Presentation on theme: "Basic Concepts in Ductile Detailing"— Presentation transcript:
1Basic Concepts in Ductile Detailing of Steel StructuresEAC 2013Michael D. EngelhardtUniversity of Texas at Austin
2Overview of Presentation What is Ductility ?Why is Ductility Important ?How Do We Achieve Ductility in Steel Structures ?Ductility in Seismic-Resistant Design
3Ductility = inelastic deformation capacity What is Ductility ?Ductility:The ability to sustain large inelastic deformations without significant loss in strength.Ductility = inelastic deformation capacityDuctility:- material response- structural component response (members and connections)- global frame response
5How is ductility developed in steel structures ? FΔDuctility = YieldingFLoss of load carrying capability:InstabilityFractureΔ
6Why is Ductility Important? Permits redistribution of internal stresses and forcesIncreases strength of members, connections and structuresPermits design based on simple equilibrium modelsResults in more robust structuresProvides warning of failurePermits structure to survive severe earthquake loading
7Why Ductility ? Permits redistribution of internal stresses and forces Increases strength of members, connections and structuresPermits design based on simple equilibrium modelsResults in more robust structuresProvides warning of failurePermits structure to survive severe earthquake loading
8General 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 repairDesign Approach: Survive earthquake by providing large ductility rather than large strength
9H Δ H Helastic Available Ductility Required Strength 3/4 *Helastic MAX
10H Δ H Helastic Observations: We can trade strength for ductility MAX
11H Δ H Helastic Observations: Ductility = Damage 3/4 *Helastic MAX
12H Δ H Helastic Observations: The maximum lateral load a structure will see in an earthquake is equal to the lateral strength of the structure1/4 *HelasticMAX
13How Do We Achieve Ductility in Steel Structures ?
14Ductile Limit States Must Precede Brittle Limit States Achieving Ductile Response....Ductile Limit States Must Precede Brittle Limit States
15Example P P gusset plate double angle tension member Ductile Limit State: Gross-section yielding of tension memberBrittle Limit States: Net-section fracture of tension memberBlock-shear fracture of tension memberNet-section fracture of gusset plateBlock-shear fracture of gusset plateBolt shear fracturePlate bearing failure in double angles or gusset
16double angle tension member PPExample: Gross-section yielding of tension member must precede net section fracture of tension memberGross-section yield:Pyield = Ag FyNet-section fracture:Pfracture = Ae Fu
17P P Pyield Pfracture Ag Fy Ae Fu double angle tension member The required strength for brittle limit states is defined by the capacity of the ductile elementPyield PfractureAg Fy Ae FuSteels with a low yield ratio are preferable for ductile behavior= yield ratio
18double angle tension member PPExample: Gross-section yielding of tension member must precede bolt shear fractureGross-section yield:Pyield = Ag FyBolt shear fracture:Pbolt-fracture = nb ns Ab Fv
19Pyield Pbolt-fracture double angle tension memberPPThe required strength for brittle limit states is defined by the capacity of the ductile elementPyield Pbolt-fractureThe ductile element must be the weakest element in the load path
20P P Example: Bolts: 3 - 3/4" A325-X double shear double angle tension memberPPExample:Bolts: /4" A325-X double shearAb = 0.44 in2 Fv = x 120 ksi = 68 ksiPbolt-fracture = 3 x 0.44 in2 x 68 ksi x 2 = 180kAngles: 2L 4 x 4 x 1/4 A36Ag = 3.87 in2Pyield = 3.87 in2 x 36 ksi = 139k
21Pyield Pbolt-fracture double angle tension memberPPPyield Pbolt-fracturePyield = 139kPbolt-fracture = 180kOKWhat 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.
22Pyield Pbolt-fracture Pyield Pbolt-fracture double angle tension memberPPPyield Pbolt-fracturePyield = 3.87 in2 x 54 ksi = 209kPbolt-fracture = 180kPyield Pbolt-fractureBolt fracture will occur before yield of anglesnon-ductile behavior
23P P Pyield Pbrittle Stronger is not better in the ductile element double angle tension memberPPPyield PbrittleStronger 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
24P P Pyield Pbrittle Pyield = Ag RyFy double angle tension memberPPPyield PbrittleThe required strength for brittle limit states is defined by the expected capacity of the ductile element (not minimum specified capacity)Pyield = Ag RyFyRy Fy = expected yield stress of angles
25Ductile Limit States Must Precede Brittle Limit States Achieving Ductile Response....Ductile Limit States Must Precede Brittle Limit StatesDefine the required strength for brittle limit states based on the expected yield capacity of ductile elementThe ductile element must be the weakest in the load pathUnanticipated overstrength in the ductile element can lead to non-ductile behavior.Steels with a low value of yield ratio, Fy / Fu are preferable for ductile elements
26Connection response is generally non-ductile..... Achieving Ductile Response....Connection response is generally non-ductile.....Connections should be stronger than connected members
32Be cautious of high-strength steels Achieving Ductile Response....Be cautious of high-strength steels
33Ref: Salmon and Johnson - Steel Structures: Design and Behavior General Trends:As FyElongation (material ductility)Fy / FuRef: Salmon and Johnson - Steel Structures: Design and Behavior
34Be cautious of high-strength steels Achieving Ductile Response....Be cautious of high-strength steelsHigh strength steels are generally less ductile (lower elongations) and generally have a higher yield ratio.High strength steels are generally undesirable for ductile elements
35Achieving Ductile Response.... Use Sections with Low Width-Thickness Ratios and Adequate Lateral Bracing
36M q Effect of Local Buckling on Flexural Strength and Ductility Mp Increasing b / t
37Mp Moment Capacity Ductility Plastic Buckling Inelastic Buckling 0.7 MyElastic BucklinghdpWidth-Thickness Ratio (b/t)rDuctility
38Mp Moment Capacity Ductility Plastic Buckling Inelastic Buckling 0.7 MyElastic BucklingSlender Element SectionshdpWidth-Thickness Ratio (b/t)rDuctility
39Mp Moment Capacity Ductility Plastic Buckling Inelastic Buckling 0.7 MyElastic BucklingNoncompact SectionshdpWidth-Thickness Ratio (b/t)rDuctility
40Mp Moment Capacity Ductility Plastic Buckling Inelastic Buckling 0.7 MyElastic BucklingCompact SectionshdpWidth-Thickness Ratio (b/t)rDuctility
41Mp Moment Capacity Ductility Plastic Buckling Inelastic Buckling 0.7 MyElastic BucklingHighly DuctileSections (for seismic design)hdpWidth-Thickness Ratio (b/t)rDuctility
42Local buckling of noncompact and slender element sections
43Local buckling of moment frame beam with highly ductile compactness ( < hd ) .....
