Page 152 - Structural Steel Designers Handbook AISC, AASHTO, AISI, ASTM, and ASCE-07 Design Standards

Brockenbrough_Ch03.qxd 9/29/05 5:05 PM Page 3.84

CONNECTIONS

3.84 CHAPTER THREE

Weld of Plate B to Column Web. This 17.75-in-long weld carries all of the beam shear. The
1
required weld size in / 16ths is thus
D = 107 = 217.
2 ×17 75. ×1 392.

3
Use / 16-in fillet weld.
Because this weld occurs on both sides of the column web, the column web should have suffi-
cient shear strength per unit length (φR v = 0.75 × 0.60 × F u t w ) to transmit the force from the welds
through its thickness. Thus, the thickness should satisfy the relationship 0.75 × 0.60 × 65t w ≥ 1.392 ×
D × 2 or t w > 0.207. Since the column web thickness is 0.485 in, the web can support the / 16-in fil-
3
lets. The same result can be achieved using AISC Manual Table 10.2.
Weld of Plate B to Plate A. There is a shear flow q = VQ/I acting on this interface, where the
shear force V = R = 107 kips, and Q is the statical moment of plate A with respect to the neutral axis
of the I section formed by plates A as flanges and plate B as web. Thus,

.
.
I = 1 × 0 375 ×19 25 3 + 0 75 ×12 5 . ×  19 25 + 0 75  2 ×= 2100 in 4
.
.
2
.
12  2 
Q = 0 75 ×12 5 . ×10 = 93 8 . in 3
.
q = 107 × 93 8 . = 478 kips/in
.
2100
Thus,
.
D = 478 = 172
.
.
2 ×1 392
3
3
Since plate B is / 8 in thick, use the AISC minimum fillet weld size, / 16 in.
The total shear flow force acting on plate A is 4.78 × 6.25 = 29.9 kips. This force does not affect
the welds of stiffener A to the column. Rather, stiffener A can be considered an extension of the beam
flange, and the shear flow force is taken as part of the flange force. Since the beam flange is full-
penetration welded to the stiffener A, no further analysis is required.
A Further Consideration. It sometimes happens in the design of this type of connection that the beam
is much stronger in bending than the column. In the example just completed, this is not the case. For the
strong-axis W21 × 62 beam, design M = 389 ft⋅kip, while for the column, φM p = 647 ft⋅kip. If the φM p
of the column were less than half the design M of the beam, then the connection should be designed for
2(φM p ) of the column because this is the maximum moment that can be developed between the beam and
column, that is, that the system can deliver. Similar conclusions can be arrived at for other arrangements.
3.7 VERTICAL BRACE DESIGN BY UNIFORM FORCE METHOD

The vertical bracing system in a structure acts as a vertical truss, providing stability for the structure
and resisting lateral loads resulting from wind and seismic forces. When the bracing system is con-
centric, lateral loads will cause only axial loads in the members. On a global level, a concentrically
braced frame is a determinate system and the force distribution is easily determined. On a local level,
however, the force distribution, or load path, through the connection is not as obvious, and assump-
tions must be made to establish a reasonable load path.
As discussed in Art. 3.1.3, an endless variety of possible load paths exist, and any design based
on a load path that satisfies equilibrium, and for which none of the limit states is exceeded, can be
considered as a lower bound to the design strength of the connection.

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