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SEITSSTRENGTHWHENITISUNDERACALCULATEDDEGREE適當(dāng)?shù)腛FTENSION,AFORCEWHICH,ASWEHAVESEEN,TENDSTO（往往）PULLAPARTMANYMATERIALSNEWALLOYSHAVEFURTHERINCREASEDTHESTRENGTHOFSTEELANDELIMINATEDSOMEOFITSPROBLEMS,SUCHASFATIGUE,WHICHISATENDENCYFORITTOWEAKENASARESULTOFCONTINUALCHANGESINSTRESS（連續(xù)的應(yīng)力變化）作為現(xiàn)代兩種最重要的建筑材料，鋼材與水泥在十九世紀(jì)得到了推廣。直到那個(gè)時(shí)候，鋼材才通過(guò)繁復(fù)的過(guò)程制造出來(lái)，基本上是鐵合金，并含有少量的碳，因而被限制在一些特殊的用途如刀刃。在1856年發(fā)明了貝塞麥煉鋼法后，鋼材才得以大量低價(jià)獲得。鋼材巨大的優(yōu)勢(shì)即是它的抗拉強(qiáng)度，也就是當(dāng)它在適當(dāng)?shù)睦ο虏粫?huì)失去強(qiáng)度，正如我們所看到的，該力往往能夠?qū)⒑芏嗖牧侠_(kāi)。新的合金進(jìn)一步提高了鋼材的強(qiáng)度，并消除了一些缺點(diǎn)，如疲勞，即在連續(xù)的應(yīng)力變化下導(dǎo)致強(qiáng)度減弱的趨勢(shì)。MODERNCEMENT,CALLEDPORTLANDCEMENT,WASINVENTEDIN1824ITISAMIXTUREOFLIMESTONE（石灰石）ANDCLAY,WHICHISHEATEDANDTHENGROUNDINTOAPOWDER（磨成粉末）ITISMIXEDATORNEARTHECONSTRUCTIONSITE（施工現(xiàn)場(chǎng)）WITHSAND,AGGREGATESMALLSTONES,CRUSHEDROCK,ORGRAVEL,ANDWATERTOMAKECONCRETEDIFFERENTPROPORTIONSOFTHEINGREDIENTS（配料）PRODUCECONCRETEWITHDIFFERENTSTRENGTHANDWEIGHTCONCRETEISVERYVERSATILEITCANBEPOURED,PUMPED,OREVENSPRAYEDINTO（噴射成）ALLKINDSOFSHAPESANDWHEREASSTEELHASGREATTENSILESTRENGTH,CONCRETEHASGREATSTRENGTHUNDERCOMPRESSIONTHUS,THETWOSUBSTANCESCOMPLEMENTEACHOTHER（互補(bǔ)）現(xiàn)代水泥發(fā)明于1824年，稱為波特蘭水泥。它是石灰石和粘土的混合物，加熱后磨成粉末。在或靠近施工現(xiàn)場(chǎng)，將水泥與砂、骨料（小石頭、壓碎的巖石或礫石）、水混合而制成混凝土。不同比例的配料會(huì)制造出不同強(qiáng)度和重量的混凝土。混凝土的用途很多，可以澆筑、泵送甚至噴射成各種形狀?；炷辆哂泻艽蟮目箟簭?qiáng)度，而鋼材具有很大的抗拉強(qiáng)度。這樣，兩種材料可以互補(bǔ)。THEYALSOCOMPLEMENTEACHOTHERINANOTHERWAYTHEYHAVEALMOSTTHESAMERATEOFCONTRACTIONANDEXPANSIONTHEYTHEREFORECANWORKTOGETHERINSITUATIONSWHERE（在情況下）BOTHCOMPRESSIONANDTENSIONAREFACTORS（主要因素）STEELRODS（鋼筋）AREEMBEDDEDIN（埋入）CONCRETETOMAKEREINFORCEDCONCRETEINCONCRETEBEAMSORSTRUCTURESWHERETENSIONWILLDEVELOP（出現(xiàn)）CONCRETEANDSTEELALSOFORMSUCHASTRONGBONDTHEFORCETHATUNITES（粘合）THEMTHATTHESTEELCANNOTSLIP（滑移）WITHTHECONCRETESTILL（還有）ANOTHERADVANTAGEISTHATSTEELDOESNOTRUSTINCONCRETEACID（酸）CORRODESSTEEL,WHEREASCONCRETEHASANALKALINECHEMICALREACTION,THEOPPOSITEOFACID它們也以另外一種方式互補(bǔ)它們幾乎有相同的收縮率和膨脹率。