design of structural connections
TRANSCRIPT
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
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Division of Structural Engineering
Modular co-ordination
Traditional detail Alternative detail
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Division of Structural Engineering
Demountability
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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
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Division of Structural Engineering
The force paths – local level
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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
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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
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Division of Structural Engineering
Avoid unfavourable crack locationsUnfavourable
Preferred
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Division of Structural Engineering
Alternative solutions
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Division of Structural Engineering
Unintended composite action
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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
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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
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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
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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
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Division of Structural Engineering
Anchorage failuresN Nmax
N N max
Splitting
Pull-out
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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
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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