Basic Concepts in Ductile Detailing
of Steel Structures
EAC 2013
Michael D. Engelhardt
University of Texas at Austin
1
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
Ductility: - material response
- structural component response
(members and connections)
- global frame response
3
Δ
F
F
Ductility
Fyield
Δ
4
How is ductility developed in steel structures ?
Δ
F
F
Ductility = Yielding
Loss of load carrying
capability:
Instability
Fracture
Δ
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
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
Δ
H
H
Helastic
3/4 *Helastic
1/2 *Helastic
Available Ductility
Required Strength
1/4 *Helastic
MAX
9
Δ
H
H
Helastic
3/4 *Helastic
Observations:
We can trade strength for
ductility
1/2 *Helastic
1/4 *Helastic
MAX
10
Δ
H
H
Helastic
3/4 *Helastic
Observations:
Ductility = Damage
1/2 *Helastic
1/4 *Helastic
MAX
11
Δ
H
H
Helastic
3/4 *Helastic
Observations:
The maximum lateral load
a structure will see in an
earthquake is equal to the
lateral strength of the
structure
1/2 *Helastic
1/4 *Helastic
MAX
12
How Do We Achieve Ductility in
Steel Structures ?
13
Achieving Ductile Response....
Ductile Limit States Must Precede Brittle Limit States
14
Example
gusset plate
double angle tension member
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
double angle tension member
P
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
16
double angle tension member
P
P
Pyield  Pfracture
The required strength for brittle limit
states is defined by the capacity of the
ductile element
Ag F y  Ae F u
Ae Fy

Ag Fu
Fy
Fu
= yield ratio
Steels with a low yield ratio are
preferable for ductile behavior
17
double angle tension member
P
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
18
double angle tension member
P
P
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
P
P
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
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.
21
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
22
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
23
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
24
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
General Trends:
As Fy
Elongation (material
ductility)
Fy / Fu
Ref: Salmon and Johnson - Steel
Structures: Design and Behavior
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
Effect of Local Buckling on Flexural Strength and Ductility
q
M
M
Mp
Increasing b / t
q
36
Moment Capacity
Ductility
Plastic Buckling
Mp
Inelastic Buckling
0.7 My
Elastic Buckling
hd p
r
Width-Thickness Ratio (b/t)
37
Moment Capacity
Ductility
Plastic Buckling
Mp
Inelastic Buckling
0.7 My
Elastic Buckling
Slender Element Sections
hd p
r
Width-Thickness Ratio (b/t)
38
Moment Capacity
Ductility
Plastic Buckling
Mp
Inelastic Buckling
0.7 My
Elastic Buckling
Noncompact Sections
hd p
r
Width-Thickness Ratio (b/t)
39
Moment Capacity
Ductility
Plastic Buckling
Mp
Inelastic Buckling
0.7 My
Elastic Buckling
Compact Sections
hd p
r
Width-Thickness Ratio (b/t)
40
Moment Capacity
Ductility
Plastic Buckling
Mp
Inelastic Buckling
0.7 My
Elastic Buckling
Highly Ductile
Sections (for seismic design)
hd p
r
Width-Thickness Ratio (b/t)
41
Local buckling of noncompact and slender element sections
42
Local buckling of moment frame beam with highly ductile
compactness (  < hd ) .....
43
5000
Bending Moment (kN-m)
4000
Mp
3000
2000
1000
0
-1000
-2000
-3000
Mp
-4000
-5000
-0.05
RBS Connection
-0.04
-0.03
-0.02
-0.01
0
0.01
0.02
0.03
0.04
0.05
Drift Angle (radian)
44
Local buckling of a shear yielding EBF link with highly
ductile compactness (  < hd ) .....
45
Link Shear Force (kips)
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)
46
Effect of Local Buckling on Ductility
For highly ductile flexural response:
bf
Example: W-Shape
tf
h
tw
47
Beam Flanges
Highly Ductile Compactness:
bf
E
 0.30
2t f
Fy
Beam Web
Highly Ductile Compactness:
E
h
 2.45
tw
Fy
48
Lateral Torsional Buckling
Lateral torsional buckling
controlled by:
Lb
ry
Lb = distance between beam lateral braces
ry = weak axis radius of gyration
Beam lateral braces
Lb
Lb
49
Effect of Lateral Torsional Buckling on Flexural Strength and Ductility:
q
M
M
Mp
Increasing Lb / ry
q
50
51
Effect of Lateral Buckling on Ductility
For highly ductile flexural response:
 E
Lb  0.086 
 Fy


 ry

For Fy = 50 ksi:
 E
0.086 
 Fy


 ry = 50 ry

52
Achieving Ductile Response....
Recognize that buckling of a compression member is
non-ductile
53
Pcr
d
P
Pcr
d
54
Experimental Behavior of Brace Under Cyclic Axial Loading
W6x20 Kl/r = 80
P
d
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
•
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
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
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.
59
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
Ductile fuse: tension yielding of braces.
64
Inelastic Response of an SCBF
Tension Brace: Yields
(ductile)
Compression Brace: Buckles
(nonductile)
65
Inelastic Response of CBFs under Earthquake Loading
Compression Brace
(previously in tension):
Buckles (nonductile)
Connections, columns and beams: designed to be
stronger than braces
Tension Brace (previously
buckled in compression):
Yields (ductile)
66
67
68
69
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)
Ductile fuse:
Tension yielding and
compression yielding of
Buckling Restrained Braces
75
Buckling-Restrained Brace
BucklingRestrained Brace:
Steel Core
+
Casing
Casing
Steel Core
76
A
A
Buckling-Restrained Brace
Casing
BucklingRestrained Brace:
Steel Core
+
Casing
Steel Core
Steel jacket
Mortar
Debonding material
Section A-A
77
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)
Ductile fuse:
Tension field yielding of web
panels
81
Inelastic Response of a SPSW
Development yielding
along tension
diagonals
Shear buckling
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 Yielding
83
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