翻译4000中文版

英文部分：

LOADS

Loads that act on structures are usually classified as dead loads or live loads.Dead loads are fixed in location and constant in magnitude throughout the life of the https://www.360docs.net/doc/812160165.html,ually the self-weight of a structure is the most important part of the structure and the unit weight of the material.Concrete density varies from about 90 to 120 pcf (14 to 19 2

KN/m)for lightweight concrete,and is about 145 pcf (23 2

KN/m)for normal concrete.In calculating the dead load of structural concrete,usually a 5 pcf (1 2

KN/m)increment is included with the weight of the concrete to account for the presence of the reinforcement.

Live loads are loads such as occupancy,snow,wind,or traffic loads,or seismic forces.They may be either fully or partially in place,or not present at all.They may also change in location.

Althought it is the responsibility of the engineer to calculate dead loads,live loads are usually specified by local,regional,or national codes and specifications.Typical sources are the publications of the American National Standards Institute,the American Association of State Highway and Transportation Officials and,for wind loads,the recommendations of the ASCE Task Committee on Wind Forces.

Specified live the loads usually include some allowance for overload,and may include measures such as posting of maximum loads will not be exceeded.It is oftern important to distinguish between the specified load,and what is termed the characteristic load,that is,the load that actually is in effect under normal conditions of service,which may be significantly less.In estimating the long-term deflection of a structure,for example,it is the characteristic load that is important,not the specified load.

The sum of the calculated dead load and the specified live load is called the service load,because this is the maximum load which may reasonably be expected to act during the service resisting is a multiple of the service load.

Strength

The strength of a structure depends on the strength of the materials from which it is made.Minimum material strengths are specified in certain standardized ways.The properties of concrete and its components,the methods of mixing,placing,and curing to obtain the required quality,and the methods for testing,are specified by the American Concrete Insititue(ACI).Included by refrence in the same document are standards of the American Society for Testing Materials(ASTM)pertaining to reinforcing and

prestressing steels and concrete.

Strength also depends on the care with which the structure is built.Member sizes may differ from specified dimensions,reinforcement may be out of position,or poor placement of concrete may result in voids.An important part of the job of the ergineer is to provide proper supervision of construction.Slighting of this responsibility has had disastrous consequences in more than one instance.

Structural Safety

Safety requires that the strength of a structure be adequate for all loads that may conceivably act on it.If strength could be predicted accurately and if loads were known with equal certainty,then safely could be assured by providing strength just barely in excess of the requirements of the loads.But there are many sources of uncertainty in the estimation of loads as well as in analysis,design,and construction.These uncertainties require a safety margin.

In recent years engineers have come to realize that the matter of structural safety is probabilistic in nature,and the safety provisions of many current specifications reflect this view.

Separate consideration is given to loads and strength.Load factors,larger than unity,are applied to the calculated dead loads and estimated or specified service live loads,to obtain factorde loads that the member must just be capable of sustaining at incipient failure.Load factors pertaining to different types of loads vary,depending on the degree of uncertainty associated with loads of various types,and with the likelihood of simultaneous occurrence of different loads.

Early in the development of prestressed concrete,the goal of prestressing was the complete elimination of concrete ternsile stress at service loads.The concept was that of an entirely new,homogeneous material that woukd remain uncracked and respond elastically up to the maximum anticipated loading.This kind of design,where the limiting tensile stressing,while an alternative approach,in which a certain amount of tensile amount of tensile stress is permitted in the concrete at full service load,is called partial prestressing.

There are cases in which it is necessary to avoid all risk of cracking and in which full prestressing is required.Such cases include tanks or reservious where leaks must be avoided,submerged structures or those subject to a highly corrosive envionment where maximum protection of reinforcement must be insured,and structures subject to high frequency repetition of load where faatigue of the reinforcement may be a consideration.

However,there are many cses where substantially improved performance,reduced cost,or both may

be obtained through the use of a lesser amount of prestress.Full predtressed beams may exhibit an undesirable amount of upward camber because of the eccentric prestressing force,a displacement that is only partially counteracted by the gravity loads producing downward deflection.This tendency is aggrabated by creep in the concrete,which magnigies the upward displacement due to the prestress force,but has little influence on the should heavily prestressed members be overloaded and fail,they may do so in a brittle way,rather than gradually as do beams with a smaller amount of prestress.This is important from the point of view of safety,because suddenfailure without warning is dangeroud,and gives no opportunity for corrective measures to be taken.Furthermore,experience indicates that in many cases improved economy results from the use of a combination of unstressed bar steel and high strength prestressed steel tendons.

While tensile stress and possible cracking may be allowed at full service load,it is also recognized that such full service load may be infrequently applied.The typical,or characteristic,load acting is likely to be the dead load plus a small fraction of the specified live load.Thus a partially predtressed beam may not be subject to tensile stress under the usual conditions of loading.Cracks may from occasionally,when the maximum load is applied,but these will close completely when that load is removed.They may be no more objectionable in prestressed structures than in ordinary reinforced.They may be no more objectionable in prestressed structures than in ordinary reinforced concrete,in which flexural cracks always form.They may be considered a small price for the improvements in performance and economy that are obtained.