55Experimental Behavior of Brace Under Cyclic Axial Loading W6x20 Kl/r = 80P
56How Do We Achieve Ductile Response in Steel Structures ? Ductile limit states must precede brittle limit statesDuctile elements must be the weakest in the load pathStronger is not better in ductile elementsDefine Required Strength for brittle limit states based on expected yield capacity of ductile elementProvide connections that are stronger than membersAvoid high strength steels in ductile elementsUse cross-sections with low b/t ratiosProvide adequate lateral bracingRecognize that compression member buckling is non-ductile
57How Do We Achieve Ductile Response in Steel Structures ?
58Ductile Detailing for Seismic Resistance High Ductility Steel Systems for Lateral Resistance:Special Moment FramesSpecial Concentrically Braced FramesEccentrically Braced FramesBuckling Restrained Braced FramesSpecial Plate Shear Walls
59Ductile Detailing for Seismic Resistance 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.
60Special Moment Frame (SMF) Ductile fuse: Flexural yielding of beams
61Inelastic Response of a Special Moment Frame Fuse: Flexural Yielding of BeamsDetail beam for ductile flexural response:no high strength steelslow b/t ratios (highly ductile )beam lateral bracing (per seismic req'ts)
62Inelastic Response of a Special Moment Frame Design all other frame elements to be stronger than the beam:ConnectionsBeam-to-column connectionsColumn splicesColumn basesColumn buckling capacityColumn flexural capacity
64Special Concentrically Braced Frame Ductile fuse: tension yielding of braces.
65Inelastic Response of an SCBF Tension Brace: Yields (ductile)Compression Brace: Buckles (nonductile)
66Inelastic 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
69An X-braced CBF with wide flange braces that have buckled in-plane. Note that depending on the orientation of the bracing member and the end connection details, braces may buckle either in-plane or out-of-plane.
70Eccentrically Braced Frames (EBFs) Ductile fuse: Shear yielding of linksEccentrically Braced Frames (EBFs) are braced frames, consisting of beams, columns, and braces. Unlike conventional concentrically braced frames, an intentional eccentricity is introduced in EBFs, so that the brace-beam-column centerlines do not meet at a single point. The framing is arranged so as to isolate a short segment of beam at one of each brace. This isolated beam segment is called a "link."EBFs are designed so that inelastic action occurs within the links. The links, in turn, can be designed and detailed to provide very high levels of ductility. The braces, columns and beam segments outside of the links are designed to remain essentially elastic, by designing these elements to be stronger than the links.A well designed EBF can combine high stiffness in the elastic range (similar to a concentrically braced frame), with high ductility in the inelastic range (similar to a moment resisting frame). Because of the high elastic stiffness of an EBF, beam and column sizes are generally much smaller than in a moment resisting frame.
80Compression Brace: Yields Tension Brace: YieldsConnections, columns and beams: designed to be stronger than braces
81Special Plate Shear Walls (SPSW) Ductile fuse: Tension field yielding of web panels
82Inelastic Response of a SPSW Development yielding along tension diagonalsShear bucklingColumns, beams and connections: designed to be stronger than web panel
83Ductile Fuses: Special Moment Frames: Beams: Flexural Yielding Special Concentrically Braced Frames:Braces: Tension YieldingEccentrically Braced Frames:Links: Shear YieldingBuckling Restrained Braced Frames:Braces: Tension and Compression YieldingSpecial Plate Shear Walls:Web Panels: Tension Field Yielding
84Summary Ductility = Inelastic Deformation Capacity Ductility Important in all StructuresDuctility Key Element of Seismic Resistance
85Summary Achieving Ductility - Simple Rules......... avoid high strength steelsuse sections with low b/t's and adequate lateral bracingdesign connections and other brittle elements to be stronger than ductile membersFor seismic-resistant structures: Follow AISC Seismic Provisions