Page 73 - Structural Steel Designers Handbook AISC, AASHTO, AISI, ASTM, and ASCE-07 Design Standards

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CONNECTIONS

CONNECTIONS 3.5

To prevent fracture at the weld of a single-plate shear connection, the plate is designed to yield
before the weld fractures. From Astaneh (1989), interaction curves relating the shear and the moment
caused by the shear can be conservatively approximated by an ellipse:
 V  2  M  2
For the weld   +   ≤1 (3.4)
 V   M 
w
w
 V  2  M  2
For the plate   +   ≤1 (3.5)
 V   M 
y
y
where V = applied shear
M = moment caused by shear = V e
0 6F
V w = shear capacity of weld = 2 1 wL(. )
2 EXX
0 9F
M w = moment capacity of weld = 2 1 w L 2 (. )
2 4 EXX
V y = shear capacity of plate = tL(0.6F y )
M y = moment capacity of plate = t L 2 (. F )
06
4 y
F EXX = electrode strength classification, ksi
Since the plate must yield before the weld fractures,
 V  2  M  2  V  2  M  2
  +   ≤   +   (3.6)
 V   M   V   M 
y
y
w
w
Solving for w, the weld size, in terms of t, the plate thickness, yields
+ .
.
tF 139 989(/ 2
e L)
w ≥ y (3.7)
F EXX 278 +16(/ 2
e L)
.
Substituting F y = 50 ksi and F EXX = 70 ksi, the above inequality yields
w ≥ 0.562t as e/L approaches infinity
w ≥ 0.505t as e/L approaches zero
Therefore, the required weld size is 0.562t. For A36 steel, a slightly smaller weld size can be
achieved, but due to the considerable overstrength typical of A36 steel, it is advisable to size the weld
based on the Grade 50 value.
To prevent bolt fracture at a single-plate shear connection, the plate must deform sufficiently to
redistribute unanticipated moments prior to bolt fracture. This can be achieved through two different
actions. First, the bolts can plow through the material. In order for such plowing to occur, the thick-
ness of the material joined cannot exceed one-half the diameter of the bolt, and sufficient edge dis-
tance must be present to prevent the bolt from tearing through the edge of the material. An edge
distance of twice the bolt diameter is usually assumed.
The second action requires that the plate yield in bending prior to the bolts failing in shear. As an
example, consider a two-row, two-column connection with 1-in-diameter A490-X bolts in single
shear and F y = 50-ksi steel. The assumed bolt strength here is 1.25 times the nominal strength =
2
1.25F n A b = 1.25[75(0.5) π] = 73.6 kips/bolt. The bolt strength is assumed to be 25% higher than the
specified nominal strength, because a 20% reduction in bolt strength is assumed in the AISC
Specification to account for nonuniform loading in end-loaded connections. Since this is not an end-
loaded connection, this reduction is not taken. The moment capacity of the connection is found to

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