Modification of seismic behavior of steel moment resisting connection with top and bottom plate with lack of penetration in the welds by analytical method

The experience of Northridge and Kobe earthquakes have demonstrated poor behavior of welded beam-column connections due to deficiency of welds. After numerous studies, proper method for welding of connection has been proposed. Yet, the proper procedure and detailing of welding usually do not follow due to insufficient supervision of welding process, therefore deficiency in the connection of a real steel structures is expected. One of the most common moment resisting connections in the steel structures is connection with top and bottom plates which is the subject of study in this paper. Among the possible deficiency in the welds, lack of penetration (LOP) and the lack of fusion (LOF) are two main deficiencies in these connections. In this study, the cyclic behavior of three steel moment resistant connection with LOP deficiency in the weld have been studied by analytical method. The deficiency was considered on the top, bottom and both top and bottom plates and results are compared with the original connection. The connections are selected from three different size (small, medium and large) to evaluate the size effect on the results. The connections are modeled by ABAQUS finite element software. The molding of the initiation and growing of cracks in the connections was conducted by the extended finite element technique (XFEM). In the beginning, to examine the efficiency and accuracy of modeling, an experimental study was used for verification. The hysteretic behavior of specimens was studied under the typical loading protocol of SAC and based on that, the constitutional behavior of connections was developed. The backbone curve of each connection was obtained and the curve of each deficiency was compared with each other and the original connection. Results show that in LOP deficiency, the crack is formed and grow in the welds which ultimately lead to rupture in the welds. In general, the deficiency reduces energy absorption, moment and ductility capacity of connections. The result in different beams size are very close which suggest that the size of the beam has not effect on the reduction of the capacity. To estimate the reduced capacity of connections with deficiency in the real cases, reduction coefficient is introduced and presented for yield and ultimate moment and yield and ultimate rotation. In general, it can be concluded that LOP cause 30% reduction in the yielding and ultimate moment capacity of connections and 20% reduction in the yielding and ultimate rotation of connection. In addition, to study the effect of different deficiencies on the reduction of absorbed energy, the area under the hysteresis curve are calculated and compared. The results indicated that the connection with LOP in both plate have the highest reduction in observed energy which follows by the LOP in top and then bottom plate. This results in addition to the value of reduction in capacity and ductility of connections indicated that LOP in the top plate cause the connection to fail in shorter cycles compare to other cases. Therefore, it is better to have not the LOP in top plate which needs special attention in welding of that plate which usually conducted in the workshop with the least possible of supervision.