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NEHRP RECOMMENDED PROVISIONSSEISMIC DESIGN OF STEEL STRUCTURES Context in NEHRP Recommended Provisions Steel behavior Reference standards and design strength Moment resisting frames Braced frames Other topics SummaryInstructional Material Complementing FEMA 451, Design ExamplesSteel Structures 10 - 1

Steel Design: Context in ProvisionsDesign basis: Strength limit stateUsing the 2003 NEHRP Recommended Provisions:Load combinationChap. 4Seismic load analysisChap. 5Components and attachments Chap. 6Design of steel structuresChap. 8AISC Seismicand othersInstructional Material Complementing FEMA 451, Design ExamplesSteel Structures 10 - 2

Seismic Resisting SystemsUnbraced Frames Joints are:Rigid/FR/PR/Moment-resisting Seismic classes are:Special/intermediate/Ordinary/not detailedBraced Frames Concentric bracing Eccentric bracingInstructional Material Complementing FEMA 451, Design ExamplesSteel Structures 10 - 3

Monotonic Stress-Strain BehaviorInstructional Material Complementing FEMA 451, Design ExamplesSteel Structures 10 - 4

Bending of Steel BeamStrainMStressφyExtreme fiber reachesyield strain and stressεyσyStrain slightly above yield strainεy ε εsh σySection near “plastic”φuεshInstructional Material Complementing FEMA 451, Design ExamplesσySteel Structures 10 - 5

Plastic Hinge FormationInstructional Material Complementing FEMA 451, Design ExamplesSteel Structures 10 - 6

Cross - section DuctilityConceptual moment - curvatureMMpMyφu φu ε u φy′ φy ε yφ y φ′yInstructional Material Complementing FEMA 451, Design ExamplesφφuSteel Structures 10 - 7

Moment CurvatureLaboratory Test -- Annealed W BeamInstructional Material Complementing FEMA 451, Design ExamplesSteel Structures 10 - 8

Behavior Modes For BeamsMrOLMOHIOJGOJEOJKElastic lateral tors. bucklingInelastic lateral tors. bucklingInelastic lateral tors. bucklingIdealized behaviorStrain hardeningInstructional Material Complementing FEMA 451, Design ExamplesSteel Structures 10 - 9

Flexural Ductility of Steel MembersPractical Limits123Lateral torsional bucklingBrace wellLocal bucklingLimit width-to-thickness ratiosfor compression elementsFractureAvoid by proper detailingInstructional Material Complementing FEMA 451, Design ExamplesSteel Structures 10 - 10

Local and Lateral BucklingInstructional Material Complementing FEMA 451, Design ExamplesSteel Structures 10 - 11

Lateral Torsional BucklingInstructional Material Complementing FEMA 451, Design ExamplesSteel Structures 10 - 12

Local BucklingbtClassical plate buckling solution:σ cr kπ 2 E12(1 μ )(b / t )22 σySubstituting μ 0.3 and rearranging:bkE 0.95tFyInstructional Material Complementing FEMA 451, Design ExamplesSteel Structures 10 - 13

Local BucklingcontinuedWith the plate buckling coefficient taken as 0.7 and an adjustmentfor residual stresses, the expression for b/t becomes:bE 0.38tFyThis is the slenderness requirement given in the AISC specificationfor compact flanges of I-shaped sections in bending. Thecoefficient is further reduced for sections to be used in seismicapplications in the AISC Seismic specificationbE 0.3tFyInstructional Material Complementing FEMA 451, Design ExamplesSteel Structures 10 - 14

Welded Beam to Column Laboratory Test - 1960sInstructional Material Complementing FEMA 451, Design ExamplesSteel Structures 10 - 15

Bolted Beam to Column Laboratory Test - 1960sInstructional Material Complementing FEMA 451, Design ExamplesSteel Structures 10 - 16

Pre-Northridge StandardInstructional Material Complementing FEMA 451, Design ExamplesSteel Structures 10 - 17

Following the 1994Northridge earthquake,numerous failures of steelbeam-to-column momentconnections were identified.This led to a multiyear,multimillion dollar FEMAfunded research effortknown as the SAC jointventure. The failures causeda fundamental rethinking ofthe design of seismicresistant steel momentconnections.Instructional Material Complementing FEMA 451, Design ExamplesSteel Structures 10 - 18

Bottom Flange Weld Fracture Propagating Through ColumnFlange and WebInstructional Material Complementing FEMA 451, Design ExamplesSteel Structures 10 - 19

Beam Bottom Flange Weld Fracture Causing a Column DivotFractureInstructional Material Complementing FEMA 451, Design ExamplesSteel Structures 10 - 20

Northridge FailureBeamweb Crack through weld Note backup barand runoff tabInstructional Material Complementing FEMA 451, Design ExamplesBottom flangeof beamSteel Structures 10 - 21

