CONNECTION 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 gt 0.375 in.
• Made with high strength bolts
• If welded account for bending moment in design

2
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

3
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

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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

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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
• Rn Kh Ks Ns Pt
• Where, Pt minimum required bolt tension
  specified in Table 1
• Kh hole factor specified in Table 1
• Ks surface condition factor specified in Table
  3

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SLIP-CRITICAL BOLTED CONNECTION
Values of Kh

• Faying surfaces
• Unpainted clean mill scale, and blast-cleaned
  surfaces with Class A coating
• Unpainted blast-cleaned surfaces with Class B
  coating
• Hot-dip galvanized surfaces roughened by wire
  brushing Class C

Values of Pt
Values of Ks
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SLIP-CRITICAL CONNECTION

• Connection subjected to tensile force (Tu), which
  reduces clamping
• Nominal slip resistance should be reduced by (1-
  Tu/Pt)
• Slip is not a catastrophic failure limit-state
  because slip-critical bolted connections behave
  as bearing type connections after slip.
• Slip-critical bolted connections are further
  designed as bearing-type bolted connection for
  the applicable factored strength limit state.

8
BEARING CONNECTION

• In a bearing-type connection, bolts are subjected
  to shear and the connecting / connected plates
  are subjected to bearing stresses

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BEARING CONNECTION

• Bearing type connection can fail in several
  failure modes
• Shear failure of the bolts
• Excessive bearing deformation at the bolt holes
  in the connected parts
• Edge tearing or fracture of the connected plate
• Tearing or fracture of the connected plate
  between two bolt holes
• Failure of member being connected due to fracture
  or block shear or ...

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BEARING CONNECTION

• Nominal shear resistance of a bolt
• Threads excluded Rn 0.48 Ab Fub Ns
• Threads included Rn 0.38 Ab Fub Ns
• Where, Ab area of the bolt corresponding to
  the nominal diameter
• Fub 120 ksi for A325 bolts with diameters 0.5
  through 1.0 in.
• Fub 105 ksi for A325 bolts with diameters
  1.125 through 1.5 in.
• Fub 150 ksi for A490 bolts.
• Ns number of shear planes
• Resistance factor for bolts in shear fs 0.80
• Equations above - valid for joints with length lt
  less than 50.0 in.
• If the length is greater than 50 in., then the
  values from the equations have to be multiplied
  by 0.8

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BEARING CONNECTION

• Effective bearing area of a bolt the bolt
  diameter multiplied by the thickness of the
  connected material on which it bears
• Bearing resistance for standard, oversize, or
  short-slotted holes in any direction, and
  long-slotted holes parallel to the bearing force
• For bolts spaced with clear distance between
  holes greater than or equal to 3.0 d
• and for bolts with a clear end distance greater
  than or equal to 2.0 d
• Rn 2.4 d t Fu
• For bolts spaced with clear distance between
  holes less than 3.0 d
• and for bolts with clear end distances less than
  2.0 d
• Rn 1.2 Lc t Fu
• Where, d nominal bolt diameter
• Lc clear distance between holes or between the
  hole and the end of the member in
• the direction of applied bearing force
• Fu tensile strength of the connected material
• The resistance factor fbb for material in bearing
  due to bolts 0.80

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BEARING CONNECTION

• SPACING REQUIREMENTS
• Minimum spacing between centers of bolts in
  standard holes shall not be less than three times
  the diameter of the bolt
• For sealing against penetration of moisture in
  joints, the spacing on a single line adjacent to
  the free edge shall satisfy s (4.0 4.0 t)
  7.0
• Minimum edge distances

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BOLTED 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.

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BOLTED CONNECTION

• Step II. Slip-critical splice connection
• Slip resistance of one fully-tensioned
  slip-critical bolt Rn Kh Ks Ns Pt
• f 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, fRn 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

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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.

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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 f 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 lt 1-1/8 in.
• Ns 1
• Therefore, fRn 16.1 kips
• The shear strength of 24 bolts 16.1 kips/bolt x
  24 386.9 kips

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BOLTED CONNECTION

• Bearing strength of 3/4 in. bolts at edge holes
  (Le 1.25 in.)
• fbb Rn 0.80 x 1.2 Lc t Fu
• Because the clear edge distance 1.25 (3/4
  1/16)/2 0.84375 in. lt 2 d
• fbb 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)
• fbb Rn 0.80 x 2.4 d t Fu
• Because the clear distance between holes 2.5
  (3/4 1/16) 1.6875 in. gt 2d
• fbb 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.)
• 8 x 52.65 x 0.465 16 x 93.6 x 0.465 892 kips

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CONNECTION STRENGTH

• Connection strength (fRn) gt applied factored
  loads (gQ).
• Therefore ok

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WELDED CONNECTIONS

• Introduction
• 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.

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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.

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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

       
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FILLET WELDED CONNECTIONS

• The shear strength of the fillet weld fe2 0.60
  Fexx
• Where, fe2 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
• fRn fe2 x 0.60 Fexx x 0.707 a Lw
• Electrode strength should match the base metal
  strength
• If yield stress (sy) of the base metal is ? 60 -
  65 ksi, use E70XX electrode
• If yield stress (sy) of the base metal is ? 60 -
  65 ksi, use E80XX electrode
• E70XX is the most popular electrode used for SMAW
  fillet welds
• For E70XX, fRn 0.80 x 0.60 x 70 x 0.707 a Lw
  0.2375 a Lw kips

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FILLET WELDED CONNECTIONS

• The shear strength of the base metal must be
  considered
• f Rn fv x 0.58 Ag Fy
• where, fv 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

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FILLET WELDED CONNECTIONS

• Limitations 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 lt 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 gt 1.5 in.

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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)

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FILLET WELD DESIGN

• Example 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 f 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 f Rn 1.0
  x 0.58 x Fy x t x Lw
• 10.44 Lw kips
•          Shear strength of weld metal governs, f
  Rn 8.9 Lw kips

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FILLET WELD DESIGN

• Design strength f Rn gt 250 kips
• Therefore, 8.9 Lw gt 250 kips
• Therefore, Lw gt 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.

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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.

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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.

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FILLET WELD DESIGN

• Consider another layout

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.
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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.

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Groove Welds

• Complete penetration groove welded connections
• Tension and compression loaded
• Factored resistance factored resistance of
  base metal
• Shear loaded on effective area ? lesser of
• Factored resistance of weld 0.6 x fe1 x Fexx
  0.6 x 0.85 x Fexx
• 60 of factored resistance of base metal in
  tension
• Partial penetration groove-welded connections
• Tension or compression parallel to the weld axis
  and compression normal to effective area ?
  factored resistance of the base metal
• Tension normal to the effective area ? lesser of
• Factored resistance of the weld 0.6 fe2 Fexx
  0.60 x 0.80 x Fexx
• Factored resistance of the base metal
• Shear loaded ? lesser of
• Factored resistance of the weld 0.6 fe2 Fexx
  0.60 x 0.80 x Fexx
• Factored resistance of base metal 0.58 Fy

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Groove Welds