因此，它們?cè)诶?、壓為主要因素時(shí)能共同工作。在出現(xiàn)拉力的混凝土梁或結(jié)構(gòu)中，將鋼筋埋入混凝土而成鋼筋混凝土。混凝土與鋼筋形成如此強(qiáng)大的結(jié)合力這個(gè)力將它們粘合在一起以致于鋼筋在混凝土中不會(huì)滑移。還有另一個(gè)優(yōu)勢(shì)是鋼筋在混凝土中不會(huì)銹蝕。酸能腐蝕鋼筋，而混凝土?xí)l(fā)生堿性的化學(xué)反應(yīng)，與酸相反。THEADOPTIONOFSTRUCTURALSTEELANDREINFORCEDCONCRETECAUSEDMAJORCHANGESINTRADITIONALCONSTRUCTIONPRACTICES（施工作業(yè)）ITWASNOLONGERNECESSARYTOUSETHICKWALLSOFSTONEORBRICKFORMULTISTORYBUILDINGS,ANDITBECAMEMUCHSIMPLERTOBUILDFIRERESISTANTFLOORS（防火地面）BOTHTHESECHANGESSERVEDTO（有利于）REDUCETHECOSTOFCONSTRUCTIONITALSOBECAMEPOSSIBLETOERECT（建造）BUILDINGSWITHGREATERHEIGHTSANDLONGERSPANS結(jié)構(gòu)鋼與鋼筋混凝土的采用使傳統(tǒng)的施工作業(yè)發(fā)生了明顯的變化。對(duì)多層建筑，再也沒(méi)必要采用厚的石墻或磚墻，且施工防火地面變?yōu)槿菀椎枚?。這些變化有利于降低建筑的成本。它也使建造高度更高和跨度更大的建筑物成為可能。SINCETHEWEIGHTOFMODERNSTRUCTURESISCARRIED（承受）BYTHESTEELORCONCRETEFRAME,THEWALLSDONOTSUPPORTTHEBUILDINGTHEYHAVEBECOMECURTAINWALLS,WHICHKEEPOUTTHEWEATHERANDLETINLIGHTINTHEEARLIERSTEELORCONCRETEFRAMEBUILDING,THECURTAINWALLSWEREGENERALLYMADEOFMASONRYTHEYHADTHESOLIDLOOKOFBEARINGWALLS（承重墻）TODAY,HOWEVER,CURTAINWALLSAREOFTENMADEOFLIGHTWEIGHTMATERIALSSUCHASGLASS,ALUMINUM,ORPLASTIC,INVARIOUSCOMBINATIONS由于現(xiàn)代結(jié)構(gòu)的重量由鋼或混凝土框架承受，墻體不再支承建筑物。它們成為幕墻，將日曬風(fēng)吹雨打阻擋在外，而讓光線進(jìn)入。在較早的鋼或混凝土框架建筑中，幕墻一般由砌體構(gòu)成；它們具有承重墻的結(jié)實(shí)外觀。但是今天，幕墻通常由輕質(zhì)材料組成，如玻璃、鋁或塑料，并形成不同的組合。ANOTHERADVANCEINSTEELCONSTRUCTION（結(jié)構(gòu)）ISTHEMETHODOFFASTENINGTOGETHER（連在一起）THEBEAMSFORMANYYEARSTHESTANDARDMETHODWASRIVETINGARIVETISABOLTWITHAHEADTHATLOOKSLIKEABLUNTSCREW（圓頭螺絲釘）WITHOUTTHREADS（螺紋）ITISHEATED,PLACEDINHOLESTHROUGHTHEPIECESOFSTEEL（鋼構(gòu)件）,ANDASECONDHEADISFORMEDATTHEOTHERENDBYHAMMERING（錘擊）ITTOHOLDITINPLACE（固定就位）RIVETINGHASNOWLARGELYBEENREPLACEDBYWELDING,THEJOININGTOGETHEROFPIECESOFSTEELBYMELTING（熔化）ASTEELMATERIALBETWEENTHEMUNDERHIGHHEAT鋼結(jié)構(gòu)中的另一個(gè)進(jìn)步是梁的連接方式。在很多年里，連接的標(biāo)準(zhǔn)方式是鉚接。