Effect of Beam Depth on the Behavior of Welded Unreinforced Flange-Welded Rigid Connection to Box Column without Continuity Plate

The Welded Unreinforced Flange-Welded (WUF-W) connection is one of the most applicable rigid connections used in moment frames. On the other hand, applying built-up box columns is conventional in Iran and Asian countries. Considering the challenges of installing the continuity plates inside the built-up box columns, developing the seismic design equation of minimum face thickness of box column in the connections without continuity plates and trying to reach some creative solutions for eliminating the continuity plates have always been hot topics for researchers, designers, and steel structure industries. It is worth mentioning that due to the lack of use of built-up box columns in European and American countries, European and American standards have not dealt with the criteria of the seismic design of the continuity plates in built-up box columns. Although the AIJ code has proposed a pair of through diaphragms instead of continuity plates to solve the challenges of installing the continuity plates in built-up box columns, and this method has been considered by Iranian researchers, it seems that because of the risk of completely cutting the column section and protruding the continuity plate from column face level, this method has low chance to be widespread used. Regarding the fact that the behavior of the panel zone of H section and box section columns are different, developing the seismic design equations for minimum thickness of column face in built-up box columns without continuity plates have been advised in AISC 341-16. The motivation of developing the seismic design equation of box column face thickness increases when considering that 5th draft edition of the 10th part of the Iranian National Building Code (Steel Building Design and Construction) clarifies that all rigid connections of I beam to box columns, regardless of the column face thickness, must have continuity plates which seems strict. On the other hand, by converting the beam extreme moment to a compression-tension couple force, it seems that the depth of the beam has influence on the nominal capacity of column face resistance.
Therefore, in this numerical study, 18 samples have been modeled to investigate the behavior of WUF-W rigid connection to built-up box columns as well as the probable effect of beam depth on the behavior of WUF-W rigid connection of I beam to box columns. The models have been loaded under cyclic loading and monotonic loading corresponding to the AISC 341-16 protocol. The loading has been continued up to 0.06 radian rotation and the frames have been simplified to a console beam and a column. To verify the modeling, the experimental study of Saniee Nia et al. has been used.
The results show that although all the models without continuity plates apparently pass the provisions of AISC 341-16 and the 4^th edition of the 10th part of the Iranian National Building Code, more attention to details displays some problems. In all samples without continuity plates, the column thickness has been almost 25% over designed, to guarantee the appropriate behavior. But focusing on the equivalent plastic strain at the beam flange edge at loading steps shows the disproportionate spread of plastic strain along the beam flange edge. The plastic hinge is located at the end of beam at the intersection of beam and column face and the column face is partially deformed, which is not desirable. The models without continuity plate have lack of enough rotational stiffness, which does not represent rigid connections but semi-rigid joints. It seems that, unlike what was expected, the beam depth has no obvious effect on the pattern of hysteresis carves of the models without continuity plates like Eurocode 3. However, by 46% decreasing in beam depth, the equivalent plastic strain at the beam flange edge becomes more uniform up to  55% and the rigidity increases up to 23%. Finally, it seems that increasing the thickness of the box column face will lead to increase the rigidity and therefore, it is recommended to develop the seismic design equations to determine the minimum face thickness of the box column in the connection without continuity plates. Finally, it is suggested to address semi-rigid connections in the AISC 341 and in the Iranian National Building Code, 10^th part.