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Michael D. Engelhardt University of Texas at Austin Basic Concepts in Ductile Detailing Basic Concepts in Ductile Detailing of Steel Structures 1 EAC 2013

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Overview of Presentation What is Ductility ? Why is Ductility Important ? How Do We Achieve Ductility in Steel Structures ? Ductility in Seismic-Resistant Design 2

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What is Ductility ? Ductility: The ability to sustain large inelastic deformations without significant loss in strength. Ductility = inelastic deformation capacity - material response - structural component response (members and connections) - global frame response Ductility: 3

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F Δ F Δ F yield Ductility 4

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F Δ F Δ Ductility = Yielding How is ductility developed in steel structures ? Loss of load carrying capability: Instability Fracture 5

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Why is Ductility Important? Permits redistribution of internal stresses and forces Increases strength of members, connections and structures Permits design based on simple equilibrium models Results in more robust structures Provides warning of failure Permits structure to survive severe earthquake loading 6

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Why Ductility ? Permits redistribution of internal stresses and forces Increases strength of members, connections and structures Permits design based on simple equilibrium models Results in more robust structures Provides warning of failure Permits structure to survive severe earthquake loading 7

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General Philosophy for Earthquake-Resistant Design Objective:Prevent loss of life by preventing collapse in the extreme earthquake likely to occur at a building site. Objectives are not to: - limit damage - maintain function - provide for easy repair Design Approach: Survive earthquake by providing large ductility rather than large strength 8

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H Available Ductility MAX H elastic 3/4 *H elastic 1/2 *H elastic 1/4 *H elastic Required Strength H Δ 9

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H MAX H elastic 3/4 *H elastic 1/2 *H elastic 1/4 *H elastic H Δ Observations: We can trade strength for ductility 10

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H MAX H elastic 3/4 *H elastic 1/2 *H elastic 1/4 *H elastic H Δ Observations: Ductility = Damage 11

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H MAX H elastic 3/4 *H elastic 1/2 *H elastic 1/4 *H elastic H Δ Observations: The maximum lateral load a structure will see in an earthquake is equal to the lateral strength of the structure 12

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How Do We Achieve Ductility in Steel Structures ? 13

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Achieving Ductile Response.... Ductile Limit States Must Precede Brittle Limit States 14

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Example double angle tension member gusset plate P P Ductile Limit State:Gross-section yielding of tension member Brittle Limit States:Net-section fracture of tension member Block-shear fracture of tension member Net-section fracture of gusset plate Block-shear fracture of gusset plate Bolt shear fracture Plate bearing failure in double angles or gusset 15

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double angle tension member P P Example:Gross-section yielding of tension member must precede net section fracture of tension member Gross-section yield: P yield = A g F y Net-section fracture: P fracture = A e F u 16

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double angle tension member P P P yield P fracture A g F y A e F u The required strength for brittle limit states is defined by the capacity of the ductile element = yield ratio Steels with a low yield ratio are preferable for ductile behavior 17

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double angle tension member P P Example:Gross-section yielding of tension member must precede bolt shear fracture Gross-section yield: P yield = A g F y Bolt shear fracture: P bolt-fracture = n b n s A b F v 18

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double angle tension member P P P yield P bolt-fracture The required strength for brittle limit states is defined by the capacity of the ductile element The ductile element must be the weakest element in the load path 19

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double angle tension member P P Example : Bolts: 3 - 3/4" A325-X double shear A b = 0.44 in 2 F v = x 120 ksi = 68 ksi P bolt-fracture = 3 x 0.44 in 2 x 68 ksi x 2 = 180 k Angles: 2L 4 x 4 x 1/4 A36 A g = 3.87 in 2 P yield = 3.87 in 2 x 36 ksi = 139 k 20

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double angle tension member P P P yield P bolt-fracture P yield = 139 k P bolt-fracture = 180 k OK What if the actual yield stress for the A36 angles is greater than 36 ksi? Say, for example, the actual yield stress for the A36 angle is 54 ksi. 21

