9.17. Welded Connections
Structural welding is a process whereby the parts to be connected are heated and fused, with supplementary molten
metal added to the joint. A relatively small depth of material will become molten, and upon cooling, the structural steel
and the weld metal will act as one continuous part where they are joined. The additional metal, sometimes referred to as
filler metal, is deposited from a special electrode, which is part of an electrical circuit that includes the connected part, or
base metal.
9.18. Advantages
1. The use of welding permits large savings in pounds of steel used. Welded structures allow the elimination of a
large percentage of gusset and splice plates necessary for bolted structures as well as the elimination of bolt heads.
2. Welding has a much wider range of application than bolting.
3. Welded structures are more rigid because the members often are welded directly to each other.
4. The process of fusing pieces together creates the most truly continuous structures.
5. It is easier to make changes in design and to correct errors during erection (and less expensive) if welding is used.
6. Relative silence in the workplace.
7. Fewer pieces are used, and as a result, time is saved in detailing, fabrication, and field erection.
9.19. Types of Welding
1. Shielded Metal Arc Welding (SMAW)
One of the oldest, simplest, and perhaps most versatile types for welding structural steel. It is often referred to as
the manual stick electrode process. Heating is accomplished by means of an electric arc between a coated
electrode and the materials being joined. The coated electrode is consumed as a metal is transferred from the
electrode to the base material during the welding process. The electrode wire becomes filler material and the
coating is converted partly into a shielding gas, partly into slag, and some is absorbed by the weld metal.
2. Submerged Arc Welding (SAW)
The arc is not visible because it is covered by a blanket of granular, fusible material. The bare metal electrode is
consumable in that it is deposited as a filler material. The end of the electrode is kept continuously shielded by the
molten flux over which is deposited a layer of unfused flux in its granular condition.
The granular flux is usually laid automatically along the seam ahead of the advancing electrode, and provides a
cover that allows the weld to be made without spatter, sparks, or smoke. This flux material protects the weld pool
against the atmosphere, serves to clean the weld, and modifies the chemical composition of the weld metal.
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3. Gas Metal Arc Welding (GMAW)
The electrode is continuous wire that is fed from a coil through the electrode holder. The shielding is entirely
from an externally supplied gas or gas mixture. The distinguishing features of this method are the high rates at
which filler metal can be transferred and the gaseous shield that is uniformly provided around the molten weld.
4. Flux Cored Arc Welding (FCAW)
It is similar to GMAW except that the continuously fed filler metal electrode is tubular and contains the flux
material within its core.
5. Electrogas Welding (EGW)
A machine process used primarily for vertical position welding. Either flux cored or solid electrodes maybe used.
This method is used to obtain a single pass weld such as for the splice in a heavy column section. Weld metal is
deposited into a cavity created by the separated plate edges on two sides and water-cooled shoes or guides to keep
the molten metal in its proper location on two sides. The gas shielding is provided either by the flux cored
electrode, by externally supplied gas, or both.
6. Electroslag Welding (ESW)
Similar to electrogas welding except that the welding is actually done by the heat produced through the resistance
of the slag to the flow of current. The molten conductive slag protects the weld and melts the filler metal and the
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plate edges. Since solid slag is not conductive, an arc is required to start the process by melting the slag and
heating the plates.
7. Stud Welding
The most commonly used process of welding a metal stud to a base material. It is an essentially automatic process
but similar in characteristics to the SMAW process. The stud serves as the electrode and an electric arc is created
from the end to the stud plate. The stud is contained in a gun which controls the timing during the process.
Shielding is accomplished by placing a ceramic ferrule around the end of the stud in the gun. The gun is placed in
a position and the arc is created, during which time the ceramic ferrule contains the molten metal. After a short
instant time, the gun drives the stud into the molten pool and the weld is completed laving a small fillet around the
stud. Full penetration across the shank of the stud is obtained and the weld is completed usually in less than one
second.
9.20. Types of Welding Joints according to
A. Type of weld
1. Fillet weld
- Use for parts lap over each other. It can also be used for tee joints. Fillet weld is the most economical
weld to use, as little preparation of the parts to be connected is necessary.
2. Butt or groove weld
- Use to connect structural members that are aligned in the same plane.
3. Plug weld /Slot weld
- A weld formed in a slot or elongated hole that joins one member to the other member through the slot.
