design of structural connections

Björn Engström

Division of Structural Engineering

Design of Structural

Connections

Björn EngströmChalmers University of Technology

Göteborg, Sweden

Björn Engström

Division of Structural Engineering

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

Division of Structural Engineering

Design of structural connections

Design aspects:

Björn Engström

Division of Structural Engineering

Structural behaviour for ordinary and excessive loads

Björn Engström

Division of Structural Engineering

Appearance and function in the service state

Björn Engström

Division of Structural Engineering

Structural fire protection

• Load bearing function• Separating function

Björn Engström

Division of Structural Engineering

Manufacture

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

precast elements

Björn Engström

Division of Structural Engineering

Mounting of precast systems Mounting should be possible

Accessibility

Tolerances

Björn Engström

Division of Structural Engineering

Modular co-ordination

Traditional detail Alternative detail

Björn Engström

Division of Structural Engineering

Demountability

Björn Engström

Division of Structural Engineering

Force paths – structural level

Björn Engström

Division of Structural Engineering

Force paths – structural level

Fixed endcolumns

Diaphragm action

Shear panels

Björn Engström

Division of Structural Engineering

Force paths –structural level

Compression

Tension

Tension

Compression

Core

Shear wallsFloor diaphragms

Shear

Shear

Compression

TensionCompression

Core

Tension

Shear walls

Floor diaphragms

Shear

Shear

Björn Engström

Division of Structural Engineering

Force paths –structural subsystems

In-plane action of precast floor

Björn Engström

Division of Structural Engineering

Force paths –structural subsystems

In-plane action of precast wall

Björn Engström

Division of Structural Engineering

The force paths – local level

Björn Engström

Division of Structural Engineering

Alternative designs – force paths

Björn Engström

Division of Structural Engineering

Alternative designs –

force paths

Björn Engström

Division of Structural Engineering

Design of the whole connection

The connection as part of the structural system

Björn Engström

Division of Structural Engineering

Design and detailingfor safe force paths

Björn Engström

Division of Structural Engineering

Flow of forces through the

connection and further away

Björn Engström

Division of Structural Engineering

Mechanical behaviour

Björn Engström

Division of Structural Engineering

Mechanical response

Björn Engström

Division of Structural Engineering

Need for movement

Björn Engström

Division of Structural Engineering

Balanced design for ductility

Björn Engström

Division of Structural Engineering

Anchorage for ductility

Avoid anchorage failures

Provide anchorage for rupture of the steel

Björn Engström

Division of Structural Engineering

Unintended restraint

Björn Engström

Division of Structural Engineering

Avoid unfavourable crack locationsUnfavourable

Preferred

Björn Engström

Division of Structural Engineering

Alternative solutions

Björn Engström

Division of Structural Engineering

Unintended composite action

Björn Engström

Division of Structural Engineering

Force transfer in connections

Björn Engström

Division of Structural Engineering

Basic force transfer mechanisms

w

NN

Compression

Tension

Shear

Björn Engström

Division of Structural Engineering

Transfer of compressionlocal compression,compressive strengthin confined concrete

stress dispersionsplitting effects

tension

Björn Engström

Division of Structural Engineering

Compression through several layers

Björn Engström

Division of Structural Engineering

Design of bearings

Björn Engström

Division of Structural Engineering

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

Björn Engström

Division of Structural Engineering

Design examples

Wall connection with mortar joint Beam support

with soft bearing

Björn Engström

Division of Structural Engineering

Design examples

Beam column connectionwith steel plates

Hollow core floor wall connection

Björn Engström

Division of Structural Engineering

Bolted connections

Björn Engström

Division of Structural Engineering

Failure modes of bolted connection

Shear failure of boltSplitting of concreteCombined bending

in bolt and crushing of concrete

Björn Engström

Division of Structural Engineering

Avoid splitting failure

Q

Björn Engström

Division of Structural Engineering

Design of splitting reinforcement

Compare with local compression

Björn Engström

Division of Structural Engineering

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

Björn Engström

Division of Structural Engineering

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

Björn Engström

Division of Structural Engineering

Dowel action – two-sided

Slip

Slip

syccR ffV ⋅φ= 2

φ⋅=

cc

R

fVx

30

x0

x0

2e

fcc

fcc

Björn Engström

Division of Structural Engineering

Different conditions

Failure mechanism

symincc ffV ⋅φ= ,

21

φ⋅=

mincc

R

fVx

., 310

x0,1

2e

fcc,max

fcc,min

symincc ffV ⋅φ= ,

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

Division of Structural Engineering

Response in shear

Shear slip

Shear force

First plastic hinge

Second plastic hinge ⇒ plastic mechanism

symincc ffV ⋅φ= ,

21

φ⋅=

mincc

R

fVx

., 310

x0,1

2e

fcc,max

fcc,min

Björn Engström

Division of Structural Engineering

Effect of restraint

syccrR ffkV ⋅φ⋅= 2

φ⋅=

cc

R

fVx

30

21 ≤≤ rk

fcc

fcc

My,red x0

Björn Engström

Division of Structural Engineering

Restraint and eccentricity syccreR ffkV ⋅φ⋅= 2

,

φ⋅=

cc

R

fVx

30

ε−ε+= 22rre kk ,

fcc

x0 e

fcc

Björn Engström

Division of Structural Engineering

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

ssredr ff σ−=,

fcc

x0

x0

fcc

Björn Engström

Division of Structural Engineering

What happened here?

