附录：中英文翻译
delilah英文部分：borough
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 structure.Usually the lf-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 )for lightweight concrete,and is about 145 pcf (23 )for normal concrete.In calculating the dead load of structural concrete,usually a 5 pcf (1 apart from)increment is included with the weight of the concrete to account for the prence of the reinforcement.
Live loads are loads such as occupancy,snow,wind,or traffic loads,or ismic forces.They may be either fully or partially in place,or not prent 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 rvice,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.
猴子的英文The sum of the calculated dead load and the specified live load is called the rvice load,becau this is the maximum load which may reasonably be expected to act during the rvice resisting is a multiple of the rvice 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 conquences in more than one instance.
跷跷板英文
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.The uncertainties require a safety margin.
billabongIn 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 rvice 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 prestresd concrete,the goal of prestressing was the complete elimination of concrete ternsile stress at rvice 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 rvice load,is called partial prestressing.
There are cas in which it is necessary to avoid all risk of cracking and in which full prestressing is required.Such cas include tanks or rervious where leaks must be avoided,submerged structures or tho 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.sufferer
