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

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

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

4. F Δ
Ductility F
Fyield Δ

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

6. 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

7. 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

8. 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

9. H Δ H Helastic Available Ductility Required Strength 3/4 *Helastic
MAX

10. H Δ H Helastic Observations: We can trade strength for ductility
MAX

11. H Δ H Helastic Observations: Ductility = Damage 3/4 *Helastic
MAX

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

13. How Do We Achieve Ductility in Steel Structures ?

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

15. Example P P gusset plate double angle tension member
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

16. double angle tension memberP P
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

17. P 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 element
Pyield  Pfracture
Ag Fy  Ae Fu
Steels with a low yield ratio are preferable for ductile behavior
= yield ratio

18. double angle tension memberP P
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

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

20. P P Example: Bolts: 3 - 3/4" A325-X double shear
double angle tension member P P
Example:
Bolts: /4" A325-X double shear
Ab = 0.44 in2 Fv = 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

21. Pyield  Pbolt-fracture
double angle tension member P P
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.

22. Pyield  Pbolt-fracture Pyield  Pbolt-fracture
double angle tension member P P
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 behavior

23. P P Pyield  Pbrittle Stronger is not better in the ductile element
double angle tension member P P
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

24. P P Pyield  Pbrittle Pyield = Ag RyFy
double angle tension member P P
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 angles

25. Ductile Limit States Must Precede Brittle Limit States
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

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

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

33. Ref: Salmon and Johnson - Steel Structures: Design and Behavior
General Trends:
As Fy
Elongation (material ductility)
Fy / Fu
Ref: Salmon and Johnson - Steel Structures: Design and Behavior

34. Be cautious of high-strength steels
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

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

36. M q Effect of Local Buckling on Flexural Strength and Ductility Mp 
Increasing b / t

37. Mp Moment Capacity Ductility Plastic Buckling Inelastic Buckling
0.7 My
Elastic Buckling
hd p
Width-Thickness Ratio (b/t) r
Ductility

38. Mp Moment Capacity Ductility Plastic Buckling Inelastic Buckling
0.7 My
Elastic Buckling
Slender Element Sections
hd p
Width-Thickness Ratio (b/t) r
Ductility

39. Mp Moment Capacity Ductility Plastic Buckling Inelastic Buckling
0.7 My
Elastic Buckling
Noncompact Sections
hd p
Width-Thickness Ratio (b/t) r
Ductility

40. Mp Moment Capacity Ductility Plastic Buckling Inelastic Buckling
0.7 My
Elastic Buckling
Compact Sections
hd p
Width-Thickness Ratio (b/t) r
Ductility

41. Mp Moment Capacity Ductility Plastic Buckling Inelastic Buckling
0.7 My
Elastic Buckling
Highly Ductile
Sections (for seismic design)
hd p
Width-Thickness Ratio (b/t) r
Ductility

42. Local buckling of noncompact and slender element sections

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

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

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

48. Beam Flanges Beam Web Highly Ductile Compactness:

49. Lateral Torsional Buckling
Lateral torsional buckling controlled by:
Lb = distance between beam lateral braces
ry = weak axis radius of gyration
Beam lateral braces Lb Lb

50. Effect of Lateral Torsional Buckling on Flexural Strength and Ductility:M  M q Mp
Increasing Lb / ry

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

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

54. Pcr P d P
Pcr d

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

56. 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
Provide connections that are stronger than members
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

57. How Do We Achieve Ductile Response in Steel Structures ?

58. 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

59. Ductile 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.

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

61. 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)

62. 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

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

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

66. 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

69. An 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.

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

71. Link
Link

72. Inelastic Response of an EBF

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

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

76. Buckling- Restrained Brace: Steel Core + Casing

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

78. Inelastic Response of BRBFs

79. Tension Brace: Yields
Compression Brace: Yields

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

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

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

83. 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

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

85. 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