CONNECTION DESIGN • Connections must be designed at the strength limit state – Average of the factored force effect at the connection and the force effect in the member at the same point – At least 75% of the force effect in the member • End connections for diaphragms, cross-frames, lateral bracing for straight flexural members - designed for factored member loads • Connections should be symmetrical about member axis – At least two bolts or equivalent weld per connection – Members connected so that their gravity axes intersect at a point – Eccentric connections should be avoided • End connections for floorbeams and girders – Two angles with thickness > 0.375 in. – Made with high strength bolts – If welded account for bending moment in design
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CONNECTION DESIGN• Connections must be designed at the strength limit state
– Average of the factored force effect at the connection and the force effect in the member at the same point
– At least 75% of the force effect in the member
• End connections for diaphragms, cross-frames, lateral bracing for straight flexural members - designed for factored member loads
• Connections should be symmetrical about member axis– At least two bolts or equivalent weld per connection
– Members connected so that their gravity axes intersect at a point
– Eccentric connections should be avoided
• End connections for floorbeams and girders– Two angles with thickness > 0.375 in.
– Made with high strength bolts
– If welded account for bending moment in design
BOLTED CONNECTIONS
• Slip-critical and bearing type bolted connections.
• Connections should be designed to be slip-critical where:
– stress reversal, heavy impact loads, severe vibration
– joint slippage would be detrimental to the serviceability of the structure
• Joints that must be designed to be slip-critical include
– Joints subject to fatigue loading or significant load reversal.
– Joints with oversized holes or slotted holes
– Joints where welds and bolts sharing in transmitting load
– Joints in axial tension or combined axial tension and shear
• Bearing-type bolted connections can be designed for joints subjected to compression or joints for bracing members
SLIP-CRITICAL BOLTED CONNECTION
• Slip-critical bolted connections can fail in two ways: (a) slip at the connection; (b) bearing failure of the connection
• Slip-critical connection must be designed to: (a) resist slip at load Service II; and (b) resist bearing / shear at strength limit states
SLIP-CRITICAL BOLTED CONNECTION• Slip-critical bolted connections can be installed with such a degree
of tightness large tensile forces in the bolt clamp the connected plates together
• Applied Shear force resisted by friction
Tightened
P
P
Tightened Tightened
P
P
P
P
Tb
N =Tb
N =Tb
N =Tb
P
F=N
Tb
N = Tb
F=N
N = Tb
N =Tb
P
Tb
N =Tb
Tb
N =Tb
N =Tb
N =Tb
P
F=N
N =Tb
N =Tb
P
F=N
Tb
N = Tb
Tb
N = Tb
F=N
N = Tb
N =Tb
P
F=N
N = Tb
N =Tb
N = Tb
N =Tb
P
SLIP-CRITICAL BOLTED CONNECTION
• Slip-critical connections can resist the shear force using friction.
– If the applied shear force is less than the friction that develops between the two surfaces, then no slip will occur between them
• Nominal slip resistance of a bolt in a slip-critical connection
• Example 1 Design a slip-critical splice for a tension member. For the Service II load combination, the member is subjected to a tension load of 200 kips. For the strength limit state, the member is subjected to a maximum tension load of 300 kips.
– The tension member is a W8 x 28 section made from M270-Gr. 50 steel. Use A325 bolts to design the slip-critical splice.
• Step I. Service and factored loads
– Service Load = 200 kips.
– Factored design load = 300 kips
– Tension member is W8 x 28 section made from M270 Gr.50. The tension splice must be slip critical (i.e., it must not slip) at service loads.
BOLTED CONNECTION
Step II. Slip-critical splice connection
• Slip resistance of one fully-tensioned slip-critical bolt = Rn = Kh Ks Ns Pt
– = 1.0 for slip-critical resistance evaluation
– Assume bolt diameter = d = ¾ in. Therefore Pt = 28 kips from Table 1
– Assume standard holes. Therefore Kh = 1.0
– Assume Class A surface condition. Therefore Ks = 0.33
– Therefore, Rn = 1.0 x 0.33 x 1 x 28 = 9.24 kips
• Therefore, number of ¾ in. diameter bolts required for splice to be slip-critical at service loads = 200 / 9.24 = 21.64.
• Therefore, number of bolts required ≥ 22
BOLTED CONNECTION
Step III: Layout of flange-plate splice connection
• To be symmetric about centerline, need the number of bolts = multiple of 8.
