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8.3 WELDED
CONNECTIONS
8.3.1
Types of Welding
In welded
connections, different elements are connected by heating their surfaces to
a plastic or fluid state. Notwithstanding the availability of both gas and
arc welding, welded connections in steel structures are ordinarily done by
arc welding. To obtain satisfactory connections, additional metal is used
for joining different elements. In electric arc welding, the additional
material is a metallic rod, which is used as the electrode. In this type of
welding, the electric arc produced between the elements being welded and
the electrode heats the elements and the electrode to the melting point.
This transformation of electrical energy into thermal energy and the
resulting high temperature (up to 10,000 °F) causes the metallic
electrode to melt off into the joint. Small droplets of the molten metallic
electrode are in fact driven onward to the joint. Thus, overhead welding is
possible by electric arc welding.
Molten steel must be protected from the surrounding air; otherwise,
gases contained in the molten steel can combine chemically with oxygen and
nitrogen in the air. This chemical reaction leaves small pockets of gases
in the weld after it has cooled down, making it porous. The resulting weld will
be brittle with very little resistance to corrosion. To prevent this
undesirable brittleness of the weld, two types of arc welding are commonly
used. One is called Shielded Metal Arc Welding
(with acronym SMAW) and the other is Submerged (or hidden) Arc
Welding (with acronym SAW).
In SMAW, the weld is protected by using an electrode covered with a layer
of mineral compounds. Melting of this layer during the welding produces an
inert gas encompassing the weld area. This inert gas shields the weld by preventing
the molten metal from having contact with the surrounding air (Fig. 8.10).
The protecting layer of the electrode leaves a slag after the mold has
cooled down. The slag can be removed by peening and brushing.
In
SAW, the surface of the weld and the electric arc are covered by some
granular fusible material and thus is protected from the surrounding air.
In this method, a bare metal electrode is used as filler material. Compared
with SMAW, SAW welds provide deeper penetration, and this fact is the
allowable shear stress values recommended by ASD J2.2a. Also, SAW welds
show good ductility and corrosion resistance and high impact strength.
8.3.2
Advantage of Welding
1. In welded connections,
in general, fewer pieces are used. This will speed up the detailing and
fabrication process.
2. In welded connections,
gusset and splice plates may be eliminated. Bolts or rivets are not needed
either. Thus, the total weight of a welded steel structure is somewhat less
than that of the corresponding bolted structure.
3. Connecting unusual
members (such as pipes) is easier by welding than by bolting.
4. Welding provides truly
rigid joint and continuous structures.
One
possible drawback of welding is the need for careful execution and
supervision. For this reason, welding is sometimes done in the shop and
bolting in the field. In other words, shop-welding is complemented by the
bolting in the field.
8.3.3
Types of Welds
The two
common types of welds in welded steel structures are groove welds and fillet
welds. Fillet welds are much more popular in structural steel design
than groove welds. Two different types of groove welds are shown in Fig.
8.11. They are the partial penetration (single vee) groove weld and full
penetration (double vee) groove weld. Groove welds can be used when the
pieces to be connected can be lined up in the same plane with small
tolerances.
Fillet welds
are shown in Fig. 8.12. Depending on the direction of the applied load and
the line of the fillet weld, fillet welds are classified as longitudinal or
transverse fillet weld. In the former, the shear force to be transferred is
parallel to the weld line; in the latter, the force to be transmitted is
perpendicular to the weld line.
Fillet welds
can be either equal-leg or unequal-leg, as shown in Fig. 8.13. The
intersection point of the original faces of the steel elements being
connected is called the root of
the weld. The surface of the weld should have a slight convexity. In
computation of the strength of the weld, however, this convexity is not
taken into account and the theoretical flat surface is used. A convex
surface for a weld is clearly superior to a concave surface. When the weld
cools down, it shrinks. This shrinkage causes surface tension in concave
welds and surface compression in convex welds (Fig. 8.14). The concave
surface in tension tends to crack, causing the separation of the weld from
the faces of the pieces being connected. The normal distance from the root
to the theoretical face of the weld is called the throat of the weld.
