《土木工程专业英语 第2版》 翻译版 课件 Unit 3 Introduction to Steel Structures

Unit3IntroductiontoSteelStructuresEnglishforCivilEngineering——Unit3IntroductiontoSteelStructures3.1StructuralDesign（结构设计）3.2PrinciplesofDesign（设计准则）3.3HistoricalBackgroundofSteelStructures（钢结构背景）3.4Loads（荷载）3.4.1DeadLoad（静荷载）3.4.2LiveLoad（活荷载）3.4.3HighwayLiveLoads（高速公路活荷载）3.4.4Impact（冲击荷载）3.4.5SnowLoad（雪荷载）3.4.6WindLoad（风荷载）3.4.7EarthquakeLoad（地震作用）3.5TypesofStructuralSteelMembers（钢结构构件）3.5.1TensionMembers（受拉构件）3.5.2CompressionMembers（受压构件）3.5.3Beams（梁）3.5.4BendingandAxialLoad（压（拉）弯构件）Structuraldesignmaybedefinedasamixtureofartandscience,combiningtheexperiencedengineer’sintuitivefeelingforthebehaviorofastructurewithasoundknowledgeoftheprinciplesofstatics,dynamics,mechanicsofmaterials,andstructuralanalysis,toproduceasafeeconomicalstructurethatwillserveitsintendedpurpose.结构设计的定义intuitive直观的；sound合理的；intended预期的3.1StructuralDesignStructuraldesignmaybedefinedasamixtureofartandscience,(combiningtheexperiencedengineer’sintuitivefeelingforthebehaviorofastructurewithasoundknowledgeoftheprinciplesofstatics,dynamics,mechanicsofmaterials,andstructuralanalysis),toproduceasafeeconomicalstructure(thatwillserveitsintendedpurpose.)结构设计可以定义为艺术和科学的结合，再加上有经验的工程师基于静力学，动力学，材料力学和结构分析方面合理的知识而对于一个结构行为的直观感受，来设计一个能够达成它的预期目的的安全经济的结构。3.1StructuralDesignUntilabout1850,structuraldesignwaslargelyanartrelyingonintuitiontodeterminethesizeandarrangementofthestructuralelements.Earlyman-madestructuresessentially

conformedtothosewhichcouldalsobeobservedinnature,suchasbeamsandarches.Astheprinciplesgoverningthebehaviorofstructuresandstructuralmaterialshavebecomebetterunderstood,designprocedureshavebecomemorescientific.早期的结构设计intuition直觉；

conformedto符合；

essentially基本上；governing控制3.1StructuralDesignUntilabout1850,structuraldesignwaslargelyanart(relyingonintuitiontodeterminethesizeandarrangementofthestructuralelements).Earlyman-madestructuresessentiallyconformedtothosewhichcouldalsobeobservedinnature,suchasbeamsandarches.(Astheprinciplesgoverningthebehaviorofstructuresandstructuralmaterialshavebecomebetterunderstood),designprocedureshavebecomemorescientific.直到1850，结构设计很大程度上是一种依赖直觉决定结构构件尺寸和分布位置的艺术。早期的人造结构基本上和那些可以在自然界中观察到的结构构件一致，比如梁和拱。随着控制结构行为和结构材料的原则变得越来越清楚，设计过程变得越来越科学。3.1StructuralDesignComputationsinvolvingscientificprinciplesshouldserveasaguidetodecisionmakingandnotbefollowedblindly.Theartorintuitiveabilityoftheexperiencedengineerisutilizedtomakethedecisions,guidedbythecomputationalresults.结构设计的原则isutilizedto利用3.1StructuralDesignComputationsinvolvingscientificprinciplesshouldserveasaguidetodecisionmakingandnotbefollowedblindly.Theartorintuitiveabilityoftheexperiencedengineerisutilizedtomakethedecisions,(guidedbythecomputationalresults).具有科学依据的计算应该起到对决策起指导作用而且应不被盲目遵从。一个有经验工程师应在计算结果的指导之下，利用设计艺术或者直觉来做决策。3.1StructuralDesignDesignisaprocessbywhichanoptimumsolutionisobtained.Inthistexttheconcerniswiththedesignofstructures—inparticular,steelstructures.Inanydesign,certaincriteriamustbeestablishedtoevaluatewhetherornotanoptimumhasbeenachieved.Forastructure,typicalcriteriamaybe(a)minimumcost;(b)minimumweight;(c)minimumconstructiontime;(d)minimumlabor;(e)minimumcostofmanufactureofowner’sproducts;and(f)maximumefficiencyofoperationtoowner.Usuallyseveralcriteriaareinvolved,eachofwhichmayrequireweighting.Observingtheabovepossiblecriteria,itmaybeapparentthatsettingclearlymeasurablecriteria(suchasweightandcost)forestablishinganoptimumfrequentlywillbedifficult,andperhapsimpossible.Inmostpracticalsituations,theevaluationmustbequalitative.设计准则的要求optimum最佳的，最佳效果；criteria原则；manufacture建造；weighting加权重；qualitative定性的3.2PrinciplesofDesignDesignisaprocess

