## 533 Moment Connections

The most commonly used moment connection is the field welded connection shown in Fig. 5.56. This connection has been in common use throughout the U.S. for many years. In current seismic design covered by the AISC ''Seismic Provisions for Structural Steel Buildings,'' it is permitted for use in ordinary moment-resisting frames (Art. 9.7) without requirements for physical testing. It is also permitted for use in special moment-resisting frames, when the member sizes used for the specific project have been tested to demonstrate that the required ductility level can be achieved. Furthermore, it is widely used in areas of low seismicity where the AISC seismic provisions do not apply, and in frames designed primarily for wind and gravity forces, such as in the following example.

Example—Three Way Moment Connection—AISC LRFD. The moment connection of Fig. 5.56a is a three-way moment connection. Additional views are shown in Figs. 5.56b and 5.56c. If the strong axis connection requires stiffeners, there will be an interaction between the flange forces of the strong and weak axis beams. If the primary function of these moment connections is to resist lateral maximum load from wind or seismic sources, the interaction can generally be ignored because the maximum lateral loads will act in only one direction at any one time. If the moment connections are primarily used to carry gravity loads, such as would be the case when stiff floors with small deflections and high natural frequencies are desired, there will be interaction between the weak and strong beam flange forces. The calculations here will be for a wind or a seismic condition in a region of low to moderate seismicity, but interaction will be included to demonstrate the method.

Following common practice based on tests, the load path assumed here is that the moment is carried entirely by the flanges, and the shear entirely by the web. Proceeding to the connection design, the strong axis beam, beam No. 1, will be designed first.

Design of Beam No. 1. Beam No. 1 is a composite W21 x 62 section of A36 steel. The flange connection is a full penetration weld so no connection design is required. The column must be checked for stiffeners and doublers.

Design of Column Stiffeners. The connection is to be designed for the full moment capacity of the beam, (Mp. Thus, the flange force Ff is

(Mp 389 x 12 Ff = d-t = (20.99 - 0.<>15) = 229 kips where d is the beam depth and tf is the flange thickness. From the column load tables of the AISC LRFD Manual, Vol. I, find the following parameters.

Web yielding: Pwy = Pwo + tf Pm = 174 + 0.615 x 24.3 = 189 kips < 229 kips, thus stiffeners are required at both flanges.

FIGURE 5.56a Example of field-welded moment connection. For section A—A, see Fig. 5.56c. For section B-B, see fig. 5.56b. (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.)

FIGURE 5.56a Example of field-welded moment connection. For section A—A, see Fig. 5.56c. For section B-B, see fig. 5.56b. (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.)

Web buckling: Pwb = 261 kips > 229 kips—no stiffener required at compression flange. Flange bending: Pfb = 171 kips < 229 kips—stiffener required at tension flange.

From the above three checks (limit states), a stiffener is required at both flanges. For the tension flange, the total stiffener force is 229 - 171 = 58 kips and for the compression flange, the stiffener force is 229 - 189 = 40 kips. Because the loads may reverse, use the larger value of 58 kips as the stiffener force for both flanges. Then, the force in each stiffener is 58/2 = 29 kips, both top and bottom.

The minimum stiffener width ws depends on the flange width of the beam, bfb, and the web thickness of the column, twc:

Use a stiffener 6V2 in wide to match column. The minimum stiffener thickness ts is

Use a stiffener at least 3/8 in thick.

Mg =*Mp = 358 k-ft (FULL MOMENT CAPACITY - TYP.) y4 =^Mp = 358 k.ft

FIGURE 5.56b Example of field-welded moment connection; section B-B of Fig. 5.56a. (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.)

Mg =*Mp = 358 k-ft (FULL MOMENT CAPACITY - TYP.) y4 =^Mp = 358 k.ft

FIGURE 5.56b Example of field-welded moment connection; section B-B of Fig. 5.56a. (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.)

The minimum stiffener length ls depends on the column depth, dc, and the flange thickness of the column, tfc:

The minimum length is for a ''half depth'' stiffener, which is not possible in this example because of the weak axis connections. Therefore, use a full depth stiffener, 12V2 in long. A final stiffener size consideration is a plate buckling check which requires that

Therefore, the minimum stiffener thickness is V2 in and the final stiffener size for the strong axis beam is V2 X 6V2 X 12V2 in. The contact area of this stiffener against the inside of the column flange is 6.5 - 0.75 = 5.75 in due to the snip to clear the column web-to-flange fillet. The stiffener design strength is thus 0.9 X 36 X 5.75 X 0.5 = 93.2 kips > 29 kips, OK.

Welds of Stiffeners to Column Flange and Web. Putting aside for the moment that the weak axis moment connections still need to be considered and will affect both the strong axis connection stiffeners and welds, the welds for the V2 x 6V2 x 12V2 strong axis stiffener are designed as follows. For the weld to the inside of the flange, the connected portion of the stiffener must be developed. Thus, the 5% in contact, which is the connected portion, is designed for 93.2 kips rather than 29 kips, which is the load the stiffener actually carries. The size of the weld to the flange in number of sixteenths of an inch is thus

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