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1、结构设计AugustineJoFredrich摘要:结构设计是选择材料和构件类型,大小和形状以安全有用的样式承担荷载。一般说来，结构设计暗指结构物如建筑物和桥或是可移动但有刚性外壳如船体和飞机框架的工厂稳定性。设计的移动时彼此相连的设备(连接件)，一般被安排在机械设计领域。关键词：结构设计结构分析结构方案工程要求Abstract：Structuredesignistheselectionofmaterialsandmembertype,size,andconfigurationtocarryloadsinasafeandserviceablefashion.Ingeneralstructura
2、ldesignimpliestheengineeringofstationaryobjectssuchasbuildingsandbridges,orobjectsthatmaybemobilebuthavearigidshapesuchasshiphullsandaircraftframes.DeviceswithpartsplannedtomovewithrelationtoeachOther(Iinkages)aregenerallyassignedtotheareaofmechanical.Keywords:StructureDesignStructuralanalysisstruct
3、uralschemeProjectrequirementsStructureDesignStructuraldesigninvolvedatleastfivedistinctphasesofwork:projectrequirements,materials,structuralscheme,analysis,anddesign0Forunusualstructuresormaterialsasixphase,testing,shouldbeincluded.Thesephasesdonotproceedinarigidprogression,sincedifferentmaterialsca
4、nbemosteffectiveindifferentschemes,testingcanresultinchangetoadesign,andafinaldesignisoftenreachedbystartingwitharoughestimateddesign,thenloopingthroughseveralcyclesofanalysisandredesign.Often,severalalternativedesignswillprovequitecloseincost,strength,andServiceability0Thestructuralengineer,owner,o
5、renduserwouldthenmakeaselectionbasedonotherconsiderationsoProjectrequirementSoBeforestartingdesign,thestructuralengineermustdeterminethecriteriaforacceptableperformance.TheloadsorforcestoberesistedmustbeprovidedoForspecializedstructures,thismaybegivendirectly,aswhensupportingaknownpieceofmachinery,o
6、racraneofknowncapacity.Forconventionalbuildings,buildingscodesadoptedonamunicipal,county,or,statelevelprovideminimumdesignrequirementsforliveloads(occupantsandfurnishings,snowonroofs,andsoon).Theengineerwillcalculatedeadloads(structuralandknown,permanentinstallations)duringthedesignprocess0Forthestr
7、ucturaltobeserviceableoruseful,deflectionsmustalsobekeptwithinlimits,sinceitispossibleforsafestructuraltobeuncomfortable“bounce“Verytightdeflectionlimitsaresetonsupportsformachinery,sincebeamsagcancausedriveshaftstobend,bearingtoburnout,partstomisalign,andoverheadcranestostall0Limitationsofsaglessth
8、anspan/1000(1/1000ofthebeamlength)arenotuncommon.Inconventionalbuildings,beamssupportingceilingsoftenhavesaglimitsofspan/360toavoidplastercracking,orspan/240toavoidoccupantconcern(keepvisualperceptionlimited)0Beamstiffnessalsoaffectsfloorbounciness,“whichcanbeannoyingifnotcontrolledoInaddition,later
9、aldeflection,sway,ordriftoftallbuildingsisoftenheldwithinapproximatelyheight/500(1/500ofthebuildingheight)tominimizethelikelihoodofmotiondiscomfortinoccupantsofupperfloorsonwindydays0Membersizelimitationsoftenhaveamajoreffectonthestructuraldesign.Forexample,acertaintypeofbridgemaybeunacceptablebecau
10、seofinsufficientunderclearanceforrivertraffic,orexcessiveheightendangeringaircraftoInbuildingdesign,ceilingheightsandfloor一to一floorheightsaffectthechoiceoffloorframing。Wallthicknessesandcolumnsizesandspacingmayalsoaffecttheserviceabilityofvariousframingschemes.Materialsselection.Technologicaladvance
