Rules of Thumb - Structural Engineering Other Technical Topics FAQ - Eng-Tips
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Preliminary design Rules of thumb 1. The cost of reinforce con crete (in place) is usually somewhe re betwee n $100/ m3 a nd $800 /m3. This illustrate s the fact that for a "rule of thumb" to be any good, the background fo r its development nee ds to be known. That in turn means that most of the "rules of thumb" are most applicable by the engineer that came up with them; and that everybody else better be careful in using them. 2. If placing conc rete directly from a truck or con crete pump, place conc rete ver tically into the face of concrete alre ady in place. Never allow the concrete to fall more than 1 to 1.5 metres. 3. The poss ible spa ns, and ass oci ate d depths, depend on the loading to which the bea m is s ubjected. The figures given assume 'normal' commercial building loads. They do not apply to more heavily loaded situations (e.g. plant rooms) or to unconventional loading scenarios. 4. This info rma tion is given withou t prej udic e and is for guidanc e purposes only. It is suitable for possibly initial sizing of str uctural elements for architectura l scheme or costing purposes for actual build ing projects the size o f str uctural eleme nts must be ver i?ed through detailed design by a quali?ed structural engineer. 5. Prepar e dr awings proper ly & a ccu rat ely if possible la bel ea ch bar a nd show its s hape for clar ity 6. Indicate proper cover-clea r cover, nomin al cover or effective cover to r einforcement. 7. Decide detailed loc ation of opening /ho le and supply adequate details for r einforcements around the opening s. 8. Use common ly available size of bars and spirals. For a single structural member the number of different size s of bars shall be kept to a minimum. 9. Show enlarged details at corners, inte rsection s of walls, bea ms and column joint and at similar situations. 10. Congestion of bars s houl d be a voided at points wher e member s inter sect and mak e cer tain that all re in. Can be properly placed. 11. In the case of bund led bar s, lapped splice of bundl ed bar s s hall be m ade by splicing on e ba r a t a time; such individual splices within the bundle shall be staggered. 12. Make sure that hoo ked a nd bent up bar s can be placed a nd have adequate con crete protection. 13. Indicate all expansion, construction and con tra ction joints on plans and provide de tails for s uch joints. 14. The location of co nstruction joints s hall be a t the poin t of minimum s hear approximate ly at mid or nea r the mid points. It shall be formed vertically and not in a sloped manner. DO NOT'S-GENERAL: 15. Bonded reinforcement shall not e xtend acr oss a n expans ion joint and the brea k betw een the s ections shall be complete. 16. Flexura l r einforcement prefer ably s hall not be ter minated in a tension z one. 17. Bars larger than 36mm dia. Shall not be bundl ed. 18. Lap s plic es shall be not be use d for bar s la rger than 36mm dia except whe re welded. 19. Where dowels ar e provided, their diameter shall not excee d the diame ter of the column bars by more than 3mm. 20. Primar y movement j oints ar e r equired to pr event cracking where buildings (or parts of buildings) are la rge, wher e a building spans different ground conditions, changes height considerably or where the shape suggests a point of natural weakness. Without detailed calculation, joints should be detailed to permit 15–25 mm movement unless seismic pounding is an issue then this should be increased to min 200mm. Advice on joint spacing for different building types can be variable and conflicting. While rules of thumb are provided be sure to seek guidance of an experienced engineer. Expansion joint is a movement (functional) joint which is installed to accommodate volume change due to temperature changes, shrinkage, and change in moisture content.. The other members of this family of joints are: o Contr ol (contraction) joints o Shrinkage str ips. Acco rding to MAR K FINT EL the use of Expansion joints in a build ing is a con trover sial issue. There is a grea t divergence of opinion concerning the importance of expansion joints in concrete construction. Some experts recommend joint spacing's as low as 30 ft while others consider expansion joints entirely unnecessary. Joint spacing's of roughly 100 to 200 ft for concrete structures seem to be typical ranges recommended by various authorities. For steel structures a spacing of 200 ft is normal. However, the spacing is most of the times also dictated by the following factors which determine the location of such joints: • New building adj oining exis ting building • Long low building abutt ing higher building • Wings adjoining main str ucture • Long low connecting wings betwee n buildings • Inter sections at wings of 'L', 'T', or 'U' sha ped build ings. • So il stra ta in length of structu re vary in nature -- better to have join ts (suc h situation is ve ry rare )