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double angle tension member P P P yield P bolt-fracture P yield = 3.87 in 2 x 54 ksi = 209 k P bolt-fracture = 180 k P yield P bolt-fracture Bolt fracture will occur before yield of angles non-ductile behavior 22

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double angle tension member P P P yield P brittle Stronger is not better in the ductile element (Ductile element must be weakest element in the load path) For ductile response: must consider material overstrength in ductile element 23

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double angle tension member P P P yield P brittle The required strength for brittle limit states is defined by the expected capacity of the ductile element (not minimum specified capacity) P yield = A g R y F y R y F y = expected yield stress of angles 24

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Achieving Ductile Response.... Ductile Limit States Must Precede Brittle Limit States Define the required strength for brittle limit states based on the expected yield capacity of ductile element The ductile element must be the weakest in the load path Unanticipated overstrength in the ductile element can lead to non-ductile behavior. Steels with a low value of yield ratio, F y / F u are preferable for ductile elements 25

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Achieving Ductile Response.... Connection response is generally non-ductile..... Connections should be stronger than connected members 26

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Achieving Ductile Response.... Be cautious of high-strength steels 32

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Ref: Salmon and Johnson - Steel Structures: Design and Behavior General Trends: As F y Elongation (material ductility) F y / F u 33

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Achieving Ductile Response.... Be cautious of high-strength steels High strength steels are generally less ductile (lower elongations) and generally have a higher yield ratio. High strength steels are generally undesirable for ductile elements 34

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Achieving Ductile Response.... Use Sections with Low Width-Thickness Ratios and Adequate Lateral Bracing 35

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M MpMp Increasing b / t Effect of Local Buckling on Flexural Strength and Ductility M 36

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0.7 M y Moment Capacity p r Width-Thickness Ratio (b/t) MpMp Plastic Buckling Inelastic Buckling Elastic Buckling hd Ductility 37

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Moment Capacity p r Width-Thickness Ratio (b/t) MpMp Plastic Buckling Inelastic Buckling Elastic Buckling hd Ductility Slender Element Sections M y

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p r Width-Thickness Ratio (b/t) MpMp Plastic Buckling Inelastic Buckling Elastic Buckling hd Noncompact Sections 39 Moment Capacity Ductility 0.7 M y

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p r Width-Thickness Ratio (b/t) MpMp Plastic Buckling Inelastic Buckling Elastic Buckling hd Compact Sections 40 Moment Capacity Ductility 0.7 M y

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p r Width-Thickness Ratio (b/t) MpMp Plastic Buckling Inelastic Buckling Elastic Buckling hd Highly Ductile Sections (for seismic design) 41 Moment Capacity Ductility 0.7 M y

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Local buckling of noncompact and slender element sections 42

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Local buckling of moment frame beam with highly ductile compactness ( < hd )

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Local buckling of a shear yielding EBF link with highly ductile compactness ( < hd )

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Effect of Local Buckling on Ductility For highly ductile flexural response: Example: W-Shape b f t f h t w 47

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Beam Flanges Beam Web Highly Ductile Compactness: 48 Highly Ductile Compactness:

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Lateral Torsional Buckling Lateral torsional buckling controlled by: L b = distance between beam lateral braces r y = weak axis radius of gyration LbLb LbLb Beam lateral braces 49

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M MpMp Increasing L b / r y Effect of Lateral Torsional Buckling on Flexural Strength and Ductility: M 50

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Effect of Lateral Buckling on Ductility For highly ductile flexural response: For F y = 50 ksi: 52

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Achieving Ductile Response.... Recognize that buckling of a compression member is non-ductile 53

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P cr P P 54

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Experimental Behavior of Brace Under Cyclic Axial Loading P W6x20 Kl/r = 80 55

