Development of Parametric Equations for Behavior Characteristics of Conventional Saddle-like Connections

Saddle-like (Khorjeeni) connections are formed in steel frames, when the beams are not cut at their intersection with columns, and they by-pass the columns continuously and are connected to them using top and bottom angles. This type of connection provides some benefits, such as ease of construction and superior behavior under gravity loads, however it has some disadvantages that can cause significant damage to the structure when subjected to seismic loads. Past studies have shown that saddle-like connections can be classified as semi-rigid connections, but they do not have the ductility capacity observed in those types of semi-rigid connections, which have been recommended in various codes for seismic resistant design.
In recent years several researchers have tried to propose details for improving the behavior of Khorjeeni connections. However, in the process of seismic evaluation and rehabilitation for existing structures, in which the conventional type of Khorjeeni connections are used, it is necessary to have typical behavior curves and acceptance criteria for different levels of structural performance. The aim of this paper is to develop parametric equations for behavior characteristics of the conventional saddle like connections.
Experiments and finite element modeling have been conducted on fourteen different specimens of the connections. The details have been selected based on what are normally found in medium rise buildings in Iran. The beam height and the angle length vary in the specimens. Six specimens have been tested in laboratory and then modeled by finite elements. FE analysis has considered the crack initiation and propagation using a micromechanical model originally proposed for predicting crack initiation in ultra-low cycle fatigue, ULCF. ABAQUS multi-purpose software has been employed for this work. As the Finite Element modeling of tested saddle-like connections has proved to be successful in predicting the behavior of this type of connection, additional samples have been modeled and analyzed using FE models. Based on the results of experiments and FE modeling, backbone curves representing moment rotation behavior of the connections have been determined following the FEMA recommendations Characteristic parameters of the backbone curve have been identified as initial stiffness, yield moment, ultimate moment and ultimate rotation of the connection. Also the parameters affecting these characteristic values have been found to be beam depth, top angle size, top angle length, and bottom angle length . Finally, using the regression methods, some relationships have been proposed for each characteristic parameter of the backbone curves. A comparison of the experimental and numerical results and the results of parametric equations shows good accuracy. The differences in initial stiffness, yield moment and ultimate moment are less than 10 percent for majority of the specimens. The differences in ultimate rotation are also less than 15 percent in majority of the cases. The proposed equations in conjunction with FEMA recommendations for acceptance criteria can be used in seismic evaluation and rehabilitation of steel structures with saddle like connections.