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Transcript of Rules of Thumb - Structural Engineering Other Technical Topics FAQ - Eng-Tips
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PreliminarydesignRulesofthumb1.Thecostofreinforceconcrete(inplace)isusuallysomewherebetween$100/m3and$800/m3.Thisillustratesthefactthatfora"ruleofthumb"tobeanygood,thebackgroundforitsdevelopmentneedstobeknown.Thatinturnmeansthatmostofthe"rulesofthumb"aremostapplicablebytheengineerthatcameupwiththemandthateverybodyelsebetterbecarefulinusingthem.2.Ifplacingconcretedirectlyfromatruckorconcretepump,placeconcreteverticallyintothefaceofconcretealreadyinplace.Neverallowtheconcretetofallmorethan1to1.5metres.3.Thepossiblespans,andassociateddepths,dependontheloadingtowhichthebeamissubjected.Thefiguresgivenassume'normal'commercialbuildingloads.Theydonotapplytomoreheavilyloadedsituations(e.g.plantrooms)ortounconventionalloadingscenarios.4.Thisinformationisgivenwithoutprejudiceandisforguidancepurposesonly.Itissuitableforpossiblyinitialsizingofstructuralelementsforarchitecturalschemeorcostingpurposesforactualbuildingprojectsthesizeofstructuralelementsmustbeveri?edthroughdetaileddesignbyaquali?edstructuralengineer.5.Preparedrawingsproperly&accuratelyifpossiblelabeleachbarandshowitsshapeforclarity6.Indicatepropercoverclearcover,nominalcoveroreffectivecovertoreinforcement.7.Decidedetailedlocationofopening/holeandsupplyadequatedetailsforreinforcementsaroundtheopenings.8.Usecommonlyavailablesizeofbarsandspirals.Forasinglestructuralmemberthenumberofdifferentsizesofbarsshallbekepttoaminimum.9.Showenlargeddetailsatcorners,intersectionsofwalls,beamsandcolumnjointandatsimilarsituations.10.Congestionofbarsshouldbeavoidedatpointswheremembersintersectandmakecertainthatallrein.Canbeproperlyplaced.11.Inthecaseofbundledbars,lappedspliceofbundledbarsshallbemadebysplicingonebaratatimesuchindividualspliceswithinthebundleshallbestaggered.12.Makesurethathookedandbentupbarscanbeplacedandhaveadequateconcreteprotection.13.Indicateallexpansion,constructionandcontractionjointsonplansandprovidedetailsforsuchjoints.14.Thelocationofconstructionjointsshallbeatthepointofminimumshearapproximatelyatmidornearthemidpoints.Itshallbeformedverticallyandnotinaslopedmanner.
DONOT'SGENERAL:15.Bondedreinforcementshallnotextendacrossanexpansionjointandthebreakbetweenthesectionsshallbecomplete.16.Flexuralreinforcementpreferablyshallnotbeterminatedinatensionzone.17.Barslargerthan36mmdia.Shallnotbebundled.18.Lapsplicesshallbenotbeusedforbarslargerthan36mmdiaexceptwherewelded.19.Wheredowelsareprovided,theirdiametershallnotexceedthediameterofthecolumnbarsbymorethan3mm.20.Primarymovementjointsarerequiredtopreventcrackingwherebuildings(orpartsofbuildings)arelarge,whereabuildingspansdifferentgroundconditions,changesheightconsiderablyorwheretheshapesuggestsapointofnaturalweakness.Withoutdetailedcalculation,jointsshouldbedetailedtopermit1525mmmovementunlessseismicpoundingisanissuethenthisshouldbeincreasedtomin200mm.Adviceonjointspacingfordifferentbuildingtypescanbevariableandconflicting.Whilerulesofthumbareprovidedbesuretoseekguidanceofanexperiencedengineer.Expansionjointisamovement(functional)jointwhichisinstalledtoaccommodatevolumechangeduetotemperaturechanges,shrinkage,andchangeinmoisturecontent..Theothermembersofthisfamilyofjointsare:oControl(contraction)jointsoShrinkagestrips.
AccordingtoMARKFINTELtheuseofExpansionjointsinabuildingisacontroversialissue.Thereisagreatdivergenceofopinionconcerningtheimportanceofexpansionjointsinconcreteconstruction.Someexpertsrecommendjointspacing'saslowas30ftwhileothersconsiderexpansionjointsentirelyunnecessary.
Jointspacing'sofroughly100to200ftforconcretestructuresseemtobetypicalrangesrecommendedbyvariousauthorities.Forsteelstructuresaspacingof200ftisnormal.