(bywhichanoptimumsolutionisobtained).Inthistexttheconcerniswiththedesignofstructures—inparticular,steelstructures.Inanydesign,certain

criteriamustbeestablished(toevaluatewhetherornotanoptimumhasbeenachieved).Forastructure,typical

criteriamaybe(a)minimumcost;(b)minimumweight;(c)minimumconstructiontime;(d)minimumlabor;(e)minimumcostofmanufactureofowner’sproducts;and(f)maximumefficiencyofoperationtoowner.Usuallyseveralcriteriaareinvolved,(eachofwhichmayrequireweighting).Observingtheabovepossiblecriteria,itmaybeapparentthatsettingclearlymeasurablecriteria(suchasweightandcost)forestablishinganoptimumfrequentlywillbedifficult,andperhapsimpossible.Inmostpracticalsituations,theevaluationmustbequalitative.设计是一个寻找最佳解决方案的过程。在这篇文章中，尤其关心的是钢结构的设计。在任何设计中，有些标准必须被建立来评价是否实现了最佳方案。对一个结构来说，典型的标准比如（a）最低花费；（b）最低重量；（c）最低建造时间；（d）最低劳动力；（e）最低制造花费；和（f）最大工作效率。通常的设计会考虑几个标准，每个标准也许都会有一定的权重。观察上述所有可能的标准，显然为了实现最佳目标（比如重量和花费）而频繁建立明确的可以量化的标准是很困难的，甚至是不可能的。在多数实际情况中，评价必须是定性的。3.2PrinciplesofDesignIfaspecificobjectivecriterioncanbeexpressedmathematically,thenoptimization

techniquesmaybeemployedtoobtainamaximumorminimumfortheobjectivefunction.Optimizationproceduresandtechniquescompriseanentiresubjectthatisoutsidethescopeofthistext.Thecriterionofminimumweightisemphasizedthroughout,underthegeneralassumptionthatminimummaterialrepresentsminimumcost.Othersubjectivecriteriamustbekeptinmind,eventhoughtheintegrationofbehavioralprincipleswithdesignofstructuralsteelelementsinthistextutilizesonlysimpleobjectivecriteria,suchasweightorcost.结构设计的优化方法criterion标准；optimization