Northridge FailureBeam flangeand webColumnflangeBackup barInstructional Material Complementing FEMA 451, Design ExamplesSteel Structures 10 - 22

Northridge FailuresWeldWeld FusionColumn FlangeHAZInstructional Material Complementing FEMA 451, Design ExamplesColumn DivotLamellar TearSteel Structures 10 - 23

Flexural Mechanics at a Joint221121Cross SectionsFw12FyFyBeam MomentFwFw Z1 Fy Z2Instructional Material Complementing FEMA 451, Design ExamplesSteel Structures 10 - 24

Welded Steel Frames Northridge showed serious flaws. Problemscorrelated with:- Weld material, detail concept and workmanship- Beam yield strength and size- Panel zone yield Repairs and new design- Move yield away from column face(cover plates, haunches, “dog bone”)- Verify through tests SAC Project: FEMA Publications 350 through 354Instructional Material Complementing FEMA 451, Design ExamplesSteel Structures 10 - 25

Moment (MN-m)Reduced Beam Section (RBS) Test SpecimenSAC Joint Venture420-2-4-4-2024Plastic Rotation (% rad)Graphics courtesy of Professor Chia-Ming Uang, University of California San DiegoInstructional Material Complementing FEMA 451, Design ExamplesSteel Structures 10 - 26

T-stub Beam-Column TestSAC Joint VenturePhoto courtesy of Professor Roberto Leon, Georgia Institute of TechnologyInstructional Material Complementing FEMA 451, Design ExamplesSteel Structures 10 - 27

T-Stub Failure MechanismsPlastic hinge formation -- flange andweb local bucklingNet section fracture in stem of T-stubPhotos courtesy of Professor Roberto Leon, Georgia Institute of TechnologyInstructional Material Complementing FEMA 451, Design ExamplesSteel Structures 10 - 28

T-Stub Connection Moment Rotation t (kN-m)Moment (k-in)FS-03 - Moment/Rotation0.06Rotation (rad)Graphic courtesy of Professor Roberto Leon, Georgia Institute of TechnologyInstructional Material Complementing FEMA 451, Design ExamplesSteel Structures 10 - 29

Extended Moment End-Plate Connection ResultsPhoto courtesy of Professor Thomas Murray, Virginia TechInstructional Material Complementing FEMA 451, Design ExamplesSteel Structures 10 - 30

25000250002000020000Moment at Column Centerline (in-kips)Moment at Column Centerline (in-kips)Extended Moment End-Plate Connection -0.08-0.06-0.04-0.020.000.020.04Total Rotation (rad)(a) Moment vs Total 8Total Plastic Rotation (rad)(b) Moment vs Plastic RotationGraphics courtesy of Professor Thomas Murray, Virginia TechInstructional Material Complementing FEMA 451, Design ExamplesSteel Structures 10 - 31

Ductility of Steel Frame JointsLimitsWelded Joints- Brittle fracture of weld- Lamellar tearing of base metal- Joint design, testing, and inspectionBolted Joints- Fracture at net cross-section- Excessive slipJoint Too Weak For Member- Shear in joint panelInstructional Material Complementing FEMA 451, Design ExamplesSteel Structures 10 - 32

Multistory FrameLaboratory TestInstructional Material Complementing FEMA 451, Design ExamplesSteel Structures 10 - 33

Flexural DuctilityEffect of Axial Loade P 0PyM pcMpL 7.0rx 1 .0W12x36e 5.10"P 0.55PyMpce 1.75"P 0.76PyMpcMpMp 0.57 0.28Instructional Material Complementing FEMA 451, Design ExamplesSteel Structures 10 - 34

Axial StrutLaboratory testL 45rInstructional Material Complementing FEMA 451, Design ExamplesSteel Structures 10 - 35

Cross Braced FrameLaboratory testInstructional Material Complementing FEMA 451, Design ExamplesSteel Structures 10 - 36

Tension Rod (Counter) BracingConceptual BehaviorHHΔ“Slapback”For cycle 2Instructional Material Complementing FEMA 451, Design ExamplesSteel Structures 10 - 37

Eccentrically Braced FrameInstructional Material Complementing FEMA 451, Design ExamplesSteel Structures 10 - 38

Eccentrically Braced FrameLab test of linkInstructional Material Complementing FEMA 451, Design ExamplesSteel Structures 10 - 39

Steel Behavior Ductility- Material inherently ductile- Ductility of structure ductility of material Damping- Welded structures have low damping- More damping in bolted structures dueto slip at connections- Primary energy absorption is yielding ofmembersInstructional Material Complementing FEMA 451, Design ExamplesSteel Structures 10 - 40

