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In this paper, the cyclic behavior and available ductility of batten columns subjected to constant axial and cyclic lateral load (seismic condition) and their failure mode are evaluated numerically using nonlinear finite element analysis. The column specimens were steel I-shape sections and were analyzed as an equivalent cantilever column. Batten columns are compression members composed of two or more similar longitudinal components (chords) that are connected at points along their length with batten plates as transverse connectors. These connectors ensure that the column behaves as one integral unit to achieve maximum axial capacity. In the past decades, many research activities were conducted on the buckling problem of batten columns. When a batten column is subjected to lateral load or bending moment about its hollow axis (axis perpendicular to battens) in addition to axial compression, the additional internal actions will be imposed to its members (chords and battens). In this case, it is expected that the batten column will have different behavior and failure modes. If the lateral load or displacement is due to seismic actions, more complexities will exist in the column behavior due to nonlinearities and its post-failure response. Few researches were reported about the behavior of batten columns in seismic conditions and their ductility. In this research, the backbone curves for batten columns have been also developed based on their cyclic response. The component backbone curve represents the nonlinear behavior of component in plastic hinge locations and was used in the nonlinear pushover analysis. The backbone curve for some structural components has been found in many standard and guidelines of seismic evaluation like as FEMA356. Using the backbone curve, the available ductility of column considering its post failure response under cyclic lateral loads, could be evaluated. The backbone curve for batten columns does not exist in any guideline or research reports. Because of differences between behavior and failure modes of batten and solid web columns under seismic action, it was expected that their backbone curves had been substantially different. In this research, cyclic response of batten columns with different geometries have been investigated subjected to cyclic lateral and 3 level of constant axial load. Using cyclic curves, the backbone curves of considered batten columns have been developed. The results show that the available ductility of batten columns is considerably low compared with solid web columns. The failure mode of batten columns is local buckling of bottom chords (in flanges and web) in combination with overall buckling of these chords symmetrically. It is also shown that the backbone curves of batten columns are different from solid web columns. The backbone curves of batten columns are semi-ductile (Type 2) based on FEMA356 classification and don’t have any residual strength. Finally, a conservative backbone curve has been proposed for engineering applications.

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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.