However,thespacingismostofthetimesalsodictatedbythefollowingfactorswhichdeterminethelocationofsuchjoints:
NewbuildingadjoiningexistingbuildingLonglowbuildingabuttinghigherbuildingWingsadjoiningmainstructureLonglowconnectingwingsbetweenbuildingsIntersectionsatwingsof'L','T',or'U'shapedbuildings.Soilstratainlengthofstructurevaryinnaturebettertohavejoints(suchsituationisveryrare)
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Differenttypeoffoundationsinstructure(i.e.buildingfoundedpilesandraft)
21.Actualconcretedeflectionsareinfluencedbymanyfactorswhichcannotbefullytakenintoaccount.Tensilestrengthofconcreteachangeinstrengthfrom2.7to2.1canincreasedeflectionsby50%Modulusofconcrete+/20%EarlyconstructionloadingShrinkagewrappingAlwaysrememberloadcanonlybeestimatedandevendeadloadscannotusuallybecalculatedtowithin5%accuracy.Withthisinmindnotcalculationneedhavemorethan2significantfigures.
22.Forhighrisksfacilitiessuchaspublicandcommercialtallbuildings,designconsiderationsagainstextremeevents(bombblast,highvelocityimpact)isveryimportant.ItisrecommendedthatguidelinesonabnormalloadcasesandprovisionsonprogressivecollapsepreventionshouldbeincludedinthecurrentBuildingRegulationsandDesignStandards.Requirementsonductilitylevelsalsohelpimprovethebuildingperformanceundersevereloadconditions.
Concrete:InitialEstimations:"Todesignevenasimplysupportedbeam,thedesignerneedstoguessthebeamsizebeforehecanincludeitsselfweightintheanalysis."
BEAMS:OVERALLDEPTHOFBEAMS:
MEMBERSPAN/OVERALLDEPTHRATIOreinforcedMaxrecommendedspanSPAN/OVERALLDEPTHRATIOPrestressed/posttensionedMaxrecommendedspanRectangularBEAMwidth>250mmorspan/15butlessthan5D.10TO14(con't2026)8m(con't12m)132012m(con't15m)Flangedbeams1218(con't1821)cantilever265mBandbeams(b?3D)orspan/51820812253014(18mcon't)FornonbandbeamsConcretebeams:2to1depthtowidthratiothewidth(b)ofarectangularbeamshouldbebetween1/3and2/3oftheeffectivelength(d).Thelargerfractionisusedforrelativelylargerdesignmoments.
BandwidthL/4toL/3maxatslabsoffit(Lforaveragetransversespan)Iwouldnormallytaperthesidesofthebandwitha1to1.5'taperfortheL/25ora2'taperfortheL/33tosaveweightbutthatdependsonformworkcosts)BanddepthL/25toL/30(Lforlongitudinalspan)
NotesNote:1.Beamsneedmoredepthtofitsufficientreinforcinginsectionsocheckdetailingearly2.Themaximumspanslistedherearenotabsolutelimits.Longerspansarepossiblewitheverytype,butmaynotbeeconomical.Asaruleofthumbforestimatesofthicknessabovethespanondeflectionratio'sshouldbemultipliedby"maximumrecommendedspan"/"actualspan")3.Thehighernumberaregivenforlightloadings(about1.5kpa)andthelowernumbersforheavyloadings(about10kpa)
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Commonbandbeamwidthsare1200,1800and2400
bForflangedsectionswiththeratiooftheflangetotheribwidthgreaterthan3,theTablevalueforbeamsshouldbemultipliedby0.8.cFormembers,otherthanflatslabpanels,whichsupportpartitionsliabletobedamagedbyexcessivedeflectionofthemember,andwherethespanexceeds7m,theTablevalueshouldbemultipliedby7/span.dForflatslabswherethegreaterspanexceeds8.5m,theTablevalueshouldbemultipliedby8.5/span.eThevaluesmaynotbeappropriatewhentheformworkisstruckatanearlyageorwhentheconstructionloadsexceedthedesignload.Inthesecasesthedeflectionmayneedtobecalculatedusingadviceinspecialistliterature.Sizing:Fornoncantilevers:d(mm)=span(mm)/26+300,roundtheresulttonearest25mm.Forcantilevers:d(mm)=span(mm)/7+300,roundtheresulttonearest25mm.Fornoncantilevers:Ifspan
