关于钢结构交错桁架体系的一个观点

ErectingtheStaggered-trussSystem:AViewfromtheFieldARTHURE.HASSLERThestaggered-steeltrusssystemhasbeenusedasthemajorsupportingelementformanyyears.Projectsusingthistrusssystemareeconomical,easytofabricateanderect.andasaresultbeatoutallotherframingsystems.Thispaperdealswithabuildingrecentlycompletedwhichusedthestaggeredsteeltrusssystem.TheprojectistheResortsInternationalHotelinAtlanticCity,N.J.FABRICATIONFabricationofthistypeofstructurecanreadilybeaccomplishedbyanyshopengagedinstructuralsteelfabrication,providedtheypossesscertifiedweldersandarewelding-oriented.Inaddition,thebuildingmustcontainoverheadcranescapableoflifting10to15-tontrussesandcolumnsforprojectsupto20stories.Fabricationinvolvesthefollowingcomponents:(1)Columns,(2)SpandrelBeams,(3)Trusses,(4)SecondaryColumnsandBeamsand(5)FloorSystem.1—ColumnsColumnfabricationisnotcomplicatedforbuildingsupto20-storieshigh.Theywillberolledwide-flangesectionsuptoW14×720withthelongestbeingapproximately25ftinlengthandwithatotalweightof9to10tonseach(baseplatesareshippedloose).Columnscontainedinbuildingsfrom20to30storieswillprobablybereinforcedwithflangeplatesasinFig.1.Above30storieswillbebuilt-uptypecolumnsconsistingofthreeplatesofvaryingthickness.OntheResortsInternationalHotelproject,wehadtofabricate(Fig.2)columnswith8-in.thickflangeplatesand6-in.thickwebplates.Thistypeofcolumnrequiresmoresophisticatedfabricationequipmentsuchaspreheatdevices,submergedarcweldingequipmentandheavyliftingcapacityof25to40tons.Inotherwords,Figure1Figure2shopshavingthecapacitytohandleheavyplatesandplatefabrication.AsinFig.3,thesecolumnscanalsocontainheavybracingconnectionsinthebottomtierwhichtransfersthewindloadtotheconcretefoundation.Asanalternatefortransferofthiswindstress,abracingtruss(Fig.4)canbeusedatthebottomifarchitecturalfeaturesarenotobstructed.2—SpandrelBeamsSpandrelbeamsaredesignedtoresistthewindmomentimposedontheendwallsinthelongitudinaldirection(Figs.5,6).Toresistthisforce,theycanbesteelbeamsmomentconnected(Fig.7)eitherbyendplateconnectionsorflangeplatestopandbottomwithaweb-shearplate.Fabricationofthistypeofbeampresentsnoproblem.However,duetorollingtolerances,thematchingholesonthecolumnflangeplatesortheflangewidthdimensionwhenusingendplateswillhavetobecheckedandmatchedwiththebeamtoinsurecorrectcenter-to-centerofcolumns.Thisbeamcontrolsthetotallengthofthebuilding.Inabuildingwith9or10baysadeviationof+¼in.perbay(withintherollingtolerance)increasestheoveralllengthby2to2½in.Thebeamwillalsorequirea2-hourfirerating(sprayon)inaccordancewithmostbuildingcodes.Figure3Figure4Figure5Figure6Figure7Figure8Asaresultofthis,andwiththeinputofthearchitectontheprojectswehavecompleted,aconcretespandrelbeamwasdeveloped(Fig.8)tonotonlyprovideanincreasedleverarmtoresisttheappliedmoment,butalsotoeliminatefireproofingandatthesametimeserveaspartoftheexteriorfacade.Thisdistancebetweenconnectionsonthecolumnproducedaconsiderabledecreaseinthequantityoffieldbolts.Inaddition,byusingconcrete,thebeamcanalsofunctionaspartoftheexteriorfacade.FortheResortsHotelthisbeamwasastructuralcomponentonlyandcontainedinsertstosecurethewindowwallfacade.However,itdideliminatefield-appliedfireproofing.Shopworkwasnoteliminatedbythissubstitutionofconcreteforsteel.Toinsuretheaccuratefit,aspreviouslydiscussed,itwasdecidedtodevelopaspandrelrebarcagetobefabricatedintheshopandshippedtoaprecastcontractor,whowouldsettheminasteelformcontainingashopsuppliedsteeljigdesignedtomaintaintolerancesonlyasteelfabricatorcouldprovide.3—TrussesTheweightofatrussisdeterminedbygravityloads,windloads,penetrationsduetocorridor,doors,pipesand/orwindowopenings(Fig.9).Thetrussweightwillvaryfrom10,000lbs.ina4-storybuildingto36tonsina43-storybuildingontheResortproject.Trussfabricationisnotdifficult.Theyshouldbesetuponjugstomaintaincorrectoveralldimensions.However,qualifiedweldersarerequiredtofollowaqualityassuranceprogram(Fig.10).4—SecondarySupportingStructuresSecondarysupportingstructuresarethosenecessarytosupportstairopenings,elevatorshaftsandotherframedopeningsrequiredbythearchitecturaldesign.Thisisimportanttooverallcosts.Ifthearchitectrecognizesthistypeofstructuralsolutionwillbeused,hecandesignwithinitsconstraintsandavoidunnecessaryadditionalframing.(Figs.11and12).TheResortsprojectandtheTowersontheParkproject(Figs.13and14)requireadditionalsupportsteelduetoarchitecturalconfigurations.Theaddedsteelframingcanbedesignedasconventional(i.e.,simpleshear-connected)ortoresistappliedwindmomentsrequiringpartialmomentconnections.Thistypeofframingisfamiliartomostfabricationshops.5—FloorSystemsFloorsystemscanbethefollowingtypes,howeverallmustbedesignedtodeliverthewindloadasadiaphram(shearload)tothesupportingtrusses.Forthesteelfabricatortheyposenoproblems.Architecturally,depthofconstructionandmechanicalrequirementscanandwilldictatethetypeofsystemtobeused.Asanexample,the8-in,hollowcorefloordeckprovidesafloor-to-floorheightequaltoaflat-plateconcretesystem.Therefore,totalheightofthebuildingwillnotbeaffected.Figure9Figure10Figure11Figure12Figure13FIELDERECTIONThistypeofstructurehastobeerectedonafloor-by-floorsequencebecauseoftheinstabilitycausedbyafloorsystemnotinplace.Also,dependingontheheightofthebuilding,atowercranehastobeusedtoreachoverthespandrelbeams.Therefore,acranemusthavetherange(reach)andcapacity(pickingload)tocovertheentirefloorareaifthebuildingistoremainstable.Usingguycablesortemporarybracinginterfereswiththeerectionprocess.Guyderricks(thoseattachedtoandclimbinguponcolumns)andinternalclimbingcranes(supportedonthefloorstructure)cannotbeused,unlessthedesignprovidessupportareas—suchasanelevatorcorewhichcanbereinforcedtosupporttheadditionalloads.Forbuildingsuptoabout20storieswithafloorheightof8ft-9in.(atotalheightofabout190ft,atowercrane,selfsupported,truckorcat-mounted(similartoaManitowoc4100W),canbeused.Above20stories,useanexternalclimbingcrane(similartoLinkBeltTG-1900)supportedonsteeltowerunitsandconnectedtothestructureatintervalsof75ft,withtheinitialtie-inat160ft.Figures15,16,17and18showcranelocation,rangeetc.forerectionofHillsd1.Setcolumns2.Setspandrelbeamstotiecolumnsalongstrongaxis3.SettrussesFigure14Note:Connectionofthebottomchordshouldbeboltedtightafterthedeadloadisimposedfromthefloorsystem.Thebottomchordshouldbeshortenedapproximately3/16in.toallowforcamberreductionandsubsequentchordlengthening)4.Boltupandtorquehigh-tensionbolts5.The8-in.hollow-coreplanksarebolted(inlieuofweldplates)tothetruss(Fig.19).Thisprovidesanimmediatebracingsystem,workingplatformetc.betweentrusses.However,groutingthekeyjointsmustproceedtoprovidethesheardistributionpreviouslymentioned.Note:Duetotheinherentstiffnessofthetrusses,concretespandrelbeamsandhollow-corefloorplanks,thesestructurescansafelybeerectedforabout8–10floorsbeforegroutinghastobeinplace.Duringextremeweatherconditions—cold,rainetc.—erectioncanproceedeventhoughconcretecannotbepoured.Figure15Figure16Figure17Figure18Figure19BUILDINGCOSTSUsingabuildingcontaining13supportedlevels,9bays@25ftc.toc.longand56ft-1in.c.toc.mainbuildingcolumnscontainingabout170,000sqftofsupportedareausing6-in,thickconcretespandrelbeamsactingalsoastheexteriorfacadeand4-in.thickprecaBuildingCostBreakdown1.Structuralsteel2.Precastconcreteelements3.Miscellaneousitems4.Erection5.UnitpricepersqftItem1—StructuralsteelMaterialcostsWeight(lbs.)1.Anchorbolts5002.Columns&loosebaseplates600,0003.Trusses580,0004.MiscellaneousbeamsA.ElevatordividerbeamsB.EndspandrelbeamsC.RoofbeamsD.Penthouse&elevatormach.roombeams167,5005.Fieldbolts12,0006.Paint(ifrequired)7.ShippingTotalcost:1,360,000@$.30/lb.average1,360,000lbs.(8lb./sqft)=$408,000.Note:NomaterialdiscountshavebeenappliedShopdrafting—1,360hrs.@=27,2$20/hr.00.Shoplabor—Shoplaboraveraged15hrs./ton680tons×15H/T=10,200hrs.