SafetyofStructures.ppt

单击此处编辑母版标题样式,单击此处编辑母版文本样式,第二级,第三级,第四级,第五级,*,Lesson Plan,Review,New Words and Phrases,Reading and TranslatingUnit Eight,Classroom Exercises,Review,a premature failure by diagonal tension,the amount of cracking(width and number of cracks)is proportional to the strain,Unit Eight,Safety of Structures,单 元 8,结构的安全度,The principal purpose of specifications is to provide general principles and computational methods in order to verify safety of structures.,The principal purpose of specifications is to provide general principles and computational methods in order to verify safety of structures.,规范的主要目的是提供一般性的设计原理和计算方法，以便验算结构的安全性。,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:,（1）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.,(1)承载力极限状态，它相当于结构能够达到的最大承载能力。例如：结构的局部屈曲和整体不稳定性；某些截面失效，随后结构转变为机构；疲劳破坏；引起结构几何形状显著变化的弹性变形或塑性变形或徐变；结构对交变荷载、火灾和爆炸的敏感性。,（2）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.,（2）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.,(2)正常使用极限状态，它对应着结构的使用功能和耐久性。例如：结构失稳前的过大变形和位移；早期开裂或过大的裂缝；较大的振动和腐蚀。,Computational methods used to verify structures with respect to the different safety conditions can be separated into:,（1）Deterministic methods,in which the main parameters are considered as nonrandom parameters.,（2）Probabilistic methods,in which the main parameters are considered as random parameters,Computational methods used to verify structures with respect to the different safety conditions can be separated into:,（1）Deterministic methods,in which the main parameters are considered as nonrandom parameters.,（2）Probabilistic methods,in which the main parameters are considered as random parameters,根据不同的安全度条件，可以把结构验算所采用的计算方法分成:,（1）确定性的方法，在这种方法中，把主要参数看作非随机参数。,（2）概率方法，在这种方法中，主要参数被认为是随机参数。,Alternatively,with respect to the different use of factors of safety,computational methods can be separated into:,（1）Allowable stress method,in which the stresses computed under maximum loads are compared with the strength of the material reduced by given safety factors.,Alternatively,with respect to the different use of factors of safety,computational methods can be separated into:,（1）Allowable stress method,in which the stresses computed under maximum loads are compared with the strength of the material reduced by given safety factors.,或者，根据安全系数的不同用途，可以把结构的计算方法分为:,(1)容许应力法：在这种方法中，把结构承受最大荷载时的,计算应力,和经过特定安全系数折减后的,材料强度,作比较。,（2）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).,（2）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.,(2)极限状态法，在这种方法中，结构的工作状态是以其最大强度为依据来衡量的。由理论分析确定的,（结构材料）最大强度,应不小于结构,承受设计荷载所需要的强度,。设计荷载等于乘以荷载系数的活载与恒载之和。,The safety factor,which according to modern trends is homogeneous and independent of the nature and combination of the materials used,can usually be defined as the ratio between the conditions yield failure of the construction and the worst predictable working conditions.This ratio is also inversely proportional to the probability of failure of the structure.,The safety factor,which according to modern trends is homogeneous and independent of the nature and combination of the materials used,can usually be defined as the ratio between the conditions yield failure of the construction and the worst predictable working conditions.This ratio is also inversely proportional to the probability of failure of the structure.,就目前的趋势而言，安全系数是单一的而且与所使用的材料性质及其组合情况无关，通常把它定义为结构发生破坏的条件与结构可预料的最不利的工作条件之比值。这个比值还与结构的破坏概率成反比。,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.,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.,至少在理论上，概率法的主要优点是可以科学地考虑所有随机安全系数，然后将这些随机安全系数组合成确定的安全系数。,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.,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.,但是，应该考虑到进行全概率分析的困难。对于这种分析来说，应该了解活载及其所引起的应力的分布规律、材料的力学性能的弥散性和截面及结构几何尺寸。,Classroom Exercises,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 that introduces some simplifying assumptions(semi-probabilistic methods).,