Hancock G.J., Murray Th.M., Ellifritt D.S. Cold-Formed Steel Structures to the AISI Specification
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LRFD is speci®ed for ®llet welds subject to transverse
loading. For ASD, a factor of safety of 2.5 is used. In Section
E2.4 of the AISI Speci®cation, the lesser of t
1
and t
2
is used
to check both sheets connected by a ®llet weld, where t
1
is
for Sheet 1 and t
2
is for Sheet 2.
A series of tests was performed at Institute TNO (Ref.
9.2) to determine the effect of single lap joints and the
welding process on the strength of ®llet weld connections.
The joints tested are shown in Figure 9.2a and were
fabricated by the TIG process for uncoated sheet and by
covered electrodes for galvanized sheet. The failure modes
observed were inclination failure, as shown in Figure 9.2a,
combined with weld shear, weld tearing, and plate tearing.
The mean test strengths (P
m
) were found to be a function of
the weld length to sheet width in addition to the para-
meters in Eq. (9.2), and are given by
P
m
tLF
u
1 ÿ 0:3
L
b
9:5
Hence, for the single lap joint, the ratio L=b appears to be
important.
The results for galvanized sheet were found to not be
signi®cantly lower than those for uncoated sheet except
that the deviation was found to be higher as a result of the
dif®culty encountered in making a sound weld.
9.2.3 Fillet Welds Subject to Longitudinal
Loading
The Cornell test data for ®llet welds, deposited from
covered electrodes, was produced for the type of double
lap joints shown in Figure 9.4b. These joints failed by
tensile tearing across the connected sheet or by weld
shear or tearing along the sheet parallel to the contour of
the weld, as shown in Figure 9.4b. Based on these tests, the
Chapter 9
258
following equations were found to predict the connection
strength:
P
n
tL 1 ÿ0:01
L
t
F
u
for
L
t
< 25 9:6
P
n
0:75tLF
u
for
L
t
5 25 9:7
P
n
is the strength of a single weld. The results of all tests
are shown in Figure 9.3b compared with the predictions of
Eqs. (9.6) and (9.7) where the least value has been used.
The values on the abscissa of Figure 9.3b are 4P
n
since the
FIGURE 9.4 Fillet welds subject to longitudinal loading: (a) single
lap joint (TNO tests); (b) double lap joint (Cornell tests).
Connections
259
joints tested were double lap joints with ®llet welds on each
side of each sheet.
Resistance factors of 0.60 and 0.55 are speci®ed for
LRFD for Eqs. (9.6) and (9.7), respectively. For ASD, a
factor of safety of 2.5 is used. In Section E2.4 of the AISI
Speci®cation, the lesser of t
1
and t
2
is used to check both
sheets connected by a ®llet weld, where t
1
is for Sheet 1 and
t
2
is for Sheet 2.
A series of tests was performed more recently at Insti-
tute TNO (Ref. 9.2) to determine the effect of single lap joint
geometry and welding process on the strength of ®llet weld
connections subject to longitudinal loading. The types of
joints tested are shown in Figure 9.4a and were manufac-
tured by the TIG process for uncoated sheet and by covered
electrodes for galvanized sheet. The failure modes observed
were tearing at the contour of the weld and weld shear
accompanied by out-of-plane distortion and weld peeling for
short-length welds. For longer welds, plate tearing oc-
curred. The mean test strengths (P
m
) were found to be a
function of the weld length (L) and sheet width (b):
P
m
2tL 0:95 ÿ 0:45
L
b
F
u
9:8
P
m
0:95tbF
u
9:9
The lesser of the values of Eq. (9.8) for weld failure and Eq.
(9.9) for plate failure should be used. As for the transverse
®llet welds, there was no signi®cant difference between the
values for the uncoated and galvanized sheets.
9.2.4 Combined Longitudinal and Transverse
Fillet Welds
Tests were performed at Institute TNO (Ref. 9.2) to ascer-
tain whether combined longitudinal and transverse ®llet
welds interacted adversely or bene®cially. The test series
showed that similar failure modes to those for longitudinal
and transverse ®llet welds were observed. Also, the defor-
Chapter 9
260
mation capacity of the individual welds allowed full coop-
eration so that the strengths of the transverse and longi-
tudinal welds can be simply added. In fact, the combined
welds were better than the sum of the two, but the addi-
tional bene®ts have not been quanti®ed.
