Basic principles of steel structures - sals.org.cn

1 1 Basic principles of steel structuresDr. Xianzhong concept & rationalemetallurgySteel fabricationMember + connectionerectionstructural systemLoadsactionStrength, stability , rigidityForce, deformationS RPerformance:Safety, serviceability, durabilityAdvantages:..Structural steelOutlines requirement of properties for structural steel factors affecting steel properties state of stress and stress concentration fatigue failure steel grade, steel products & steel selection recent advances of structural steelRequirement of steel propertiesuni-axial tensile test: preparation;tensile-test specimenSample location: transverse or longitudinalSpecimen: shape and dimension;loading methodRate of stressing;temperatureroom temperature/A 0/ limitLinear elasticYield stressPerfectly plasticStrain-hardeningUltimate stressLateral contractionNeckingFractureModulus of elasticityProof stressTrue stress,,,,yuyffE ,/,/,/py u tcffE d dE ==, p Requirement of steel propertiesuni-axial tensile test: stress-strain diagramYield strain is 10 to 15 times proportional strain for mild steel or low-alloy steel .

2 How about the ultimate strain?Nominal stress= conventional stress= engineering stressTrue stress2 Requirement of steel properties;strengthProportional limit, yield point, tensile strengthYield-to-tensile strength ratio ( ~ for mild steel );ductilityPercentage elongation at failure, reduction of area;toughnessstatic toughness, impact toughness;cold-formingcold bent test;weldabilityfor construction durabilitycorrosion resistance, fatigue resistanceRequirement of steel propertiesductility: elongation & reduction of area;ductility: occur remarkable residual strain (plastic deformation) without fracture after stress exceeding yield point;percentage elongation at failure;percentage reduction of areatrue assessment criteria but difficult to measurethrough-thickness propertyRequirement of steel propertiestoughness: static & impact toughness;impact toughness: measure of impact resistance or the ability to absorb sudden increase in stress at notch.

3 Charpy V-notch testtemperature-dependentstrength + plasticityRequirement of steel propertiescold-forming ability;cold-forming property: ability to resist crack while producing plastic deformation under cold-forming work;Cold-bent testassess ductility and weldabilityRequirement of steel propertiesweldability;weldabilityConstru ction: no crack in welds and HAZ area under normal weld conditionUsage: mechanical properties of welds and HAZ area are not less than the base metal;Carbon EquivalentRequirement of steel propertiesdurability;corrosion resistance30-40% demolishedCorrosion-resist steel : Cu-P-Ti-Re;fatigue resistanceNo statement here3 Factors affecting steel properties;chemical compositionFe, C, Si, Mn, V, S, P, O, N, others;process of metallurgySmelt, cast, rolling, residual stress while cooling;time effecttime - hardening;cold workhistory of strain & stress, cold work;temperatureelevated temperature and low temperature;rate of stressingthe higher rate, the higher stressFactors affecting steel propertieschemical composition nitrogen oxygen phosphorus Sulfur --- vanadium-( )-( )-- manganese---- silicon Carbon Irondurabilityweldabilitytoughnessductil itystrengthFactors affecting steel properties process of steelmakingFactors affecting steel properties process of steelmakingFactors affecting steel propertiesprocess of steelmaking;produce - smeltBasic oxygen furnace, electric-arc furnace;casting (ladle treatment)Rimming steel (Mn)Semikilled steelKilled steel (Si)Ultra-killed steel ;hot rolling1200-1300 degreetransverse & longitudinal;residual stress while coolingFactors affecting steel propertiestime-effect and cold work hardening;time effectStrength increase, ductility decrease, crispyHeat treatment;cold work hardeningHistory of stressingCold work: cut, punch,roll, press, fold, drill,plane, strike.

4 Rate of stressingthe higher rate, the higher stress4 Factors affecting steel propertiestemperature;elevated temperature250 C, brittle600 C, softFactors affecting steel propertiestemperature;low temperaturetoughness decrease suddenlyBrittle failuretransitionductileEnergy absorbed AkTemperatureTransition temperatureat steepest slopeState of stress and stress concentrationcombined stress and criteria of yield;equivalent stress (von Mises stress))(3222222zxyzxyxzzyyxzyxzs +++ ++=()()()[]21323222121 + + =zsPlane stress?Stress in beam?;criteria of elastic failure1st: The maximum-principal-stress theory2nd: The maximum-principal-strain theory3rd: The maximum-shear-stress theoryThe maximum-strain-energy theory4th: The maximum-distortion-energy theoryvon Mises theory (4th):ElasticityPlasticity(perfect elasto-plastic model)State of stress and stress concentrationcombined stress and criteria of yieldyzsf< yzsf= yfufState of stress and stress concentrationyielding under biaxial or triaxial stresses1 1 yfUni-axial stressBi-/tri-axial stress with same signBi-/tri-axial stress with different signEffects of biaxial or triaxial stresses on yieldingState of stress and stress concentrationyielding under biaxial or triaxial stressesYielding under biaxial stresses: any stress is smaller than yield strength ?

