The BFP moment connection requires plate steel for the flange plate, shear plate, and possibly panel zone doubler plates. Past tests have been performed with plates fabricated both from ASTM A36/A36M and A572/A572M Grade 50 (Grade 345) steels. Therefore, the prequalification extends to both plate types. The designer should be aware of potential pitfalls with the material selection for the flange plate design. The flange plate must develop tensile yield strength over the gross section and ultimate tensile fracture resistance over the effective net section. A36/A36M steel has greater separation of the nominal yield stress and the minimum tensile strength, and this may simplify the satisfaction of these dual requirements. However, variation in expected yield stress is larger for A36/A36M steel, and design calculations may more accurately approximate actual flange plate performance with A572/A572M steel.
The flange plate welds are shop welds, and these welds are subject to potential secondary yielding caused by strain hardening at the primary yield location in the beam. As a result, the welds are required to be demand-critical CJP groove welds. If backing is used, it must be removed, and the weld must be backgouged to sound material and backwelded to ensure that the weld can sustain yielding of the flange plate. Because the welds are shop welds, considerable latitude is possible in the selection of the weld process as long as the finished weld meets the demand-critical weld requirements stipulated in the AISC Seismic Provisions. In the test specimens used to prequalify this connection, electroslag, gas shielded metal arc, and flux cored arc welding have been used.
The BFP moment connection places somewhat less severe demands on the web con- nection than most FR connections because of the somewhat greater flexibility of the bolted flange connection. As a result, the shear plate may be welded with CJP groove welds, PJP groove welds, or fillet welds.
Bolts in the flange plate are limited to two rows of bolts, and the bolt holes must be made by drilling or subpunching and reaming. These requirements reflect testing used to prequalify the BFP moment connection, but they also reflect practical limitations in the connection design. Net section rupture is a clear possibility in the beam flange and flange plates, and it is very difficult to meet the net section rupture criteria if more than two rows of bolts are employed.
A single row of bolts causes severe eccentricity in the connection and would lead to an excessively long connection. Punched bolt holes without reaming and thermally cut bolt holes are not permitted because the surface roughness in the hole may initiate cracking of the net section under high tensile stress. As noted earlier, the connection is prequalified only for A490 or A490M bolts with threads excluded from the shear plane. Bolt diameter is limited to a maximum of 1 1/8 in. (28 mm) because larger bolts are seldom used and the 1 1/8 in. (28 mm) diameter is the maximum used in past BFP tests. The bolt diameter must be selected to ensure that flange yielding over the gross area exceeds the net section capacity of the beam flange.
Oversized bolt holes in the flange plates were included in some past tests because the oversized holes permit easier alignment of the bolts and erection of the connection and resulted in good performance of the connection. Consequently, oversize holes are permitted in the flange plates. When oversize holes are used, the bolted joint may be designed as for bearing type joints and need not be designed as slip critical. However, as required in the AISC Seismic Provisions, all bolts should be installed as pretensioned high strength bolts, and faying surfaces should be prepared to provide a Class A slip coefficient or higher.
The beam must fit between two welded flange plates with full consideration of rolling and fabrication tolerances. As a result, shims may be used to simplify erection while ensuring a tight connection fit.