4. Spot weld
B. Position
1. Flat weld - On the floor
2. Horizontal weld - On the wall
3. Vertical weld - On the wall
4. Overhead weld - Under the roof
C. Type of joint
1. Butt weld
2. Lap weld
3. Tee weld
4. Corner weld
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9.21. Advantages of welded connections
1. Welded connections can be adapted to almost any connection configuration in which bolts are used.
2. The full design strength of a member can be more easily developed with a welded connection with an all-around
weld.
3. Welded connections take up less material and space.
4. Welded connections are more rigid and are subject to less deformation than bolted connections.
5. Welded connections are often preferred in exposed conditions where aesthetics are of concern because they can be
modified to have a smoother and cleaner appearance.
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9.22. Disadvantages of welded connections
1. Welded connections require greater skill.
2. Welded connections often require more time to construct than equivalent bolted connections.
3. The inspection of welded connections is more extensive than that of bolted connections.
4. In existing structures, welded connections may be difficult or even impossible due to the use of the structure.
9.23. Fillet Welds
The design and analysis of fillet welds is based on the assumption that the cross section of the weld is a 45° right
triangle, as shown in the figure below. Any reinforcement (buildup outside the hypotenuse of the triangle) or penetration
is neglected. The size of a fillet weld is denoted w and is the length of one of the two equal sides of this idealized cross
section. Although a length of weld can be loaded in any direction in shear, compression, or tension, a fillet weld is
weakest in shear and is always assumed to fail in this mode. Specifically, failure is assumed to occur in shear on a
plane through the throat of the weld. For fillet welds made with the shielded metal arc process, the throat is the
perpendicular distance from the corner, or root, of the weld to the hypotenuse and is equal to
t  0.707w
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w = size of weld.
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The effective throat thickness for a weld made with the submerged arc welding process is larger. Thus, for a given
length of weld L subjected to a load of P, the critical shearing stress is
fv 
P
0.707 wL
where:
fv  critical shear stress
P  applied load
w  size of weld
L  length of weld
When welds are made, the electrode material should have properties of the base metal. If the properties are
comparable, the weld metal is referred to as matching base metal. (That is, nominal strengths are similar)
Table below provides nominal strengths for various types of welds including fillet welds, plug and slot welds, and
complete-penetration and partial-penetration groove welds.
The design strength of a particular weld  Rn  and the allowable strength Rn  of welded joints shall be the lower
value of the base material strength determined according to the limit state of tensile rupture and shear rupture, and the
weld metal strength determine according to the limit state of rupture by the expressions:
for base metal:
where:
Rn  FnBM ABM
for weld metal:
Rn  Fnv Awe
FnBM  nominal stress of the base metal
Fnv  nominal stress of the weld metal
ABM  effective area of the base metal
Awe  effective area of weld
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9.24. Effective Area
The effective area of a fillet weld shall be the effective length multiplied by the effective throat. The effective throat
of a filet weld shall be the shortest distance from the root to the face of the diagrammatic weld .An increase in effective
throat is permitted if consistent penetration beyond the root of the diagrammatic weld is demonstrated by test using the
production process and procedure variables.
9.25.. Minimum size of fillet weld
The minimum size of fillet welds shall be not less than the size required to transmit calculated forces, nor the size as
shown in the table below. These provisions do not apply to fillet weld reinforcements of partial- or complete-jointpenetration groove welds.
Material Thickness of Thinner Part
Minimum Size of Fillet
Loined, mm
Weld(a) , mm
To 6 inclusive
3
Over 6 to 13
5
Over 13 to 19
6
Over 19
8
(a)
Leg Dimensions of fillet welds. Single pass welds must be used.
Note: see NSCR 2010 Section 510.2.2b for maximum size of fillet welds.
9.26. Maximum size of fillet welds
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The maximum size of fillet welds of connected parts shall be:
1. Along edges of material less than 6 mm thick, not greater than the thickness of the material.
2. Along edges of material 6 mm or more in thickness, not greater thanthe thickness of the material minus 2 mm,
unless the weld is especially designated on the drawings to be built out to obltain full-throat thickness. In the aswelded conditions, the distane between the edge of the base metal and the toe of the weld is permitted to be less
than 2 mm provided the weld size is clearly verifiable.
3. the minimum effective length of fillet welds designed on the basis of strength shall be not less than four times the
nominal size, or else the size of the weld shall be conisdered not to exceed ¼ of its effective length. If longitudinal
fillet welds are use 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. For the effect of the longitudinal fillet weld length in end
connections upon the effective area of the connected member, see NSCP 2010 section 504.3.3.
