《土木工程專業(yè)英語 第2版》 翻譯版 課件 Unit 3 Introduction to Steel Structures

Unit3IntroductiontoSteelStructuresEnglishforCivilEngineering——Unit3IntroductiontoSteelStructures3.1StructuralDesign（結構設計）3.2PrinciplesofDesign（設計準則）3.3HistoricalBackgroundofSteelStructures（鋼結構背景）3.4Loads（荷載）3.4.1DeadLoad（靜荷載）3.4.2LiveLoad（活荷載）3.4.3HighwayLiveLoads（高速公路活荷載）3.4.4Impact（沖擊荷載）3.4.5SnowLoad（雪荷載）3.4.6WindLoad（風荷載）3.4.7EarthquakeLoad（地震作用）3.5TypesofStructuralSteelMembers（鋼結構構件）3.5.1TensionMembers（受拉構件）3.5.2CompressionMembers（受壓構件）3.5.3Beams（梁）3.5.4BendingandAxialLoad（壓（拉）彎構件）Structuraldesignmaybedefinedasamixtureofartandscience,combiningtheexperiencedengineer’sintuitivefeelingforthebehaviorofastructurewithasoundknowledgeoftheprinciplesofstatics,dynamics,mechanicsofmaterials,andstructuralanalysis,toproduceasafeeconomicalstructurethatwillserveitsintendedpurpose.結構設計的定義intuitive直觀的；sound合理的；intended預期的3.1StructuralDesignStructuraldesignmaybedefinedasamixtureofartandscience,(combiningtheexperiencedengineer’sintuitivefeelingforthebehaviorofastructurewithasoundknowledgeoftheprinciplesofstatics,dynamics,mechanicsofmaterials,andstructuralanalysis),toproduceasafeeconomicalstructure(thatwillserveitsintendedpurpose.)結構設計可以定義為藝術和科學的結合，再加上有經(jīng)驗的工程師基于靜力學，動力學，材料力學和結構分析方面合理的知識而對于一個結構行為的直觀感受，來設計一個能夠達成它的預期目的的安全經(jīng)濟的結構。3.1StructuralDesignUntilabout1850,structuraldesignwaslargelyanartrelyingonintuitiontodeterminethesizeandarrangementofthestructuralelements.Earlyman-madestructuresessentially

conformedtothosewhichcouldalsobeobservedinnature,suchasbeamsandarches.Astheprinciplesgoverningthebehaviorofstructuresandstructuralmaterialshavebecomebetterunderstood,designprocedureshavebecomemorescientific.早期的結構設計intuition直覺；

conformedto符合；

essentially基本上；governing控制3.1StructuralDesignUntilabout1850,structuraldesignwaslargelyanart(relyingonintuitiontodeterminethesizeandarrangementofthestructuralelements).Earlyman-madestructuresessentiallyconformedtothosewhichcouldalsobeobservedinnature,suchasbeamsandarches.(Astheprinciplesgoverningthebehaviorofstructuresandstructuralmaterialshavebecomebetterunderstood),designprocedureshavebecomemorescientific.直到1850，結構設計很大程度上是一種依賴直覺決定結構構件尺寸和分布位置的藝術。早期的人造結構基本上和那些可以在自然界中觀察到的結構構件一致，比如梁和拱。隨著控制結構行為和結構材料的原則變得越來越清楚，設計過程變得越來越科學。3.1StructuralDesignComputationsinvolvingscientificprinciplesshouldserveasaguidetodecisionmakingandnotbefollowedblindly.Theartorintuitiveabilityoftheexperiencedengineerisutilizedtomakethedecisions,guidedbythecomputationalresults.結構設計的原則isutilizedto利用3.1StructuralDesignComputationsinvolvingscientificprinciplesshouldserveasaguidetodecisionmakingandnotbefollowedblindly.Theartorintuitiveabilityoftheexperiencedengineerisutilizedtomakethedecisions,(guidedbythecomputationalresults).具有科學依據(jù)的計算應該起到對決策起指導作用而且應不被盲目遵從。一個有經(jīng)驗工程師應在計算結果的指導之下，利用設計藝術或者直覺來做決策。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.設計準則的要求optimum最佳的，最佳效果；criteria原則；manufacture建造；weighting加權重；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.設計是一個尋找最佳解決方案的過程。在這篇文章中，尤其關心的是鋼結構的設計。在任何設計中，有些標準必須被建立來評價是否實現(xiàn)了最佳方案。對一個結構來說，典型的標準比如（a）最低花費；（b）最低重量；（c）最低建造時間；（d）最低勞動力；（e）最低制造花費；和（f）最大工作效率。通常的設計會考慮幾個標準，每個標準也許都會有一定的權重。觀察上述所有可能的標準，顯然為了實現(xiàn)最佳目標（比如重量和花費）而頻繁建立明確的可以量化的標準是很困難的，甚至是不可能的。在多數(shù)實際情況中，評價必須是定性的。3.2PrinciplesofDesignIfaspecificobjectivecriterioncanbeexpressedmathematically,thenoptimization

