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8/12/2019 Duct Lity
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Michael D. Engelhardt
University of Texas at Austin
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
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What is Ductility ?
Ductility: The ability to sustain large inelasticdeformations without significant loss in
strength.
Duct i l ity = inelast ic deformat ion capaci ty
- material response
- structural component response
(members and connections)
- global frame response
Ductility:
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F
F
Fyield
Ductility
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F
F
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 andforces
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
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Why Ductility ?
Permits redistribution of internal stresses andforces
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
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General Philosophy for
Earthquake-Resistant Design
Objective: Prevent loss of life by preventing collapsein 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
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H
Available Ductility
MAX
Helastic
3/4 *Helastic
1/2 *Helastic
1/4 *HelasticRequired Strength
H
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H
MAX
Helastic
3/4 *Helastic
1/2 *Helastic
1/4 *Helastic
H
Observations:
We can trade strength for
ductility
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H
MAX
Helastic
3/4 *Helastic
1/2 *Helastic
1/4 *Helastic
H
Observations:
Ductility = Damage
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H
MAX
Helastic
3/4 *Helastic
1/2 *Helastic
1/4 *Helastic
H
Observations:
The maximum lateral load
a structure will see in an
earthquake is equal to thelateral strength of the
structure
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How Do We Achieve Ductility in
Steel Structures ?
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Achieving Ductile Response....
Ductile Limit States Must Precede Brittle Limit States
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Example
double angle tension member
gusset 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
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double angle tension member
PP
Example: Gross-section yielding of tension
member must precede net section
fracture of tension member
Gross-section yield: Pyield= AgFy
Net-section fracture: Pfracture= AeFu
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double angle tension member
PP
PyieldPfracture
AgFyAeFu
u
y
g
e
F
F
A
A
The required strengthfor 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
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double angle tension member
PP
Example: Gross-section yielding of tension member
must precede bolt shear fracture
Gross-section yield: Pyield= AgFy
Bolt shear fracture: Pbolt-fracture= nbnsAbFv
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double angle tension member
PP
PyieldPbolt-fractureThe required strengthfor brittle limit
states is defined by the capacity of the
ductile element
The ductile element must be the
weakest element in the load path
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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 in2x 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
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double angle tension member
PP
PyieldPbolt-fracturePyield= 139
k 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
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double angle tension member
PP
PyieldPbolt-fracturePyield= 3.87 in
2x 54 ksi = 209k
Pbolt-fracture= 180k
PyieldPbolt-fractureBolt fracture will occur before yield of angles
non-ductilebehavior
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double angle tension member
PP
PyieldPbrittleStronger 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
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double angle tension member
PP
PyieldPbrittle
The required strengthfor brittle limit
states is defined by the expected
capacity of the ductile element (not
minimum specified capacity)
Pyield=Ag RyFy RyFy= expected yield stress of angles24
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Achieving Ductile Response....
Ductile Limit States Must Precede Brittle Limit States
Define the required strengthfor 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 canlead to non-ductile behavior.
Steels with a low value of yield ratio, Fy/ Fuare preferable
for ductile elements25
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Achieving Ductile Response....
Connection response is generally non-ductile.....
Connections should be stronger than connected
members
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Achieving Ductile Response....
Be cautious of high-strength steels
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Ref: Salmon and Johnson - SteelStructures: Design and Behavior
General Trends:
As Fy
Elongation (material
ductility)
Fy/ Fu
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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
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Achieving Ductile Response....
Use Sections with Low Width-Thickness Ratios andAdequate Lateral Bracing
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M
q
Mp
Increasing b / t
Effect of Local Buckling on Flexural Strength and Ductility
M
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0.7 My
M
omentCapacity
p r Width-Thickness Ratio (b/t)
Mp
Plastic Buckling
Inelastic Buckling
Elastic Buckling
hd
Duc
tility
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M
omentCapacity
p r Width-Thickness Ratio (b/t)
Mp
Plastic Buckling
Inelastic Buckling
Elastic Buckling
hd
Duc
tility
Slender Element Sections
38
0.7 My
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p r Width-Thickness Ratio (b/t)
Mp
Plastic Buckling
Inelastic Buckling
Elastic Buckling
hd
Noncompact Sections
39
M
omentCapacity
Duc
tility
0.7 My
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p r Width-Thickness Ratio (b/t)
Mp
Plastic Buckling
Inelastic Buckling
Elastic Buckling
hd
Compact Sections
40
M
omentCapacity
Duc
tility
0.7 My
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p r Width-Thickness Ratio (b/t)
Mp
Plastic Buckling
Inelastic Buckling
Elastic Buckling
hd
Highly Ductile
Sections (for seismic design)
41
M
omentCapacity
Duc
tility
0.7 My
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Local buckling of noncompact and slender element sections42
Local buckling of moment frame beam with highly ductile
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Local buckling of moment frame beam with highly ductile
compactness ( < hd) .....
