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Michael D. Engelhardt

University of Texas at Austin

Basic Concepts in Ductile Detailing

of Steel Structures

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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

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

Connections should be stronger than connected

members

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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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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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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


Mp

Plastic Buckling

Inelastic Buckling

Elastic Buckling

hd

Duc

tility

Slender Element Sections

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0.7 My

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Mp

Plastic Buckling

Inelastic Buckling

Elastic Buckling

hd

Noncompact Sections

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M

omentCapacity

Duc

tility

0.7 My

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Mp

Plastic Buckling

Inelastic Buckling

Elastic Buckling

hd

Compact Sections

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M

omentCapacity

Duc

tility

0.7 My

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Mp

Plastic Buckling

Inelastic Buckling

Elastic Buckling

hd

Highly Ductile

Sections (for seismic design)

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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

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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)

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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:

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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

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Achieving Ductile Response

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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

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How Do We Achieve Ductile Response in Steel Structures ?

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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

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Ductile Detailing for Seismic Resistance

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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

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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.


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Special Moment Frame (SMF)

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Special Moment Frame (SMF)

Ductile fuse:

Flexural yielding of beams

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Inelastic Response of a

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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)

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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)

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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

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Link

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Link

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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

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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

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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

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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

Documents

Duct Lity