@$15/hr.=153,000.Totalitem1—steelfurnished&delivered$588,200.Item2Precastconcretecomponents1.Spandrelbeams(a)Rebarcages234pcs.@$400=$93,600(b)Casting22,340sqft@$9.50=212,230Totaldeliveredprice$305,830=$350,830.2.4-in.precastendwalls,12,826sqft@$6.50/sqft=83,369.3.8-in.hollow-coreplank,165,000sqft@$3.50/sqft=577,500.4.Connectionangles,plank&spandrelties,fieldbolts=10,000.$976,699.5.Groutprecastplanks(a)Plankjoints(b)Plankends,165,000sqft@$.50/sqft=82,500.6.CaulkingExteriorjointsInteriorplankjoints=25,000.TotalItem2—precastconcrete$1,084,199.Item3Miscellaneousitems(a)Fieldexpenses(b)Shop&fieldtesting=25.000.Item4Fieldlabor—EquipmentcraneManitowoc4100W(a)1.Cranesetup&removal2.Shippingto&fromsite3.70-tontruckcrane&crewtoassemble=40,000.Hoisting(b)Forthissizebuilding—shouldcompleteabout13,000sqft/floorinfourdays,includesa10%downfactorforweatherandmaintenance.Crane—3monthsrentalCranecrew—2men×60days=120man-daysHoist"—6men×60days=360""Bolt"—6men×60days=360""Stone"—4men×60days=240""Surveys—1man×60days=60""Super—1man×60days=60""Total1,200""@$315/day=378,000.TotalItem4—Fieldlabor418,000.TotalItems1,2,3,4Totalcost$2,115,399.×O&P15%317,310.Total$2,432,709.+SalestaxifrequiredItem5Unitpricepersquarefoot(a)Structuralframeconsistingof:1.Structuralsteel2.Concretespandrelbeams3.8-in.hollow-coreplank,includinggrouting(b)Architecturalcomponent(c)4-in.Precastendwalls,includingcaulking1.Totalcostofthesupportedframe=$2,284,343=$13.44sqft2.""""exteriorfacade=148,266=.87Total$2,432,609=$14.31sqftErectionallowance—approx.$31,760COSTFORFRAMINGTHERESORTSINTERNATIONALHOTELBuildingDescriptionAspreviouslyshown,thisbuildingcontained43supportedlevelsapprox.1,116,000sqft;420fthigh.Center(elev.core)—35ft×114ftTwoadjacentwings—68ft×157ft-6inTotalsteelweight—13,700tons,24.6lbs./sqft0Totalcostofthisproject,basedonacontractpriceof$22,100,000(includingperformancebondandsalestax),amountedto$19.80/sqft.Notes:1.FielderectioncommencedonMarch18,1985,withthefinalliftonMarch31,1986.Projectwasestimatedtotakeoneyearusingtwocranes,andincludingholidays,weatheranddowntime.2.Thestructureisplumbwithin5/8in.fromtopfloortogroundlevel—quiteanachievement(Fig.20).SUMMARYIthasbeenshowntheuseofthissystemforapartmentsandhotelsisfastandeconomical.Astructuralframe,includingtheexteriorfacade,canbecompletedasshownintwomonthsforacostequaltowhatwouldprovideonlyastructuralsupportframe,suchasflat-plateconcrete.Materialcostsarebasicallythesamethroughoutthecountry.However,theycanvaryduetomilldiscounts,freightetc.Thecostofshopproductionandfieldlaborcanvaryconsiderablyduetofactorssuchasunionvs.non-union,fringebenefits,insuranceetc.Toanalyzetheimpactofthissystemlocally,careshouldbeexercisedinconvertingtolocalwages.ResortsInternationalAtlanticCity,NewJerseyFigure20172ENGINEERINGJOURNAL/AMERICANINSTITUTEOFSTEELCONSTRUCTION关于钢结构交错桁架体系的一个观点钢结构交错桁架体系成为主要的支撑体系已经很多年了，工程中使用这种桁架体系是经济的，易于制造和安装，因此优于所有其他的支撑体系。