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Note:Themaximumspanslistedherearenotabsolutelimits.Longerspansarepossiblewitheverytype,butmaynotbeeconomical.forflatplatesandflatslabswithdroppanels,thelongerofthetwoorthogonalspansisusedinthedeterminationofthespantodepthratio,whileforedgesupportedslabs,theshorterspanisused.minimumfireresistancenormallyrequireadepthofatleast125mm,150isbestforfittingthereoin.Thehigherspantodepthratiosareforlightloadings(about1.5kPa)andthelowerspantodepthratio'sforheavyloadings(about10kpa).Thespansassumeroughly1.50kPaforsuperimposeddeadloading(SDL).NotesFortwowayspanningslabs(supportedonbeams),thecheckontheratioofspan/effectivedepthshouldbecarriedoutontheshorterspan.Forflatslabs,thelongerspanshouldbetaken.Forflangedsectionswiththeratiooftheflangetotheribwidthgreaterthan3,theTablevalueforbeamsshouldbemultipliedby0.8.Formembers,otherthanflatslabpanels,whichsupportpartitionsliabletobedamagedbyexcessivedeflectionofthemember,andwherethespanexceeds7m,theTablevalueshouldbemultipliedby7/span.Forflatslabswherethegreaterspanexceeds8.5m,theTablevalueshouldbemultipliedby8.5/span.Thevaluesmaynotbeappropriatewhentheformworkisstruckatanearlyageorwhentheconstructionloadsexceedthedesignload.Inthesecasesthedeflectionmayneedtobecalculatedusingadviceinspecialistliterature.
1.Provideamaxspacingof250mm(8")formainreinforcementinordertocontrolthecrackwidthandspacing.2.Amin.of0.24%shallbeusedfortheroofslabssinceitissubjectedtohighertemperature.Variationsthanthefloorslabs.Thisisrequiredtotakecareoftemp.Differences.3.Spansaredefinedasbeingfromcentrelineofsupporttocentrelineofsupport.Althoughsquarebaysaretobepreferredongroundsofeconomy,architecturalrequirementswillusuallydictatethearrangementoffloorlayoutsandthepositioningofsupportingwallsandcolumns.Pinnedsupportsareassumed.4.Particularattentionisdrawntotheneedtoresolvelateralstability,andthelayoutofstairandservicecores,whichcanhaveadramaticeffectonthepositionofverticalsupports.Servicecorefloorstendtohavelargeholes,greaterloadsbutsmallerspansthanthemainareaoffloorslab.5.Eliminatingdropsresultsinsimplerfalseworkandformworkarrangements,enablesrapidfloorconstructionandgivingmaximumflexibilitytotheoccupier.6.Thebenefitsofusinginsituconcreteflatslabconstructionshouldbeinvestigatedattheconceptualdesignstage.Considernotonlythebenefitsintermsofpotentialdesignefficienciesbutalsothemajoradvantagesfortheoverallconstructionprocess,notablyinsimplifyingtheinstallationofservicesandthesavingsinconstructiontime.7.Tooptimisetheslabthickness,considerallfactorssuchasthemethodofdesign,thepresenceorabsenceofholes,theimportanceofdeflections,andpreviousexperience.8.Deflectionswillgenerallybegreatestatthecentreofeachpanel.However,aspartitionsmaybeplacedalongcolumnlines,itisusualtocheckdeflectionsherealso.Thepossibleeffectofdeflectionsoncladdingshouldalsobeconsideredcarefully.Edgethickenings,upstandanddownstandbeamsshouldbeavoided,astheydisrupttheconstructionprocess.9.Thereisevidencethatearlystrikingandearlyloadingthroughrapidfloorconstructionhassomeimpactonlongtermdeflections.10.Thinflatslabconstructionwillalmostcertainlyrequirepunchingshearreinforcementatcolumns.11.Minimumrecommendedthicknessforslabsforfireis120mm12.Drapeslabtendonstohighpointsatthefacesofthebandsattheslabsoffitandrunflatoverbandwidthatminimumtopcover,exceptatendcolumnswheretheyaredrapedtothecenterlineofthecolumnandcentroidoftheslab(Dslab/2fromthetopsurface).13.Whenopeningsinfloorsorroofsarerequiredsuchopeningsshouldbetrimmedwherenecessarybyspecialbeamsorreinforcementsothatthedesignedstrengthofthesurroundingfloorisnotundulyimpairedbytheopening.Dueregardshouldbepaidtothepossibilityofdiagonalcracksdevelopingatthecornersofopenings.Theareaofreinforcementinterruptedbysuchopeningsshouldbereplacedbyanequivalentamount,halfofwhichshouldbeplacedalongeachedgeoftheopening.Forflatslabs,openingsinthecolumnstripsshouldbeavoided.14.Whenisitanadvantagetousefabricmeshinsuspendedconcreteslabs?Designingslabswithmeshreinforcementisapropositionthatcanproducesubstantialcostsavingswhenconsiderationisgiventothefollowingpoints.
a)Meshshouldbesufficientforshrinkagecontrolwithoutadditionalreinforcement.