techniquesmaybeemployedtoobtainamaximumorminimumfortheobjectivefunction.Optimizationproceduresandtechniquescompriseanentiresubjectthatisoutsidethescopeofthistext.Thecriterionofminimumweightisemphasizedthroughout,underthegeneralassumptionthatminimummaterialrepresentsminimumcost.Othersubjectivecriteriamustbekeptinmind,eventhoughtheintegrationofbehavioralprincipleswithdesignofstructuralsteelelementsinthistextutilizesonlysimpleobjectivecriteria,suchasweightorcost.結構設計的優(yōu)化方法criterion標準；optimization

techniques優(yōu)化方法；scope范圍；comprise

包含assumption假定；integration整合；utilize利用3.2PrinciplesofDesignIfaspecificobjectivecriterioncanbeexpressedmathematically,thenoptimizationtechniquesmaybeemployedtoobtainamaximumorminimum(fortheobjectivefunction).Optimizationproceduresandtechniquescompriseanentiresubjectthatisoutsidethescopeofthistext.Thecriterionofminimumweightisemphasizedthroughout,(underthegeneralassumptionthatminimummaterialrepresentsminimumcost).Othersubjectivecriteriamustbekeptinmind,eventhoughtheintegrationofbehavioralprincipleswithdesignofstructuralsteelelementsinthistextutilizesonlysimpleobjectivecriteria,suchasweightorcost.如果一個明確客觀的標準可以用數(shù)學公式表達，那么可以采用優(yōu)化技術來獲取目標函數(shù)的最大或最小值。優(yōu)化過程和技術包含一個主體，它已經(jīng)超出了本文討論的范圍。最小重量的標準從頭到尾都在強調(diào)，在通常的假設下最少的材料代表最小花費。盡管這篇文章中提到的設計原理和鋼結構單元的綜合只用到了簡單客觀的標準，其它主觀的標準必須被記住，比如重量或者花費。3.2PrinciplesofDesignThedesignproceduremaybeconsideredtobecomposedoftwoparts—functionaldesignandstructuralframeworkdesign.Functionaldesignensuresthatintendedresultsareachieved,suchas(a)adequateworkingareasandclearances;(b)properventilationand/orairconditioning;(c)adequatetransportationfacilities,suchaselevators,stairways,andcranesormaterialshandlingequipment;(d)adequatelighting;and(e)aesthetics.設計過程包含兩部分——功能設計和結構框架設計adequate充足的；clearance空隙；ventilation通風系統(tǒng)；crane吊車；aesthetics美觀3.2PrinciplesofDesignThedesignproceduremaybeconsideredtobecomposedoftwoparts—functionaldesignandstructuralframeworkdesign.Functionaldesignensuresthatintendedresultsareachieved,suchas(a)adequateworkingareasandclearances;(b)properventilationand/orairconditioning;(c)adequatetransportationfacilities,suchaselevators,stairways,andcranesormaterialshandlingequipment;(d)adequatelighting;and(e)aesthetics.設計過程一般認為包含兩部分——功能設計和結構框架設計。功能設計確保預期結果可以達成，例如（a）充足的工作區(qū)域和空隙；（b）合適的通風設備和空調(diào)；（c）足夠的運輸設備，例如電梯，樓梯，吊車和材料處理設備；（d）充足的照明；和（e）美觀。3.2PrinciplesofDesignThestructuralframeworkdesignistheselectionofarrangementandsizesofstructuralelementssothatserviceloadsmaybesafelycarried,anddisplacementsarewithinacceptablelimits.結構框架設計3.2PrinciplesofDesignThestructuralframeworkdesignistheselectionofarrangementandsizesofstructuralelements(sothatserviceloadsmaybesafelycarried,anddisplacementsarewithinacceptablelimits).結構框架設計是選擇結構單元的尺寸和順序的過程，由此工作荷載可以安全的加載并且位移也在可接受的限制內(nèi)。3.2PrinciplesofDesignTheiterativedesignproceduremaybeoutlinedasfollows:1.Planning.Establishmentofthefunctionsforwhichthestructuremustserve.Setcriteriaagainstwhichtomeasuretheresultingdesignforbeinganoptimum.2.Preliminarystructuralconfiguration.Arrangementoftheelementstoservethefunctionsinstep1.3.Establishmentoftheloadstobecarried.4.Preliminarymemberselection.Basedonthedecisionsofsteps1,2,and3selectionofthemembersizestosatisfyanobjectivecriterion,suchasleastweightorcost.設計過程的具體步驟1—4iterative重復的；preliminary初步的；configuration配置；satisfy達到3.2PrinciplesofDesignTheiterativedesignproceduremaybeoutlinedasfollows:1.Planning.Establishmentofthefunctionsforwhichthestructuremustserve.Setcriteriaagainstwhichtomeasuretheresultingdesignforbeinganoptimum.2.Preliminary

