CHAPTER TWENTY ONE
WELDED CONNECTIONS
21.1 Welded Connections
Welding is a process of joining two pieces of metal by heating their surfaces to a plastic or fluid state and allow the parts to fuse together usually with the addition of other molten metal. The bond between the members is completed after the molten metal solidifies. Structural welds are usually made either by the Shielded-Metal-Arc Welding Process (SMAW) or by the Submerged-Arc Welding Process (SAW).
In both process the heat of an electric are simultaneously melts the welding electrode and the adjacent steel parts being joined. The electrode is deposited in the weld as a filler material.
21.2 Types of Welding Process
A. Shielded Metal Arc Welding Process (SMAW)
This is the most common welding method using metallic rod which is used as the electrode. In arc welding an electric is formed between the pieces being welded and an electrode weld by the welder with a special type of holder. The electrodes produce a continuous spark which upon contact brings the electrode and the pieces being welded to the melting point. At the end of the electrodes, small droplets of molten metal are formed and are forced by the arc across the metal being connected, penetrating the molten metal to become part of the weld.
B. Submerged Arc Welding Process (SAW)
This method of welding is most often used in fabrication shop. The joint is aligned and covered with a blanket of granular fusible material and the electrode is inserted into the granular material, the arc produced and the melting of electrode and base metal takes place. The base metal electrode is kept continuously shielded by the molten flux over which is deposited a layer of infused flux in its granular condition. Welds made by the submerged arc process are found to have uniformly high quality, exhibiting, good ductility, high impact strengths and good corrosion resistance.
Three Classifications of Welds:
Based upon the types of welds.
Fillet welds
Groove welds
Plugged welds
Slot welds
Fillet Weld
Fillet welds are usually used for structural connections but they are weaker compared to the groove welds, which are welds made in grooves between members to be joined are they are used when the members to be connected are lined up in the same plane.
Groove Weld
Type of Groove welds:
Complete-penetration welds which extend for the full thickness of the part being connected.
Partial-penetration welds which extend for only part of the member thickness.
Plug Weld
Plug weld is a circular weld passing through one member to another and joining the two together.
Slot Weld
Slot weld is a weld form in a slot or elongated hole which joins one member to the other member through the slot. The slot maybe partly or fully filled with the weld material.
Based upon the type of joint used.
Butt joint
Lap joint
Tee joint
Edge
Corner
Based on the position of weld:
Flat weld
Horizontal weld
Vertical weld
Overhead weld
21.3 Fillet-Weld Nomenclature
a) Fillet-weld nomenclature for equal legs.
The strength of the fillet weld is equal to the allowable shearing stress times the theoretical throat area of the weld. The throat area equals the theoretical throat distance times the length of the weld.
The strength of the fillet weld is equal to the allowable shearing stress times the theoretical throat area of the weld. The throat area equals the theoretical throat distance times the length of the weld.
b) Fillet-weld nomenclature for unequal legs.
21.4 Values of Theoretical Throat for a Filet Weld
A. For SMAW Process (Shielded-metal-arc welding)
1. Te=0.707 t (for equal legs of nominal size "t") (See Figure 21.3-a)
t = size of fillet weld in mm
2. Te=aba²+b² for unequal legs of nominal size "a" and "b" (See Figure 21.3-b)
B. For SAW Process (Submerged Arc Welding)
1. When t < 10 mm
Te=t
2. When t > 10 mm
Te=0.707 t+3 mm
21.5 NSCP Specifications for Fillet Welds
1) Effective area:
The effective area of fillet welds shall be taken as the effective length times the effective throat thickness:
2) Effective length of fillet welds; except fillet welds in holes and slots shall be the overall length of full size fillets including returns.
3) The effective throat thickness of a fillet weld shall be the shortest distance from the root of the joint to the face of the diagrammatic weld, except that for fillet welds made by the submerged arc process (SAW), the effective throat thickness shall be taken equal to the leg size for 10 mm and smaller fillet welds and equal to the theoretical throat plus 3 mm for fillet welds larger than 10 mm.
Te=t for t 10mm
Te=0.707t+3mm for t >10mm SAW Process
4) For fillet welds in holes and slots, the effective length shall be the length of the center line of the weld along the center of the plane through the throat. In the case of over lapping fillets, the effective area shall not exceed the nominal cross sectional area of the hole slot in the plane of the faying surface.