It has been observed that reinforced concrete is but a special case of prestressed concrete in which the prestressing force is zero.The behavior of reinforced and prestressed concrete beams,as the failure load is approached,is essentially the same.

The Joint European Committee on Concrete establishes threee classes of prestressed beams.

Class 1:Fully prestressed,in which no tensile stress is allowed in the concrete at service load.

Class 2:Partially prestressed, in which occasional temporary cracking is permitted under infrequent high loads.

Class 3:Partially prestressed,in which there may be permanent cracks provided that their width is suitably limited.

The choise of a suitable amount of prestress is governed by a variety of factors.These include the nature of the loading (for exmaple,highway or railroad bridged,storage,ect.),the ratio of live to dead

load,the frequency of occurrence of loading may be reversed,such as in transmission poles,a high uniform prestress would result ultimate strength and in brittle failure.In such a case,partial prestressing provides the only satifactory solution.

The advantages of partial prestressing are important.A smaller prestress force will be required,permitting reduction in the number of tendons and anchorages.The necessary flexural strength may be provided in such cases either by a combination of prestressed tendons and non-prestressed reinforcing bars,or by an adequate number of high-tensile tendons prestredded to level lower than the prestressing force is less,the size of the bottom flange,which is requied mainly to resist the compression when a beam is in the unloaded stage,can be reduced or eliminated altogether.This leads in turn to significant simplification and cost reduction in the construction of forms,as well as resulting in structures that are mor pleasing esthetically.Furthermore,by relaxing the requirement for low service load tension in the concrete,a significant improvement can be made in the deflection characteristics of a beam.Troublesome upward camber of the member in the unloaded stage fan be avoeded,and the prestress force selected primarily to produce the desired deflection for a particular loading condition.The behavior of partially prestressed beamsm,should they be overloaded to failure,is apt to be superior to that of fully prestressed beams,because the improved ductility provides ample warning of distress.

英译汉：

荷 载

作用在结构上的荷载通常分为恒载或活载。在结构的整个使用寿命期间，恒载的位置是固定的，大小是不变的。通常，结构的自重是恒载的最重要部分。它可以根据结构的尺寸和材料的单位重量进行精确计算。混凝土的密度是变化的，对于轻质混凝土大约从90～120pcf （14～19 ），对于标准混凝土大2KN/m 约为145pcf （23 2

KN/m ）。在计算结构混凝土的恒载时，考虑到钢筋的存在，通常除了混凝土的重量以外还计入5pcf （1 2KN/m ）的增加量。

荷载就是诸如居住、雪、风、车辆荷载或地震力等荷载。它们可能全部或部分地出现，或者根本不出现。这些荷载的位置也是会变化的。

计算恒载时工程师的职责，然而活载通常由当地的、地区的或国家的规范和准则所规定。标准的来源是美国国家标准学会、美国州际公路与运输工作者协会主办的刊物，对于风荷载采用美国土木工程学会风力专题委员会的建议。

规定活载通常包含某些容许的超载，并可以明显的或隐含地计入动态影响。活载可以采用标明楼板或桥梁最大荷载那样的措施在某种程度上加以控制，但是也不能肯定这些荷载不会被超过。将规定荷载和所谓特征荷载区别开来往往是很重要的，也就是说，后者是正常使用情况下实际起作用的荷载，它可能很小。例如在计算结构的长期挠度时，重要的是特征荷载，而不是规定荷载。

计算得到的荷载和规定活载的和称为使用荷载，因为这是在结构使用寿命期间可预料到的要作用在其上的最大荷载。使用荷载乘以一个系数就是计算荷载，即破坏荷载，它就是结构必须恰好能承受的荷载。

强度

结构的强度取决于建造它的材料的强度。材料的最小强度都以一些标准的方式来规定。美国混凝土学会对混凝土的性质及其成分、满足质量要求的拌和、浇筑和养生方法以及试验方法均作了规定。在同一文件中，作为参考也列入了美国材料试验协会关于普通钢筋、预应力钢筋和混凝土的标准。

强度也取决于结构施工的精心程度。构建的大小可能与规定的尺寸有所不同，钢筋的位置可能发生移动，或者由于混凝土浇筑得不好可能会造成空洞。工程师工作的重要职责是要保证应有的施工监督。工程师的失职曾经不止一次产生了造成巨大损失的后果。

结构安全度

安全性要求结构的强度足以承受可以预料到的，作用在结构上的全部荷载。如果强度能够精

确地预先计算出来而且荷载也可以同样确切地知道的话，则所提供的强度只要稍微超过荷载的要求就能保证安全。可是有许多因素会导致在荷载的估算以及分析、设计和施工等方面的不确定性。这些不确定因素要求具有安全储备。