9.2.5 Flare Groove Welds
Flare welds are of two common types. These are ¯are-bevel
welds, as shown in Figure 9.1e and in cross section in
Figure 9.5a, and ¯are V-welds, as shown in cross section
in Figure 9.5b.
As for ®llet welds, the ¯are welds may be loaded
transversely or longitudinally, and their modes of failure
are similar to ®llet welds described in Sections 9.2.2 and
9.2.3. The primary mode of failure is sheet tearing along
the weld contour.
For transverse ¯are-bevel groove welds, the nominal
shear strength in Section E2.5 of the AISI Speci®cation is
the same as for a ®llet weld subject to transverse loading
[Eq. (9.4)] except that it is factored by 5=6 0:833:
P
n
0:833tLF
u
9:10
FIGURE 9.5 Flare groove weld cross sections: (a) ¯are-bevel
groove weld; (b) ¯are V-groove weld.
Connections
261
The resistance factor to be used in LRFD with Eq. (9.10) is
f 0:55. For ASD a factor of safety of 2.5 is used.
For ¯are groove welds subject to longitudinal loading,
either on a ¯are-bevel groove weld as in Figure 9.5a or a
¯are V-groove weld as in Figure 9.5b, the nominal shear
strength depends on the effective throat thickness (t
w
)of
the ¯are weld and the lip height (h) in Figure 9.5a. If
t 4 t
w
< 2t or the lip height (h) is less than the weld length
(L), the nominal strength is the same for a ®llet weld
subject to longitudinal loading given by Eq. (9.7), so that
P
n
0:75tLF
u
9:11
If t
w
5 2t and h 5 L, then
P
u
1:5tLF
u
9:12
The resistance factor to be used in both cases in LRFD is
0.55, and the factor of safety in ASD is 2.5.
In the AISI Speci®cation, Section E2.5 de®nes t
w
for a
range of cases including ¯are groove welds ®lled ¯ush to
surface and ¯are groove welds not ®lled ¯ush to surface, as
shown in Figure 9.5. For t > 0:15 in., failure through the
throat thickness t
w
is checked based on the weld metal
strength of F
xx
.
9.2.6 Arc Spot Welds (Puddle Welds)
Arc spot welds are for welding sheet steel to thicker
supporting members in a ¯at position. Arc spot welds
have three different diameters at different levels of the
weld, as shown in Figure 9.6a for a single thickness of
attached sheet and in Figure 9.6b for a double thickness of
attached sheet. The diameter d is the visible width of the
arc spot weld, the diameter d
a
is the average diameter at
mid-thickness, and the diameter d
e
is the effective diameter
of the fused area. In general, arc-spot-welded connections
are applied in sheet steel with thicknesses from about
0.02 in. to 0.15 in. Weld washers must be used when the
thickness of the sheet is less than 0.028 in. In many ways,
Chapter 9
262
the design of an arc spot weld is similar to an equivalent
bolted connection since the failure modes are similar to
those of mechanical connections. The principal modes of
failure are shown in Figure 9.7. Only the mode shown in
(b), tearing and bearing at weld contour, does not have an
exact equivalent in bolted connection design.
The Cornell test data for spot welds is shown in Figure
9.8a. Those joints which failed by weld shear, as shown in
Figure 9.7e, were used to determine the effective diameter
(d
e
) of the fused area:
d
e
0:70d ÿ 1:5t 4 0:55d 9:13
The minimum allowable value of d
e
is
3
8
in.
Weld shear failure loads can be successfully predicted
by
V
w
p
4
d
e
2
0:75F
xx
9:14
where F
xx
is the ®ller metal strength for the AWS Electrode
Classi®cation. A resistance factor of 0.60 for LRFD is
speci®ed in the AISI Speci®cation for use with Eq. (9.14).
For ASD a factor of safety of 2.5 is used.