Minimum Size of Fillet Welds
Material Thickness of Thicker Part Joined (mm)
Minimum Size of Fillet Weld (mm)
To 6 mm
3 mm
Over 6 to 12 mm
5 mm
Over 12 to 20 mm
6 mm
Over 20 mm
8 mm
Maximum Size of Fillet Welds
Material Thickness
Max. Size of Fillet Welds
< 6 mm
Not greater than thickness of material
> 6 mm
Not greater than thickness of material minus 1.6 mm
Minimum weld size is dependent upon the thickness of the two parts joined, except that the weld size need not to exceed the thickness of the thinner part. Base metal thickness is that of the thicker part being joined.
Maximum size of Fillet welds that is permitted along edged of connected parts shall be:
If the material is less than 6 mm or 6 mm thick the max. size of the weld should not be greater than the thickness of the material.
If the material is greater then 6 mm in thickness, the max. size of the fillet weld should not be greater than the thickness of material minus 1.6 mm., unless the weld is specially designated on the drawings to be built out to obtain full throat thickness.
Minimum effective length of the fillet welds
The minimum effective length of fille welds designated on the basis of strength shall not be less than 4 times the nominal size or else the size of the weld shall be considered not to exceed ¼ of its effective length.
Min. length of Longitudinal fillet welds:
If longitudinal fillet welds are used alone in end connections of flat bar tension members, the length of each fillet weld shall be not less than the perpendicular distance between them. If the load is transmitted to a plate by longitudinal welds along both ends at the end of the plate, the length of the welds shall not be less than the width of the plate.
The transverse spacing of longitudinal fillet welds used in end connections of tension members shall not exceed 200 mm. unless the member is designed on the basis of effective net area.
Effective Net Area of Fillet Welds
When the load is transmitted by transverse welds to some but not all of the cross-sectional elements of W, M or S shapes and structural tees cut from these shapes, the effective net area shall be taken as the area of the directly connected elements.
When the load is transmitted by welds through some but not all of the cross-sectional elements of the member, the effective net area Ae shall be computed as:
Ae = U Ag
Where: Ae = net area of member
Ag = gross area of member
U = reduction coefficient
Values of Reduction Coefficient U
When L > 2W----- U=1.0
When 2W > L and L > 1.5W----- U=0.87
When 1.5W > L and L > W----- U=0.75
Where L = weld length
W = plate width (distance between welds)
21.7 Intermittent Fillet Welds
Intermittent fillet welds are permitted to transfer calculated stress across a joint or faying surface when the strength required is less than the developed by a continuous fillet weld of the smallest permitted size, and to join components of built up members.
The effective length of any segment of intermittent weld shall not be less than 4 times the weld size, with a minimum of 38 mm.
21.8 Lap Joints
In lap joints, the minimum lap shall be 5 times the thickness of the thinner part joined, but not less than 25mm. Lap joints joining plates or bars subjected to axial stress shall be fillet welded along the end of both lapped parts except where the deflection of the lapped part is sufficiently restrained to prevent opening of the joint under maximum loading.
21.9 End Returns of Fillet Welds
(Sometimes called Boxing)
Slide of end fillet welds terminating at ends or sides respectively, or parts or members shall, wherever practicable, be returned continuously around the corners for a distance not less than 2 times the nominal size of the weld.
This provision shall apply to side and top fillet welds connecting brackets, beam seats, and similar connections, on the plane about which bending moments are computed. For framing angles and simple end-plate connections which depend upon flexibility of the outstanding legs for connection flexibility, end returns shall not exceed 4 times the nominal size of weld. End returns are useful in reducing the high concentrations which occurs at the ends of welds.
Fillet welds in holes or slots are permitted to transmit shear in lap joints or to prevent the buckling or separation of lapped parts and to join components of built up members. Such fillet welds may overlap subject to the provisions of fillet welds. Fillet welds in holes or slots are not to be considered plug or slot welds.
21.10 Basic Weld Symbols
Basic Weld Symbols
Back
Fillet
Plug or Slot
Groove or butt
Square
V
Bevel
U
J
Flare V
Supplementary Weld Symbols
Buckling
Spacer
Weld all around
Field Weld
Contour
Flush
Convex
Standard Location of Elements of a Welding Symbol
Note. Size, weld symbol, length of weld and spacing must read in that order from left to right along the reference line. Neither orientation of reference line nor location of the arrow alters this rule.