techniques优化方法；scope范围；comprise

包含assumption假定；integration整合；utilize利用3.2PrinciplesofDesignIfaspecificobjectivecriterioncanbeexpressedmathematically,thenoptimizationtechniquesmaybeemployedtoobtainamaximumorminimum(fortheobjectivefunction).Optimizationproceduresandtechniquescompriseanentiresubjectthatisoutsidethescopeofthistext.Thecriterionofminimumweightisemphasizedthroughout,(underthegeneralassumptionthatminimummaterialrepresentsminimumcost).Othersubjectivecriteriamustbekeptinmind,eventhoughtheintegrationofbehavioralprincipleswithdesignofstructuralsteelelementsinthistextutilizesonlysimpleobjectivecriteria,suchasweightorcost.如果一个明确客观的标准可以用数学公式表达，那么可以采用优化技术来获取目标函数的最大或最小值。优化过程和技术包含一个主体，它已经超出了本文讨论的范围。最小重量的标准从头到尾都在强调，在通常的假设下最少的材料代表最小花费。尽管这篇文章中提到的设计原理和钢结构单元的综合只用到了简单客观的标准，其它主观的标准必须被记住，比如重量或者花费。3.2PrinciplesofDesignThedesignproceduremaybeconsideredtobecomposedoftwoparts—functionaldesignandstructuralframeworkdesign.Functionaldesignensuresthatintendedresultsareachieved,suchas(a)adequateworkingareasandclearances;(b)properventilationand/orairconditioning;(c)adequatetransportationfacilities,suchaselevators,stairways,andcranesormaterialshandlingequipment;(d)adequatelighting;and(e)aesthetics.设计过程包含两部分——功能设计和结构框架设计adequate充足的；clearance空隙；ventilation通风系统；crane吊车；aesthetics美观3.2PrinciplesofDesignThedesignproceduremaybeconsideredtobecomposedoftwoparts—functionaldesignandstructuralframeworkdesign.Functionaldesignensuresthatintendedresultsareachieved,suchas(a)adequateworkingareasandclearances;(b)properventilationand/orairconditioning;(c)adequatetransportationfacilities,suchaselevators,stairways,andcranesormaterialshandlingequipment;(d)adequatelighting;and(e)aesthetics.设计过程一般认为包含两部分——功能设计和结构框架设计。功能设计确保预期结果可以达成，例如（a）充足的工作区域和空隙；（b）合适的通风设备和空调；（c）足够的运输设备，例如电梯，楼梯，吊车和材料处理设备；（d）充足的照明；和（e）美观。3.2PrinciplesofDesignThestructuralframeworkdesignistheselectionofarrangementandsizesofstructuralelementssothatserviceloadsmaybesafelycarried,anddisplacementsarewithinacceptablelimits.结构框架设计3.2PrinciplesofDesignThestructuralframeworkdesignistheselectionofarrangementandsizesofstructuralelements(sothatserviceloadsmaybesafelycarried,anddisplacementsarewithinacceptablelimits).结构框架设计是选择结构单元的尺寸和顺序的过程，由此工作荷载可以安全的加载并且位移也在可接受的限制内。3.2PrinciplesofDesignTheiterativedesignproceduremaybeoutlinedasfollows:1.Planning.Establishmentofthefunctionsforwhichthestructuremustserve.Setcriteriaagainstwhichtomeasuretheresultingdesignforbeinganoptimum.2.Preliminarystructuralconfiguration.Arrangementoftheelementstoservethefunctionsinstep1.3.Establishmentoftheloadstobecarried.4.Preliminarymemberselection.Basedonthedecisionsofsteps1,2,and3selectionofthemembersizestosatisfyanobjectivecriterion,suchasleastweightorcost.设计过程的具体步骤1—4iterative重复的；preliminary初步的；configuration配置；satisfy达到3.2PrinciplesofDesignTheiterativedesignproceduremaybeoutlinedasfollows:1.Planning.Establishmentofthefunctionsforwhichthestructuremustserve.Setcriteriaagainstwhichtomeasuretheresultingdesignforbeinganoptimum.2.Preliminary

structural

configuration.Arrangementoftheelementstoservethefunctionsinstep1.3.Establishmentoftheloadstobecarried.4.Preliminary

