附录：中英文翻译英文部分：LOADSLoads that act on structures are usually classified as dead loads or live loads are fixed inlocation and constant in magnitude throughout the life of the the self-weight of astructure is the most important part of the structure and the unit weight of the densityvaries from about 90 to 120 pcf (14 to 19 KN/m)for lightweight concrete,and is about 145 pcf (23KN/m)for normal calculating the dead load of structural concrete,usually a 5 pcf (1KN/m)increment is included with the weight of the concrete to account forthe presence of the reinforcement.Live loads are loads such as occupancy,snow,wind,or traffic loads,or seismic may be either fully or partially in place,or not present at may also change in location.Althought it is the responsibility of the engineer to calculate deadloads,live loads are usuallyspecified by local,regional,or national codes and sources are the publications of the American National Standards Institute,the American Association of State Highway andTransportation 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 measuressuch as posting of maximum loads will not be is oftern important to distinguish between thespecified load,and what is termed the characteristic load,that is,the load that actually is in effect undernormal conditions of service,which may be significantly 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,becausethis is the maximum load which may reasonably be expected to act during the service resisting is a multiple of the service load.StrengthThe strength of a structure depends on the strength of the materials from which it ismaterial strengths are specified in certain standardized properties of concrete and itscomponents,the methods of mixing,placing,and curing to obtain the requiredquality,and the methods fortesting,are specified by the American Concrete Insititue(ACI).Included by refrence in the same documentare 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 sizes may differ fromspecified dimensions,reinforcement may be out of position,or poor placement of concrete may result inimportant part of the job of the ergineer is to provide proper supervision ofof this responsibility has had disastrous consequences in more than one instance.Structural SafetySafety requires that the strength of a structure be adequate for all loads that may conceivably act onstrength could be predicted accurately and if loads were known with equal certainty,then safelycould be assured by providing strength just barely in excess of the requirements of the there aremany sources of uncertainty in the estimation of loads as well as in analysis,design,and 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 factors,larger than unity,are applied tothe calculated dead loads and estimated or specified service live loads,to obtain factorde loads that themember must just be capable of sustaining at incipient factors pertaining to different typesof 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 completeelimination of concrete ternsile stress at service concept was that of an entirelynew,homogeneous material that woukd remain uncracked and respond elastically up to the maximumanticipated kind of design,where the limiting tensile stressing,while an alternativeapproach,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 iscases include tanks or reservious where leaks must be avoided,submerged structures orthose subject to a highly corrosive envionment where maximum protection of reinforcement must beinsured,and structures subject to high frequency repetition of load where faatigue of the reinforcementmay be a consideration.However,there are many cses where substantially improved performance,reduced cost,or both maybe obtained through the use of a lesser amount of predtressed beams may exhibit anundesirable amount of upward camber because of the eccentric prestressing force,a displacement that isonly partially counteracted by the gravity loads producing downward tendency isaggrabated by creep in the concrete,which magnigies the upward displacement due to the prestressforce,but has little influence on the should heavily prestressed members be overloaded and fail,they maydo so in a brittle way,rather than gradually as do beams with a smaller amount of isimportant from the point of view of safety,because suddenfailure without warning is dangeroud,and givesno opportunity for corrective measures to be ,experience indicates that in many casesimproved 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 serviceload,it is also recognizedthat such full service load may be infrequently typical,or characteristic,load acting is likelyto be the dead load plus a small fraction of the specified live a partially predtressed beam maynot be subject to tensile stress under the usual conditions of may from occasionally,whenthe maximum load is applied,but these will close completely when that load is may be nomore objectionable in prestressed structures than in ordinary may be no moreobjectionable in prestressed structures than in ordinary reinforced concrete,in which flexural cracksalways 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 ofprestressed concrete in whichthe prestressing force is behavior of reinforced and prestressed concretebeams,as the failure load is approached,is essentially the same.The Joint European Committee on Concrete establishes threee classes of prestressed beams.Class1: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 ofinclude thenature of the loading (for exmaple,highway or railroad bridged,storage,ect.),the ratio of live to deadload,the frequency of occurrence of loading may be reversed,such as in transmission poles,a highuniform prestress would result ultimate strength and in brittle such acase,partial prestressing provides the only satifactory solution.The advantages of partial prestressing are smaller prestress force will berequired,permitting reduction in the number of tendons and necessary flexural strengthmay be provided in such cases either by a combination of prestressed tendons and non-prestressedreinforcing bars,or by an adequate number of high-tensile tendons prestredded to level lower than theprestressing force is less,the size of the bottom flange,which is requied mainly to resist the compressionwhen a beam is in the unloaded stage,can be reduced or eliminated leads inturn tosignificant simplification and cost reduction in the construction of forms,as well as resulting in structuresthat are mor pleasing ,by relaxing the requirement for low service load tension inthe concrete,a significant improvement can be made in the deflection characteristics of aupward 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 behaviorof 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。