29

For the web welds, fv = 29 = 3.5 ksi < 27 ksi OK. Jv .75 x 11

Associated Shear Connections—Beams 3 and 4. The specified shear for these beams is R = 107 kips. First consider the weld to the beam web. As with the strong axis beam web connection, this is a field welded connection with bolts used for erection only. The design load (required strength) is R = 107 kips. The beam web shear R is essentially constant in the area of the connection and is assumed to act at the edge of Plates A (Section A-A of Fig. 5.56b). This being the case, there will be a small eccentricity on the C shaped field weld. Following AISC LRFD Manual Table 8.42, l = 17 in, kl = 4 in, k = 0.24, x = 0.04, xl = 0.68 in, al = 4.25 - .68 = 3.57 in. From Table 8.42 by interpolation, c = 2.10. The weld size required is

which indicates that a 3/16 fillet weld should be used.

Plate B is a shear plate and will be sized for gross shear. Try a 3/8-in thick plate of A36 steel. Then

(RV = 0.9 x 0.6 x 36 x 0.375 x 17 = 124 kips > 107 kips OK.

Consider the weld of plate B to the column web. This weld caries all of the beam shear, R = 107 kips. The length of this weld is 17.75 in. Thus, the required weld size is

A 3/16-in fillet weld is indicated. Because this weld occurs on both sides of the column web, the column web thickness should satisfy the relationship 0.9 x 0.6 x tw > 1.392 x D x 2 or tw > 0.103 x 2.17 = 0.22 in. Since the column web thickness is 0.485 in, the web can support the 3/16-in fillets. The same result can be achieved using AISC LRFD Manual Table 9.3.

Now consider the weld of plate B to plates A. There is a shear flow q = VQ /1 acting on this interface, where V = R = 107 kips, Q is the statical moment of each Plate A with respect to the neutral axis of the I section formed by Plates A as flanges and Plate B as web, and I is the moment of inertia of Plates A and Plate B. Thus,

I = 12 x 0.375 x (19.25)3 + 0.75 x 12.5 x ( ' „ ' ) x 2 = 2100 in4 Q = 0.75 x 12.5 x 10 = 93.8 in3

107 x 93.8

The required weld size is

Since plates A are % in thick, the AISC minimum fillet weld of V4 in prevails.

A reconsideration of welds for Plates A is now required. In the course of this example, these welds have already been considered twice. The first time was as stiffener welds for the strong axis beam. The second time was for the combination of forces from the weak and strong axis beam flange connections. Now, additional forces are added to plates A from the weak axis beam web connections. The additional force is 4.78 x 6.25 = 30 kips. Figure 5.60 shows this force and its distribution to the plate edges. Rechecking welds for Plates A for the flange weld,

FIGURE 5.60 Additional forces acting on Plates A of Fig. 5.56c. (From W. A. Thornton and T. Kane, ''Connections,'' Chapter 7, Steel Design Handbook—LRFD Method, A. R. Tamboli, Ed., 1997, McGraw-Hill, 1997 with permission.)

_ V292 + (53 + 7.5)2 _ Df = 2 x 6.25 x 1.392 = 3.86

which indicates that a V4-in fillet is required. A 5/i6-in fillet has already been determined. For the web weld,

_ V(107 + 15)2 + 292 _ Dw = 2 x 11.0 x 1.392 = 4'08

This weld, which was previously determined to be a Vi-in fillet weld, must now be increased to a 5/16-in fillet weld.

This completes the calculations required to design the moment connections. Load paths of sufficient strength to carry all loads from the beams into the column have been provided. However, note that 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 x 62 beam, faMp = 389 ft-kip while for the column, 4>Mp = 647 ft-kip. If the faMp of the column were less than half the faMp of the beam, then the connection should be designed for 2(faMp) of the column because this is the maximum moment that can be developed between the beam and column (maximum moment the system can deliver). Similar conclusions can be arrived at for other arrangements.

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Renewable Energy 101

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