For plate tearing around the weld, as shown in Figure
9.7b, the following formulae for the nominal shear strength
of an arc spot weld have been developed:
P
n
2:2td
a
F
u
for
d
a
t
4 0:815
E
F
u
s
9:15
P
n
1:4td
a
F
u
for
d
a
t
5 1:397
E
F
u
s
9:16
and
P
n
0:280 1
5:59
E=F
u
p
d
a
=t
!
td
a
F
u
in the transition region 9:17
Connections
263
where d
a
is the average diameter of the arc spot weld, F
u
is
the tensile strength of the sheet material, and t is the total
combined sheet thickness of steels involved in shear trans-
fer above the plane of maximum shear transfer. The value
of d
a
is d ÿ t for a single sheet and d ÿ 2t for multiple
sheets.
The results of all of the tests are shown in Figure 9.8a
compared with the predictions of Eqs. (9.13)±(9.19) where
the least value has been used. The values on the abscissa of
Figure 9.8a are 2P
n
since the joints tested were double lap
joints. The variability of the test results in Figure 9.8a
occurred because all of the ®eld-welded specimens were
poorly made. Resistance factors in LRFD of 0.60, 0.50, 0.50
apply to Eqs. (9.15)±(9.17), respectively, as speci®ed in
Section E2.2.1 of the AISI Speci®cation. For ASD a factor
of safety of 2.5 is used throughout.
The failure mode of net section failure shown in Figure
9.7d must be checked separately by using the net section
capacity of a tension member as given by Section C2 of the
AISI Speci®cation. For the purpose of this calculation, the
net section is the full section width (b) of the plate.
FIGURE 9.6 Arc spot and arc seam weld geometry: (a) single
thickness of sheet; (b) double thickness of sheet; (c) minimum
edge distance (arc spot welds); (d) geometry and minimum edge
distance (arc seam welds).
Chapter 9
264
The failure mode of edge failure shown in Figure 9.7c
is limited in Section E2.2.1 of the AISI Speci®cation by
limiting the edge distance. The minimum distance e
min
from the centerline of a weld to the nearest edge of an
adjacent weld or the end of the connected point toward
which the force is directed is
e
min
P
u
fF
u
t
for LRFD 9:18
FIGURE 9.7 Failure modes in arc spot welds: (a) inclination
failure; (b) tearing and bearing at weld contour; (c) edge failure;
(d) net section failure; (e) weld shear failure.
Connections
265
FIGURE 9.8 Arc spot and arc seam weld tests (Cornell): (a) arc
spot welds; (b) arc seam welds.
Chapter 9
266
where P
u
is the required strength (factored) transmitted by
the weld.
e
min
PO
F
u
t
for ASD 9:19
where P is the required strength (unfactored) transmitted
by the weld. In Eqs. (9.18) and (9.19), t is the thickness of
the thinnest connected sheet. The values of resistance
factor (f) and factor of safety (O) depend on the ratio of
F
u
=F
sy
: when it is less than 1.08, f and O are 0.70 and 2.0,
respectively; otherwise they are 0.60 and 2.22, respectively.
Arc spot welds in tension are speci®ed in Section
E2.2.2 of the AISI Speci®cation. Either weld failure or
sheet tear around the weld can occur. For weld failure,
the nominal tensile strength (P
n
) of each concentrically
loaded arc spot weld is given by
P
n
p
4
d
2
e
F
xx
9:20
such that F
xx
is the ®ller metal strength, which must be
greater than 60 ksi and F
u
. For sheet tear the nominal
tensile strength (P
n
) of each concentrically loaded arc spot
weld is given by
P
n
6:59 ÿ
3150F
u
E
td
a
F
u
4 1:46tdaF
u
for F
u
< 0:00187E 9:21
P
n
0:70td
a
F
u
for F
u
5 0:00187E 9:22
where F
u
4 82 ksi. The values of resistance factor for LRFD
and safety factor for ASD are 0.60 and 2.50, respectively.
Further, if eccentric loading can be applied to the arc spot
weld, then it is prudent to reduce the tensile capacity of an
arc spot weld to 50% of the values given by Eqs. (9.20)±
(9.22).
Connections
267