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b)Thestructuralsystemshouldbepredominantlya"onewayslab"system,withextrabarsatthemaximummoment.
c)Lappingofmeshshouldbeminimal.
d)Lappinglocationsshouldbeclearlydocumentedsoastoeliminateanypossibilityoftopandbottomlapsbeingcoincidentandsoastomaximiseusageandminimisecutting.
e)Lappingshouldbeachievedusingbarsplicessothateachmeshremainsinthesameplane.
f)Meshlengthsshouldbefactorofthesheetlengthpreferablyusingfullsheetstominimisewastage.
PRESTRESS:1.Maximumlengthofslab50m,bondedorunbounded,stressedfrombothends.25m,bonded,stressedfromoneendonly.2.MeanprestressTypicallyP/A:Slabs:0.72.5mPaBeams:1.03.0mPa3.CoverTakeminimumcovertobe25mm.4.Allowsufficientcoverfor(atleast)nominalbendingreinforcementoverthecolumns,inbothdirections(typicallyT16barsineachdirection).5.EffectofrestrainttofloorshorteningAllconcreteelementsshrinkduetodryingandearlythermaleffectsbut,inaddition,prestressingcauseselasticshorteningandongoingshrinkageduetocreep.Stiffverticalmemberssuchasstabilitywallsrestrainthefloorslabfromshrinking,whichpreventstheprestressfromdevelopingandthusreducingthestrengthofthefloor.Thisshouldbeconsideredinthedesignofthestabilitysystemorallowedforinthemethodofconstruction
COLUMNS:1.Usehighergradeofconcretewhentheaxialloadispredominant.2.Goforhighersectionpropertieswhenthemomentispredominant.9.Restrictthemaximum%ofreinforcementtoabout3%.Inaninsitucolumntheabsolutemaximumreinforcementis6%or10%atlaps.3.Approximatemethodforallowingformoments:multiplytheaxialloadfromthefloorImmediatelyabovethecolumnbeingconsidered)by:1.25interiorcolumns(allowsforpatternloading)1.50edgecolumns2.00cornercolumnsButtrykeepthecolumnstoconstantsizeforthetoptwostoreys.4.Preliminarysizingbesttoaimforcoulmnswith1to2%reinfcoing
oColumnH/1020,oedgecolumnsH/79,ocornercolumnH/68oAccanbeestimatedforstockycolumnsbyAc=N/15(1%reo),Ac=18(2%reo)orN/20(3%reo)forN32concrete.(Ninnewtons)5.Acolumnshouldhaveminimumsection200250sq,ifitisnotanobligatorysizecolumn.6.Inaddition,allcolumnsshallbedesignedforminimumeccentricityequalto[(unsupportedlengthofcolumn/500)+(lateraldimension/30]subjecttominimumeccentricityof20mm.7.Areinforcedcolumnshallhaveatleastsixbarsoflongitudinalreinforcementforusingintransversehelicalreinforcement.forCIRCULARsections.8.Aminfourbarsoneateachcornerofthecolumninthecaseofrectangularsections.9.Keepouterdimensionsofcolumnconstant,asfaraspossible,forreuseofforms.10.Whendoesacolumnchangetowall?generallythisconsideredatabout4timesthethickness,howeverforfirepurposesifthefirecangettoallfoursidesitshouldbeconsideredasacolumn.11.Forserviceloadkeepthetotalstressabout0.3fc'forgravityloadkeepthisabout0.15f'c.12.REINFORCEDCONCRETECOLUMNS:Sizing:Forpreliminarydesignusesquarecolumns.Ifthebuildingheightis3storiesorless:
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Ifbeamspan