structural

configuration.Arrangementoftheelementstoservethefunctionsinstep1.3.Establishmentoftheloadstobecarried.4.Preliminary

member

selection.Basedonthedecisionsofsteps1,2,and3selectionofthemembersizestosatisfyanobjectivecriterion,suchasleastweightorcost.重復的設計過程可以被概括如下：1.計劃。建立結構必須具有的功能。為實現(xiàn)最佳效果的設計制定標準。2.初始的結構配置。排列結構單元來實現(xiàn)第一步中的功能。3.建立要加載的荷載。4.初步的單元選定。根據(jù)第一，第二，第三步中的決定選擇單元的尺寸大小來滿足客觀的標準，例如最小的重量或最少的花費。3.2PrinciplesofDesign5.Analysis.Structuralanalysisinvolvingmodelingtheloadsandthestructuralframeworktoobtaininternalforcesandanydesireddeflections.6.Evaluation.Areallstrengthandserviceabilityrequirementssatisfiedandistheresultoptimum?Comparetheresultwithpredeterminedcriteria.7.Redesign.Repetitionofanypartofthesequence1through6foundnecessaryordesirableasaresultofevaluation.Steps1through6representaniterativeprocess.Usuallyinthistextonlysteps3through6willbesubjecttothisiterationsincethestructuralconfigurationandexternalloadingwillbeprescribed.8.Finaldecision.Thedeterminationofwhetherornotanoptimumdesignhasbeenachieved.設計過程的具體步驟5—8internalforces內(nèi)力；serviceabilityrequirements使用要求；prescribed規(guī)定3.2PrinciplesofDesign5.Analysis.Structuralanalysisinvolvingmodelingtheloadsandthestructuralframeworktoobtaininternalforcesandanydesireddeflections.6.Evaluation.Areallstrengthandserviceabilityrequirementssatisfiedandistheresultoptimum?Comparetheresultwithpredeterminedcriteria.7.Redesign.Repetitionofanypartofthesequence1through6foundnecessaryordesirableasaresultofevaluation.Steps1through6representaniterativeprocess.Usuallyinthistextonlysteps3through6willbesubjecttothisiterationsincethestructuralconfigurationandexternalloadingwillbeprescribed.8.Final