鉚釘是個(gè)有頭的螺栓，看上去象個(gè)沒(méi)有螺紋的圓頭螺絲釘。鉚釘加熱后穿過(guò)鋼構(gòu)件之間的孔洞，并通過(guò)錘擊另一端而形成第二個(gè)鉚釘頭，從而將其固定就位。如今鉚接已大量地被焊接所替代，鋼構(gòu)件間的連接通過(guò)在高熱下熔化它們之間的鋼材料（即焊條）進(jìn)行。PRESTRESSEDCONCRETEISANIMPROVEDFORMOFREINFORCEMENT（加強(qiáng)方法）STEELRODSAREBENTINTOTHESHAPESTOGIVETHEMTHENECESSARYDEGREEOFTENSILESTRENGTHTHEYARETHENUSEDTOPRESTRESS（對(duì)預(yù)加應(yīng)力）CONCRETE,USUALLYBYONEOFTWODIFFERENTMETHODSTHEFIRSTISTOLEAVECHANNELSINACONCRETEBEAMTHATCORRESPONDTO（相應(yīng)于）THESHAPESOFTHESTEELRODSWHENTHERODSARERUNTHROUGHTHECHANNELS,THEYARETHENBONDEDTOTHECONCRETEBYFILLINGTHECHANNELSWITHGROUT,ATHINMORTARORBINDINGAGENTINTHEOTHERANDMORECOMMONMETHOD,THEPRESTRESSEDSTEELRODSAREPLACEDINTHELOWERPARTOFAFORM（模板）THATCORRESPONDSTOTHESHAPEOFTHEFINISHEDSTRUCTURE（成品結(jié)構(gòu)）,ANDTHECONCRETEISPOUREDAROUNDTHEMPRESTRESSEDCONCRETEUSESLESSSTEELANDLESSCONCRETEBECAUSEITISSOECONOMICAL,ITISAHIGHLYDESIRABLE（非常理想）MATERIAL預(yù)應(yīng)力混凝土是加強(qiáng)法的改進(jìn)形式。將鋼筋彎成一定的形狀以使它們具有必要的抗拉強(qiáng)度，然后用該鋼筋對(duì)混凝土施加預(yù)應(yīng)力，通?？刹捎脙煞N不同方法中的任何一種。第一種方法是在混凝土梁中按鋼筋的形狀留下孔道，當(dāng)鋼筋穿過(guò)孔道后，通過(guò)在孔道內(nèi)灌注薄砂漿（一種稀薄的砂漿或粘合劑）將鋼筋與混凝土粘結(jié)在一起。另一種（更常用的）方法是將預(yù)應(yīng)力鋼筋置于按成品結(jié)構(gòu)的形狀設(shè)置的模板的較低部位，然后將混凝土倒入（模板）而包圍著鋼筋。預(yù)應(yīng)力混凝土使用了較少的鋼筋和混凝土，由于它是如此的經(jīng)濟(jì)，因此是一種非常理想的材料。PRESTRESSEDCONCRETEHASMADEITPOSSIBLETODEVELOP（建造）BUILDINGSWITHUNUSUALSHAPES,LIKESOMEOFTHEMODERNSPORTSARENAS,WITHLARGESPACEUNBROKENBYANYOBSTRUCTINGSUPPORTS（阻礙的支撐物）THEUSESFORTHISRELATIVELYNEWSTRUCTURALMETHODARECONSTANTLYBEINGDEVELOPED（不斷地?cái)U(kuò)大）預(yù)應(yīng)力混凝土使建造獨(dú)特形狀的建筑物成為可能，象一些現(xiàn)代的運(yùn)動(dòng)場(chǎng)，它具有不受任何支撐物阻擋視線的大空間。這種較新的結(jié)構(gòu)方法的使用正在不斷地被擴(kuò)大。THECURRENTTENDENCYISTODEVELOP（采用）LIGHTERMATERIALS,ALUMINUM,FOREXAMPLE,WEIGHSMUCHLESSTHANSTEELBUTHASMANYOFTHESAMEPROPERTIESALUMINUMBEAMSHAVEALREADYBEENUSEDFORBRIDGECONSTRUCTIONANDFORTHEFRAMEWORKOFAFEWBUILDINGS目前的趨勢(shì)是采用較輕的材料。例如，鋁的重量比鋼輕得多，但具有很多相同的性能。鋁材梁已經(jīng)用于橋梁建筑和一些建筑的框架。