The perpendicular leg of weld symbols must be at left.
Arrow and Other Side welds are of the same size unless otherwise shown. Dimensions of fillet welds must be shown on both the Arrow Side and the Other Side Symbol.
Flag of field-weld symbol shall be placed above and right angle to reference line of junction with the arrow.
Symbols apply between abrupt changes in direction of welding unless governed by the All Around symbol or otherwise dimensioned.
These symbols do not explicitly provide for the case that frequently occurs in structural work, where duplicate material (such as stiffeners) occurs on the far side of a web or gusset plate. The fabricating industry has adopted this convention; that when the billing of the detail material discloses the existence of a member on the far side as well as on the near side, the welding shown for the near side shall be duplicated on the far side.
21.11 Basic Weld Symbols
Fillet weld on near side (arrow side). Side of weld (6mm) is given to left of weld symbol and length (150 mm) to right of symbol.
12.5 mm fillet weld on far side, 50 mm long intermittent welds 150 mm on centers.
6 mm fillet weld on both sides. 150 mm long. As welds same on both sides, not necessary, but permissible, to give sizes on both sides on line. The flag indicates it to be a field weld.
A staggered intermittent 10 mm fillet weld 50 mm long, 150 mm on centers both sides.
Weld-all-around joint symbol.
The tail used to indicate reference to a certain specification or process.
21.12 Allowable Stresses for Filet Weld
Type of Weld and Stress
Allowable Stress
Required Weld Strength Level
Complete-penetration Groove Welds
Tension normal to effective area
Same as base metal
"Matching" weld metal shall be used
Compression normal to effective area
Same as base metal
Weld metal with a strength level equal to or less than "matching" weld metal is permitted.
Tension or compression parallel to axis of weld
Same as base metal
Shear on effective area
0.30* nominal tensile strength of weld metal (MPa)
Partial-penetration Groove Welds
Compression normal to effective area
Same as base metal
Weld metal with a strength level equal to or less than "matching" weld metal is permitted.
Tension or compression parallel to axis of weld
Same as base metal
Shear parallel to axis of weld
0.30 * nominal tensile strength of weld metal (MPa)
Tension normal to effective area
0.30 * nominal tensile strength of weld metal (MPa) except tensile stress on base metal shall not exceed 0.60 x yield stress of base metal
Fillet Welds
Shear on effective area
0.30 * nominal tensile strength of weld metal (MPa)
Weld metal with a strength level equal to or less than "matching" weld metal is permitted.
Tension or compression parallel to axis of weld
Same as base metal
Plug and Slot Welds
Shear parallel to faying surfaces (on effective area)
0.30* nominal tensile strength of weld metal (MPa)
Weld metal with a strength level equal to or less than "matching" weld metal is permitted.
Allowable Stresses
Tension or compression for weld metal.
Ft = 0.60 Fy
Shear stress for fillet welds.
Fv = 0.30 Fu
21.13 Welding Electrodes
A variety of electrodes are available so that proper match of base metal strength and metallurgical characteristics to the weld metal can be made. The AWS in cooperation with ASTM has established an electrode numbering system which classifies welding electrodes or rods as follows.
E xxx b c
Where: E = electrode
xxx = two or three digit number indicating the ultimate tensile strength of the weld metal such as 60 ksi (415 MPa)
b = digit to indicate the suitability of welding position, which maybe flat, horizontal, vertical or overhead.
b = 1 for suitable for all position
b= 2 suitable for horizontal fillets and flat position of work
c= digit indicating current supply and welding technique
c = 1 for use with either ac or dc current
c = 2 for use with dc straight polarity
c =3 for use with dc reversed polarity
Example: E 6011 Exxbc
It is an electrode with Fu = 60 ksi (450 MPa) which is an all position welding and for use with either ac or dc current.
Example: E7021
It is an electrode with Fy = 70 ksi (485 MPa) which is suitable for horizontal fillets and flat position of work and for use of with either ac or dc current.
21.14 Minimum Tensile Strength of Electrodes (Fu)
Electrodes
E 60
E70
E 80
E 90
E 100
E 110
E 120
Minimum Tensile Strength (Fu)
415 MPa
485 MPa
550 MPa
620 MPa
690 MPa
760 MPa
825 MPa
