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    Welded connection

    Chapter-2

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    What is welded connectionsWelding consist of joining two pieces of metal by

    establishing a metallurgical bond between them. The

    elements to be connected are brought closer and the metal

    is melted by means of electric arc or oxy-acetylene flame

    along with weld rod which add metal to the joint. After

    cooling the bond is established between the two elements

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    Advantage of Welded connections1. No use of gusset plates, connecting angles etc., Welded structures

    are lighter.2. Welding process is easier as compare to making holes for

    fasteners.

    3. Welding is more adaptable than bolting or riveting. For example,

    even circular tubes can be easily connected by welding

    4. 100 % efficiency can be achieved whereas in bolted connection it

    can reach a maximum of 70-80 % only

    5. Noise produced in welding process is relatively less.

    6. Have good aesthetic appearance

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    Advantages8) Welded connection is airtight and watertight because of that less

    danger of steel structures and welded connection is preferredfor water tanks

    9) Welded joint are rigid

    10) There is no problems in mismatching of holes in welded

    connections whereas in bolted connections mismatching of bolt

    holes creates considerable problem.

    11) Alterations in connection can be easily made in the design of

    welded connections.

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    Disadvantages of welded connections1. Due to uneven heating and cooling members are likely to

    distort in the process of welding.

    2. Possibility of brittle fracture in welding.

    3. Welded joint fails earlier than a bolted joint, if the structure is

    under fatigue stresses.

    4. The inspection of welded joint is difficult and expensive. it

    needs non destructive testing

    5. Highly skilled person is required for welding

    6. Proper welding in field conditions is difficult

    7. Welded joint are over rigid

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    Types of Welded joints

    Butt Weld

    Fillet weld

    Slot weld and plugweld

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    Types of butt weldSquare butt weld on one side

    Square butt weld, both sides

    Single V butt joint

    Double V butt joint

    Single U butt joint

    Single J-butt joint

    Single bevel butt joint

    Similarly there can be double U, double J and double bevel butt joints

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    Fillet Weld

    It is a weld of approximately triangular cross section joining two

    surfaces approximately right angles to each other in lap joint, tee

    joint or corner joint

    When the cross section of fillet weld is isoscele triangle with faceat 45, it is known as standard fillet weld. In special circumstances

    60 and 30 angles are also used.

    A fillet weld is known as concave fillet weld, convex fillet weld or

    as mitre fillet weld depending upon the shape of weld face.

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    fillet weld

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    Slot weld or plug Weld

    A weld in which a plate with circular hole is kept with another

    plate to be joined and then fillet welding is made along the

    periphery of the hole.

    A plug weld in which small holes are made in one plate and is

    kept over another plate to be connected and then the entire hole isfilled with filler material

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    Important Specifications For welding

    Welding shall conform to IS 816 and IS 9595.some important

    specifications regarding butt weld, fillet weld and plug and

    slot weld as per IS 800-2007

    Butt Weld

    It shall be specified by the effective throat thickness.

    Complete Penetration- thickness of the thinner part joint

    taken as a size of butt weld ( Double U, Double V, Double Jand Double bevel butt welds may be generally regarding as

    complete penetration butt weld

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    Effective throat thickness in case of incomplete penetration

    The minimum thickness of the weld metal common to the

    part joined, excluding reinforcment. in the absence of actual

    data it may be taken as 5/8 of thickness of thinner material.

    The effective length of butt weld shall be taken as the length

    of full size weld.

    The minimum length of butt weld shall be four times the

    size of the weld

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    If intermittent but welding is

    used, it shall have an effective

    length of not less than four

    times the weld size and space

    between the two welds shall not be more than 16 times the

    thickness of the thinner part

    joined.

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    Fillet weldSize of fillet weld

    The size of normal fillet weld shall be taken as the

    minimum weld leg size.

    For Deep penetration welds with penetration not less than2.4mm, size of weld is minimum leg size+2.4mm

    For fillet welds made by semi automatic or automatic

    processes with deep penetration more than 2.4mm, if

    purchaser and contractor agree

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    S = minimum leg size + actual penetration

    2. Minimum size of fillet weld specified is 3m. To avoid therisk of cracking in the absence of preheating the

    minimum size specified are

    For less than 10 mm plate 3mm

    For 10 to 20 mm plate 5mm

    For 20 to 32 mm plate 6mmFor 32 to 50 mm plate 8mm

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    3. Effective throat thickness

    it shall not be less than 3mm and shall not genrally

    exceed 0.7t.t is thickness of the thinner plate of the

    element being welded. If the faces of plates being welded

    are inclined to each other, the effective throat thickness

    shall be taken as K times the fillet size where K depend

    upon the angle between fusion faces

    4) Effective length should not be less than four times the sizeof the weld.

