The welded unreinforced flange-welded web (WUF-W) moment connection is an all-welded moment connection, wherein the beam flanges and the beam web are welded directly to the column flange. A number of welded moment connections that came into use after the 1994 Northridge earthquake, such as the reduced beam section and connections provided with beam flange reinforcement, were designed to move the plastic hinge away from the face of the column. In the case of the WUF-W moment connection, the plastic hinge is not moved away from the face of the column. Rather, the WUF-W moment connection employs design and detailing features that are intended to permit the connection to achieve SMF performance criteria without fracture. Key features of the WUF-W moment connection that are intended to control fracture are as follows:
- The beam flanges are welded to the column flange using CJP groove welds that meet the requirements of demand critical welds in the AISCE Seismic Provisions, along with the requirements for treatment of backing, weld tabs, and welding quality control and quality assurance requirements, as specified in Chapter 3.
- The beam web is welded directly to the column flange using a CJP groove weld that extends the full depth of the web—that is, from weld access hole to weld access hole. This is supplemented by a single-plate connection, wherein a single plate is welded to the column flange and is then fillet welded to the beam web. Thus, the beam web is attached to the column flange with both a CJP groove weld and a welded single-plate connection. The single-plate connection adds stiffness to the beam web connection, drawing stress toward the web connection and away from the beam flange-to-column flange connections. The single plate also serves as backing for the CJP groove weld connecting the beam web to the column flange.
- Instead of using a conventional weld access hole detail as specified in AISC Specification Section J1.6 (AISC, 2022b), the WUF-W moment connection employs a special seismic weld access hole with requirements on size, shape, and finish that reduce stress concentrations in the region around the access hole detailed in AWS D1.8/D1.8M (AWS, 2021).
Prequalification of the WUF-W moment connection is based on the results of two major research and testing programs. Both programs combined large-scale tests with extensive finite-element studies. Both are briefly described herein.
The first research program on the WUF-W moment connection was conducted at Lehigh University as part of the SAC-FEMA program. Results are reported in several publications (Ricles et al., 2000, 2002). This test program formed the basis of prequalification of the WUF-W moment connection in FEMA 350 (FEMA, 2000e). As part of the Lehigh program, tests were conducted on both interior and exterior type specimens. The exterior specimens consisted of one beam attached to a column. The interior specimens consisted of a column with beams attached to both flanges. One of the interior specimens included a composite floor slab. All specimens used W36×150 (W920×223) beams. Three different column sizes were used: W14×311, W14×398, and W27×258 (W360×463, W360×592, and W690×384). All WUF-W moment connection specimens tested in the Lehigh program satisfied the rotation criteria for SMF connections (±0.04 rad total rotation). Most specimens significantly exceeded the qualification criteria. Considering that the interior type specimens included two WUF-W moment connections each, 12 successful WUF-W moment connections were tested in the Lehigh program. This research program included extensive finiteelement studies that supported the development of the special seismic weld access hole and the details of the web connection.
The second major research program on the WUF-W moment connection was conducted at the University of Minnesota. The purpose of this research program was to examine alternative doubler plate details, continuity plate requirements, and effects of a weak panel zone. All test specimens used the WUF-W moment connection. Results are reported in several publications (Lee et al., 2002, 2005a, 2005b). Six interior type specimens were tested in the Minnesota program. All specimens used W24x94 (W610x140) beams. Three column sizes were used: W14x283, W14x176, and W14x145 (W360x421, W360x262, and W360x216). All specimens were designed with panel zones weaker than permitted by the AISC Seismic Provisions. Two of the test specimens, CR1 and CR4, were inadvertently welded with low-toughness weld metal. This resulted in premature weld failure in specimen CR4 (failure occurred at about 0.015 rad rotation). With the exception of CR4, all specimens achieved a total rotation of ±0.04 rad, and sustained multiple cycles of loading at ±0.04 rad prior to failure. All successful specimens exhibited substantial panel zone yielding, due to the weak panel zone design. This test program was also supported by extensive finiteelement studies.
Considering the WUF-W moment connection research programs at both Lehigh and the University of Minnesota, WUF-W moment connection specimens have shown excellent performance in tests. There is only one reported failed test, due to the inadvertent use of low-toughness weld metal for beam flange CJP groove welds (Minnesota specimen CR4). Of all of the WUF-W moment connection specimens that showed good performance (achieved rotations of at least ), approximately one-half had panel zones weaker than permitted by the AISC Seismic Provisions. The other half satisfied the panel zone strength criteria of the AISC Seismic Provisions. This suggests that the WUF-W moment connection performs well for both strong and weak panel zones; therefore, the connection is not highly sensitive to panel zone strength.
The protected zone for the WUF-W moment connection is defined as the portion of the beam extending from the face of the column to a distance, , from the face of the column, where is the depth of the beam. Tests on WUF-W moment connection specimens show that yielding in the beam is concentrated near the face of the column but extends to some degree over a length of the beam approximately equal to its depth.