عنوان مقاله [English]
Brittle failure can prevent structural connections from reaching their peak performance. It is therefore considered as one of the most destructive forms of failure. The prevalence of different failure in rigid connections of steel frames in the aftermath of the Northridge and Kobe earthquakes brought the performance of these connections under question. Research into rigid connections with complete penetrating welding revealed that it is highly probable for the welds to undergo premature brittle failure at low drifts. To address this problem, the use of Reduced Beam Sections (RBS) was recommended by the scientific community after the Northridge earthquake. In this connection, the beam’s flanges are cut (reduced) so that it can take on the form of a fuse, making it possible for the plastic hinge to be driven toward the inside of the beam, thereby preventing the panel zone from failing. RBSs, which are categorized as “prequalified connections”, have been the subject of extensive investigations and have suitable energy absorption and ductility under cyclic loadings. They are not, nonetheless, without flaws and are accompanied by problems such as the need for replacement after average or severe earthquakes due to severe inelastic deformations in the reduced area. This problem is compounded by the connection of secondary beams to primary beams in the ceiling of the structures in which they are used.
The objective of this investigation is the numerical evaluation of RBS connections with replaceable fuses. Numerical simulations on three models – namely, a conventional reduced beam section connection (RBS), a reduced-flange connection with a replaceable fuse (RBS-F), and a reduced-web connection with a replaceable fuse (RWS-F) – were carried out using ABAQUS, with material and geometric nonlinearities having been considered. Also, the materials of the columns, beams, and plates, stiffeners, doubler and continuity plates, seat plates, and bolts have been defined based precisely on experimental data. Loading and support conditions of the numerical models were the same as those of the experimental samples.
In the numerical models, the bolts were first pre-stressed to a sufficient degree. Then, lateral cyclic loading was applied to the beam of each model. The hysteretic curves of the numerical models are in good agreement with those of the experimental samples, indicating that the numerical models can reliably be used for the evaluation of other sections. Seven different profiles were selected from IPB sections (IPB140 to IPB340) for the beam. Suitable columns and endplates were designed for every beam size. For every set, three RBS, RBS-F, RDS-F, and RWS-FR models were constructed, bring the total analyzed models to 28. The dimensions of the RWS model were selected so that its plastic section modulus would be the same as that of the RBS sample. Similar to the tests, the analyses continued until a draft of 8% and the hysteretic moment-rotation diagram of each sample was obtained. Since in tall buildings beams and columns with variable dimensions are used in the experiment was carried out for beams and columns with one size, performing extensive numerical analyses can offer a better comparison of the performance reduced-depth sections and reduced-flange sections.
The results of more than 28 numerical analyses showed that in the RBS and RBS-F models, increasing the size of the beam reduces ductility. However, for the RWS-F sample, not only does increasing the size of the beam maintains the beam’s ductility, it also keeps it, noticeably, above those of the other two samples. The ultimate strength of the sample, however, is less than the other two samples. By increasing the web’s thickness and its plastic section modulus, an ultimate strength on par with those of the other samples can be achieved. Therefore, the modified RWS-F sample can be a suitable replacement for RBS connections.