Exprimental Evaluation On The Behaviors Of Side-plate Reinforced Section And Widen Flange Section Of Steel Frame

Widespread and unpredicted brittle fractures have been found in weld steel beam-column connections (weld-flange-bolted-web connections) of steel frames shaken during the Northridge earthquake and the Kobe earthquake. Such fractures were most often initiated at the bottom flange weld and propagated into the column flange and the beam web. The post-earthquake studies have shown that the traditional seismic behavior of beam-column connections is not good because of the brittle failure prevents the welded moment connections from exhibiting the inelastic behavior expected. Extensive experimentally and numerically studies were therefore conducted to the improved beam-column connections in the worldwide. The beam-to-column connection with beam-end horizontal haunch is a kind of reinforced beam-to-column which is one of the typical forms to move plastic hinge outward from the beam and column interface connections. It includes two categories of connections. One of the two categories is the side plate-reinforced section. The other category is the widen flange section. By widening beam flanges, the plastic hinge will form the welds that have much fracture proneness due to stress concentration and welding sensitivity, reducing the damage of brittle fracture of connections and improving the ductility of the structure.The details and design formula of the beam-to-column connections with the horizontal haunch were provided based on the suggestion of "Recommendation for Design of Connections in Steel Structures" published in Japan and FEMA-350 published in U.S.A. On the basis of above suggestions, 4 one-half scale specimens of the beam-to-column connections with the horizontal haunch were designed, among which the influences of length of the haunch and different reinforced types, compared with a traditional specimen.The connections with beam-end horizontal haunch under cyclic load were conducted to investigate their ductility behavior and hysteretic behavior. And the experiment was the effect for failure mode, ultimate load, plastic rotation angles and the hysteresis curves. The experiment was also plotted in the form of a stress distribution of beam flanges. The results demonstrated that the connections with beam-end horizontal haunch have a better plastic deformation and capacity of consuming energy, its hysteretic curves were well-shaped with large-area hysteretic loops, however, hysteretic curves of common connection was flat with small-area hysteretic loops. The plastic rotation angles of the 4 connections with beam-end horizontal haunch specimens exceeded 3%rad and their ductility coefficient exceeded 3.0 which meeting the requirement for plastic rotation angles and ductility coefficient. Also, the seismic performances of the widen flange connections and side-plate reinforced connections were compared to know that both connections can form the desired plastic hinge at the beam section away from the column face, but the side-plate reinforced connections have a worse ductility due to the heat effect of welding to connect the side plates and the beam flanges, which caused brittle tear of beam flanges in the heat effect zone outside of reinforced part. It is suggested that the widen flange connections which can satisfied the demand of plastic deformation.