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-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)
Bending
Moment(kN-
RBS Connection
Mp
Mp
44
L l b kli f h i ldi EBF li k ith hi hl
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Local buckling of a shear yielding EBF link with highly
ductile compactness ( < hd) .....
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-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, (rad)
Link
ShearForce
(kips)
46
Eff t f L l B kli D tilit
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Effect of Local Buckling on Ductility
For highly ductile flexuralresponse:
Example: W-Shape
b f
t f
h
tw
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Beam Flanges
Beam Web
2.45
w y
Eh
t F
0.302
f
f y
b E
t FHighly Ductile Compactness:
48
Highly Ductile Compactness:
Lateral Torsional Buckling
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Lateral Torsional Buckling
Lateral torsional buckling
controlled by:
y
b
r
L
Lb= distance between beam lateral braces
ry= weak axis radius of gyration
L b L b
Beam lateral braces
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M
q
Mp
Increasing Lb/ ry
Effect of Lateral Torsional Buckling on Flexural Strength and Ductility:
M
50
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Effect of Lateral Buckling on Ductility
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Effect of Lateral Buckling on Ductility
For highly ductile flexural response:
b y
y
EL 0.086 r
F
For Fy= 50 ksi:
y y
y
E0.086 r = 50 r
F
52
Achieving Ductile Response
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Achieving Ductile Response....
Recognize that buckling of a compression member isnon-ductile
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Experimental Behavior of Brace Under Cyclic Axial Loading
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d
P
W6x20 Kl/r = 80
55
How Do We Achieve Ductile Response in Steel Structures ?
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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 Strengthfor 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 ?
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How Do We Achieve Ductile Response in Steel Structures ?
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Ductile Detailing for Seismic Resistance
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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
Ductile Detailing for Seismic Resistance
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Choose frame elements ("fuses") that will yield in anearthquake.
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)
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Special Moment Frame (SMF)
Ductile fuse:
Flexural yielding of beams
60
Inelastic Response of a
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Inelastic Response of a
Special Moment Frame
Fuse: Flexural Yielding
of Beams
Detail beam for ductileflexural response:
no high strength steels
low b/t ratios (highly
ductile )
beam lateral bracing
(per seismic req'ts)
61
Inelastic Response of a
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Inelastic Response of a
Special Moment Frame
Design all other frameelements to be stronger
than the beam:
Connections
Beam-to-column
connections
Column splices
Column bases
Column buckling capacity
Column flexural capacity
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Special Concentrically Braced Frame
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p y
Ductile fuse: tension yielding of braces.
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Inelastic Response of an SCBF
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Tension Brace: Yields(ductile)
Compression Brace: Buckles(nonductile)
65
Inelastic Response of CBFs under Earthquake Loading
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Compression Brace(previously in tension):
Buckles (nonductile)
Tension Brace (previouslybuckled in compression):
Yields (ductile)
Connections, columns and beams: designed to be
stronger than braces 66
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Eccentrically Braced Frames (EBFs)
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y ( )
Ductile fuse:
Shear yielding of links
70
Link
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Link
71
Inelastic Response of an EBF
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p
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Braces, beam segments outside of link, columns,
connections: designed to be stronger than link
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Buckling-Restrained Braced Frames (BRBFs)
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Ductile fuse:
Tension yielding and
compression yielding ofBuckling Restrained Braces
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Buckling-Restrained Brace
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Buckling-
Restrained Brace:
Steel Core+
Casing
Casing
Steel Core
76
Buckling-Restrained Brace
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Buckling-
Restrained Brace:
Steel Core+
CasingA
A
Section A-A
Steel Core
Debonding material
Casing
Steel jacket
Mortar
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Tension Brace: Yields Compression Brace: Yields
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Compression Brace: Yields Tension Brace: Yields
Connections, columns and beams: designed to be
stronger than braces
80
Special Plate Shear Walls (SPSW)
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Ductile fuse:
Tension field yielding of web
panels
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Shear buckling
Development yieldingalong tension
diagonals
Inelastic Response of a SPSW
Columns, beams and connections: designed to be
stronger than web panel
82
Ductile Fuses:
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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
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Ductility = Inelastic Deformation Capacity
Ductility Important in all Structures
Ductility Key Element of Seismic Resistance
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Summary
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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 bestronger than ductile members
For seismic-resistant structures:
Follow AISC Seismic Provisions