LIGHTWEIGHTCONCRETES,ANOTHEREXAMPLE,ARENOWRAPIDLYDEVELOPING（發(fā)展）THROUGHOUTTHEWORLDTHEYAREUSEDFORTHEIRTHERMALINSULATION絕熱性THETHREETYPESAREILLUSTRATEDBELOW（舉例說(shuō)明如下）ACONCRETESMADEWITHLIGHTWEIGHTAGGREGATESBAERATEDCONCRETESUSGASCONCRETESFOAMED（起泡）BYWHISKING（攪拌）ORBYSOMECHEMICALPROCESSDURINGCASTINGCNOFINESCONCRETES另一個(gè)例子是輕質(zhì)混凝土，如今已在全世界快速地發(fā)展，因它們的絕熱性而被采用，其三種類(lèi)型舉例說(shuō)明如下A輕質(zhì)骨料制成的混凝土；B通過(guò)澆筑時(shí)攪拌或一些化學(xué)方法起泡而成的加氣混凝土（US加氣混凝土）；C無(wú)細(xì)骨料混凝土。ALLTHREETYPESAREUSEDFORTHEIRINSULATINGPROPERTIES（絕熱性）,MAINLYINHOUSING,WHERETHEYGIVEHIGH（非常）COMFORTINCOLDCLIMATESANDALOWCOSTOFCOOLING（降溫成本）INHOTCLIMATESINHOUSING,THERELATIVEWEAKNESSOFLIGHTWEIGHTCONCRETEWALLSISUNIMPORTANT,BUTITMATTERS（有重大關(guān)系）INROOFSLABS,FLOORSLABSANDBEAMS這三種類(lèi)型的混凝土都是由于它們的絕熱性而被使用，主要用于房屋，使其在寒冷的氣候中非常舒服，在炎熱的氣候中降溫的成本不高。在房屋中，墻采用較薄弱的輕質(zhì)混凝土不重要，但是屋面板、樓面板和梁（采用輕質(zhì)混凝土）則有重大關(guān)系。INSOMELOCATIONS,SOMELIGHTWEIGHTAGGREGATESCOSTLITTLEMORETHAN（幾乎等于）THEBESTDENSE（致密）AGGREGATESANDALARGENUMBEROF（大量）FLOORSLABSHAVETHEREFOREBEENBUILTOFLIGHTWEIGHTAGGREGATECONCRETEPURELYFORITSWEIGHTSAVING,WITHNOTHOUGHTOF（沒(méi)考慮）ITSINSULATIONVALUE在某些地區(qū)，一些輕質(zhì)骨料的費(fèi)用幾乎等于最致密的骨料，因此大量的樓面板采用輕骨料混凝土制作純粹是節(jié)約重量，而沒(méi)考慮它的絕熱價(jià)值。THELIGHTWEIGHTAGGREGATEREDUCESTHEFLOORDEADLOAD（恒載）BYABOUT20PERCENTRESULTINGIN（導(dǎo)致）CONSIDERABLESAVINGSINTHEFLOOR（樓蓋結(jié)構(gòu)）STEELINEVERYFLOORANDTHEROOF,ASWELLASINTHECOLUMNSTEELANDLESSINTHEFOUNDATIONSONELONDONCONTRACTOR（承包商）PREFERSTOUSELIGHTWEIGHTAGGREGATEBECAUSEITGIVESHIMTHESAMEWEIGHTREDUCTIONINTHEFLOORSLABASTHEUSEOFHOLLOWTILES,WITHSIMPLERORGANIZATIONANDTHEREFOREHIGHERSPEEDANDPROFITTHEINSULATIONVALUEOFTHELIGHTWEIGHTAGGREGATEISONLYIMPORTANTINTHEROOFINSULATION,WHICHISGREATLYIMPROVED（改進(jìn)）輕質(zhì)骨料使樓面的恒載減少了約20，因而大量的節(jié)約了每層樓面以及屋面的樓蓋結(jié)構(gòu)中的鋼材和柱子與基礎(chǔ)中（較少）的鋼材使用量。一位倫敦的承包商寧愿使用輕質(zhì)骨料，因?yàn)檫@使樓面板上減少的重量與用空心磚相同，且組織更簡(jiǎn)單，因而速度和利潤(rùn)更高。輕質(zhì)骨料的絕熱價(jià)值只在屋面絕熱時(shí)顯得重要，它已被大大地改進(jìn)了。作業(yè)練習(xí)通過(guò)兩篇READINGMATERIALS的學(xué)習(xí)，進(jìn)一步了解建筑材料中最常用的混凝土材料的一些特點(diǎn)、種類(lèi)和性能等，從而更多地掌握一些專業(yè)詞匯和句法。