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Basic principles of steel structures
Dr. Xianzhong ZHAO
[email protected]
www.sals.org.cn
Members + connections = systemtransfer forces supported
by a member to others
ConnectionsOutlines
types of connections and their characteristics
butt weld connections: details and calculation
fillet weld connections: details and calculation
bolted connections: details and calculation
high-strength bolted connections: details andcalculation
Types of structural connectionsbasic types of connections
welded connectionsmolten parent metals are fused with each other being togetherelectric-arc/slag/resistance welding, gas welding
riveted connections
bolted connectionsordinary structural bolt/ high strength bolt
other connectionsscrew, glue
weld rivet bolt
Types of structural connectionswelded connections: types of welding
electric arc welding: molten weld metal (welding wire or electrode) is fused with the base metal of the members being connected
shielded metal arc welding (SMAW)Q235: E43 electrode / Q345: E50 / Q390, Q420: E55electrode matches with lower yield strength steel
submerged arc welding (SAW) : auto-/ semi-automaticH08 welding wire, with Mn flux
gas metal-arc welding (GMA): CO2shielding gas (indoor weld)
Types of structural connectionswelded type: shielded metal arc welding
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Types of structural connectionswelded type: submerged arc welding
Types of structural connectionswelded type: gas metal-arc welding
Types of structural connectionswelded connections: types of welding
electric slag weldingmolten slag + base metal + welding wire
electric resistance weldingMolten base metal + pressure
gas weldingAcetylene + oxygen + electrode
Types of structural connectionsclassification of welds
Types of joint used: position of base metalsbutt, lap, tee, edge, corner
Types of weld madebutt weld: straight / bevel welds
fillet weld: end / side welds
Types of structural connectionsclassification of welds
Types of weld madeContinuous weld
Intermittent weld
Welding positionFlat, horizontal, vertical, overhead
Types of structural connectionsadvantage and disadvantage of weld connections
Efficiency: material saving and time saving
Wider range of application
More rigid, most truly continuous structures
Residual stress: rigid, stability and fatigue
Weld deformation
HAZ: brittle failure
Crack: propagation to members
Qualified: skill dependent/ qualification of welding procedurecrack, blow hole, slag inclusion, undercut, overlapincomplete penetration / fusion / filled groove
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Types of structural connectionsresidual stress
Self balance system
Not affect the static performance
Decrease the stiffness?
Decrease fatigue?
Decrease stability?
P
u
P/ yA f=
P=u=
0.6rt yf =
0.3rc yf =
0.4 yf =
0.3 0.4 0.1 + =
0.6 0.4 1+ =
0.8 yf =
0.1
1
0.4 3 / 2 0.7+ =
yf =
0.7
1
0.2 3 / 2 1+ =
Types of structural connectionsweld deformation
Types of structural connectionsHAZ and weld crack
Butt weld connectionsdetailing
Backup strip, back gouging and weld mending
1:2.51:2.5
Grooves and welding symbols
Run-out plate
Transition of thickness and width
Butt weld connectionsdesign of butt welds
design resistance of butt weldsQuality grade I & II : equal to the design strength of base metalQuality grade III : decrease to 85% design strength of base metal
how to classify the quality grade of butt weldQuality grade III: visual inspectionQuality grade II: visual inspection + ultrasonic testing (20%)Quality grade I: visual inspection + ultrasonic + radiographic (100%)cross-section of butt weld(1) Area = thickness of plate (t) X effective length of weld (L)(2) With run-out plate: L = length of weld(3) Without run-out plate: L = length of weld 2t
Butt weld connectionsdesign of butt welds
design principle of butt weldsa. Butt weld subject to compressive force: NO NEEDb. Butt weld under repeated load: Quality grade Ic. Butt weld under tension load: Quality grade II + run-out plated. Set the butt weld in the vicinity of lower stress
Steps to design of butt weld(1) Determine the internal force at the section to be checked(2) Calculate the section properties of A, S, W, I(3) Calculate the stress(4) Check the strength of weld