I II III

Björn Engström

Division of Structural Engineering

Shear in joints

Björn Engström

Division of Structural Engineering

Shear friction

Shear slip

Crackwidth

Crack width

Shear slip

Björn Engström

Division of Structural Engineering

Self-generated friction

Björn Engström

Division of Structural Engineering

Influence of bond and anchorage

s

w Fv

Fv

deformed barsstrain localisationhigh steel stress

yields for small slip,friction dominates

Björn Engström

Division of Structural Engineering

Influence of bond and anchorage

shear slipjoint separation

inducedtension

plain barsuniform strainlow steel stress

s

w Fv

Fv

yields for large slip,friction + dowel

Björn Engström

Division of Structural Engineering

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

Björn Engström

Division of Structural Engineering

Different responses

External bars Internal reinforcement

Björn Engström

Division of Structural Engineering

When will the transverse bars yield?

s

w Fv

Fv

Depends on:joint roughnessbond resistance of

transverse bar

Björn Engström

Division of Structural Engineering

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

Björn Engström

Division of Structural Engineering

Increased joint separation

52

6

Joint profile with wave-shaped undulations

Björn Engström

Division of Structural Engineering

Joints with shear keys

with shear keys

plain joints

shear stress

slip

Björn Engström

Division of Structural Engineering

Shear transfer – clamping is needed

Björn Engström

Division of Structural Engineering

Distributed ties provides clamping

Björn Engström

Division of Structural Engineering

Concentrated ties provide clamping

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

Björn Engström

Division of Structural Engineering

Distributed ties are needed here

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

between floor and wall

Björn Engström

Division of Structural Engineering

Shear resistance of joints depends on overall design of the subsystem

Björn Engström

Division of Structural Engineering

Design examples

Frictional resistance of concreteinterface

High compression High

bending stress

Fv

Response of plain dowel

Björn Engström

Division of Structural Engineering

Connections between wall elements

monolithic

with shear keys

plainmonolithic plain with

shear-keys

Björn Engström

Division of Structural Engineering

Connections between floor elements

Uncracked jointCracked joints

Vertical shearcapacity

Björn Engström

Division of Structural Engineering

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

Björn Engström

Division of Structural Engineering

Transfer of tension

l ty l t,pl

suF plτ

yτ

2 φ

Anchorage with bondAnchor head

Björn Engström

Division of Structural Engineering

Headed bar

Concrete capacitydesign approach

Björn Engström

Division of Structural Engineering

Design of connection zoneThe force must go further

Local compression Anchorage of headed bar

Björn Engström

Division of Structural Engineering

Hollow core floor connection

Tensile capacity isneeded for:

diaphragm action in floor

shear friction resistance of joints

Björn Engström

Division of Structural Engineering

Bond stress development

Björn Engström

Division of Structural Engineering

Bond stress development

Björn Engström

Division of Structural Engineering

Inclined bond forces

αα

α

b τ

tanα bτ .

N

Björn Engström

Division of Structural Engineering

Splitting cracks

Björn Engström

Division of Structural Engineering

Anchorage failuresN Nmax

N N max

Splitting

Pull-out

Björn Engström

Division of Structural Engineering

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

Björn Engström

Division of Structural Engineering

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

Björn Engström

Division of Structural Engineering

End slip

Slip

Steel bar in tension Steel bar

Reference point

F

Björn Engström

Division of Structural Engineering

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

Björn Engström

Division of Structural Engineering

Yield penetration

lty lt,pl

Fsuτpl

2φ

τy

Plastic zoneElastic zone

Björn Engström

Division of Structural Engineering

Response of connections

w

NN

N

F su

w y

F sy

w u w w u0,5

Ruptureof bar

Björn Engström

Division of Structural Engineering

This information was needed

s

w Fv

Fv

Depends on:joint roughnessbond resistance of

transverse bar

When will the transverse bars yield?

Björn Engström

Division of Structural Engineering

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

Björn Engström

Division of Structural Engineering

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

Björn Engström

Division of Structural Engineering

ExamplesDesign of anchorage allowingfor full yield penetration

Effect of local weakening

Björn Engström

Division of Structural Engineering

Examples

w

N N

Estimation of tie bar stiffness

Design of loop connection

Björn Engström

Division of Structural Engineering

Prevention of progressive collapse

• Withstand accidental loading

• Reducing the risk of accidental loading

• Increase redundancy and prevent propagation of initial damage

Björn Engström

Division of Structural Engineering

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

Björn Engström

Division of Structural Engineering

Transfer of bending moment

Beam columnconnections

Björn Engström

Division of Structural Engineering

Transfer of bending moment

Column spliceconnection

Column baseconnections

Björn Engström

Division of Structural Engineering

Transfer of bending moment

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

Björn Engström

Division of Structural Engineering

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

Björn Engström

Division of Structural Engineering

Transfer of torsional moment

Simply supported Firmly connected

Torsional restraintat beam support

q

Björn Engström

Division of Structural Engineering

fib Bulletin on Structural Connections

• Encourage good practice in design of structural connections

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

mechanisms