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

Michael D. EngelhardtMichael D. Engelhardt

University of Texas at AustinUniversity of Texas at Austin

Basic Concepts in Ductile DetailingBasic Concepts in Ductile Detailingof Steel Structuresof Steel Structures

1

EAC 2013

Overview of Presentation

• What is Ductility ?

• Why is Ductility Important ?

• How Do We Achieve Ductility in Steel

Structures ?

• Ductility in Seismic-Resistant Design

2

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

F

Δ

F

Δ

Fyield

Ductility

4

F

Δ

F

Δ

Ductility = Yielding

How is ductility developed in steel structures ?

Loss of load carrying capability:

Instability

Fracture

5

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

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

General Philosophy for General Philosophy for Earthquake-Resistant DesignEarthquake-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

H

Available Ductility

MAX

Helastic

3/4 *Helastic

1/2 *Helastic

1/4 *Helastic

Required Strength

H

Δ

9

H

MAX

Helastic

3/4 *Helastic

1/2 *Helastic

1/4 *Helastic

H

Δ

Observations:

We can trade strength for ductility

10

H

MAX

Helastic

3/4 *Helastic

1/2 *Helastic

1/4 *Helastic

H

Δ

Observations:

Ductility = Damage

11

H

MAX

Helastic

3/4 *Helastic

1/2 *Helastic

1/4 *Helastic

H

Δ

Observations:

The maximum lateral load a structure will see in an earthquake is equal to the lateral strength of the structure

12

How Do We Achieve Ductility in Steel Structures ?

13

Achieving Ductile Response....

Ductile Limit States Must Precede Brittle Limit States

14

ExampleExample

double angle tension membergusset plate

PP

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 gusset15

double angle tension member

PP

Example: Gross-section yielding of tension member must precede net section fracture of tension member

Gross-section yield: Pyield = Ag Fy

Net-section fracture: Pfracture = Ae Fu

16

double angle tension member

PP

Pyield Pfracture

Ag Fy Ae Fu

u

y

g

e

F

F

A

A

The required strength for brittle limit states is defined by the capacity of the ductile element

u

y

F

F= yield ratio Steels with a low yield ratio are

preferable for ductile behavior

17

double angle tension member

PP

Example: Gross-section yielding of tension member must precede bolt shear fracture

Gross-section yield: Pyield = Ag Fy

Bolt shear fracture: Pbolt-fracture = nb ns Ab Fv

18

double angle tension member

PP

Pyield Pbolt-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

double angle tension member

PP

Example: Bolts: 3 - 3/4" A325-X double shear

Ab = 0.44 in2 Fv = 0.563 x 120 ksi = 68 ksi

Pbolt-fracture = 3 x 0.44 in2 x 68 ksi x 2 = 180k

Angles: 2L 4 x 4 x 1/4 A36

Ag = 3.87 in2

Pyield = 3.87 in2 x 36 ksi = 139k

20

double angle tension member

PP

Pyield Pbolt-fracture

Pyield = 139k Pbolt-fracture = 180k 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

double angle tension member

PP

Pyield Pbolt-fracture

Pyield = 3.87 in2 x 54 ksi = 209k

Pbolt-fracture = 180k

Pyield Pbolt-fracture

Bolt fracture will occur before yield of angles non-ductile behavior22

double angle tension member

PP

Pyield Pbrittle

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

double angle tension member

PP

Pyield Pbrittle

The required strength for brittle limit states is defined by the expected capacity of the ductile element (not minimum specified capacity)

Pyield = Ag RyFy Ry Fy = expected yield stress of angles24

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, Fy / Fu are preferable for ductile elements

25

Achieving Ductile Response....

Connection response is generally non-ductile.....

Connections should be stronger than connected members

26

27

28

29

30

31

Achieving Ductile Response....

Be cautious of high-strength steels

32

Ref: Salmon and Johnson - Steel Structures: Design and Behavior

General Trends:

As Fy

Elongation (material ductility)

Fy / Fu

33

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

Achieving Ductile Response....