教學(xué)目標(biāo)了解構(gòu)件（主要為梁）的設(shè)計(jì)過(guò)程了解單軸應(yīng)力與多軸應(yīng)力對(duì)失效理論的影響熟悉材料力學(xué)中涉及的專業(yè)詞匯熟悉科技類(lèi)文獻(xiàn)常用句型熟悉INGENERAL、USUALLY、FREQUENTLY的不同含義與BEREFERREDTOAS、BEKNOWNAS、THATIS、BEDEFINEDAS、INOTHERWORDS等的用法INTRODUCTION介紹MECHANICSOFMATERIALSDEALSWITH（研究）THERESPONSEOFVARIOUSBODIES,USUALLYCALLEDMEMBERS（構(gòu)件）,TOAPPLIEDFORCES（施加力）INMECHANICSOFENGINEERINGMATERIALSTHEMEMBERSHAVESHAPESTHATEITHEREXISTINACTUALSTRUCTURESORAREBEINGCONSIDEREDFORTHEIRSUITABILITY（根據(jù)其需要）ASPARTSOFPROPOSED（擬建的）ENGINEERINGSTRUCTURESTHEMATERIALSINTHEMEMBERSHAVEPROPERTIESTHATARECHARACTERISTICOFCOMMONLYUSED（常用的）ENGINEERINGMATERIALSSUCHASSTEEL,ALUMINUM,CONCRETE,ANDWOOD材料力學(xué)用以研究不同物體（通常稱為構(gòu)件）對(duì)施加力的響應(yīng)。在工程材料力學(xué)中，構(gòu)件的形狀可以是實(shí)際結(jié)構(gòu)中存在的，也可以根據(jù)其需要而進(jìn)行考慮（設(shè)計(jì)），作為擬建工程結(jié)構(gòu)的部件。構(gòu)件中材料的性能即是常用的工程材料如鋼材、鋁材、混凝土和木材的特性。ASYOUCANSEEALREADYFROMTHEVARIETYOFMATERIALS,FORCES,ANDSHAPESMENTIONED,MECHANICSOFENGINEERINGMATERIALSISOFINTERESTTO（對(duì)有價(jià)值）ALLFIELDSOFENGINEERINGTHEENGINEERUSESTHEPRINCIPLESOFMECHANICSOFMATERIALSTODETERMINEIFTHEMATERIALPROPERTIESANDTHEDIMENSIONSOFAMEMBERAREADEQUATETO（足以）ENSURETHATITCANCARRYITSLOADSSAFELYANDWITHOUTEXCESSIVEDISTORTIONINGENERAL（通常）,THEN,WEAREINTERESTEDINBOTHTHESAFELOADTHATAMEMBERCANCARRYANDTHEASSOCIATED（相關(guān)的）DEFORMATIONENGINEERINGDESIGNWOULDBEASIMPLEPROCESSIFTHEDESIGNERCOULDTAKEINTOCONSIDERATION（考慮）THELOADSANDTHEMECHANICALPROPERTIESOFTHEMATERIALS,MANIPULATE（利用）ANEQUATION,ANDARRIVEAT（得到）SUITABLEDIMENSIONSDESIGNISSELDOMTHATSIMPLEUSUALLY（通常）,ONTHEBASISOF（根據(jù)）EXPERIENCE,THEDESIGNERSELECTSATRIAL（試算）MEMBERANDTHENDOESANANALYSISTOSEEIFTHATMEMBERMEETSTHESPECIFIEDREQUIREMENTSFREQUENTLY（常常）,ITDOESNOTANDTHENANEWTRIALMEMBERISSELECTEDANDTHEANALYSISREPEATEDTHISDESIGNCYCLE（設(shè)計(jì)周期）CONTINUESUNTILASATISFACTORYSOLUTIONISOBTAINEDTHENUMBEROFCYCLES（循環(huán)次數(shù)）REQUIREDTOFINDANACCEPTABLEDESIGNDIMINISHESASTHEDESIGNERGAINSEXPERIENCE正如你已經(jīng)從提到的各種各樣的材料、力和形狀所看到的，工程材料力學(xué)對(duì)所有的工程領(lǐng)域都有價(jià)值。