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    4. Lap jointMinimum lap should be four times the thickness of thinner

    part joined or 40 mm whichever is more. The length of weldalong either edge should not be less than the transversespacing of welds

    5. Intermittent welds

    Length shall not be more than 4 times the weld size or 40mm whichever is more.Minimum clear spacing------ 12t for compression joint

    16t for tensile jointt is the thickness of the thinner plate joined.it will not be used in positions subject to dynamic, repetitiveand alternating stress.

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    Plug weld

    The effective area of a plug weld shall be considered the

    nominal area of the hole

    Design Stress in weld

    Butt weld

    It shall be treated as parent metal with a thickness equal to the

    throat thickness and the stresses shall not exceed those

    permitted in the parent metal.

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    Fillet Weld, Slot or Plug weldDesign strength shall be based on its throat area and shall be

    given by f wd = f wn / mw

    f wn = fu/3 Fu smaller of the ultimate stress of the weld or of t parent metalmw = 1.25 for shop weld and 1.5 for filled weld

    The following provisions are made in the code for the fillet wapplied to the edge of a plate or section

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    1. If a fillet weld is to the square edge of a part, the specified size of

    the weld should generally be at least 1.5 mm less than the edge

    thickness

    2. If fillet weld is to the rounded toe of a rolled section, the specified

    size of the weld should generally not exceed 3/4 th of the thickness

    of the toe.3. In members subject to dynamic loading, the fillet weld shall be of

    full size with its leg length equal to the thickness of the plate

    4. End fillet weld , normal to the direction shall be of unequal size

    with throat thickness not less than 0.5t.the difference in the

    thickness of weld shall be negotiated at a uniform slope.

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    Reduction in design stresses for long joints

    If the length of the welded joint lj is greater than 150t ,where t is

    throat thickness, the design capacity of weld fwd shall be redu

    by the factor

    l ld d

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    Example on welded jointA 18 mm thick plate is joined to a 16 mm plate by

    200 mm long (effective) butt weld. Determine thestrength of joint ifi. A double V butt weld is usedii. A single V butt weld is used

    Assume that Fe 410 grade plates and shop welds areused

    Case (i) double V butt weldThroat thickness= thickness of thinner plate t= 16mmEffective lengthLw = 200mm, fu = 410 N/mm, shop weld =

    1.25Effective area of weld = effective length x Throat thickness

    Design strength of weld =( Lw x t x fu/3)/

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    (200 x 16 x 410/3)/1.25 = 605987N (605.987kN)

    Case (ii) Single V butt weld jointSince penetration is not complete, effective throat

    thickness t = (5/8) x 16=10 mm

    Design strength = =( Lw x t x fu/3)/ mv(200 x 10 x 410/3)/1.25 = 378.742kN

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    A tie member in a truss girder is 250 mm x 14 mmin size. It is welded to a 10 mm thick gusset plateby a fillet weld. The overlap of the member is 300mm and the weld size is 6mm. Determine thedesign strength of the joint, if welding is done asshown in fig below. What is the increase instrength of the joint, if welding is done all around.Assume shop welding.

    300 mm

    250

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    For Fe 410 grade steel fu = 410 Mpa and fy = 250 MpaFor shop weld: partial safety factor for material= 1.25

    Effective length of the weld Lw = 2x 300+250 =850 mmEffective throat thickness t = KS= 0.7x 6= 4.2mmDesign strength of the weld, fwd = (L w x t x(fu /3 x )) ( 850 x 4.2 x(410/ 3x1.25))

    676050 N , 676.05 kNWhen the weld ing i s d one a ll aroun d

    300 mm

    250

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    Effective length of the weld Lw = 2x (300+250) = 1100mm

    (L w x t x(fu /3 x )) ( 1100 x 4.2 x(410/ 3x1.25))

    874890 N , 874.890 k N

    Inc rease in s treng th o f th e joint = 874.89- 676.05

    198.85k N

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    A tie member 75mm x 8mm is to transmit a factoredload of 145kN. Design fillet weld and necessaryoverlaps for the case shown in figure below The steelused is of grade Fe 410. Assume gusset plate to be 12mm thick.