decision.Thedeterminationofwhetherornotanoptimumdesignhasbeenachieved.5.分析。結構分析包括為荷載和結構框架建模來獲取內(nèi)力和任何需要的變形。6.評估。是否所有的強度和使用性要求都被滿足了？是否是最優(yōu)解決方案？比較結果和之前決定的標準。7.重新設計。重復步驟一到六的任何一部分，找到必要的或需要的作為評估的結果。步驟一到步驟六代表一個重復的過程。通常在這篇文章中，只有步驟三到步驟六受這個循環(huán)過程支配因為結構配置和外部荷載會被規(guī)定。8.最終決定。決定是否為最佳設計方案。3.2PrinciplesofDesignMetalasastructuralmaterialbeganwithcastiron,usedona100-ft(30-m)archspanwhichwasbuiltinEnglandin1777-1779.Anumberofcast-ironbridgeswerebuiltduringtheperiod1780-1820,mostlyarch-shapedwithmaingirdersconsistingofindividualcast-ironpiecesformingbarsortrusses.Castironwasalsousedforchainlinksonsuspensionbridgesuntilabout1840.鑄鐵的應用castiron鑄鐵；girder梁；bar條形；truss桁架；chainlinks鎖鏈；suspensionbridges懸索橋3.3HistoricalBackgroundofSteelStructuresMetalasastructuralmaterialbeganwithcastiron,(usedona100-ft(30-m)archspanwhichwasbuiltinEnglandin1777-1779).Anumberofcast-ironbridgeswerebuiltduringtheperiod1780-1820,(mostlyarch-shapedwithmaingirdersconsistingofindividualcast-ironpiecesformingbarsortrusses).Castironwasalsousedforchainlinksonsuspensionbridgesuntilabout1840.金屬作為一種結構材料始于鑄鐵，在1777—1779年英格蘭建造的30m的拱橋中得到應用。很多鑄鐵橋在1780—1820這段時間修建起來，大多數(shù)是拱形的帶有由單個鑄鐵塊組成條狀或桁架的主梁。鑄鐵也被應用于懸索橋上的鎖鏈連接直到大約1840年。3.3HistoricalBackgroundofSteelStructuresThedevelopmentoftheBessemerprocess(1855),theintroductionofabasiclinerintheBessemerconverter(1870),andtheopen-hearthfurnacebroughtwidespreaduseofironoreproductsinbuildingmaterials.Since1890,steelhasreplacedwroughtironastheprincipalmetallicbuildingmaterial.Currently(1989),steelshavingyieldstressesvaryingfrom24,000to100,000poundspersquareinch,psi(165to690megapascals,MPa),andavailableforstructuraluses.金屬材料從鍛鐵向鋼鐵轉變Bessemerprocess貝塞麥（轉爐）煉鋼法；Bessemerconverter貝氏轉爐；open-hearthfurnace平爐；ironore鐵礦石；wroughtiron鍛鐵（熟鐵）3.3HistoricalBackgroundofSteelStructuresThedevelopmentoftheBessemerprocess(1855),theintroductionofabasiclinerintheBessemerconverter(1870),andtheopen-hearthfurnacebroughtwidespreaduseofironoreproductsinbuildingmaterials.Since1890,steelhasreplacedwroughtironastheprincipalmetallicbuildingmaterial.Currently(1989),steelshavingyieldstressesvaryingfrom24,000to100,000poundspersquareinch,psi(165to690megapascals,MPa),andavailableforstructuraluses.貝塞麥（轉爐）煉鋼法的發(fā)展，貝氏轉爐基本班輪的介紹，還有平爐在建筑材料方面給鐵礦石產(chǎn)品帶來了廣泛應用。自從1890年，鋼材代替熟鐵成為主要金屬建筑材料。目前（1989），剛才的屈服應力可達到165—690MPa，可以勝任結構使用。3.3HistoricalBackgroundofSteelStructuresTheaccuratedeterminationoftheloadstowhichastructureorstructuralelementwillbesubjectedisnotalwayspredictable.Eveniftheloadsarewellknownatonelocationinastructure,thedistributionofloadfromelementtoelementthroughoutthestructureusuallyrequiresassumptionsandapproximations.Someofthemostcommonkindsofloadsarediscussedinthefollowingsections.荷載概述distribution分布；approximations近似3.4LoadsTheaccuratedeterminationoftheloads(towhichastructureorstructuralelementwillbesubjected)isnotalwayspredictable.Eveniftheloadsarewellknownatonelocationinastructure,thedistributionofload(fromelementtoelementthroughoutthestructure)usuallyrequiresassumptionsandapproximations.Someofthemostcommonkindsofloadsarediscussedinthefollowingsections.一個結構或結構單元所承受的精確的荷載并不總是可預測的。即便結構中一個位置的荷載可以很清楚的知道，在結構中的從單元到單元的荷載分布通常需要假設和近似。下面部分討論一些常見荷載。3.4LoadsDeadloadisafixed-positiongravityserviceload,socalledbecauseitactscontinuouslytowardtheearthwhenthestructureisinservice.Theweightofthestructureisconsidereddeadload,aswellasattachmentstothestructuresuchaspipes,electricalconduit,air-conditioningandheatingducts,lightingfixtures,floorcovering,roofcovering,andsuspendedceilings;thatis,allitemsthatremainthroughoutthelifeofthestructure.靜荷載定義attachments附屬物3.4.1DeadLoadDeadloadisafixed-positiongravityserviceload,(socalledbecauseitactscontinuouslytowardtheearthwhenthestructureisinservice).Theweightofthestructureisconsidereddeadload,aswellasattachments