这篇论文研究了一栋近期建造的完全使用了交错桁架建筑体系，该工程是新泽西州,大西洋城的一家国际度假酒店。制造任何从事结构钢生产的企业,只要他们拥有合格的焊工及焊接方面的技术,特们就能轻而易举地制造出这种结构.此外,建筑还必须具备架空吊车,能够将10至15吨重的桁架及柱吊至20层高处.该结构的制造包括下列构件(1)柱(2)拱侧梁(3)桁架(4)次柱及次梁(5)楼盖1-柱高达20层的建筑物,其柱子的制造并不复杂.它们被轧制成宽翼缘截面大可达W14×720,最长近25英尺,总重量达9至10吨每块(底版分散安装).30层以上建筑物的柱子是组合式柱,由3块厚度不一的板组成.我们所建的国际度假酒店,其翼缘板厚8英寸,腹板厚6英寸.这种柱需要精密的制造设备,如,预热装置,水下焊接设备以及具备25-40吨的携重能力.换句话说,企业必须具有处理重板及板面制造的能力.如图3,这些柱子底层具有重型支撑连接,此连接可将风荷转移到混泥土基座上.支撑桁架(如图4)作为一种转移风压的方法,如果对建筑特色不造成影响,可以用于底部.2-拱侧梁拱侧梁是用于抵抗衡附压在纵向尾墙上的风矩(见图5\6).为抵抗这股力量拱侧梁可以是钢板梁矩,由尾板的连接部分来连接的,或者带有焊剪板的翼缘板顶和底连接起来的.这种梁的制造不会出现任何问题.但是,由于辗轧耐度不同,在使用尾板时,柱上的匹配孔,翼缘板,或者是翼缘宽围必须经过校对,与梁相匹配,以确保柱心与柱心之间无误.这种梁控制着整个建筑物的长度.一栋建筑物有九个或十个壁洞(偏差为±1\4英寸),每个壁洞将使总长度从2英尺增加到2.5英尺.根据大多数建筑物的规则,梁也要求有2小时的耐火定额度。因此，再加上建筑设计师的努力，一根混泥土拱侧梁就不仅是提供了一根杠杆来抗拒应用矩，并能根除防火装置的使用，同时还可作为外用面的一部分。柱上的这种连接距离促使域螺栓的使用大大减少。此外，通过使用混泥土，此梁还可以作为外正面的一部分。拱侧梁只是度假酒店的一个构件，它包含了一些插入件以确保窗墙的正面。但是，它确实消除了防火装置的使用。用混泥土代替钢板并不能消除车间工作。为确保能够精确匹配，如前所述，决定在车间内研制一个拱侧箱，并将它运送给预制承包商。承包商会将它安入一个钢板格7，此钢板格具有工厂提供的用来保持韧性的钻模（只有钢板生产商才能提供）。3—桁架桁架的重量是由物荷、风荷以及由于走廊、门、管道及窗户的敞开而引起的穿透率所决定的（如图9）。其大小变化可以从4层楼建筑的10，000Ibs变到43层的度假酒店工程的36吨。桁架制造并不困难。桁架应该是竖立在壶状物上以保持准确的总面积大小。但是，合格的焊接工必须遵循质量保证程序，（如图10）4—次支撑结构次支撑结构就是那些在建筑设计上要求用来支撑楼梯接隙、电梯轴、以及其它框架物的附属物。次支撑结构对总成本至关重要。如果设计师意识到这种结构方法会被使用到，他的设计就能保持在限度范围之内，从而避免不必要的附加设计。度假酒店工程及公园工程中的塔（如图13、14），由于建筑设计构造都要求附加支撑钢板。附加支撑钢板结构可以设计成传统式的或者设计成抗风矩的。这种框架与大多数生产厂里的是相似的。5—楼盖楼盖可以是如下几种类型，但是设计上都必须能作为一种隔板传递风荷带支撑桁架上。这对钢板制造者并不会产生任何影响，从建筑上说，建设的深度及机械要求能够也会决定所使用的楼盖种类。例如，那块8英寸的空心楼板就能提供一种楼层与楼层间的高度（等同于混泥土楼盖）。因此，建筑物的总体高度不会受到影响。领域建筑物这种类型的结构必须按照一层接一层的顺序建筑，因为不适合的楼盖会造成不稳定所以必须这么做。也取决于建筑物的高度。一因为高楼起重机的使用，所以会超过拱侧梁。因此，如果想保持建筑物的稳定，一台起重机必须要有覆盖整个搂层面积的范围和能力。使用钢索电缆成临时的支撑装置介入建筑物进程。钢索起重机（那些附属的和向上的柱状物）以及内部上升起重机（支撑搂层结构）不能使用，除非设计提供支撑区域---比方说电梯中心，它能支撑额外的负荷。对于高达20层，每层高度为8ft-9in的建筑物来说（总高度大约为190ft）高楼起重机、自我支撑物、卡车或猫形拖架（类似于一个maintowoc4100w）可以被使用。20层以上，可使用一个外部上升起重机（类似于LinkBeltTG1900），它由钢筋塔单元支撑，在间隔为75ft原1）建柱2）建立拱侧梁用来沿着强轴系柱状物3）建立构架注意：在冷负荷从楼层体系中被加附以后，底部弦应该用螺栓紧紧连接。考虑到木材，底部弦应缩短到大约3\16in。建筑成本建筑一个包含13支撑程度，9堤@25ftctoc长和56ft-1建筑成本的分解1钢材2预制混泥土成分3各种条款4建筑物5每个sqft的单价条款1—钢材材料花费重量（1bs）1猫形螺栓5002柱及松木板600，0003构架580，0004各种梁A电梯隔离梁B端面拱侧梁C屋梁D电梯机器房＆电梯机器房梁167，5005领域螺栓12，0006油漆（如果有要求）7运输总成本：1，360，000＠＄.301lb平均1,360,000lbs(8lb./sqft)=$408,000.注意：没有任何材料折扣可供应用工厂制图----1,360hrs.@=27,2$20