    (a) Shop welding

    75mm

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    (b) Shop Welding

    75mm

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    (c) Site Welding

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    Eccentric connection-Plane of momentand the Plane of welds is the same

    The eccentric load P is equivalent to A direct load P at the centre of gravity of the group ofweld A twisting momentP x e

    d

    e

    X X

    Y

    Y

    rmax

    G

    b

    P

    q 1q 2

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    Let a w eld o f u n i fo rm s ize be p rov ided th rou gh ou t

    and t b e the e ffec t ive throa t th ick ness . if d i s d epth

    of th e w eld and b i s the w id th . The d i rec t sh ear

    s t ress in th e w eld i s

    q 1 = (p /(2b +d )x t)

    Due to tw is t ing m om ent the s t r es s in the we ld i s

    m axim um in the w eld a t the ex t rem e d i s t ance f rom

    C. G of the gro up of w eld and ac ts in the d i rec t ion

    perpendic ular to th e rad iu s vec to r. The m axim um

    st ress d ue to the mom ent

    =

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    C bi d i l d h t

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    Combined axial and shear stressCombined axial and shear stress

    If weld is subjected to axial stresses, compression ortension due to axial force or bending momentsimultaneously with shear, IS 800:2007 made the followingprovisions

    Fillet Weld : equivalent stress e = fa + 3q u/3mw

    fa- axial stress, direct due to bending.

    q- shear stress due to shear force or tension

    Check for the combination of stresses need not to bedone for :

    (a) side fillet weld joining cover plates and flange plates,

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    Eccentric connection- Moment at right angles tothe plane of weld

    P is a factored load at an eccentricity eh is the effective depth of the fillet weldFillet welding is on both sides of the bracket plate effectivearea of the weld 2ht e

    X X

    Y

    Y

    G

    P

    hh

    Throatthickness

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    Direct shear stress q= p/2htBending stress at the extreme edge of the weld f =m/z P.e/((2ht)x1 = 6pe/2th

    Equivalent stress- e = fa + 3q u/3mwFor the purpose of finding the effective depth hrequired, firs depth required for bending only may befound. To take care of shear a;so. Increase this valueby about 10 %.

    h = 6M/(2tfw d)Hence try h = 1.1h

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    Load = 100 kN

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    Load = 100 kN

    Factored load, p = 100 x 1.5= 150 kN

    Thickness of the flane pf ISHB 300 @ 618 N/m is 10.6mm

    Minimum size of the weld= 5mm

    Use 8mm fillet weld on each side of bracket plate.

    Throat thickness t = 0.7x8

    Resistance of weld fwd = (fu/3)/(1/1.25)

    (410/3) /(1/1.25) = 189.37 N/mm Depth of weld required to resist bending alone = h =(6x150x1000x150)/(2x0.7x8x189.37) = 252.3mm

    About 10% extra depth is to be provided.

    Let h= 280mm

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    Check for the stressesDirect shear stress q = p/(2th)

    150x1000/(2x0.7x8x280)Bending stress f = M/Z = (6M/2th) (6x150x1000x150)/2x0.7x8x280)

    =153.744 n/mm

    f +3q = 153.744 +3x47.83 = 174.6 N/mm < 189.37 N/mm Hence design is safe.

    E l

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    ExampleDesign a fillet weld for the bracket shown in fig below,

    eccentricity e= 250mm. Thickness of the bracket plateis 12mm and the column used is ISHB 300 @ 618 N/m

    Throatthickness

    X XG

    w

    400mm

    250mm

    400mm

    250mm

    165mm

    A i l fill ld

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    Assuming normal fillet welds

    Throat thickness of the weld=0.7x10=7mm

    Throat thickness of the web weld=0.7x5=3.5mmTotal throat thickness of web welds= 165x7x2+250x3.5x2=4060mm

    Ixx= (3.5x250)/12 +((165x7)x200) x2 = 101514583mm4. Bending moment to be transferred= p x 2250kN-mm, if P isthe factored load in kN

    Consider flange weld which is subjected to maximum stress=qv =Px1000/4060 = 0.2463N/mm

    Due to bending = qh= ((250x1000P)/(101514583)x200) =

    h

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    q= qv+qh P x0.2463+0.49254 =0.5507P

    0.5507P=(410/3)x(1/1.5) equating it to design stress p = 286.57kN

    Working load that can be permitted= P/1.5 = 191.04kN

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