• Therefore, choose 24 fully tensioned 3/4 in. A325 bolts with layout above. – Slip-critical strength of the connection = 24 x 9.24 kips = 221.7 kips
• Minimum edge distance (Le) = 1 in. from Table 4.
– Design edge distance Le = 1.25 in.
• Minimum spacing = s = 3 x bolt diameter = 3 x ¾ = 2.25 in.– Design spacing = 2.5 in.
BOLTED CONNECTION
Step IV: Connection strength at factored loads
• The connection should be designed as a normal shear/bearing connection beyond this point for the factored load of 300 kips
• Shear strength of high strength bolt = Rn = 0.80 x 0.38 x Ab x Fub Ns
– Equation given earlier for threads included in shear plane.
– Ab = 3.14 x 0.752 / 4 = 0.442 in2
– Fub = 120 ksi for A325 bolts with d < 1-1/8 in.
– Ns= 1
– Therefore, Rn = 16.1 kips
• The shear strength of 24 bolts = 16.1 kips/bolt x 24 = 386.9 kips
BOLTED CONNECTION
• Bearing strength of 3/4 in. bolts at edge holes (Le = 1.25 in.)
– bb Rn = 0.80 x 1.2 Lc t Fu
Because the clear edge distance = 1.25 – (3/4 + 1/16)/2 = 0.84375 in. < 2 d
– bb Rn = 0.80 x 1.2 x 0.84375 x 65 kips x t = 52.65 kips / in. thickness
• Bearing strength of of 3/4 in. bolts at non-edge holes (s = 2.5)
– bb Rn = 0.80 x 2.4 d t Fu
Because the clear distance between holes = 2.5 – (3/4 + 1/16) = 1.6875 in. > 2d
– bb Rn = 0.80 x 2.4 x 0.75 x 65 kips x t = 93.6 kips / in. thickness
• Bearing strength of bolt holes in flanges of wide flange section W8 x 28 (t = 0.465 in.)
– The shielded metal arc welding (SMAW) process for field welding.
– Submerged metal arc welding (SAW) used for shop welding – automatic or semi-automatic process
– Quality control of welded connections is particularly difficult because of defects below the surface, or even minor flaws at the surface, will escape visual detection.
– Welders must be properly certified, and for critical work, special inspection techniques such as radiography or ultrasonic testing must be used.
WELDED CONNECTIONS
• Two most common types of welds are the fillet and the groove weld.
– lap joint – fillet welds placed in the corner formed by two plates
– Tee joint – fillet welds placed at the intersection of two plates.
• Groove welds – deposited in a gap or groove between two parts to be connected e.g., butt, tee, and corner joints with beveled (prepared) edges
– Partial penetration groove welds can be made from one or both sides with or without edge preparation.
Fillet weld
Fillet weld
Fillet weldFillet weld
Fillet weldFillet weld
WELDED CONNECTIONS• Design of fillet welded connections
– Fillet welds are most common and used widely
– Weld sizes are specified in 1/16 in. increments
– Fillet welds are usually fail in shear, where the shear failure occurs along a plane through the throat of the weld
– Shear stress in fillet weld of length L subjected to load P
fv =
a
aThroat = a x cos45o
= 0.707 a
a
aThroat = a x cos45o
= 0.707 a
Failure Plane
L
wLa707.0P
FILLET WELDED CONNECTIONS• The shear strength of the fillet weld = e2 0.60 Fexx
– Where, e2 = 0.80
– Fexx is the tensile strength of the weld electrode used in the welding process. It
can be 60, 70, 80, 90, 100, 110, or 120 ksi. The corresponding electrodes are specified using the nomenclature E60XX, E70XX, E80XX, and so on.
• Therefore, the shear strength of the fillet weld connection
– Rn = e2 x 0.60 Fexx x 0.707 a Lw
• Electrode strength should match the base metal strength
– If yield stress (y) of the base metal is 60 - 65 ksi, use E70XX electrode
– If yield stress (y) of the base metal is 60 - 65 ksi, use E80XX electrode
• E70XX is the most popular electrode used for SMAW fillet welds
– For E70XX, Rn = 0.80 x 0.60 x 70 x 0.707 a Lw = 0.2375 a Lw kips
FILLET WELDED CONNECTIONS• The shear strength of the base metal must be considered:
– Rn = v x 0.58 Ag Fy
where, v = 1.0
Fy is the yield strength of the base metal and Ag is the gross area in shear
Strength of weld in shear Strength of base metal
= 0.80 x 0.60 x Fexx x 0.707 x a x Lw = 1.0 x 0.58 x Fy x t x Lw
Smaller governs the strength of the weld
T
Elevation Plan
T
Elevation Plan
FILLET WELDED CONNECTIONSLimitations on weld dimensions
• Minimum size (amin)
– Weld size need not exceed the thickness of the thinner part joined.
