Design of Structural Connections

Björn Engström

Division of Structural Engineering

Design of Structural

Connections

Björn EngströmChalmers University of Technology

Göteborg, Sweden

Björn Engström


Content• Design philosophy

– Structural purpose– Force paths at different levels– Mechanical behaviour – design aspects

• Basic force transfer mechanisms– Compression– Shear – Tension– Bending - torsion

• fib Bulletin on –Structural connections

Björn Engström


Design of structural connections

Design aspects:

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Structural behaviour for ordinary and excessive loads

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Appearance and function in the service state

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Structural fire protection

• Load bearing function• Separating function

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Manufacture

• Production of precast elements• Handling, storage and transportation of

precast elements

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Mounting of precast systems Mounting should be possible

Accessibility

Tolerances

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Modular co-ordination

Traditional detail Alternative detail

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Demountability

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Force paths – structural level

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Force paths – structural level

Fixed endcolumns

Diaphragm action

Shear panels

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Force paths –structural level

Compression

Tension

Tension

Compression

Core

Shear wallsFloor diaphragms

Shear

Shear

Compression

TensionCompression

Core

Tension

Shear walls

Floor diaphragms

Shear

Shear

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Force paths –structural subsystems

In-plane action of precast floor

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Force paths –structural subsystems

In-plane action of precast wall

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The force paths – local level

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Alternative designs – force paths

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Alternative designs –

force paths

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Design of the whole connection

The connection as part of the structural system

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Design and detailingfor safe force paths

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Flow of forces through the

connection and further away

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Mechanical behaviour

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Mechanical response

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Need for movement

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Balanced design for ductility

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Anchorage for ductility

Avoid anchorage failures

Provide anchorage for rupture of the steel

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Unintended restraint

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Avoid unfavourable crack locationsUnfavourable

Preferred

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Alternative solutions

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Unintended composite action

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Force transfer in connections

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Basic force transfer mechanisms

w

NN

Compression

Tension

Shear

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Transfer of compressionlocal compression,compressive strengthin confined concrete

stress dispersionsplitting effects

tension

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Compression through several layers

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Design of bearings

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Design of soft bearings

Consider vertical resistanceLimit shear deformation for horizontal loadsCompression over the entire face of the bearing padAvoid direct contact in case of rotationAvoid that the bearing protrudes outside the edges

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Design examples

Wall connection with mortar joint Beam support

with soft bearing

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Design examples

Beam column connectionwith steel plates

Hollow core floor wall connection

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Bolted connections

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Failure modes of bolted connection

Shear failure of boltSplitting of concreteCombined bending

in bolt and crushing of concrete

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Avoid splitting failure

Q

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Design of splitting reinforcement

Compare with local compression

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Dowel action – one-sided

syccR ffV ⋅φ= 2

φ⋅

=cc

R

fVx

30

Failure mechanism

High compression

High bending stress

Q Q

x0

Bending failure in boltcrushing of concrete

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Effect of eccentricity

1

0.18322

k e1 e( )

k e2 e( )

2.50 ζ0 0.5 1 1.5 2 2.5

0

0.2

0.4

0.6

0.8

1

eccentricity factor ke

1

0,8

0.6

0,4

0,2

0 0 0,5 1,0 1,5 2,0 2,5

e/φ

sycceR ffkV ⋅φ= 2

φ⋅

=cc

R

fVx

30

Q

x0

e

20/500

50/320

fck/fyk

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Dowel action – two-sided

Slip

Slip

syccR ffV ⋅φ= 2

φ⋅=

cc

R

fVx

30

x0

x0

2e

fcc

fcc

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Different conditions

Failure mechanism

symincc ffV ⋅φ= ,

21

φ⋅=

mincc

R

fVx

., 310

x0,1

2e

fcc,max

fcc,min


21

φ⋅=

mincc

R

fVx

., 310

symaxccR ffV ⋅φ= ,2

φ⋅=

max,,

cc

R

fVx

320

x0,2

x0,1

2e

fcc,max

fcc,min

Björn Engström


Response in shear

Shear slip

Shear force

First plastic hinge

Second plastic hinge ⇒ plastic mechanism


21

φ⋅=

mincc

R

fVx

., 310

x0,1

2e

fcc,max

fcc,min

Björn Engström


Effect of restraint

syccrR ffkV ⋅φ⋅= 2

φ⋅=

cc

R

fVx

30

21 ≤≤ rk

fcc

fcc

My,red x0

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Restraint and eccentricity syccreR ffkV ⋅φ⋅= 2

,

φ⋅=

cc

R

fVx

30

ε−ε+= 22rre kk ,

fcc

x0 e

fcc

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Effect of anchorage redsccssR ffAV ,)( ⋅φ+σ⋅µ= 2

ssredr ff σ−=,

fcc

x0

x0

fcc

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What happened here?