11、shavecreatedmanynovelmaterialssuchascarbonfiberandboronfiber-reinforcedcomposites,whichhaveexcellentstrength,stiffness,andStrength-to-weightproperties.However,becauseofthehighcostanddifficultorunusualfabricationtechniquesrequired,theyareusedonlyinverylimitedandspecializedapplications.Glassreinforced
12、compositessuchasfiberglassaremorecommon,butarelimitedtolightlyloadedappIicationsoThemainmaterialsusedinstructuraldesignaremoreprosaicandincludesteel,aluminum,reinforcedconcrete,wood,andmasonry.Structuralschemes.Inanactualstructural,variousforcesareexperiencedbystructuralmembers,includingtension,comp
13、ression,flexure(bending),shear,andtorsion(twist).However,thestructuralschemeselectedwillinfluencewhichoftheseforcesoccursmostfrequently,andthiswillinfluencetheprocessofmaterialsSelection0Tensionisthemostefficientwaytoresistappliedloads,sincetheentiremembercrosssectionisactingtofullcapacityandbucking
14、isnotaconcernoAnytensionschememustalsoincludedanchoragesforthetensionmembers.Inasuspensionbridge,forexample,theanchoragesareusuallymassivedeadweightsattheendsofthemaincables.Toavoidundesirablechangesingeometryundermovingorvaryingloads,tensionschemesalsogenerallyrequirestiffeningbeamsortrusses0Compre
15、ssionisthenextmostefficientmethodforcarryingloadsoThefullmembercrosssectionisused,butmustbedesignedtoavoidbucking,eitherbymakingthememberstockyorbyaddingsupplementarybracing.Domedandarchedbuildings,archbridgesandcolumnsinbuildingsframesarecommonschemes.Archescreatelateraloutwardthrustswhichmustberes
16、isted0Thiscanbedonebydesigningappropriatefoundationsor,wherethearchoccursabovetheroadwayorfloorline,byusingtensionmembersalongtheroadwaytotiethearchendstogether,keepingthemfromspreading0Compressionmembersweakendrasticallywhenloadsarenotappliedalongthememberaxis,somoving,variable,andunbalancedloadsmu
17、stbecarefullyConsidered0Schemesbasedonflexurearelessefficientthantensionandcompression,sincetheflexureorbendingisresistedbyonesideofthememberactingintensionwhiletheothersideactsincompression。Flexuralschemessuchasbeams,girders,rigidframes,andmoment(bending)connectedframeshaveadvantagesinrequiringnoex
18、ternalanchoragesorthrustrestrainsotherthannormalfoundations,andinherentstiffnessandresistancetomoving,variable,andunbalancedloads.Trussesareaninterestinghybridoftheaboveschemes.Theyaredesignedtoresistloadsbyspanninginthemannerofaflexuralmember,butacttobreakuptheloadintoaseriesoftensionandcompression
19、forceswhichareresistedbyindividuallydesignedtensionandhaveexcellentstiffnessandresistancetomovingandvariableloadsoNumerousmember-to-memberconnections,supplementarycompressionbraces,andasomewhatclutteredappearancearetrussdisadvantages。Platesandshellsincludedomes,archedvaults,sawtoothroofs,hyperbolicp
20、araboloids,andsaddleshapes0Suchschemesattempttodirectallforcealongtheplaneofthesurface,andactlargelyinshear0Whilepotentiallyveryefficient,suchschemeshaveverystrictlimitationsongeometryandarepoorinresistingpoint,moving,andunbalancedloadsperpendiculartothesurface0Stressed-skinandmonologueconstructionu