4. for end-loaded fillet welds with a length up to 100 times the leg dimension, it is permitted to take the effective
length equal to the actual length. When the length of the end-loaded fillet weld exceeds 100 times the weld size, the
effective length shall be determined by multiplying the actual length by the reduction factor,  .
where:
L
  1.2  0.002    1.0
L  actual length of end-loaded weld, mm
 w
w  weld leg size, mm
When the length of the weld exceeds 300 times the leg size, the values of  shall be taken as 0.60.
5. Intermittent fillet welds are permifted to transfer calculated stress across a joint or faying surface when the strength
required is less than that 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 fillet welding shall be not less
than 4 times the weld size, with a minimum of 38 mm
6. In lap joints, the minimum lap shall be 5 times the thickness of the thinner part joined, but not less than 25 mm. 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.
7. Fillet weld terminations are permitted to be stopped short or extend to the ends or sides of parts or be boxed except
as limited by the following:
a. For lap joints in which one connected part extends beyond an edge of another connected part that is subject
to calculated tensile stress, fillet welds shall terminated not less than the size of the weld from that edge.
b. For connections where flexibility of the outstanding elements is required, when end returns are used, the
length of the return shall not exceed four times the nominal size of the weld not half the width of the part.
c. Fillet weld joining transverse stiffeners to plate girder webs 19 mm thick or less shall end not less than four
times nor more than six times the thickness of the web from the web toe of the web-to-flange welds, except
where ends of stiffeners are welded to the flange.
d. Fillet welds that occur on opposites sides of a common plane, shall be interrupted to the corner common to
both welds.
Problems:
1. Determine the designed strength of a
a. 25 mm length
b. 500 mm length
c. 750 mm length
of a 6.25 mm fillet weld formed by the shielded metal arc process (SMAW) and E70 electrodes with a minimum
tensile strength of 70 ksi (483 MPa). Assume that load is to be applied parallel to the weld length.
2. What is the design strength of the connection shown below if the plates consist of A572 Grade 50 steel (
Fu  448MPa )? E70 electrodes were used, and the 11 mm fillet welds were made by the SMAW process.
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3. Using a 345 MPa steel and E70 electrodes, design SMAW fillet welds to resist a full-capacity load on the 9 mm x 150
mm member shown.
9.27. Design of Connection Members with both Longitudinal and Transverse Fillet Welds
To determine the total nominal strength of the welds for this type of connection, the AISC in its Section J2.4c states that
the total nominal strength of a connection with side and transverse welds s to equal the larger of the values obtained
with the following two equations:
Rn  Rnwl  Rnwt
Rn  0.85Rnwl  Rnwt
where:
Rnwl  total nominal strength of the longitudinal or side
fillet weld
Rnwl  Rnwt  Fnw Awe
Problem:
Determine the total LRFD design strength and the total ASD allowable strength of the 8 mm E70 fillet welds shown
below.
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9.28. Shear and Torsion
Fillet welds are frequently loaded with eccentrically applied loads, with the result that the welds are subjected to
either shear and torsion or to shear and bending.
Td
J
Tv
Fh 
J
F
Fv 
Th
J
where:
F  force due to torsion
T  applied torsion
d  distance from the center of the weld to the point being considered.
J  polar moment of inertia of the weld.
Fh  horizontal component of F
Fv  vertical component of F
h  horizontal component of d
v  vertical component of d
Problem Set:
For the A36 bracket shown in the figure below, determine the fillet size required if E70 electrodes and the SMAW process
are used.
9.29. Shear and Bending
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For short welds of this type, the usual practice is to consider a uniform variation of shearing stress. If, however, the
bending stress is assumed to be given by the flexure formula, the shear does not vary uniformly for vertical welds, but as a
1
2
parabola with a maximum value of 1 times the average value.
The reader should carefully note that the maximum shearing stresses and the maximum bending stresses occur at
different locations. Therefore, probably it is not necessary to combine the two stresses at any point. If the weld is capable
of withstanding the worst shear and the worst moment individually, it is probably satisfactory.
Problem Set:
Using E70 electrodes, the SMAW process, and the LRFD Specification, determine the weld size required for the
connection of the figure shown above. PD  44.4 kN , PL  88.96 kN , e  62.5 mm , and L  200 mm . Assume that
the member thicknesses do not control weld size.
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