member

selection.Basedonthedecisionsofsteps1,2,and3selectionofthemembersizestosatisfyanobjectivecriterion,suchasleastweightorcost.重复的设计过程可以被概括如下：1.计划。建立结构必须具有的功能。为实现最佳效果的设计制定标准。2.初始的结构配置。排列结构单元来实现第一步中的功能。3.建立要加载的荷载。4.初步的单元选定。根据第一，第二，第三步中的决定选择单元的尺寸大小来满足客观的标准，例如最小的重量或最少的花费。3.2PrinciplesofDesign5.Analysis.Structuralanalysisinvolvingmodelingtheloadsandthestructuralframeworktoobtaininternalforcesandanydesireddeflections.6.Evaluation.Areallstrengthandserviceabilityrequirementssatisfiedandistheresultoptimum?Comparetheresultwithpredeterminedcriteria.7.Redesign.Repetitionofanypartofthesequence1through6foundnecessaryordesirableasaresultofevaluation.Steps1through6representaniterativeprocess.Usuallyinthistextonlysteps3through6willbesubjecttothisiterationsincethestructuralconfigurationandexternalloadingwillbeprescribed.8.Finaldecision.Thedeterminationofwhetherornotanoptimumdesignhasbeenachieved.设计过程的具体步骤5—8internalforces内力；serviceabilityrequirements使用要求；prescribed规定3.2PrinciplesofDesign5.Analysis.Structuralanalysisinvolvingmodelingtheloadsandthestructuralframeworktoobtaininternalforcesandanydesireddeflections.6.Evaluation.Areallstrengthandserviceabilityrequirementssatisfiedandistheresultoptimum?Comparetheresultwithpredeterminedcriteria.7.Redesign.Repetitionofanypartofthesequence1through6foundnecessaryordesirableasaresultofevaluation.Steps1through6representaniterativeprocess.Usuallyinthistextonlysteps3through6willbesubjecttothisiterationsincethestructuralconfigurationandexternalloadingwillbeprescribed.8.Final

decision.Thedeterminationofwhetherornotanoptimumdesignhasbeenachieved.5.分析。结构分析包括为荷载和结构框架建模来获取内力和任何需要的变形。6.评估。是否所有的强度和使用性要求都被满足了？是否是最优解决方案？比较结果和之前决定的标准。7.重新设计。重复步骤一到六的任何一部分，找到必要的或需要的作为评估的结果。步骤一到步骤六代表一个重复的过程。通常在这篇文章中，只有步骤三到步骤六受这个循环过程支配因为结构配置和外部荷载会被规定。8.最终决定。决定是否为最佳设计方案。3.2PrinciplesofDesignMetalasastructuralmaterialbeganwithcastiron,usedona100-ft(30-m)archspanwhichwasbuiltinEnglandin1777-1779.Anumberofcast-ironbridgeswerebuiltduringtheperiod1780-1820,mostlyarch-shapedwithmaingirdersconsistingofindividualcast-ironpiecesformingbarsortrusses.Castironwasalsousedforchainlinksonsuspensionbridgesuntilabout1840.铸铁的应用castiron铸铁；girder梁；bar条形；truss桁架；chainlinks锁链；suspensionbridges悬索桥3.3HistoricalBackgroundofSteelStructuresMetalasastructuralmaterialbeganwithcastiron,(usedona100-ft(30-m)archspanwhichwasbuiltinEnglandin1777-1779).Anumberofcast-ironbridgeswerebuiltduringtheperiod1780-1820,(mostlyarch-shapedwithmaingirdersconsistingofindividualcast-ironpiecesformingbarsortrusses).Castironwasalsousedforchainlinksonsuspensionbridgesuntilabout1840.金属作为一种结构材料始于铸铁，在1777—1779年英格兰建造的30m的拱桥中得到应用。