(tothestructuresuchaspipes,electricalconduit,air-conditioningandheatingducts,lightingfixtures,floorcovering,roofcovering,andsuspendedceilings);thatis,allitemsthatremainthroughoutthelifeofthestructure.靜荷載是一種固定位置的重力荷載，這么叫是因為它在當結構服役的時候方向總是指向地球。結構的重量被認為是靜荷載，還有結構的附屬物，例如管子，電子導管，空調(diào)和暖氣管，照明設備，樓板，屋蓋和吊頂，也就是說所有在結構生命周期內(nèi)一直存在的東西。3.4.1DeadLoadDeadloadsareusuallyknownaccuratelybutnotuntilthedesignhasbeencompleted.Understeps3through6ofthedesignprocedure,theweightofthestructureorstructuralelementmustbeestimated,preliminarysectionselected,weightrecomputed,andmemberselectionrevisedifnecessary.Thedeadloadofattachmentsisusuallyknownwithreasonableaccuracypriortothedesign.靜荷載的確定過程revise修改；priorto在……之前3.4.1DeadLoadDeadloadsareusuallyknownaccurately(butnotuntilthedesignhasbeencompleted).(Understeps3through6ofthedesignprocedure),theweightofthestructureorstructuralelementmustbeestimated,preliminary

sectionselected,weightrecomputed,andmemberselectionrevisedifnecessary.Thedeadloadofattachmentsisusuallyknown(withreasonableaccuracypriortothedesign).靜荷載通常能夠精確計算，但是要等到設計全部完成。在設計過程第三步到第六步，結構和結構單元的重量必須進行估算，初步的截面選擇，重量重新計算，如果需要的話還要重新選擇構件。附屬部分的靜荷載通常是可以在設計之前足夠精確的計算出來。3.4.1DeadLoadGravityloadsactingwhenthestructureisinservice,butvaryinginmagnitudeandlocation,aretermedliveloads.Examplesofliveloadsarehumanoccupants,furniture,movable

equipment,vehicles,andstoredgoods.Someliveloadsmaybepracticallypermanent,othersmaybehighlytransient.Becauseoftheunknownnatureofthemagnitude,location,anddensityofliveloaditems,realisticmagnitudesandthepositionsofsuchloadsareverydifficulttodetermine.活荷載的定義magnitude大?。籺ermed被稱為；occupants居住者；movableequipment移動設備；permanent永久的；transient短暫的3.4.2LiveLoad(Gravityloadsactingwhenthestructureisinservice),butvarying