I II III

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Shear in joints

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Shear friction

Shear slip

Crackwidth

Crack width

Shear slip

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Self-generated friction

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Influence of bond and anchorage

s

w Fv

Fv

deformed barsstrain localisationhigh steel stress

yields for small slip,friction dominates

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Influence of bond and anchorage

shear slipjoint separation

inducedtension

plain barsuniform strainlow steel stress

s

w Fv

Fv

yields for large slip,friction + dowel

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Compare with bolted connection redsccssR ffAV ,)( ⋅φ+σ⋅µ= 2

ssredr ff σ−=,

fcc

x0

x0

fcc

s

w Fv

Fv

Shear friction in jointPlain bars with end anchors

Bolted connectionPlain bolt with end anchors

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Different responses

External bars Internal reinforcement

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When will the transverse bars yield?

s

w Fv

Fv

Depends on:joint roughnessbond resistance of

transverse bar

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Maximum crack width vs. end slip response of transverse bar

Maximum crack widthBar yields in shear friction

Force in bar Force in bar

Maximum crack widthBar yields not in shear friction

Force in bar

Crack width Crack width

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Increased joint separation

52

6

Joint profile with wave-shaped undulations

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Joints with shear keys

with shear keys

plain joints

shear stress

slip

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Shear transfer – clamping is needed

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Distributed ties provides clamping

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Concentrated ties provide clamping

Adequate between elements with high in-plane stiffness that are arranged in the same plane

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Distributed ties are needed here

Inclined forces separatethe elements arranged at a corner Example of distributed ties

between floor and wall

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Shear resistance of joints depends on overall design of the subsystem

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Design examples

Frictional resistance of concreteinterface

High compression High

bending stress

Fv

Response of plain dowel

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Connections between wall elements

monolithic

with shear keys

plainmonolithic plain with

shear-keys

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Connections between floor elements

Uncracked jointCracked joints

Vertical shearcapacity

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Applications

longitudinal tie

transversal ties

transversal ties

reinforcement in concrete

filled sleeves

joint fill

cast in-situ concrete

Shear transfer in hollowcore floor

1

beff

2

3 4

6 7

5

Shear transfer in compositebeams

2 3 4

beff

1

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Transfer of tension

l ty l t,pl

suF plτ

yτ

2 φ

Anchorage with bondAnchor head

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Headed bar

Concrete capacitydesign approach

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Design of connection zoneThe force must go further

Local compression Anchorage of headed bar

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Hollow core floor connection

Tensile capacity isneeded for:

diaphragm action in floor

shear friction resistance of joints

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Bond stress development

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Inclined bond forces

αα

α

b τ

tanα bτ .

N

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Splitting cracks

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Anchorage failuresN Nmax

N N max

Splitting

Pull-out

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Tensile force developmentInfluence of embedment length

0

20

40

60

80

100

120

140

0 100 200 300 400 500Position x [mm]

Tensile force[kN]

N220N290aN500H90bH170H210H250

Tensile capacityYield capacity

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Bond stress - slip relation

.

.

..

..

.

..

.

..

. ..

. . . . .

...

.

Slip

Force

Bond, τb40 mm

.

.

..

..

.

..

.

..

. ..

. . . . .

...

.

Slip

Force

Bond, τb40 mm

05

101520253035404550

0 5 10 15 20 25 30Passive end-slip [mm]

Bond stress [MPa]

HSCNSC

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End slip

Slip

Steel bar in tension Steel bar

Reference point

F

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Elastic response1.58184

0

w 6 σ s

w 8 σ s

w 10 σ s

w 16 σ s

w 20 σ s

w 25 σ s

w 32 σ s

5 108.0 σ s0 1 108 2 108 3 108 4 108 5 108

0

0.5

1

1.5

2

Steel stress [MPa]

Crack width [mm]

0 100 200 300 400 500

2,0

1,5

1,0

0,5

0

φ32φ25φ20φ16φ12φ10φ8φ6

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Yield penetration

lty lt,pl

Fsuτpl

2φ

τy

Plastic zoneElastic zone

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Response of connections

w

NN

N

F su

w y

F sy

w u w w u0,5

Ruptureof bar

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This information was needed

s

w Fv

Fv

Depends on:joint roughnessbond resistance of

transverse bar

When will the transverse bars yield?

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Maximum crack width vs. end slip response of transverse bar

Maximum crack widthBar yields in shear friction

Force in bar Force in bar

Maximum crack widthBar yields not in shear friction

Force in bar

Crack width Crack width

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Examples

101109

N [kN]

send [mm]

send,y = 0,468

0,5send,u =0,795

send,u = 1,59

Prediction of end-slipresponse of anchor bar

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ExamplesDesign of anchorage allowingfor full yield penetration

Effect of local weakening

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Examples

w

N N

Estimation of tie bar stiffness

Design of loop connection

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Prevention of progressive collapse

• Withstand accidental loading

• Reducing the risk of accidental loading

• Increase redundancy and prevent propagation of initial damage

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Analysis of collapse mechanisms

l

A B

d

Q

[ m ]

6,0

4,5

5,0 5,0

3,0

3,0

3,0

N

F su

w y

F sy

w u w w u 0,5

Tie force

Crack width

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Transfer of bending moment

Beam columnconnections

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Column spliceconnection

Column baseconnections

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Floor connections: no restraint, unintended restraint, full restraint, partial continuity in the service state

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Example

60100

30Thin plasticsheet

2 rebars d=12 L = 5300

Rebars d 10 c/c 300, L = 200+300

200

10070Plywood board

30x50x60under webs,2 pc./slab end

100

50

40100

Fs

w

d

ϕ

Moment – rotation response of connection at end support

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Transfer of torsional moment

Simply supported Firmly connected

Torsional restraintat beam support

q

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fib Bulletin on Structural Connections

• Encourage good practice in design of structural connections

• Design philosophy• Connections ⇔ Structural system• Understanding of basic force transfer

mechanisms

Design of Structural Connections

Documents

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