很多铸铁桥在1780—1820这段时间修建起来，大多数是拱形的带有由单个铸铁块组成条状或桁架的主梁。铸铁也被应用于悬索桥上的锁链连接直到大约1840年。3.3HistoricalBackgroundofSteelStructuresThedevelopmentoftheBessemerprocess(1855),theintroductionofabasiclinerintheBessemerconverter(1870),andtheopen-hearthfurnacebroughtwidespreaduseofironoreproductsinbuildingmaterials.Since1890,steelhasreplacedwroughtironastheprincipalmetallicbuildingmaterial.Currently(1989),steelshavingyieldstressesvaryingfrom24,000to100,000poundspersquareinch,psi(165to690megapascals,MPa),andavailableforstructuraluses.金属材料从锻铁向钢铁转变Bessemerprocess贝塞麦（转炉）炼钢法；Bessemerconverter贝氏转炉；open-hearthfurnace平炉；ironore铁矿石；wroughtiron锻铁（熟铁）3.3HistoricalBackgroundofSteelStructuresThedevelopmentoftheBessemerprocess(1855),theintroductionofabasiclinerintheBessemerconverter(1870),andtheopen-hearthfurnacebroughtwidespreaduseofironoreproductsinbuildingmaterials.Since1890,steelhasreplacedwroughtironastheprincipalmetallicbuildingmaterial.Currently(1989),steelshavingyieldstressesvaryingfrom24,000to100,000poundspersquareinch,psi(165to690megapascals,MPa),andavailableforstructuraluses.贝塞麦（转炉）炼钢法的发展，贝氏转炉基本班轮的介绍，还有平炉在建筑材料方面给铁矿石产品带来了广泛应用。自从1890年，钢材代替熟铁成为主要金属建筑材料。目前（1989），刚才的屈服应力可达到165—690MPa，可以胜任结构使用。3.3HistoricalBackgroundofSteelStructuresTheaccuratedeterminationoftheloadstowhichastructureorstructuralelementwillbesubjectedisnotalwayspredictable.Eveniftheloadsarewellknownatonelocationinastructure,thedistributionofloadfromelementtoelementthroughoutthestructureusuallyrequiresassumptionsandapproximations.Someofthemostcommonkindsofloadsarediscussedinthefollowingsections.荷载概述distribution分布；approximations近似3.4LoadsTheaccuratedeterminationoftheloads(towhichastructureorstructuralelementwillbesubjected)isnotalwayspredictable.Eveniftheloadsarewellknownatonelocationinastructure,thedistributionofload(fromelementtoelementthroughoutthestructure)usuallyrequiresassumptionsandapproximations.Someofthemostcommonkindsofloadsarediscussedinthefollowingsections.一个结构或结构单元所承受的精确的荷载并不总是可预测的。即便结构中一个位置的荷载可以很清楚的知道，在结构中的从单元到单元的荷载分布通常需要假设和近似。下面部分讨论一些常见荷载。3.4LoadsDeadloadisafixed-positiongravityserviceload,socalledbecauseitactscontinuouslytowardtheearthwhenthestructureisinservice.Theweightofthestructureisconsidereddeadload,aswellasattachmentstothestructuresuchaspipes,electricalconduit,air-conditioningandheatingducts,lightingfixtures,floorcovering,roofcovering,andsuspendedceilings;thatis,allitemsthatremainthroughoutthelifeofthestructure.静荷载定义attachments附属物3.4.1DeadLoadDeadloadisafixed-positiongravityserviceload,(socalledbecauseitactscontinuouslytowardtheearthwhenthestructureisinservice).Theweightofthestructureisconsidereddeadload,aswellasattachments