Experiments
performed on fillet welds indicate that they are weaker in shear than in
tension and compression. Also, equal-leg fillet welds fail in shear through
the throat (at angles of about 45 degree with the legs of the weld). For
equal-leg fillet welds, the relation between the dimensions of the leg w and the throat t is
Thus, shear stress is
the controlling factor in the design of fillet welds; it is customarily
calculated by dividing the force P
acting on the weld by the effective throat area of the weld. The effective
throat area is computed by multiplying the throat thickness by the length
of the fillet weld. This method of finding average shear stress is used for
both longitudinal and transverse fillet welds.
The ASD code
does not recognize the fact that transverse fillet welds are stronger
(about one-third) than the longitudinal fillet welds. Experiments indicate
that transverse fillet welds fail in planes somewhat different from the
45-degree plane. The size of a fillet weld is indicated by the size of its
leg. For example, a 7/8 in. fillet weld means a fillet weld with a leg size
of w = 7/8 in.
As
pointed out previously, SAW welds provide a deeper penetration than the
SMAW welds. This fact is recognized in the ASD code by allowing a larger
throat area for SAW welds. According to ASD J2.2a for SAW welds with size
3/8 in. or smaller, the effective throat thickness is taken as equal to leg
size, and for fillet welds greater than 3/8 in., the effective throat is
taken equal to the theoretical throat plus 0.11 in.
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t =
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w
0.707w +
0.11
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for w
£ 3/8 in.
for w
> 3/8 in.
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(8.12)
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8.3.4
Allowable Stress on Welds
Allowable stresses for different types of welds
are given in Table J2.5 of the ASD code. The allowable shear stress on the
effective area of fillet welds is 0.30 times the nominal tensile strength
of the weld metal. Electrodes are designated as E60, E70 and so on, where E
stands for electrode and the number following the letter E is the minimum
tensile strength of the weld, in ksi.
8.3.5
Minimum and Maximum sizes of Fillet Welds
The minimum size of fillet weld is determined
on the basis of the thicker of the two pieces connected, as given in Table
8.5. (ASD J2.2b)
The
maximum size of fillet welds along edges of an element less than ¼ in.
thick is equal to the thickness of the element. Along edges of an elements
with thickness of ¼ in. or more, the maximum size of the fillet weld is
equal to the thickness of the element minus 1/16 in. (ASD J2.2b)
TABLE
8.5 MINIMUM SIZE OF FILLET WELDS (ASD TABLE J2.4)
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Thickness of thicker
part connected (in.)
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Minimum leg size of
fillet weld (in.)
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To ¼
inclusive
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1/8
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Over ¼
to ½
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3/16
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Over ½
to ¾
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¼
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Over ¾
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5/16
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8.3.6
Minimum Length of Fillet Weld
The minimum effective length of a fillet weld
shall be at least equal to four times its nominal size; otherwise weld size
shall be limited to ¼ of its effective lengths (ASD J2.2b).
The effective length of any segment of
intermittent fillet weld shall be at least 1.5 in. and four times the weld
size (ASD J2.2b)
8.3.7
Eccentrically Loaded Welded Connections
The
analysis of eccentrically loaded welded connections is similar to the
analysis of eccentrically loaded bolts, as covered in Sec. 8.2.3. Consider
a bracket welded to the flange of a column, as shown in Fig. 8.15. The size
of the fillet weld is assumed to be the same on the three edges of the
bracket. Suppose that the connection is subjected to horizontal and
vertical shears of Fx
and Fy that produce a
moment Mc about the
centroid C of weld lines. Thus,
the total components of shear force per unit length at point A with
coordinate xA and yA are
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(8.13)
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(8.14)
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Finally, the resultant shear force per unit
length of weld at point A is
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(8.15)
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Note that in Eqs. (8.13) and (8.14) it is assumed
that the positive x-axis points to the right and the positive y-axis points
downward.