21、sestheskinbetweenstiffeningribs,spars,orcolumnstoresistshearoraxialforcesoSuchdesigniscommoninairframesforplanesandrockets,andinshiphulls0ithasalsobeenusedtoadvantageinbuildingSoSuchadesignispracticalonlywhentheskinisalogicalpartofthedesignandisnevertobealteredorremoved0Forbridges,shortspansarecommo
22、nlygirdersinflexure.Asspansincreaseandgirderdepthbecomesunwieldy,trussesareoftenused,aswellasCablestayedschemes0Longerspansmayusearcheswherefoundationconditions,underclearance,orheadroomrequirementsarefavorable.Thelongestspansarehandledexclusivelybysuspensionschemes,sincetheseminimizethecrucialdeadw
23、eightandcanbeerectedwirebywire.Forbuildings,shortspansarehandledbyslabsinflexure。Asspansincrease,beamsandgirdersinflexureareused.Longerspansrequiretrusses,especiallyinindustrialbuildingswithpossiblehungloads0Domes,arches,andcable-suspendedandair-supportedroofscanbeusedoverconventionhallsandarenastoa
24、chieveclearareas.Structuralanalysis.Analysisofstructuresisrequiredtoensurestability(staticequilibrium),findthememberforcestoberesisted,anddeterminedeflections。Itrequiresthatmemberconfiguration,approximatemembersizes,andelasticmodulus;linearity;andcurvatureandplanesections0Variousmethodsareusedtocomp
25、letetheanalysis0Finaldesign。onceastructuralhasbeenanalyzed(byusinggeometryaloneiftheanalysisisdeterminate,orgeometryplusassumedmembersizesandmaterialsifindeterminate),finaldesigncanproceed.Deflectionsandallowablestressesorultimatestrengthmustbecheckedagainstcriteriaprovidedeitherbytheownerorbythegov
26、erningbuildingcodes。Safetyatworkingloadsmustbecalculated.Severalmethodsareavailable,andthechoicedependsonthetypesofmaterialsthatwillbeusedoPuretensionmembersarecheckedbydividingloadbycross-sectionareaoLocalstressesatconnections,suchasboltholesorwelds,requirespecialattention.Whereaxialtensioniscombin
27、edwithbendingmoment,thesumofstressesiscomparedtoallowancelevels.Allowable:stressesincompressionmembersaredependentonthestrengthofmaterial,elasticmodulus,memberslenderness,andlengthbetweenbracingpoints0Stockymembersarelimitedbymaterialsstrength,whileslendermembersarelimitedbyelasticbucking.Designofbe
28、amscanbecheckedbycomparingamaximumbendingstresstoanallowablestress,whichisgenerallycontrolledbythestrengthofthematerial,butmaybelimitedifthecompressionsideofthebeamisnotwellbracedagainstbucking.Designofbeamcolumns,orcompressionmemberswithbendingmoment,mustconsidertwoitems.First,whenamemberisboweddue
29、toanappliedmoment,addingaxialcompressionwillcausethebowtoincrease.Ineffect,theaxialloadhasmagnifiedtheoriginalmoment.Second,allowablestressesforcolumnsandthoseforbeamsareoftenquitedifferent.Membersthatareloadedperpendiculartotheirlongaxis,suchasbeamsandbeam-columns,alsomustcarryshear.Shearstresseswi
30、lloccurinadirectiontoopposetheappliedloadandalsoatrightanglestoittotiethevariouselementsofthebeamtogetheroTheyarecomparedtoanallowableshearstressoTheseprocedurescanalsobeusedtodesigntrusses,whichareassembliesoftensionandcompressionmembersoLastly,deflectionsarecheckedagainsttheprojectcriteriausingfin