– amin depends on the thickness of the thicker part joined
– If the thickness of the thicker part joined (T) is less than or equal to ¾ in. amin = ¼ in.
– If T is greater than ¾ in. amin = 5/16 in.
• Maximum size (amax)
– Maximum size of fillet weld along edges of connected parts
– for material with thickness < 0.25 in., amax = thickness of the material
– for plates with thickness 0.25 in., amax = thickness of material - 1/16 in.
• Minimum length (Lw)
– Minimum effective length of fillet weld = 4 x size of fillet weld
– Effective length of fillet weld > 1.5 in.
FILLET WELDED CONNECTIONS
• Weld terminations and end returns
– End returns must not be provided around transverse stiffeners
– Fillet welds that resist tensile forces not parallel to the weld axis or proportioned to withstand repeated stress shall not terminate at corners of parts or members
– Where end returns can be made in the same plane, they shall be returned continuously, full size around the corner, for a length equal to twice the weld size (2a)
FILLET WELD DESIGNExample 1 Design the fillet welded connection system for a double
angle tension member 2L 5 x 3½ x 1/2 made from A36 steel to carry a factored ultimate load of 250 kips.
Step I. Design the welded connection
Considering only the thickness of the angles; amin = 1/4 in.
Considering only the thickness of the angles; amax = 1/2 - 1/16 in. = 7/16 in.
Design, a = 3/8 in. = 0.375 in.
Shear strength of weld metal = Rn = 0.80 x 0.60 x FEXX x 0.707 x a x Lw
= 8.9 x Lw kips
Strength of the base metal in shear = Rn = 1.0 x 0.58 x Fy x t x Lw
= 10.44 Lw kips
Shear strength of weld metal governs, Rn = 8.9 Lw kips
FILLET WELD DESIGN
• Design strength Rn > 250 kips
– Therefore, 8.9 Lw > 250 kips
– Therefore, Lw > 28.1 in.
• Design length of 3/8 in. E70XX fillet weld = 30.0 in.
• Shear strength of fillet weld = 267 kips
• Connection layout
– Connection must be designed to minimize eccentricity of loading. Therefore, the center or gravity of the welded connection must coincide with the center of gravity of the member.
(d)
Tu
f L2
f L1
L1
L2
3.4 in.
(d)
Tu
f L2
f L1
L1
L2
3.4 in.
FILLET WELD DESIGN
• Connection layout
– Connection must be designed to minimize eccentricity of loading.
– The c.g. of the welded connection must coincide with c.g. of the member
– Total length of weld required = 30 in.
– Two angles assume each angle will have weld length of 15 in.
(d)
Tu
f L2
f L1
L1
L2
3.4 in.
(d)
Tu
f L2
f L1
L1
L2
3.4 in.
FILLET WELD DESIGN
• The tension force Tu acts along the c.g. of the member, which is
1.65 in. from the top and 3.35 in. from the bottom (AISC manual).
– Let, f be the strength of the fillet weld per unit length.
Therefore, fL1 + fL2 = Tu
And fL2 x 3.35 - fL1 x 1.65 = 0 - taking moments about the member c.g.
– Therefore, L1 = 2.0 L2
But, L1 + L2 = 15.0 in.
– Therefore, L1 = 10 in. and L2 = 5 in.
Design: L1 = 10.0 in. and L2 = 5.0 in.
FILLET WELD DESIGN
• Consider another layout
(e)
Tu
f L2
L1
L2
f L1
5f 3.4 in.
(e)
Tu
f L2
L1
L2
f L1
5f 3.4 in.
fL1 + fL2 + 5f = Tu
fL2 x 3.5 + 5f x 0.85 - fL1 x 1.65 = 0 - Moment about member c.g. Additionally, L1 + L2 + 5 = 15.0 in. Therefore, L1 = 7.6 in. and L2 = 2.4 in.
Design: L1 = 8.0 in. and L2 = 3.0 in.
Groove Welded Connections• Connects structural members that are aligned in the same plane
• Basic Types:
– Complete joint penetration groove weld: transmits full load of the member they join and have the same strength as the base metal.
– Partial penetration groove weld: Welds do not extend completely through the thickness of the pieces being joined.