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How Do We Achieve Ductile Response in Steel Structures ? Ductile limit states must precede brittle limit states Ductile elements must be the weakest in the load path Stronger is not better in ductile elements Define Required Strength for brittle limit states based on expected yield capacity of ductile element Avoid high strength steels in ductile elements Use cross-sections with low b/t ratios Provide adequate lateral bracing Recognize that compression member buckling is non-ductile Provide connections that are stronger than members 56

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How Do We Achieve Ductile Response in Steel Structures ? 57

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Ductile Detailing for Seismic Resistance High Ductility Steel Systems for Lateral Resistance: Special Moment Frames Special Concentrically Braced Frames Eccentrically Braced Frames Buckling Restrained Braced Frames Special Plate Shear Walls 58

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Choose frame elements ("fuses") that will yield in an earthquake. Detail "fuses" to sustain large inelastic deformations prior to the onset of fracture or instability (i.e., detail fuses for ductility). Design all other frame elements to be stronger than the fuses, i.e., design all other frame elements to develop the capacity of the fuses. Ductile Detailing for Seismic Resistance 59

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Special Moment Frame (SMF) Ductile fuse: Flexural yielding of beams 60

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Inelastic Response of a Special Moment Frame Fuse: Flexural Yielding of Beams Detail beam for ductile flexural response: no high strength steels low b/t ratios (highly ductile ) beam lateral bracing (per seismic req'ts) 61

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Inelastic Response of a Special Moment Frame Design all other frame elements to be stronger than the beam: Connections Beam-to-column connections Column splices Column bases Column buckling capacity Column flexural capacity 62

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Special Concentrically Braced Frame Ductile fuse: tension yielding of braces. 64

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Inelastic Response of an SCBF Tension Brace: Yields ( ductile ) Compression Brace: Buckles ( nonductile ) 65

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Inelastic Response of CBFs under Earthquake Loading Compression Brace (previously in tension): Buckles ( nonductile ) Tension Brace (previously buckled in compression): Yields ( ductile ) Connections, columns and beams: designed to be stronger than braces 66

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Eccentrically Braced Frames (EBFs) Ductile fuse: Shear yielding of links 70

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Link 71

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Inelastic Response of an EBF 72

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Braces, beam segments outside of link, columns, connections: designed to be stronger than link 74

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Buckling-Restrained Braced Frames (BRBFs) Ductile fuse: Tension yielding and compression yielding of Buckling Restrained Braces 75

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Buckling-Restrained Brace Buckling- Restrained Brace: Steel Core + Casing Casing Steel Core 76

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Buckling-Restrained Brace Buckling- Restrained Brace: Steel Core + Casing A A Section A-A Steel Core Debonding material Casing Steel jacket Mortar 77

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Inelastic Response of BRBFs 78

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Tension Brace: Yields Compression Brace: Yields 79

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Compression Brace: Yields Tension Brace: Yields Connections, columns and beams: designed to be stronger than braces 80

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Special Plate Shear Walls (SPSW) Ductile fuse: Tension field yielding of web panels 81

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Shear buckling Development yielding along tension diagonals Inelastic Response of a SPSW Columns, beams and connections: designed to be stronger than web panel 82

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Ductile Fuses: Special Moment Frames: Beams: Flexural Yielding Special Concentrically Braced Frames: Braces: Tension Yielding Eccentrically Braced Frames: Links: Shear Yielding Buckling Restrained Braced Frames: Braces: Tension and Compression Yielding Special Plate Shear Walls: Web Panels: Tension Field Yielding 83

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Summary Ductility = Inelastic Deformation Capacity Ductility Important in all Structures Ductility Key Element of Seismic Resistance 84

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Summary Achieving Ductility - Simple Rules avoid high strength steels use sections with low b/t's and adequate lateral bracing design connections and other brittle elements to be stronger than ductile members For seismic-resistant structures: Follow AISC Seismic Provisions 85

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