Steel Behavior Buckling- Most common steel failure under earthquake loads- Usually not ductile- Local buckling of portion of member- Global buckling of member- Global buckling of structure Fracture- Nonductile failure mode under earthquake loads- Heavy welded connections susceptibleInstructional Material Complementing FEMA 451, Design ExamplesSteel Structures 10 - 41

NEHRP Recommended ProvisionsSteel Design Context in NEHRP Recommended Provisions Steel behavior Reference standards and design strengthInstructional Material Complementing FEMA 451, Design ExamplesSteel Structures 10 - 42

Instructional Material Complementing FEMA 451, Design ExamplesSteel Structures 10 - 43

Using Reference StandardsStructural SteelBoth the AISC LRFD and ASD methodologies arepresented in a unified format in both theSpecification for Structural Steel Buildings and theSeismic Provisions for Structural Steel Buildings.Instructional Material Complementing FEMA 451, Design ExamplesSteel Structures 10 - 44

Instructional Material Complementing FEMA 451, Design ExamplesSteel Structures 10 - 45

Other Steel MembersSteel Joist InstituteStandard Specifications, 2002Steel CablesASCE 19-1996Steel Deck InstituteDiaphragm Design Manual, 3rd Ed., 2005Instructional Material Complementing FEMA 451, Design ExamplesSteel Structures 10 - 46

NEHRP Recommended ProvisionsSteel Design Context in NEHRP Recommended Provisions Steel behavior Reference standards and design strength Moment resisting framesInstructional Material Complementing FEMA 451, Design ExamplesSteel Structures 10 - 47

Steel Moment Frame structional Material Complementing FEMA 451, Design ExamplesSteel Structures 10 - 48

Steel Moment Frame Jointsa bMu M p bFy* Ry Fya b 1 1.7 Fy*Fu F Z bAf d*yInstructional Material Complementing FEMA 451, Design ExamplesSteel Structures 10 - 49

Panel ZonesSpecial and intermediate momentframe: Shear strength demand:Basic load combinationφRyMp of beamsor Shear capacity equation Thickness (for buckling) Use of doubler platesInstructional Material Complementing FEMA 451, Design ExamplesSteel Structures 10 - 50

Steel Moment Frames Beam shear: 1.1RyMp gravity Beam local buckling- Smaller b/t than LRFD for plastic design Continuity plates in joint per tests Strong column - weak beam rule- Prevent column yield except in panel zone- Exceptions: Low axial load, strong stories, topstory,and non-SRS columnsInstructional Material Complementing FEMA 451, Design ExamplesSteel Structures 10 - 51

Steel Moment Frames Lateral support of column flange- Top of beam if column elastic- Top and bottom of beam otherwise- Amplified forces for unrestrained Lateral support of beams- Both flanges- Spacing 0.086ryE/FyInstructional Material Complementing FEMA 451, Design ExamplesSteel Structures 10 - 52

Prequalified ConnectionsSee FEMA 350: Recommended Seismic Design Criteria forNew Steel Moment-Frame Buildings-Welded Unreinforced Flange-Welded Free Flange Connection-Welded Flange Plate Connection-Reduced Beam Section Connections-Bolted Unstiffened End Plate Connection-Bolted Stiffened End Plate Connection-Bolted Flange Plate ConnectionSee ANSI/AISC 358-05, Prequalified Connections for Special andIntermediate Steel Moment Frames for Seismic Applications-Reduced Beam Section Connections-Bolted Stiffened and Unstiffened Extended Moment End Plate ConnectionsInstructional Material Complementing FEMA 451, Design ExamplesSteel Structures 10 - 53

Welded CoverplatesInstructional Material Complementing FEMA 451, Design ExamplesSteel Structures 10 - 54

Reduced Beam Section (RBS)Instructional Material Complementing FEMA 451, Design ExamplesSteel Structures 10 - 55

Extended End PlateInstructional Material Complementing FEMA 451, Design ExamplesSteel Structures 10 - 56

Excellent Moment Frame BehaviorInstructional Material Complementing FEMA 451, Design ExamplesSteel Structures 10 - 57

Excellent Moment Frame BehaviorInstructional Material Complementing FEMA 451, Design ExamplesSteel Structures 10 - 58

Excellent Moment Frame BehaviorInstructional Material Complementing FEMA 451, Design ExamplesSteel Structures 10 - 59

Special Moment FramesExample5 at 25'-0"7 at 25'-0"NInstructional Material Complementing FEMA 451, Design ExamplesSteel Structures 10 - 60

Special Moment FramesThe following design steps will be reviewed: Select preliminary member sizes Check member local stability Check deflection and drift Check torsional amplification Check the column-beam moment ratio rule Check shear requirement at panel zone Select connection configurationInstructional Material Complementing FEMA 451, Design ExamplesSteel Structures 10 - 61

Special Moment Fra