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1.Withthefollowingreinforcement:100mmonelayerSL72WWF6x6W1.4xW1.4125150mmonelayerSL82WWF6x6W2.9xW2.9180mmonelayerSL92WWF6x6W2.9xW2.9200mmtwolayers(SL72)WWF6x6W1.4xW1.42.Jointsspacing:Ruleofthumbis24to36timesthethickness(givesthespacinginmm/inches).Keepthethicknessvariationto5mm(1/4")/+10mm(3/8"),andthesubgradeasflataspossiblewithnoabruptchangegreaterthan15mm(1/2")inabout1.2m(4').Thiskeepsthe"sudden"restraintpotentialdown.3.Forslabs180mmgreateracrackinitiatorissuggested.4.Nogreaterthan21m(70')betweenDowelsawnjoints5.Nogreaterthan8m(27')betweenjoints6.Makeyourjointpatternassquareaspracticableandwithnomorethana2to1ratiooflengthtowidth.7.AlwayscheckwithclientattopreferredJointssystem8.ForsmallmediumprojectsMaxareainpour:600m^2(60000sqft)Maxjointsspacingforexternalexpansionjoints4050m,internalstoppourslabjoints:60m(200').9.ForlargeprojectMaxareainpour:800m^2aimfor600m^2(60000sqft)Maxjointsspacingforexternalexpansionjoints60m(200'),internalstoppourslabjoints:80m(250')
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STEELBUILDINGS(NONCOMPOSITE)ChoiceofbeamsystemElementTypicalSpan/depthTypicalSpan(m)Span/depthMaximumspanPlategirder101225m(80')FloorJoist(steelonly)1769m(2030')CastellatedUB's*14171220m(4070')Rooftrusses(pitch>20)141517m(55')SpaceFrames1530100m(300')Primarybeams(supportedbycolumns)101512m(40')Secondarybeams(supportedbyotherbeams)152510m(33')Portalframeleg354060m(200')Simplespanrafter2430m(100')Simplespanroofbeam1525m(80')Con'tbeamorjoist.85*simplespanvalue*AvoidifhighpointloadsincreaseIreqby1.3Asaruleofthumb,thedefectionofacastellatedbeamisabout25%greaterthanthedeflectionofanequivalentbeamwiththesamedepthbutwithoutwebopenings.
ExpansionjointsSteelindustrialbuildings100mtypical150mmaximumc/c.Steelcommercialbuildings50mtypical100mmaximumc/cSteel/Tiltupbuilding50mtypicalSteelroofsheeting20mc/cdowntheslope,nolimitalongtheslope.1.Deeperischeaper.Allotherthingsbeingequal,adeeperwideflangebeamwillbemoreeconomicalthanashallowerone.Thereareexceptionstothisrule,butitisgenerallycorrect.2.Onecommonmisconceptionisthatallsteelmembershaveanactualdepthequaltotheirnominaldepth.3.Thebracesmaybedesignedforacapacityof2%oftheforceresistedbythecompressionportionofthebeam.
4.Cantilever"onetothree".Cantileveredsteelbeamsarecommonlyusedtosupportarchitecturalfeatures,givingtheillusionofanunsupportedor"flying"edge.Howfaristoofarforacantilevertostickout?Generallyspeaking,ifacantileverexceeds1/3ofthetotalbackspan,economyislostandmayleadtodesigndifficulties.Soifyourbeamhasa9m(30')backspan,trytokeepanadjacentcantilevertolessthan3m(10')long.
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5.TRUSS:Anoptimaldepth/spanratioforaplanartrussisapproximately1/10.AlthoughforcesintheCHORDSdecreasewithincreasingdepth,forcesintheWEBarepracticallyUNCHANGEDandincreasingthedepthincreasesthelengthsofthesemembers.ApproximatelyhalfthewebmembersareinCOMPRESSIONandincreasingtheirlengthsreducestheirefficiencyduetotheincreasedsusceptibilitytoBUCKLING.
6.StrutsandtiesSlendernesslimits:Membersresistingloadotherthanwind:8#180Membersresistingselfweightandwindonly:8#250Membersnormallyactingasatiebutsubjecttoloadreversalduetowind:8#350
STORIESLATERALLOADRESISTINGSYSTEM
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(a)Connectionsimplicity(b)Numberofmembers(c)Numberofboltsand/oramountoffieldwelding(d)Sizeandefficiencyoferectioncrew,andtheequipmentattheirDisposal(e)Timelysupplyofsteel.
17.Theuseofthetransportationlengthmayhavetobecurtailedtoavoiddamageduringtransportationtrytokeepmemberlengthslessthan15m.18.Oneruleofthumbfor?lletweldsonbothfacesoppositeeachotheristomakethegussetthicknesstwicetheweldsize.
19.Simpleconnectionsusegrade8.8,20mmdiameterboltsfinplates}t=8mmforUB's