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Butt weld connectionsdesign of butt welds
Typical problem using butt welds
(1) butt-welded plates subject to axial load
(2) butt-welded plates subject to axial load (inclined welds)
(3) butt welds under shear force (plates and bracket)
(4) butt welds under combined shear and moment
equivalent stress
(5) butt welds under combined tensile, shear and moment
Fillet weld connectionsdetailing
Orthogonal fillet weld
Oblique (angle) fillet weld
End weld: transversely loaded fillet weldSide weld: fillet weld loaded parallel to the welds axis
hf
hf hfhfhf
hf
hf
hf
hf
normal fillet weld concave fillet weldunequal leg fillet weld
Fillet weld connectionsdetailing
Leg size of fillet weldMinimum: 1.5Xsqrt(tthick), prevent weld crack Maximum: 1.2tthin, prevent burn through
Length of fillet weldMinimum: 8hf & 40mm, avoid mass imperfectionMaximum: 60hf ,, avoid uneven stress distribution
Distance between two longitudinal fillet welds: shear lag
Weld symbolsFillet weld on one side / on both sideFillet weld all around joint (L, 3 or 4 sides)Fillet weld in the field
8
8
8
8
8
Fillet weld connectionsfailure mode
Stress distributionEnd weld: tri-axial stress
(brittle failure)Side weld: mainly shear stress
(ductile failure)
Failure plane (assumption)Effective plane = failure plane
(45 degree through the throat)Effective thickness = 0.7 leg size
(weld throat)
Fillet weld connectionsfailure mode
Failure plane and theoretical throatOrthogonal fillet weldOblique-angle fillet weld
Fillet weld connectionsfailure mode
Failure plane and stress distribution (assumption)Normal stress perpendicular to the throat plane
Shear stress (in the plane of the throat) perpendicular to the weld axisShear stress (in the plane of the throat) parallel to the weld axis
wff3)(3
2//
22 =++
//
1)75.0()75.0()( 2
2//
2
2
2
2
=++ wu
wu
wu fff
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Fillet weld connectionsfailure mode
2 2 2 2 w w3 0.5 1.5 2 3 1.22f f + + = = =
Failure plane and stress distribution (assumption)
wff3)(3
2//
22 =++
2 2 w w//3 3 3 f f = = =
// =
//
End weld: larger strength and rigid, less deformation ability
Side weld: 22% less than strength of end weldlarger deformation ability
Fillet weld connectionssimplified method
wff3)(3
2//
22 =++
simplified method for design resistance of fillet weld
amplification factor for weld strength perpendicular to the weld axis, taken as 1.22 for static loading and 1.0 for dynamic loading
wf
2f
2
f
f )( f+
f
wff design strength of fillet weld (same for shear, tension and compression)
For applied force N perpendicular to the weld axis
stress on the failure plane
f w e/N l h =
f w e/V l h =For applied force V parallel to the weld axis
//
fN
fV
w f2l l h= e f0.7h h=
Fillet weld connectionsprocedure of fillet weld design
Focus on the distinguishing of stress perpendicular to the weld axisand stress parallel to the weld axis
Calculation of weld section properties, A, S, I, W (weld length)
Centroid of welds coincides with that of members
Axial force, shear force or combined axial and shear forceCombined bending moment, axial and shear forcesCombined torsional moment, axial and shear forces
Stress calculation under single force
wf
2f
2
f
f )( f+
Analysis of internal forces at weld connection
Superposition of stress components at critical point, then check with practical equation
Fillet weld connectionstypical problem (1)
Axially loaded weld connections
wf
2f
2
f
f )( f+
N
(1) Internal force1N
V
sin1 NN =
cosNV =
(2) Weld stress
f
11f A
Nlh
N
we
==
ff A
Vlh
V
we
==
(3) Stress check
wf
f
0 ,0 fAN
=
wff
f
0 ,90 fAN =
Fillet weld connectionstypical problem (2)
Axially loaded weld connections ( C & Angle)
(1) 3 sides around welds (cover plate of flange)w
ff 1 e1 2 f2 e22( )
N fl h l h h
+ 1l2
l2l
NN
(2) 2 sides welds
(4) L-shape welds (angle) ?
NN1 f1,l h
2 f2,l h
1e2e b
1 2 1( / )N e b N k N= =Internal force2 1 2( / )N e b N k N= =
0.7
0.3 0.25
0.750.65
0.35
(root)(toe)
1k2k
(3) 3 sides around welds (angle)NN
1 1 30.5N k N N= 2 2 30.5N k N N=
Internal force
Fillet weld connectionstypical problem (3)
, , ,N V N V M
Nfx
f
NA
=
Vfy
fw
VA
==
weld connections subject to bending moment, axial and shear forces
(1) Internal force
(2) Weld stress
(3) Stress check
N
V
M
V
x