Use Sections with Low Width-Thickness Ratios and Adequate Lateral Bracing

35

M

Mp

Increasing b / t

Effect of Local Buckling on Flexural Strength and Ductility

M

36

0.7 My

Mom

ent C

apac

ity

p rWidth-Thickness Ratio (b/t)

Mp

Plastic Buckling

Inelastic Buckling

Elastic Buckling

hd

Duc

tility

37

Mom

ent C

apac

ity

p rWidth-Thickness Ratio (b/t)

Mp

Plastic Buckling

Inelastic Buckling

Elastic Buckling

hd

Duc

tility

Slender Element Sections

38

0.7 My

p rWidth-Thickness Ratio (b/t)

Mp

Plastic Buckling

Inelastic Buckling

Elastic Buckling

hd

Noncompact Sections

39

Mom

ent C

apac

ityD

uctil

ity

0.7 My

p rWidth-Thickness Ratio (b/t)

Mp

Plastic Buckling

Inelastic Buckling

Elastic Buckling

hd

Compact Sections

40

Mom

ent C

apac

ityD

uctil

ity

0.7 My

p rWidth-Thickness Ratio (b/t)

Mp

Plastic Buckling

Inelastic Buckling

Elastic Buckling

hd

Highly Ductile Sections (for seismic design)

41

Mom

ent C

apac

ityD

uctil

ity

0.7 My

Local buckling of noncompact and slender element sections42

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

43

-5000

-4000

-3000

-2000

-1000

0

1000

2000

3000

4000

5000

-0.05 -0.04 -0.03 -0.02 -0.01 0 0.01 0.02 0.03 0.04 0.05

Drift Angle (radian)

Ben

ding

Mom

ent (

kN-m

)

RBS Connection

Mp

Mp

44

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

45

-200

-150

-100

-50

0

50

100

150

200

-0.08 -0.06 -0.04 -0.02 0 0.02 0.04 0.06 0.08

Link Rotation, g (rad)

Lin

k S

hea

r F

orc

e (k

ips)

46

Effect of Local Buckling on Ductility

For highly ductile flexural response:

Example: W-Shape

bf

t f

h

tw

47

Beam Flanges

Beam Web

2.45w y

Eht F

0.302

f

f y

b Et F

Highly Ductile Compactness:

48

Highly Ductile Compactness:

Lateral Torsional BucklingLateral Torsional Buckling

Lateral torsional buckling controlled by:

y

b

r

L

Lb = distance between beam lateral braces

ry = weak axis radius of gyration

Lb Lb

Beam lateral braces

49

M

Mp

Increasing Lb / ry

Effect of Lateral Torsional Buckling on Flexural Strength and Ductility:Effect of Lateral Torsional Buckling on Flexural Strength and Ductility:

M

50

51

Effect of Lateral Buckling on Ductility

For highly ductile flexural response:

b y

y

EL 0.086 r

F

For Fy = 50 ksi:

y yy

E0.086 r = 50 r

F

52

Achieving Ductile Response....

Recognize that buckling of a compression member is non-ductile

53

Pcr

P

Pcr

P

54

Experimental Behavior of Brace Under Cyclic Axial LoadingExperimental Behavior of Brace Under Cyclic Axial Loading

P

W6x20 Kl/r = 80

55

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

How Do We Achieve Ductile Response in Steel Structures ?

57

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

• 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

Special Moment Frame (SMF) Special Moment Frame (SMF)

Ductile fuse:Flexural yielding of beams

60

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

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

63

Special Concentrically Braced Frame Special Concentrically Braced Frame

Ductile fuse: tension yielding of braces.

64

Inelastic Response of an SCBFInelastic Response of an SCBF

Tension Brace: Yields(ductile)

Compression Brace: Buckles(nonductile)

65

Inelastic Response of CBFs under Earthquake LoadingInelastic 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

67

68

69

Eccentrically Braced Frames (EBFs)Eccentrically Braced Frames (EBFs)

Ductile fuse:Shear yielding of links

70

Link

Link

71

Inelastic Response of an EBF

72

73

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

74

Buckling-Restrained Braced Frames (BRBFs)Buckling-Restrained Braced Frames (BRBFs)

Ductile fuse:Tension yielding and compression yielding of Buckling Restrained Braces

75

Buckling-Restrained Brace Buckling-Restrained Brace

Buckling- Restrained Brace:

Steel Core+

Casing

Casing

Steel Core

76

Buckling-Restrained Brace Buckling-Restrained Brace

Buckling- Restrained Brace:

Steel Core+

CasingAA

Section A-A

Steel Core

Debonding material

Casing

Steel jacket

Mortar

77

Inelastic Response of BRBFs Inelastic Response of BRBFs

78

Tension Brace: Yields Compression Brace: Yields

79

Compression Brace: Yields Tension Brace: Yields

Connections, columns and beams: designed to be stronger than braces

80

Special Plate Shear Walls (SPSW)Special Plate Shear Walls (SPSW)

Ductile fuse:Tension field yielding of web panels

81

Shear buckling

Development yielding along tension diagonals

Inelastic Response of a SPSWInelastic Response of a SPSW

Columns, beams and connections: designed to be stronger than web panel

82

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 Yielding83

Summary

• Ductility = Inelastic Deformation Capacity

• Ductility Important in all Structures

• Ductility Key Element of Seismic Resistance

84

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