(inmagnitudeandlocation,)aretermedliveloads.Examplesofliveloadsarehumanoccupants,furniture,movableequipment,vehicles,andstoredgoods.Someliveloadsmaybepracticallypermanent,othersmaybehighlytransient.Becauseoftheunknownnatureofthemagnitude,location,anddensityofliveloaditems,realisticmagnitudesandthepositionsofsuchloadsareverydifficulttodetermine.重力荷載當結構服役時起作用，但是荷載的大小和作用位置會發(fā)生變化的荷載稱為活荷載?；詈奢d的例子包括居住者，家具，可移動的設備，車輛和儲存貨物。有些活荷載也許是幾乎永久的，其它的也許是很短暫的。由于對荷載大小，作用位置和活荷載物品密度的未知性，真實的大小和作用位置是很難確定的。3.4.2LiveLoadBecauseofthepublicconcernforadequatesafety,liveloadstobetakenasserviceloadsindesignareusuallyprescribedbystateandlocalbuildingcodes.Theseloadsaregenerallyempiricalandconservative,basedonexperienceandacceptedpracticeratherthanaccuratelycomputedvalues.Whereverlocalcodesdonotapply,ordonotexist,theprovisionsfromoneofseveralregionalandnationalbuildingcodesmaybeused.OnesuchwidelyrecognizedcodeistheAmericannationalStandardMinimumDesignLoadsforBuildingsandOtherStructuresANSIA58.1oftheAmericanNationalStandardsInstitute(ANSI),fromwhichsometypicalliveloadsarepresented.ThecodewillhenceforthbereferredtoastheANSIStandard.Thisstandardisupdatedfromtimetotime,mostrecentlyin1982.活荷載的確定方法adequate足夠的；serviceloads使用荷載；empirical憑經(jīng)驗的；conservative保守的；provision規(guī)范；regional區(qū)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.由于公眾對于足夠安全性的關心，活荷載作為使用荷載在設計中通常由州或者地方規(guī)范來規(guī)定。這些荷載通常是經(jīng)驗的，保守的，以經(jīng)驗和可接受的實例為依據(jù)而不是用精確計算的值。在地方規(guī)范不適用的情況下，或者不存在的情況下，那些地區(qū)性的或者全國規(guī)范中的規(guī)定可能被采納。一個被如此廣泛應用的規(guī)范就是《美國建筑和其它結構最低設計荷載標準》，全美標準協(xié)會A58.1條，其中一些典型的活荷載在表5.4.1中體現(xiàn)。這個規(guī)范從此之后被稱為“ANSI標準“。這個標準經(jīng)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:確定取得最大效應的活荷載分布partialloading局部荷載；alternatespanloading交替跨荷載；fullspanloading全跨荷載；fulluniformloading均布荷載；appropriate適當?shù)?；simultaneously同時3.4.2LiveLoadLiveload(whenappliedtoastructure)shouldbepositionedtogivethemaximumeffect,(includingpartialloading,alternatespanloading,orfullspanloadingasmaybenecessary).Thesimplifiedassumptionoffulluniformloadingeverywhereshouldbeusedonly(whenitagreeswithrealityorisanappropriateapproximation).Theprobability(ofhavingtheprescribedloadingapplieduniformlyoveranentirefloor,oroverallfloorsofabuildingsimultaneously,)isalmostnonexistent.Most

codesrecognizethisbyallowingforsomepercentagereductionfromfullloading.Forinstance,forliveloadsof100psformoreANSIStandardallowsmembershavinganinfluenceareaof400ft2ormoretobedesignedforareducedliveloadaccordingtoEq.3-1，asfollows:當活荷載作用在結構上的時候應該指明達到最大荷載效應的活荷載位置，包括局部荷載，交替跨荷載或者需要的話全跨荷載。關于全部均勻分布荷載的簡化假設只有當符合實際情況或者合理近似的時候才能應用。讓規(guī)定的荷載均勻的分布在整個樓板上或者同時分布在所有樓板上幾乎是不可能的。大多數(shù)的規(guī)范通過容許對全部均布荷載進行一定程度的折減。例如，對于不小于100psf的活荷載，ANSI標準允許結構單元在影響區(qū)域不小于400ft2時，對于活荷載按照等式3-1進行折減，方程如下：3.4.2LiveLoadHighwayvehicleloadingintheUnitedStateshasbeenstandardizedbytheAmericanAssociationofStateHighwayandTransportationOfficials(AASHTO)intostandardtruckloadsandlaneloadsthatapproximateaseriesoftrucks.Therearetwosystems,designatedHandHS,thatareidentifiedbythenumberofaxlespertruck.TheHsystemhastwoaxles,whereastheHSsystemhasthreeaxlespertruck.Thereareseveralclassesofloading;however,theusualonesareknownasH20andHS20,showninFig.3-1.高速公路活荷載的分類laneloads車道荷載；designated指定的；axle車軸3.4.3HighwayLiveLoadsHighwayvehicleloadingintheUnitedStateshasbeenstandardizedbytheAmericanAssociationofStateHighwayandTransportationOfficials(AASHTO)intostandardtruckloadsandlaneloadsthatapproximateaseriesoftrucks.Therearetwosystems,designatedHandHS,thatareidentifiedbythenumberofaxlespertruck.TheHsystemhastwoaxles,whereastheHSsystemhasthreeaxlespertruck.Thereareseveralclassesofloading;however,theusualonesareknownasH20andHS20,showninFig.3-1.在美國的高速公路車輛荷載由全美國家高速公路和交通運輸協(xié)會進行規(guī)定，歸類為卡車荷載和車道荷載，其中車道荷載近似為一列卡車。有兩個系統(tǒng)，H和HS，他們是由每輛卡車的車軸數(shù)量確定的。H系統(tǒng)每輛車有兩個車軸，HS系統(tǒng)每輛車有三個車軸。有幾種不同等級的荷載。然而，通常熟知的是H20和HS20，如圖3-1所示。3.4.3HighwayLiveLoadsIndesigningagivenbridge,eitheronetruckloadingisappliedtotheentirestructure,orthelaneloadingisapplied.Whenthelaneloadingisused,theuniformportionisdistributedoverasmuchofthespanorspansaswillcausethemaximumeffect.Inaddition,theoneconcentratedload(formaximumnegativemomentoncontinuousspans,asecondconcentratedloadisalsoused)ispositionedforthemostsevereloadingeffect.Theloaddistributionacrossthewidthofabridgetoitsvarioussupportingmembersistakeninaccordancewithsemiempiricalrulesthatdependonthetypeofbridgedeckandsupportingstructure.作用于橋梁上的車輛荷載如何考慮load

