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CONNECTION DESIGN

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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 – PowerPoint PPT presentation

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Title: CONNECTION DESIGN


1
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

4
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

5
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

6
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
7
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

9
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 ...

10
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

11
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

12
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

13
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.

14
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

15
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.

16
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

17
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

18
CONNECTION STRENGTH
  • Connection strength (fRn) gt applied factored
    loads (gQ).
  • Therefore ok

19
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.

20
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.

21
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

       
22
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

23
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

24
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.

25
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)

26
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

27
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.

28
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.

29
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.

30
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.
31
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.

32
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

33
Groove Welds
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