工程師利用材料力學(xué)的原理來(lái)確定是否該材料的性能和構(gòu)件尺寸足以保證它能安全地承受荷載且沒(méi)有過(guò)多的變形。通常，我們關(guān)心的是構(gòu)件能承受的安全荷載及其相應(yīng)的變形。如果設(shè)計(jì)者能通過(guò)考慮荷載和材料的力學(xué)性能，并利用公式得到合適的構(gòu)件尺寸，那么工程設(shè)計(jì)將是一個(gè)簡(jiǎn)單的過(guò)程。然而設(shè)計(jì)很少那么簡(jiǎn)單。通常，根據(jù)經(jīng)驗(yàn)，設(shè)計(jì)者選擇一個(gè)試算構(gòu)件，然后進(jìn)行分析，看它是否滿足規(guī)定的要求。它常常不會(huì)滿足要求，則再選擇一個(gè)新的試算構(gòu)件，再進(jìn)行分析。這樣的設(shè)計(jì)不斷重復(fù)，直至得到一個(gè)滿意的結(jié)果。當(dāng)設(shè)計(jì)師擁有一定的經(jīng)驗(yàn)后，為得到一個(gè)可接受的設(shè)計(jì)所需要的循環(huán)次數(shù)會(huì)減少。DESIGNOFAXIALLYLOADEDMEMBERS軸向力構(gòu)件的設(shè)計(jì)TOGIVEYOUSOMEINSIGHTINTO（使有一些了解）THEDESIGNCYCLE,ANEXTREMELYSIMPLEMEMBERWILLBEDEALTWITHFIRSTTHATMEMBERISAPRISMATICBARWITHAFORCE,P,ACTINGALONGITSLONGITUDINALAXISINTHEDIRECTION（縱軸向）SUCHTHATITTENDSTOELONGATETHEBARSUCHAFORCEISREFERREDTOAS（稱為）ANAXIALTENSILELOAD（軸向拉力）,ANDWECANREADILYIMAGINEITTRYINGTO（努力）PULLTHEFIBERSAPARTANDTOCAUSEFAILUREONATRANSVERSEPLANE（橫向平面）ITISSAFETOASSUMETHATALLFIBERSOFTHEBAR,INREGIONSREMOTEFROM（遠(yuǎn)離）THEPOINTOFAPPLICATIONOFTHELOAD,AREBEINGPULLEDAPARTWITHTHESAMELOADINTENSITY（荷載強(qiáng)度）WITHTHISASSUMPTION,THELOADINTENSITYORSTRESSISUNIFORMONATRANSVERSEPLANEANDISGIVENBYWHENPISIN（以為單位）NEWTONSANDAISINSQUAREMETRES,STRESS,ISINNEWTONSPERSQUAREMETREN/M2,WHICHISBYDEFINITION（根據(jù)定義）PASCALSPA為了使你對(duì)設(shè)計(jì)周期有一些了解，首先研究一個(gè)非常簡(jiǎn)單的構(gòu)件。構(gòu)件是個(gè)棱形的桿件，其上沿著它的縱軸向作用一個(gè)力P，這樣往往使桿件在該方向上伸長(zhǎng)。這樣的力稱為軸向拉力，我們能容易地想象它在努力地將纖維拉開(kāi)，導(dǎo)致橫向平面的破壞。安全地假定桿件的所有纖維在遠(yuǎn)離荷載施加點(diǎn)的區(qū)域以相同的荷載強(qiáng)度被拉開(kāi)。在此假定下，荷載強(qiáng)度或應(yīng)力在橫向平面上是均勻的，為當(dāng)P的單位為牛頓、A的單位為平方米時(shí)，應(yīng)力的單位為牛頓每平方米（N/M2），根據(jù)定義為帕斯卡（PA）。