Investigation of nonlinear behavior of continuous steel girder and CFT column connection under cyclic loading

Concrete Filled steel Tubes (CFT) have several advantages when used as columns in buildings. These advantages include efficient structural section benefiting from combined action of concrete and steel , drastic reduction of the need for steel rebars, confinement of concrete by steel tube permitting higher compressive strains, increasing resistance of steel tube against local buckling, eliminating a separate formwork for concrete and reduction of construction time. On the other hand, connection of steel beam to round steel columns including CFT has been a major challenge.The main complexity arises from difficulty of accessing the inside of tubular columns for installing continuity plates to transfer tension or compression between opposing beam flanges. Different methods have been proposed in the past to alleviate this problem. Solutions presented so far include external collar stiffener plates connecting opposing beam flanges, extending only the web of beam through steel tube column, extending beam flanges through column, using steel rebars embedded in concrete core and attached to beam flanges and finally, complete passage of steel beam through steel tube. Cyclic behavior of beam-column connection plays a vital role in seismic response of moment frame structures. Literature survey shows that despite numerous methods proposed for steel beam-CFT column connection, not any organized attempt has been undertaken to explore the cyclic behavior of these connections. In this paper, numerical modeling has been utilized to investigate the cyclic behavior of continuous steel girder and circular CFT connection. Before achieving the main analyzes, the modeling and analysis assumptions and techniques were verified using available pushover experiments on typical connections of similar arrangement. In current study, effect of column diameter, thickness of steel tube and thickness of girder flange and girder web have been considered. Fourteen samples with different values of column to beam moment capacity ratios ranging from 0.78 to 1.95 have been analyzed. Actual dimensions of specimens were selected to be compatible with production of steel tubes and have reasonable lateral load bearing strength. Beam and column dimensions were checked to comply with AISC360 compact section limitations. Two diameters of 405 and 505 mm were specified for columns while beams had a fixed depth of 428 mm. Thickness of St37 grade steel tube ranged between 4.5 to 8.5mm. Beams were considered to be of St52 grade with flanges of 12.5 to 16.5mm thickness and webs between 7 to 11mm thick. Some specimens slightly violated seismic compact section regulations of column and girder. Meanwhile, all specimens showed stable and large load-drift cycles and could tolerate 0.04 story drift ratio as stipulated by AISC code for special moment connections. Specimens which fully complied with code reqirements could undergo 0.05 story drift ratio while still maintaining at least 80 percent of their maximum strength. Analysis results show that in specimens with same columns, increasing girder flange or web thickness is accompanied with increased energy absorption. The fraction of energy absorbed by elastic deformation increased more than the energy dissipated through hysteretic nonlinear deformations. As a result, a reduction in hysteretic damping accompanied the increase in beam flange or web thickness.