīng)力與規(guī)定值（使用極限狀態(tài)）相比較。根據(jù)前兩種方法和后兩種方法的四種可能組合，我們可以得到一些實用的計算方法。通常采用下面兩種計算方法：

(1)確定性的方法，這種方法采用容許應(yīng)力。

(2)概率方法，這種方法采用極限狀態(tài)。

The main advantage of probabilistic approaches is that, at least in theory, it is possible to scientifically take into account all random factors of safety; which are then combined to define the safety factor.?Probabilistic approaches depend upon:

(a) Random distribution of strength of materials with respect to?the conditions of fabrication and erection (scatter of the values of mechanical properties throughout the structure);

(b)Uncertainty of the geometry of the cross-sections and of the structure (faults and imperfections due to fabrication and erection of the structure);

(c)Uncertainty of the predicted live loads and dead loads acting on the structure;

(d)Uncertainty related to the approximation of the?computational method used (deviation of the actual stresses from computed stresses).?

至少在理論上，概率法的主要優(yōu)點是可以科學(xué)地考慮所有隨機安全系數(shù)，然后將這些隨機安全系數(shù)組合成確定的安全系數(shù)。概率法取決于：

(1)制作和安裝過程中材料強度的隨機分布（整個結(jié)構(gòu)的力學(xué)性能值的分散性）；

(2)截面和結(jié)構(gòu)幾何尺寸的不確定性（由結(jié)構(gòu)制作和安裝造成的誤差和缺陷而引起的）；

(3)對作用在結(jié)構(gòu)上的活載和恒載的預(yù)測的不確定性；

(4)與所采用的近似計算方法有關(guān)的不精確性（實際應(yīng)力與計算應(yīng)力的偏差）。

Furthermore, probabilistic theories mean that the allowable risk can be based on several factors, such as:

(a) Importance of the construction and gravity of the damage by its failure;

(b) Number of human lives which can be threatened by this failure;

(c) Possibility and/ or likelihood of repairing the structure;

(d) Predicted life of the structure.

此外，概率理論意味著允許的后險率可以以若干因素為依據(jù)，例如：

(1)建筑物的重要性和建筑物破壞造成的危害性；

(2)由于建筑物破壞使生活受到威脅的人數(shù)；

(3)修復(fù)建筑物的可能性；

(4)建筑物的預(yù)期壽命。

All these factors are related to economic and social considerations such as:

(a) Initial cost of the construction;

(b) Amortization funds for the duration of the construction;

(c) Cost of physical and material damage due to the failure of the construction;

(d) Adverse impact on society;

(e) Moral and psychological views.

所有這些因素均與經(jīng)濟和社會條件有關(guān)，例如：

(1)建筑物的初始建設(shè)費；

(2)建筑物使用期限內(nèi)的折舊費；

(3）由于建筑物破壞而造成的物質(zhì)和材料損失費，

(4)在社會上造成的不良影響?。?/span>

(5)精神和心理上的考慮。

The definition of all these parameters, for a given safety factor, allows construction at the optimum cost. However, the difficulty of carrying out a complete probabilistic analysis has to be taken into?account. For such an analysis the laws of the distribution of the live load and its induced stresses, of the scatter of mechanical properties of materials, and of the geometry of the cross-sections and the structure have to be known. Furthermore, it is difficult to interpret the interaction between the law of distribution of strength and that of stresses because both depend upon the nature of the material, on the cross-sections and upon the load acting on the structure.

These?practical difficulties can be overcome in two ways. The first is to apply different safety factors to the material and to the loads, without necessarily adopting the probabilistic criterion. The second is an approximate probabilistic method which introduces some simplifying assumptions (semi-probabilistic methods).

就給定的安全系數(shù)而論，所有這些參數(shù)的確定都是以建筑物的最佳成本為依據(jù)的。但是，應(yīng)該考慮到進行全概率分析的困難。對于這種分析來說，應(yīng)該了解活載及其所引起的應(yīng)力的分布規(guī)律、材料的力學(xué)性能的分散性和截面及結(jié)構(gòu)幾何尺寸的分散性。此外，由于強度的分布規(guī)律和應(yīng)力的分布規(guī)律取決定于材料的性質(zhì)、截面的形式、作用在結(jié)構(gòu)上的荷載，因此，闡明這兩種分布規(guī)律之間的相互關(guān)系是困難的。這些實際困難可以采用兩種方法來克服。第一種方法對材料和荷載采用不同的安全系數(shù)，而不需要采用概率準則；第二種方法是引入一些簡化假設(shè)的近似概率方法（半概率方法）。