5 Pure shear = 5n State of stress and stress concentrationstress concentration: definitionThe uniform stress pattern is disruption and the intensity of stress increases greatly within a very short distance. The condition is described as STRESS is due to the abrupt changes in geometry caused by imperfection of structural steel or manufacture. It includes holes, grooves, notches, keyways, threads, or abrupt changes in plate width or concentration factor:o m nmK =State of stress and stress concentrationstress concentration and brittle failureConstant thicknessxx,, Anxnxn,, ASteel bar with abrupt change of width at middleAA<nxnxxnx, >>xx,, Ayxnxny = < =0,0nynx>> xySubjected to axial tensile load in x-direction, thenLateral strain is obtained by poisson s ratio,Lateral contraction at middle is larger than that at the ends, therefore, the lateral stress at middle must be tensile stressAbrupt change in width at middle stress concentration biaxial tensile stress field brittle failureState of stress and stress concentrationstress concentration and change of The more abrupt the geometry change,the severer the stress concentration,the larger the increase of tensile strength .

6 The worse the capacity of plastic deformationState of stress and stress concentrationstress concentration and perfect plasticity of steelFor a material with perfect elastoplastic constitutive relation, once the peak stress reaches yield point, its stress will remain this value and the stress concentration factor will 1212o2 o1 m1 ym2f 1p2p222111 KKnmnm=>= Idealized stress-strain plot for an elastoplastic materialState of stress and stress concentrationstress beyond yield point: low-cycle fatigue yfyfy2fyfyf ;hysteretic curves of steel ;stress/cycle hardening;Bauschinger Effect;plastic energy absorbedFatigue failurehigh-cycle fatigue: Basic concepts;fatigue failure: steel material subjected to dynamic loads is likely to fail at a lower stress than when the same loads are applied statically, especially when the loads are repeated for alarge number of cycles. tNumber of cycles to failureNominal stress at failure6 Fatigue failurehigh-cycle fatigue: failure mechanism;Progressive fracture:imperfection of material (local damage) microscopic crack forms (crack initiation) crack gradually enlarges (crack propagation) crack becomes unstable andsudden fracture of steel occurs(unstable crack growth and fracture);Characteristic of fractureFatigue failurehigh-cycle fatigue: Basic concepts;fatigue life: number of stress cycles to failure under certain cycle symbol.

7 ;Factors governing fatigue failuretypes of stress (tensile, compressive, shear or combined stress)cycle symbolnumber of cycles to failurestress concentrationresidual stresssurface conditionrange of stressFatigue failurefatigue research I: max. stress methodp ;research backgroundSmall steel specimen for fatigue tests;be applicableNon-welded elements;main conceptsfatigue life = cycle numbers to failurefatigue strength : ( ) the max. stress (absolute value) fatigue life under specified cycle symbolendurance limit = threshold stressstress ratio: t + 1 t + t + 1 t + max t + min max max min min 1 = 10 < <0 =01 <<1 =maxmin =Definition ofmax maxmin/ =max min 1 = 1 =1 0 yfPmax =min 011K =Pmax0minK ==+P0max1K == While design, fatigue life N is a definite value for a designated structural detailpmax Fatigue failurefatigue research I: max. stress method;Engineering application:Goodman diagramfor a specified stress ratioand fatigue life:NP Endurance limit (threshold stress)(fatigue strength ).