(tothestructuresuchaspipes,electricalconduit,air-conditioningandheatingducts,lightingfixtures,floorcovering,roofcovering,andsuspendedceilings);thatis,allitemsthatremainthroughoutthelifeofthestructure.静荷载是一种固定位置的重力荷载，这么叫是因为它在当结构服役的时候方向总是指向地球。结构的重量被认为是静荷载，还有结构的附属物，例如管子，电子导管，空调和暖气管，照明设备，楼板，屋盖和吊顶，也就是说所有在结构生命周期内一直存在的东西。3.4.1DeadLoadDeadloadsareusuallyknownaccuratelybutnotuntilthedesignhasbeencompleted.Understeps3through6ofthedesignprocedure,theweightofthestructureorstructuralelementmustbeestimated,preliminarysectionselected,weightrecomputed,andmemberselectionrevisedifnecessary.Thedeadloadofattachmentsisusuallyknownwithreasonableaccuracypriortothedesign.静荷载的确定过程revise修改；priorto在……之前3.4.1DeadLoadDeadloadsareusuallyknownaccurately(butnotuntilthedesignhasbeencompleted).(Understeps3through6ofthedesignprocedure),theweightofthestructureorstructuralelementmustbeestimated,preliminary

sectionselected,weightrecomputed,andmemberselectionrevisedifnecessary.Thedeadloadofattachmentsisusuallyknown(withreasonableaccuracypriortothedesign).静荷载通常能够精确计算，但是要等到设计全部完成。在设计过程第三步到第六步，结构和结构单元的重量必须进行估算，初步的截面选择，重量重新计算，如果需要的话还要重新选择构件。附属部分的静荷载通常是可以在设计之前足够精确的计算出来。3.4.1DeadLoadGravityloadsactingwhenthestructureisinservice,butvaryinginmagnitudeandlocation,aretermedliveloads.Examplesofliveloadsarehumanoccupants,furniture,movable

equipment,vehicles,andstoredgoods.Someliveloadsmaybepracticallypermanent,othersmaybehighlytransient.Becauseoftheunknownnatureofthemagnitude,location,anddensityofliveloaditems,realisticmagnitudesandthepositionsofsuchloadsareverydifficulttodetermine.活荷载的定义magnitude大小；termed被称为；occupants居住者；movableequipment移动设备；permanent永久的；transient短暂的3.4.2LiveLoad(Gravityloadsactingwhenthestructureisinservice),butvarying

(inmagnitudeandlocation,)aretermedliveloads.Examplesofliveloadsarehumanoccupants,furniture,movableequipment,vehicles,andstoredgoods.Someliveloadsmaybepracticallypermanent,othersmaybehighlytransient.Becauseoftheunknownnatureofthemagnitude,location,anddensityofliveloaditems,realisticmagnitudesandthepositionsofsuchloadsareverydifficulttodetermine.重力荷载当结构服役时起作用，但是荷载的大小和作用位置会发生变化的荷载称为活荷载。活荷载的例子包括居住者，家具，可移动的设备，车辆和储存货物。有些活荷载也许是几乎永久的，其它的也许是很短暂的。由于对荷载大小，作用位置和活荷载物品密度的未知性，真实的大小和作用位置是很难确定的。3.4.2LiveLoadBecauseofthepublicconcernforadequatesafety,liveloadstobetakenasserviceloadsindesignareusuallyprescribedbystateandlocalbuildingcodes.Theseloadsaregenerallyempiricalandconservative,basedonexperienceandacceptedpracticeratherthanaccuratelycomputedvalues.Whereverlocalcodesdonotapply,ordonotexist,theprovisionsfromoneofseveralregionalandnationalbuildingcodesmaybeused.OnesuchwidelyrecognizedcodeistheAmericannationalStandardMinimumDesignLoadsforBuildingsandOtherStructuresANSIA58.1oftheAmericanNationalStandardsInstitute(ANSI),fromwhichsometypicalliveloadsarepresented.ThecodewillhenceforthbereferredtoastheANSIStandard.Thisstandardisupdatedfromtimetotime,mostrecentlyin1982.活荷载的确定方法adequate足够的；serviceloads使用荷载；empirical凭经验的；conservative保守的；provision规范；regional区域的；henceforth今后3.4.2LiveLoad(Becauseofthepublicconcernforadequatesafety),liveloads(tobetakenasserviceloadsindesign)areusuallyprescribedbystateandlocalbuildingcodes.Theseloadsaregenerallyempiricalandconservative,(basedonexperienceandacceptedpracticeratherthanaccuratelycomputedvalues).(Whereverlocalcodesdonotapply,ordonotexist),theprovisionsfromoneofseveralregionalandnationalbuildingcodesmaybeused.OnesuchwidelyrecognizedcodeistheAmericannationalStandardMinimumDesignLoadsforBuildingsandOtherStructuresANSIA58.1(oftheAmericanNationalStandardsInstitute(ANSI),fromwhichsometypicalliveloadsarepresented).ThecodewillhenceforthbereferredtoastheANSIStandard.Thisstandardisupdatedfromtimetotime,mostrecentlyin1982.由于公众对于足够安全性的关心，活荷载作为使用荷载在设计中通常由州或者地方规范来规定。这些荷载通常是经验的，保守的，以经验和可接受的实例为依据而不是用精确计算的值。在地方规范不适用的情况下，或者不存在的情况下，那些地区性的或者全国规范中的规定可能被采纳。一个被如此广泛应用的规范就是《美国建筑和其它结构最低设计荷载标准》，全美标准协会A58.1条，其中一些典型的活荷载在表5.4.1中体现。这个规范从此之后被称为“ANSI标准“。这个标准经常被更新，最近一次在1982年。3.4.2LiveLoadLiveloadwhenappliedtoastructureshouldbepositionedtogivethemaximumeffect,includingpartialloading,alternatespanloading,orfullspanloadingasmaybenecessary.Thesimplifiedassumptionoffulluniformloadingeverywhereshouldbeusedonlywhenitagreeswithrealityorisanappropriateapproximation.Theprobabilityofhavingtheprescribedloadingapplieduniformlyoveranentirefloor,oroverallfloorsofabuildingsimultaneously,isalmostnonexistent.Mostcodesrecognizethisbyallowingforsomepercentagereductionfromfullloading.Forinstance,forliveloadsof100psformoreANSIStandardallowsmembershavinganinfluenceareaof400ft2ormoretobedesignedforareducedliveloadaccordingtoEq.3-1，asfollows:确定取得最大效应的活荷载分布partialloading局部荷载；alternatespanloading交替跨荷载；fullspanloading全跨荷载；fulluniformloading均布荷载；appropriate适当的；simultaneously同时3.4.2LiveLoadLiveload(whenappliedtoastructure)shouldbepositionedtogivethemaximumeffect,(includingpartialloading,alternatespanloading,orfullspanloadingasmaybenecessary).Thesimplifiedassumptionoffulluniformloadingeverywhereshouldbeusedonly(whenitagreeswithrealityorisanappropriateapproximation).Theprobability(ofhavingtheprescribedloadingapplieduniformlyoveranentirefloor,oroverallfloorsofabuildingsimultaneously,)isalmostnonexistent.Most