31、almembersizes0Onceasatisfactoryschemehasbeenanalyzedanddesignedtobewithinprojectcriteria,theinformationmustbepresentedforfabricationandConstruction0Thisiscommonlydonethroughdrawings,whichindicateallbasicdimensions,materials,membersizes,theanticipatedloadsusedindesign,andanticipatedforcestobecarriedt
32、hroughconnectionso结构设计包含至少5个不同方面的工作：工程要求，材料,结构方案，分析和设计。对于不一般的结构或材料,又包含一个方面:试验。这些方面不是严格按步骤进行,因为不同材料在不同方案大多数是有效的,试验会导致设计变更，最终设计由初步估计设计开始，然后经过分析和再设计几个循环后完成。通常，可替代的设计证明在费用，强度和使用性上十分接近。结构工程师，业主或最后住户基于其它的考虑选择一种。工程要求.在开始设计前,结构工程师必须决定容易接受的执行标准。必须提供承担的荷载或力。对于一些专门结构，当支持一台已知载重的机器或起重机时，这可能直接给出，对于普通建筑物，采用市政，县,州的
33、建筑规范，提供了设计所需活载（人群荷载和设备，屋顶雪荷载,等等）的最小值。工程师将计算出设计期间的恒载（结构和已知永久性设备）。对要正常使用的结构，也必须控制其挠度，因为安全的结构可能会存在令人不安的振动。机器的支座有严格的变形限制，因为梁下沉会导致驱动轴弯曲，烧毁，部件错位和上面的吊车熄火。挠度限制在跨度/1000（梁长的1/1000）以下是很普通的。在传统建筑里,支持板的梁挠度限制在跨度1/360以避免粉刷开裂或跨度1/240以避免人的担忧（保持在可感知的变动范围内）。梁的刚度也影响板“振动”，如果不能控制会令人很头疼。另外，高层建筑的侧面变形,位移或摇摆通常限定在高度/500（建筑物高度
34、的1/500）里,把在有风的日子里上面楼层的人移动的不舒服降到最小.构件尺寸在结构设计里起主要作用.例如，由于下面留作水上交通的净空不够或过高威胁到飞机的特定类型的桥是不可接受的。在建筑设计里，天花板高度和楼板之间高度影响楼板框架的选择。墙厚和柱子尺寸和跨度也影响不同框架方案的适用性。选择材料。技术的进步创造了许多新材料,如碳纤维加强复合材料和硼纤维加强复合材料，它们都具有极好的强度，刚度和强度重量比特性。然而，由于费用高和非通常的制造要求,它们仅用在有限特殊领域。强化玻璃合成物如玻璃纤维是很普遍，但被限制应用在小荷载情况下。用在结构设计上的主要材料更多是普通的，包括钢材，铝，钢筋混凝土，木材
35、，砌体.结构方案。在一个实际方案里,结构构件承担很多力，包括拉，压，弯,剪和扭。然而所选择的方案将会影响这些力产生的概率，也会影响材料选择过程.抗拉是有效的承担荷载的方法，整个构件的横截面性能得到发挥，并且不涉及到弯曲变形.任何抗拉方案必须也对抗拉构件的锚固。例如,在悬索桥里,锚固体通常是位于主要绳索尾段的强大自重。为了避免在荷载移动或变形时有不期望的几何变形,抗拉方案通常要求是刚性梁和桁架。抗压是另一个很有效的承担荷载方法.全部杆件截面发挥了作用，但是设计时必须避免弯曲，或者是做成粗短构件或者是增加附加支撑。圆顶和拱形建筑，拱桥和柱是很普遍的建筑方案。拱产生了必须抵挡住的水平外推力。这靠设计
36、合适的基础或建在车道或楼板的上面的拱解决，靠沿着车道用抗拉构件把两端的拱连接起来，阻止他们拉开。当荷载不是作用在构件轴线上时，抗压构件显著地被削弱。所以，必须认真考虑移动，变化和不平衡的荷载。基于受弯的方案的效率比受拉和压低，因为弯曲是靠构件一边受拉另一边受压来抵抗。受弯方案如主梁，次梁，刚架和受弯框架在外部锚固或推力限制，与一般基础不同,靠内部刚度阻挡可移动,变化和不平衡的荷载的情况下有利。桁架是上面方案的混合体。它们设计成荷载横跨在受弯构件上，但是分解成一系列拉力和压力，由抗拉和抗压构件承担。桁架方案设计时不需要特殊锚固或推力的限制，并且有很好的刚度抵抗移动或变化的荷载。大量的构件之间连结
37、和抗压构件的附加支撑,看起来有点杂乱,这就是桁架的不利处.板和壳包括圆顶，拱顶，有齿屋顶，双曲抛物面和马鞍形。这样的方案把所有的力直接作用在平板表面并且作用有巨大的剪力。尽管可能效率很高，但是这样的方案对几何有严格的限制,并且在移动,和不平衡垂直作用在表面的荷载的能力很弱。薄壳结构和硬壳结构利用加劲肋,梁之间的壳板抵抗剪力和轴向力。这样的设计在飞机机体和火箭，船体方面很普遍。它在建筑方面也是有利的.这样的设计仅仅在壳是设计的逻辑部分并且永远不会被替代和移除时才实际些。对于桥梁，短跨是很普遍受弯的梁。当跨度增加和梁高变得很大时，通常用桁架和斜拉结构.更长跨时也许用拱，要考虑基础条件和净空要求。最
38、长的跨靠悬索方案处理，因为这可把关键性的自重降到最小并且能索连索地建造起来。对于桥，短跨靠板承担弯矩.当跨度增加时，主梁和次梁被用来承担弯曲。更长的跨要求用桁架,尤其是在工业建筑有吊车荷载时，圆顶,拱和悬索和充气屋顶被用在传统的大厅和竞技场里以获得净面积。结构分析。结构分析要求确定稳定性（静力平衡），构件承担的力和变形。它需要构件形状,大概尺寸，已知或假设的材料特性.分析包括：平衡，应力，应变和弹性模量，线形，塑性和弯曲和板截面。很多方法可以完成分析过程。最终设计。一旦结构分析完成（如果分析是正确的，只用几何方法;反之附加构件尺寸和材料假设）。最终设计可以进行，必须对照业主或政府建筑规范标准来
39、检查变形和允许应力或极限强度。必须计算工作荷载下的安全性。一般方法是可行的，依据所使用的材料类型做出选择。纯抗拉构件检查横截面应力。特别注意螺栓孔或焊接处的应力。拉弯构件中,用应力之和与分析应力作比。受压构件中的允许应力取决于构件强度，弹性模量，长细比和支点间距离。粗短构件由材料强度决定，然而长细构件由弹性弯曲决定。梁的设计由对于最大弯曲应力和允许应力来检验，通常由材料强度控制，但是如果受压一边没有侧向支撑就会被限制。梁，柱或有弯矩的受压构件的设计必须考虑两项。首先，当构件由于承受弯矩而弯曲时，轴力会增加弯曲量，实际上,轴压放大了原始弯矩。其次,对于柱和梁的允许应力是不同的.承受垂直于长轴的荷载的构件.如梁和梁一柱，也必须承担剪力.剪应力和荷载的方向相反并且在其右边，把梁的不同部分连接起来.它们与允许剪应力作对比。这些步骤也能用来设计由受拉和受压构件组成的桁架。最后,用工程标准检验变形，使用最后的构件。一旦被分析和在工程标准内的设计方案是令人满意的，必须提出制造和建立信息.通过作图，指明所以基本尺寸,材料和构件大小。设计中预期荷载和节点承担的预期力。