8 Const =Fatigue failurefatigue research II: stress range method;research backgroundFull-scale structural specimen for fatigue tests;be applicablewelded elements;main conceptsfatigue failure is governed bystress range of the part of theelement, rather than stress ratiostress range: t + 1 t + t + 1 t + t + 1 = 10 <<0 =01 <<1 = 0 =0 minmax = :stress ratio:yield stress of steel ;structural detailsFatigue failurefatigue research II: stress range method;Engineering application: /1)/(][Nc= t + 1 t + t + 1 t + t + 1 = 10 <<0 =01 <<1 = 0 =0 Built-up I beam withsingle-layer flange platelgNlg lgBA lg N = 1B=A lg c, A =Built-up I beam withdouble-layer flange plates7 Fatigue failurefatigue research II: stress range method;Mechanism of stress range methodStress range and residual stress = = =cr, =Fatigue failurefatigue failure under variable amplitude stress cycles;fatigue under constant and variableamplitude stress cycles;Miner criteria:1N= iint +i j iniNjnN t + jStructural steelclassification & steel grade;carbon structural steelQ195, Q215, Q235, Q255, Q275;low alloy structural steelQ295, Q345, Q390, Q420, Q490;quality carbon structural steel31 types, 20, 45;quality structural steel wireshigh-strengthStructural steelsteel grade: carbon structural steel ;common used: Q235;product quality documentationMechanical properties: yield strength , tensile strength , elongationChemical composition: C, Mn, Si, S, P;expressed by:quality grade: A, B, C, Ddeoxidization: F, b, Z, TZQ235A[F,b,(z)] No Ce GuaranteeQ235B[F,b,(z)] 20 C, Ak 27JQ235C [(z)] 0 C, Ak 27JQ235D [(TZ)] -20 C, Ak 27 Jfy= 235 MPafu= 400 MPafvy= ?

9 E = ?Structural steelsteel grade: high- strength low alloy structural steel ;common used: Q345, Q390, Q420 (alloy <5%);product quality documentationMechanical properties: yield & tensile strength , elongation, cold workChemical composition: C, Mn, Si, S, P, V, Nb, Ti;expressed by:quality grade: A, B, C, D, Edeoxidization: Z, TZ, plus heat treatmentQ345/390/420A No toughness GuaranteeQ345/390/420B 20 C, Ak 34JQ345/390/420C 0 C, Ak 34JQ345/390/420D -20 C, Ak 34JQ345/390/420E-40 C, Ak 27JQ345:fy= 345 MPafu= ? 470~630 MPafvy= ?E = ?Structural steelsteel grade: quality carbon steel (wires);quality carbon steelHeat treatment: thermal refining, temperingAdvantages: less impurity, less imperfectionClassification: 31 types, 20, 45 for high- strength bolts;quality structural steel wires (rope)wires: quality carbon steel cold workhigh strength : 1570~1770 MPaexpressed by: 6 7, 8 198 Structural steelsteel products (shapes); steel plateSteel sheet: ~ 1mm ~ 4mmThicker steel plate: rolled steel plate ~20mm, thicker plate 20~60mmSuper-thick steel plate: >60mmFlat steel : 12~200mm width;(hot rolled) shaped steelI-section, channel, angle, H-section, T-section, tube;cold-formed thin-wall sectionsangle, channel, Z-section, hat-section, tubular;welded sections (built-up)I-section, box-sectionStructural steeldesign value of steel strength (design index);expression of design indexASD (Allowable Stress Design):LRFD (limit states design - factor).

10 Determination of design valueAs mentioned in Introduction Part;design value of structural steelsteel plate: steel grade, thicknesscast- steel : just authorizedweld: butt weld / fillet weldbolt: ultimate strengthKf/][y= Ry/ ff=Structural steelprinciples for selection of structural steel ;structural types and importanceImportant / normal / secondary;characteristic of loadsStatic load / dynamic loadPeriod of actions;connection methodsweld / bolt;temperature ( steel located)north / south, indoor / outdoor;stress conditiontension / compression, through-thickness loadingRecent advances ofStructural steel ;fire-resistant structural steel ;corrosion-resistant structural steel ;high- strength structural steelHigh- strength , weldability (Ceq)Through-thickness steel ;ultra-low yield point steel ;structural casting ( steel );high friction factor steel ~ advances of Structural steelfire-resistant structural steel ;traditional structural steel600 C:remain one-third strength ;fire-resistant steel600 C:remain two-third strengthRecent advances of Structural steelcorrosion-resistant structural steel ;additional chemical constituentCu, Cr, Mn, RE;USA Cro-Ten steel2~8 times corrosion resistance;Baosteel 09 CuPTiRE2~3 times corrosion resistance;stainless steelStrength, ductility, weldability9 Recent advances of Structural steelthrough-thickness structural steel ;thickness >100mm;excellent toughnesscolumnbeamRecent advances of Structural steelultra-low yield point structural steel ;low yield point: 100~120 MPa;tensile strength : 250~260 MPa;elongation: approximate 60%;aim: keep key members out of plasticityRecent advances of Structural steelstructural casting steel