codesrecognizethisbyallowingforsomepercentagereductionfromfullloading.Forinstance,forliveloadsof100psformoreANSIStandardallowsmembershavinganinfluenceareaof400ft2ormoretobedesignedforareducedliveloadaccordingtoEq.3-1，asfollows:当活荷载作用在结构上的时候应该指明达到最大荷载效应的活荷载位置，包括局部荷载，交替跨荷载或者需要的话全跨荷载。关于全部均匀分布荷载的简化假设只有当符合实际情况或者合理近似的时候才能应用。让规定的荷载均匀的分布在整个楼板上或者同时分布在所有楼板上几乎是不可能的。大多数的规范通过容许对全部均布荷载进行一定程度的折减。例如，对于不小于100psf的活荷载，ANSI标准允许结构单元在影响区域不小于400ft2时，对于活荷载按照等式3-1进行折减，方程如下：3.4.2LiveLoadHighwayvehicleloadingintheUnitedStateshasbeenstandardizedbytheAmericanAssociationofStateHighwayandTransportationOfficials(AASHTO)intostandardtruckloadsandlaneloadsthatapproximateaseriesoftrucks.Therearetwosystems,designatedHandHS,thatareidentifiedbythenumberofaxlespertruck.TheHsystemhastwoaxles,whereastheHSsystemhasthreeaxlespertruck.Thereareseveralclassesofloading;however,theusualonesareknownasH20andHS20,showninFig.3-1.高速公路活荷载的分类laneloads车道荷载；designated指定的；axle车轴3.4.3HighwayLiveLoadsHighwayvehicleloadingintheUnitedStateshasbeenstandardizedbytheAmericanAssociationofStateHighwayandTransportationOfficials(AASHTO)intostandardtruckloadsandlaneloadsthatapproximateaseriesoftrucks.Therearetwosystems,designatedHandHS,thatareidentifiedbythenumberofaxlespertruck.TheHsystemhastwoaxles,whereastheHSsystemhasthreeaxlespertruck.Thereareseveralclassesofloading;however,theusualonesareknownasH20andHS20,showninFig.3-1.在美国的高速公路车辆荷载由全美国家高速公路和交通运输协会进行规定，归类为卡车荷载和车道荷载，其中车道荷载近似为一列卡车。有两个系统，H和HS，他们是由每辆卡车的车轴数量确定的。H系统每辆车有两个车轴，HS系统每辆车有三个车轴。有几种不同等级的荷载。然而，通常熟知的是H20和HS20，如图3-1所示。3.4.3HighwayLiveLoadsIndesigningagivenbridge,eitheronetruckloadingisappliedtotheentirestructure,orthelaneloadingisapplied.Whenthelaneloadingisused,theuniformportionisdistributedoverasmuchofthespanorspansaswillcausethemaximumeffect.Inaddition,theoneconcentratedload(formaximumnegativemomentoncontinuousspans,asecondconcentratedloadisalsoused)ispositionedforthemostsevereloadingeffect.Theloaddistributionacrossthewidthofabridgetoitsvarioussupportingmembersistakeninaccordancewithsemiempiricalrulesthatdependonthetypeofbridgedeckandsupportingstructure.作用于桥梁上的车辆荷载如何考虑load