Denoting
the total length of fillet weld by L,
the horizontal shear force per unit length of the weld at any point A due to the direct shear Fx is
and the vertical shear
force per unit length of the weld due to the direct shear Fy is
The shear
force per unit length at point A (with coordinates xA and yA)
due to couple Mc are
where
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J = Ix + Iy
= polar
moment of inertia of the weld of unit width about point C
Ix = moment of inertia
of the weld of unit width about the x-axis
Iy = moment of inertia
of the weld of unit width about the y-axis
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8.3.8
Examples
Example 4
A welded built-up girder
is made of a W24x94 and a C12x25 section, as shown in Fig 8.16. The
maximum shear force in the girder is V
= 150 Kips. Design the intermittent fillet weld for connecting the two
sections, as shown in Fig 8.16. Use submerged arc welding (SAW) and E70
electrodes.
Solution
Properties of W24 x 94
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A = 27.7 in.2
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bf = 9.065 in.
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tf = 0.875 in.
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Ix = 2700 in.4
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d = 24.31 in.
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Properties of C12x25:
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A = 7.35 in.2
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d = 12.0 in.
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tw = 0.387 in.
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 = 4.47 in.4
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We first locate the neutral (centroidal) axis of the
cross section of the built-up girder (x-axis in Fig 8.16).
The moment of inertia of the built-up section
about the x-axis is
The two lines of fillet weld must carry the
longitudinal shear on the plane between the channel and the W shape. The total
shear force between the channel and the flange of the W shape for a unit
length of the beam is equal to
where Q is the first moment of the area of
the channel about the centroidal axis of the built-up section.
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Minimum size of the fillet weld (Sec. 8.3.5): 5/16 in.
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Maximum size of the fillet weld along the flange edge of the W
shape (Sec. 8.3.5):
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The minimum length of a segment of intermittent welds (Sec. 8.3.6):
the larger of 4w = 1.25 and 1.5
in.
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Use
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L1 = 1.5 in.
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Length of a segment of weld
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The effective throat thickness (Eq. (8.12)):
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The allowable shear stress of the weld is
If the longitudinal
spacing of intermittent welds is denoted by a, the total horizontal shear between the channel and the
flange of W shape over a length of a
is aq. The shear capacity of the
two lines of fillet weld over the same length is 2L1Fvw. Therefore,
·
The maximum longitudinal spacing of intermittent welds connecting
two rolled shapes in contact (ASD E4): amax
= 24 in.
Example 5
Determine the size of the submerged arc fillet weld
for the connection of a C12 x 25 beam to a W14 x 120 column as shown is
Fig. 8.17. Note that the beam is connected to the column at its end and at
the edge of the column flange. Use E70 electrodes.
Solution
We design the weld on the basis of the maximum shear
stress, which is at point A (Fig. 8.17).
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xA = -3 in.
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yA = 6 in.
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Fx = 0
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Fy = 30 Kips
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MC = -30(9) = -270 K-in.
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Total length of the weld: L = 2(12) = 24 in.
For SAW and E70 electrode, the allowable shear
stress is
The required effective thickness of the weld
throat is
Therefore,
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The minimum size of the fillet weld for 0.94 in.-thick plate
(column flange) (Sec. 8.3.5): 5/16 in.
·
The maximum size of the fillet weld along the edge of the beam web
(Sec. 8.3.5): 0.387 – 1/16 = 0.325 in.
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Use
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5/16 in.
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SAW fillet weld.
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Example 6
To make a moment-resisting joint in a rigid frame, a
W21x147
girder is directly welded to a W14x145 column as shown in Fig.
8.18. The beam, made of A36 steel with yield stress of 36 ksi, has an end
reaction of 194 Kips and an end bending moment of 210 K-ft. Using E70
electrodes, find the size of the submerged arc fillet weld (SAW) for the
connection of beam flanges and web to the column flange.
Solution
Assume that the web welds carry all of the shear and
the flange welds carry all of the moment.
Properties of the beam:
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d = 22.06 in.
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tf = 1.15 in.
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tw = 0.72 in.
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bf = 12.51 in.
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The allowable shear stress of the weld: Fv = 0.30(70) = 21 ksi.
(a)
Web Welds
If the effective throat
thickness is denoted by t, the
shear stress in the weld is
The required throat thickness:
For w
> 3/8 in.:
Minimum weld size (Sec. 8.3.5): wmin = 5/16 in.
(b)
Flange Welds
The force T to be carried by the weld is found
by dividing the beam end bending moment by the center-to-center distance of
flanges:
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