近些年来，工程师们已经开始认识到结构安全度这个问题在实质上就是概率统计问题，因此许多现行规范的安全规定都反映了这一观点。

荷载和强度分别加以考虑。将大于1的荷载系数乘以算得到的恒载和估算或规定的使用活载，可以得到构件在开始破坏时恰好能承受的计算荷载。对于不同类型的荷载，荷载系数是不相同的，它取决于各种不同荷载和不同荷载可能同时出现的不确定程度。

在预应力混凝土发展的早期，预加应力的目的是要完全消除在使用荷载作用下混凝土中的拉应力。这曾经是一种全新的匀质材料的概念，认为这种材料能够不开裂并且保持弹性工作状态，直至达到其最大的设计荷载。在全部使用荷载作用下，混凝土的极限拉应力值为零的这种设计，通常称为之全预应力设计；而另一种在全部荷载作用下容许混凝土内产生一定大小的拉应力的设计方法，称为部分预应力设计。

有些场合必须避免任何产生裂缝的危险，此时需要采用预应力。这些场合包括：不能产生渗漏的容器或水库，必须保证具有最大钢筋保护层的水下结构和在强腐蚀环境中的结构，必须考虑钢筋疲劳问题的承受高频重复荷载的结构。

但是，在许多场合中，只要施加少量的预应力就可以显著地改善结构的工作性能，降低造价，或者二者兼有之。施加全预应力的梁，由于偏心预张拉力作用，可能出现不希望有的、较大的拱度，产生向下挠度的重力荷载只能抵消其中一部分的位移量。混凝土的徐变加剧了这种趋势，它加大了由于预张拉力引起的向上位移，但是对于只可能间歇作用的活载引起的向下挠度影响极小。而且，施加很大预应力的构件如果超载而导致破坏，则构件会呈现脆性破坏，而不是像具有较小预应力的梁那样逐渐地产生破坏。从安全角度来说这个问题是很重要的，因为没有预兆的突然破坏是危险的，并且没有时间采取补救措施。此外，经验表明，非预应力钢筋与高强度预应力钢筋的结合使用在许多情况下可以产生更好的经济效益。

尽管在全部使用荷载作用下容许出现拉应力和可能的裂缝，但是也要认识到全部使用荷载并不是经常出现的。典型的或特征性的作用荷载可能就是恒载加上一小部分设计活载。因此部分预应力的梁在一半荷载情况下不会承受拉应力。当最大荷载作用时偶尔可能产生裂缝，但在该荷载移去后，裂缝将完全闭合。与始终带有由于承受弯曲应力而产生裂缝的普通钢筋混凝土相比，预应力结构中的裂缝就不会由什么问题了。偶尔的开裂可以看作是为得到工作性能上的改善所付出的小小代价。

可以说，钢筋混凝土只不过是预应力混凝土中预张拉力为零的一个特例。在接近破坏荷载是，钢筋混凝土梁和预应力混凝土梁的工作情况基本上是相同的。

欧洲混凝土委员会规定了三类预应力梁：

第一类：全预应力梁，在使用荷载作用下，混凝土内不容许由拉应力产生。

第二类：部分预应力梁，在不经常出现的大荷载作用下，容许出现偶然的暂时性裂缝。

第三类：部分预应力梁，在裂缝宽度受到限制的情况下，容许有永久性裂缝。

对于适量预张拉力的选择取决于多种因素。它们包括：荷载性质（例如，公路和铁路桥梁，贮罐，等等），活载与恒载的比例，满载的出现频率以及腐蚀性介质的存在。独语荷载方向可能变更的结构物，例如在输电线路中的电杆，高而且均匀的预张拉力会降低其极限强度和导致脆性破坏。在这种情况下，部分预应力提供了唯一满意的解决方法。

部分预应力有很大的有点，它需要较小的预张拉力，因此可以减少预应力筋和锚具的数量。在此种情况下，必要的抗弯强度或者由预应力钢筋和非预应力钢筋共同提供，或者由预张拉力至低于容许值的足够数量的高强钢筋来保证。在某些情况下，可以同时使用张拉的和非张拉的钢筋。因为预张拉力较小，主要为承受梁在未加荷载阶段压应力所需的底面翼缘尺寸就可以减小或完全取消。这样又使得模板结构得到显著的简化同时可以减少模板费用，并且使结构更加美观。此外，由于减少了对混凝土中在使用荷载下的拉应力要求，梁的挠度特性可以得到显著的改善。构件可以避免产生在未加荷载阶段过大的上拱度，而且对于特定的荷载情况，可以通过选择预张拉力来获得所要求的挠度。部分预应力梁如遇超载而破坏，其工作性能也往往优于全预应力梁，因为得到改善了的延性能够为事故提供充分的预兆。