FORAGIVENAXIALLOADANDGIVENDIMENSIONS,THESTRESSCANBECALCULATEDFROM41ANDCOMPAREDWITH（與相比）THESTRESSTHATCANBESAFELYCARRIEDBYTHEMATERIALTHESAFESTRESS,KNOWNAS（稱為）THEDESIGNSTRESSORALLOWABLESTRESS（許用應(yīng)力）,ISDETERMINEDBYTESTSPERFORMEDONMATERIALMADETO（按照）THESAMESPECIFICATIONSASTHEPARTBEINGCONSIDEREDASAFETYFACTOR（安全系數(shù)）,FREQUENTLYIMPOSEDBYALEGALLYESTABLISHEDCODE（法規(guī)）,ISAPPLIEDTOTHESTRENGTH,ASDETERMINEDBYTESTS,TOGIVETHEALLOWABLESTRESSTHEALLOWABLESTRESS,A,ISGIVENBYWHEREFISTHESTRESSATWHICHTHEMATERIALFAILSFAILURETOBEDEFINEDLATERANDNISTHESAFETYFACTOR對(duì)已知的軸向力和（構(gòu)件）尺寸，可根據(jù)公式（41）計(jì)算出應(yīng)力，并與材料能安全承受的應(yīng)力作比較。安全應(yīng)力，稱為設(shè)計(jì)應(yīng)力或許用應(yīng)力，它是通過(guò)對(duì)材料的試驗(yàn)來(lái)確定的，該（試驗(yàn)）材料按照與所考慮（驗(yàn)算）的桿件相同的規(guī)范制作。根據(jù)法規(guī)規(guī)定，通常對(duì)試驗(yàn)所確定的強(qiáng)度考慮安全系數(shù)后得到許用應(yīng)力。許用應(yīng)力A為這里，F(xiàn)為材料失效（失效在下文有定義）時(shí)的應(yīng)力，而N為安全系數(shù)。BEFOREAPPROVING（核準(zhǔn)）TRIALDIMENSIONS,THEDESIGNERMAKESCERTAIN（確信）THATTHEDESIGNISSAFEBYDETERMININGTHATTHEINEQUALITY（不等式）ISSATISFIEDTHEINEQUALITYISUSUALLYMORECONVENIENTINTHEFORM不等式常常以更合適的形式出現(xiàn)，即在核準(zhǔn)試算的尺寸之前，設(shè)計(jì)者通過(guò)確定不等式成立而確信設(shè)計(jì)的安全，即ITMIGHTATFIRST（起先）SEEMTHATTHEDESIGNERWOULDALWAYSDIMENSION（選定的尺寸）THECROSSSECTION（橫截面）SOTHATTHESTRESSWOULDEXACTLYEQUALTHEALLOWABLESTRESSHOWEVER,ITMAYBEVERYCOSTLYTOPRODUCEPARTSTHATHAVENONSTANDARDSIZES,SOITISUSUALLYMOREECONOMICALTOWASTESOMEMATERIALBYSELECTINGTHENEXT（接近的）LARGERSTANDARDSIZEABOVETHATREQUIREDBYTHEALLOWABLESTRESSDEPARTUREFROM（背離）STANDARDSIZESISJUSTIFIED（合理的）INCASESWHERETHEPENALTY（不利后果）FOREXCESSWEIGHTISVERYSEVERE,ASINAIRCRAFT（航天器）ORSPACESHIP（宇宙飛船）DESIGN起先似乎設(shè)計(jì)者總是在選定橫截面的尺寸，以使應(yīng)力恰好等于許用應(yīng)力。但是，生產(chǎn)非標(biāo)準(zhǔn)尺寸部件的成本可能很高，因此，通常人們會(huì)選擇比按許用應(yīng)力要求的尺寸大一些的標(biāo)準(zhǔn)尺寸部件，這樣盡管浪費(fèi)了一些材料，但總體上更經(jīng)濟(jì)。但不選擇標(biāo)準(zhǔn)尺寸的做法在諸如航天器和宇宙飛船的設(shè)計(jì)中證明是合理的，因?yàn)槎嘤嘀亓慨a(chǎn)生的不利后果是很?chē)?yán)重的。DESIGNOFBEAMS梁的設(shè)計(jì)UPTOTHISPOINT（至此）WEHAVELOOKEDAT（考慮）THEBEAMPROBLEMASAPROBLEMINANALYSISTHATIS（即）,FORAGIVENSETOFLOADS,SPAN,ANDCROSSSECTIONWEHAVEBEENCALCULATINGTHESTRESSTHEMORECOMMONLYENCOUNTEREDPROBLEMISTOSELECTASTANDARDSECTION,ORDESIGNAMEMBER,FORAGIVENSPANANDLOADSWITHOUTEXCEEDINGACERTAINALLOWABLESTRESSUNDERSOMECONDITIONSTHEALLOWABLESTRESSMAYBEDEPENDENTUPONTHEDIMENSIONSANDSHAPEOFTHECROSSSECTION,INWHICHCASETHESELECTIONOFTHEMEMBERBECOMESMOREDIFFICULTFORTHEPRESENT（暫時(shí)）WEWILLTAKETHEALLOWABLESTRESSASTHOUGH（似乎）ITDEPENDSONLYONTHESTRENGTHOFTHEMATERIALANDTHESAFETYFACTOR至此，我們已經(jīng)考慮了梁的問(wèn)題而進(jìn)行了（問(wèn)題）分析，即對(duì)給定的一組荷載、跨度和橫截面，我們已經(jīng)計(jì)算了應(yīng)力。更常遇到的問(wèn)題是在不超過(guò)某個(gè)許用應(yīng)力下對(duì)一個(gè)給定的跨度和荷載選擇一個(gè)（構(gòu)件的）標(biāo)準(zhǔn)截面，或設(shè)計(jì)一個(gè)構(gòu)件。在某些條件下，許用應(yīng)力可能依賴于橫截面的尺寸和形狀，這種情況下的構(gòu)件選擇會(huì)變得比較困難。暫時(shí)我們將采用許用應(yīng)力法，似乎它只取決于材料的強(qiáng)度和安全系數(shù)。ATRIALMEMBERWILLBEACCEPTABLE（合格）WHENTHESTRESSISEQUALTO,ORLESSTHAN,THEALLOWABLESTRESS,THATIS,IFFORDESIGNPURPOSESTHISINEQUALITYISMOREUSEFULINTHEFORMINTHEUSUALDESIGNPROCESSTHEMAXIMUMBENDINGMOMENTISTAKENFROM（取自于）THEBENDINGMOMENTDIAGRAM（彎矩圖）ANDTHEALLOWABLESTRESSISDETERMINEDQUITEFREQUENTLYINACCORDANCEWITH（根據(jù)）THERULESOFSOMELEGALLYCONSTITUTEDCODEFROMSTANDARDSTRENGTHTESTSINCOMBINATIONWITH（與結(jié)合）ASAFETYFACTORTHERIGHTHANDSIDEOF46ISTHENKNOWN,ANDITREMAINS（仍然是）TOSELECTORDESIGNAMEMBERTHATWILLSATISFYTHEINEQUALITYWHENASTANDARDSECTIONISTOBEUSED,THETABLES（表格）COULDBESEARCHEDUNTILAMEMBERISFOUNDSUCHTHATTHECOMBINATIONOFIANDCSATISFIES46THISTAKESMORETIMETHANISREALLYNECESSARY,SINCETHETABLESALSOPROVIDETHEVALUEOFI/CFOREACHMEMBERUNDERTHEHEADING（標(biāo)題）S,THESECTIONMODULUS（截面模量）當(dāng)試算構(gòu)件的應(yīng)力等于或小于許用應(yīng)力時(shí)，也就是說(shuō)，如果在通常的設(shè)計(jì)過(guò)程中，最大的彎距從彎距圖上取得，而許用應(yīng)力通過(guò)標(biāo)準(zhǔn)強(qiáng)度試驗(yàn)并考慮安全系數(shù)后確定（往往是根據(jù)一些法規(guī)的規(guī)則）。這樣，已知式（46）右手邊的值，則仍然是選擇或設(shè)計(jì)構(gòu)件以滿足該不等式。當(dāng)使用標(biāo)準(zhǔn)截面時(shí)可以查找表格，直至找到的構(gòu)件其I和C值的組合能滿足式（46）。這樣花費(fèi)的時(shí)間比實(shí)際需要的多，因?yàn)楸砀裰性诮孛婺Ａ縎的標(biāo)題下也提供了每一個(gè)構(gòu)件的I/C的值。試算構(gòu)件即為合格。根據(jù)設(shè)計(jì)的需要，（上述）不等式以下列形式出現(xiàn)更有用，即THATIS,THESECTIONMODULUSISDEFINEDAS（定義為）TOSELECTAMEMBER,THESCOLUMN（列）ISCONSULTED（查閱）ANDANYMEMBERTHATSATISFIES48COULDBEUSEDTHEMEMBERSWITHVERYHIGHVALUESOFSWILLOBVI