distribution荷载分布；inaccordancewith符合；semiempirical半经验的；bridgedeck桥面板3.4.3HighwayLiveLoadsIndesigningagivenbridge,eitheronetruckloadingisappliedtotheentirestructure,orthelaneloadingisapplied.Whenthelaneloadingisused,theuniformportionisdistributed(overasmuchofthespanorspansaswillcausethemaximumeffect).Inaddition,theoneconcentratedload(formaximumnegativemomentoncontinuousspans,asecondconcentratedloadisalsoused)ispositionedforthemostsevereloadingeffect.Theloaddistributionacrossthewidthofabridgetoitsvarioussupportingmembersistaken(inaccordancewithsemiempiricalrulesthatdependonthetypeofbridgedeckandsupportingstructure).设计一个给定的桥梁中，要么是将一个卡车的荷载加到整个结构上，要么是将车道荷载施加到结构上。当使用车道荷载时，均布的部分要尽可能覆盖多跨度，以到达最大的响应。除此之外，将一个集中荷载（对于在连续跨度上的最大负弯矩，第二个集中荷载也被用到）施加到使得结构产生最大荷载响应的位置。荷载在桥面上的分布和在桥的各种支撑部件的分布符合半经验的规则，而这是由桥面板的类型和支承结构决定的。3.4.3HighwayLiveLoadsThesingletruckloadingprovidestheeffectofaheavyconcentratedloadandusuallygovernsonrelativelyshortspans.Theuniformlaneloadistosimulatealineoftraffic,andtheaddedconcentratedloadistoaccountforthepossibilityofoneextraheavyvehicleinthelineoftraffic.Theseloadshavebeenusedwithnoapparentdifficultysince1944,beforewhichtimealineoftruckswasactuallyusedfortheloading.Ontheinterstatesystemofhighways,amilitaryloadingisalsousedthatconsistsoftwo24kip(107kN)axleloadsspaced4ft(1.2m)apart.均布车道荷载模拟一列车辆荷载governs控制；simulate模拟；interstatesystemofhighways州际高速公路系统3.4.3HighwayLiveLoadsThesingletruckloadingprovidestheeffectofaheavyconcentratedloadandusuallygovernsonrelativelyshortspans.Theuniformlaneloadistosimulatealineoftraffic,andthe

addedconcentratedloadistoaccountforthepossibilityofoneextraheavyvehicleinthelineoftraffic.Theseloadshavebeenusedwithnoapparentdifficultysince1944,beforewhichtimealineoftruckswasactuallyusedfortheloading.Ontheinterstatesystemofhighways,amilitaryloadingisalsousedthatconsistsoftwo24kip(107kN)axleloadsspaced4ft(1.2m)apart.单个的卡车荷载产生一个大的集中荷载的效应并且在相对短的跨度情况下起控制作用。均布的车道荷载用来模拟一列交通工具，附加的集中荷载用来考虑在一列车辆中可能出现的额外较重的车辆。这些荷载自从1944年来一直沿用，并且没遇到什么困难。而在此之前，事实上用一列卡车来计算荷载。在州际的高速公路系统中，也会用到军事（车辆）荷载，这是由两个间距1.2m的107kN车轴荷载组成的。3.4.3HighwayLiveLoadsThetermimpactasordinarilyusedinstructural