distribution荷載分布；inaccordancewith符合；semiempirical半經(jīng)驗的；bridgedeck橋面板3.4.3HighwayLiveLoadsIndesigningagivenbridge,eitheronetruckloadingisappliedtotheentirestructure,orthelaneloadingisapplied.Whenthelaneloadingisused,theuniformportionisdistributed(overasmuchofthespanorspansaswillcausethemaximumeffect).Inaddition,theoneconcentratedload(formaximumnegativemomentoncontinuousspans,asecondconcentratedloadisalsoused)ispositionedforthemostsevereloadingeffect.Theloaddistributionacrossthewidthofabridgetoitsvarioussupportingmembersistaken(inaccordancewithsemiempiricalrulesthatdependonthetypeofbridgedeckandsupportingstructure).設計一個給定的橋梁中，要么是將一個卡車的荷載加到整個結構上，要么是將車道荷載施加到結構上。當使用車道荷載時，均布的部分要盡可能覆蓋多跨度，以到達最大的響應。除此之外，將一個集中荷載（對于在連續(xù)跨度上的最大負彎矩，第二個集中荷載也被用到）施加到使得結構產(chǎn)生最大荷載響應的位置。荷載在橋面上的分布和在橋的各種支撐部件的分布符合半經(jīng)驗的規(guī)則，而這是由橋面板的類型和支承結構決定的。3.4.3HighwayLiveLoadsThesingletruckloadingprovidestheeffectofaheavyconcentratedloadandusuallygovernsonrelativelyshortspans.Theuniformlaneloadistosimulatealineoftraffic,andtheaddedconcentratedloadistoaccountforthepossibilityofoneextraheavyvehicleinthelineoftraffic.Theseloadshavebeenusedwithnoapparentdifficultysince1944,beforewhichtimealineoftruckswasactuallyusedfortheloading.Ontheinterstatesystemofhighways,amilitaryloadingisalsousedthatconsistsoftwo24kip(107kN)axleloadsspaced4ft(1.2m)apart.均布車道荷載模擬一列車輛荷載governs控制；simulate模擬；interstatesystemofhighways州際高速公路系統(tǒng)3.4.3HighwayLiveLoadsThesingletruckloadingprovidestheeffectofaheavyconcentratedloadandusuallygovernsonrelativelyshortspans.Theuniformlaneloadistosimulatealineoftraffic,andthe

addedconcentratedloadistoaccountforthepossibilityofoneextraheavyvehicleinthelineoftraffic.Theseloadshavebeenusedwithnoapparentdifficultysince1944,beforewhichtimealineoftruckswasactuallyusedfortheloading.Ontheinterstatesystemofhighways,amilitaryloadingisalsousedthatconsistsoftwo24kip(107kN)axleloadsspaced4ft(1.2m)apart.單個的卡車荷載產(chǎn)生一個大的集中荷載的效應并且在相對短的跨度情況下起控制作用。均布的車道荷載用來模擬一列交通工具，附加的集中荷載用來考慮在一列車輛中可能出現(xiàn)的額外較重的車輛。這些荷載自從1944年來一直沿用，并且沒遇到什么困難。而在此之前，事實上用一列卡車來計算荷載。在州際的高速公路系統(tǒng)中，也會用到軍事（車輛）荷載，這是由兩個間距1.2m的107kN車軸荷載組成的。3.4.3HighwayLiveLoadsThetermimpactasordinarilyusedinstructural