نشریه سازه و فولاد
سال هفدهم شماره 41 (پاییز 1402)

In previous earthquakes, many steel frames suffered damage to their ordinary rigid moment beam-to-column connections. As a result, the use of multi-level control systems in structures has gained attention from engineers in recent years. The main idea in this system is to combine two separate control systems with different stiffness and strength, thus creating dual seismic behaviors. In this study, a two-level friction-yielding damper passive control system is presented for beam-to-column connections, and its cyclic behavior is evaluated using nonlinear static analysis with the finite element method using ABAQUS software. The evaluation results demonstrate that the performance of the samples improves with an optimal ratio of length to thickness. This improvement is reflected in increased stability and area of the hysteresis curves, ultimately leading to higher energy absorption. Additionally, the hysteresis curves indicate a ductile behavior for the proposed damper in the moment connection. Moreover, the obtained hysteresis curves show that the system reliably dissipates energy at different earthquake levels, satisfying the seismic criteria for special moment resistant connections based on the AISC code. The proposed damper serves as an energy dissipating device, effectively dissipating energy at different levels of earthquakes by facilitating slip and yielding, thereby controlling the seismic response of the structure.

The performance evaluation of structures, particularly steel structures against blast loads, is of great importance. One important prediction for dealing with destructive lateral forces in structures is the application of structural steel framing systems. In this study, seven samples of lateral load-resisting systems were developed in a five-story steel structure, considering the provisions of Standard 2800. Using ABAQUS and SAP2000 software, numerical analysis was performed on these systems under blast loads. An aerial explosion was considered, with an explosive material quantity of 200 kg in terms of T.N.T, located at a distance of 2 meters from the centerline of the beam-column connection, in a height of 2.8 meters from the ground level. The results showed that in all models, the adjacent column to the explosion source experienced severe deformation, and plastic damage occurred in the connection region. Additionally, the stresses in the beam web and connection plates at the base of the column reached their maximum values. This occurrence was less observed in moment frame models, while special moment frames and intermediate moment frames demonstrated better performance. In various shear wall models, no plastic damage was observed at the base of the adjacent column to the explosion source, and the energy absorption per unit weight of the structure was obtained. Consequently, the special moment frame had the highest energy absorption capacity, and the lowest was associated with the concentric moment frame.

Previous studies have demonstrated the occurrence of brittle and early stage cracks in the connection between the link beam and the column, specifically in the vicinity of the groove welds  that connect  the flanges of the link beam to the column, when subjected to seismic activity. A similar type of rupture has also been observed in the end-plate connections of the link beam to the external link beam. This research explors the consept of utilizing a box-shaped link beam with a reduced cross-section in the Abaqus finite element software to magnitude the plastic strain demand generated at the flange ends of the box link beam. The objective is to prevent rupture in this specific region. The results indicate that the maximum equivalent plastic strain at the end of the link beam flanges can be reduced by up to 38% for short link beams, up to 41% for intermediate link beams, and up to 68% for long link beams. Consequently, this approach effectively prevents premature ruptures in the area adjacent to the groove weld connecting the beam flange to the end plate. The magnitude of the equivalent plastic strain at the end of the box link beam decreases as parameters "a" and "b" decrease, and parameter "c" (geometric parameters of the reduced cross-section) increases. Parameter "c" exhibits the most significant influence on this parameter, while parameter "b" has the least significant impact. Additionally, the adoption of a reduced cross-section in the box link beam results in an increase of up to 35% in the energy dissipation of the link beam due to enhanced participation of the flanges in energy dissipation for short link beams, up to 158% for intermediate link beams, and up to 250% for long link beams.

Reinforced-Concrete Shear Wall (RCSW) is a common lateral resisting system especially in mid and high-rise structures. This popularity among engineers is due to high ductility and in-plane stiffness of the RCSWs. RCSWs have shown good performance as seismic load-bearing systems in past earthquakes. However, the main disadvantage of RCSWs is the crushing of concrete at the foot of the wall and as a result the buckling of the reinforcement in this area, which makes it difficult and, in some cases, impossible to repair after the earthquake. As a solution, using Buckling-Restrained Brace (BRB) as boundary elements at the base of the RCSWs is proposed and investigated in this research. For this purpose, two sets of 6 and 12-storey structures with conventional RCSW and BRB-RCSW as lateral resisting system were designed, and their seismic response were assessed through static (monotonic and cyclic), dynamic Nonlinear Time History (NLTH) and Incremental Nonlinear Dynamic Analyses (NIDA). The results indicate improved structural performance and an increase of response modification factor from 4.4 in conventional RCSW to 5.47 in RCSW-BRB. Furthermore, the proposed system prevents the induced damage at the foot of the RCSW while the replaceable BRBs act as the fuses, increasing the resiliency of the structure. So that the behavior factor of the frame equipped with buckling-restrained braces increased by 30 percent in the 6-story building and 20 percent in the 12-story building.

Due to economic efficiency, members with variable cross-sections are widely used in steel structures, especially industrial sheds. But with the increase in temperature caused by thermal changes, the strength and hardness characteristics of industrial frame members decrease rapidly. It is necessary to check stability (thermal buckling) in industrial frames for a safe and complete design. In this article, thermal buckling and vibrations caused by thermal changes of non-radiative elastic columns are investigated by the Rayleigh-Ritz method. In the next step, the weak form of the differential equation is calculated and the Chebyshev series is used as the transverse displacement function and the weight function. In the last step, after extracting material stiffness matrices, geometric stiffness, and mass matrix, the eigenvalues of the equation are checked. The results show that the simultaneous increase of the slope of the section and the coefficient of thermal changes significantly affect the effective length coefficient and reduce the buckling load capacity in all the boundary conditions of different supports. Also, the simultaneous increase of the slope of the cross-section and the coefficient of thermal variation, depending on the type of boundary conditions of the support, cause an increase or decrease in the dimensionless natural frequency. Aligned curves are used to present results and display graphs to apply results in engineering calculations. The results of previous research are used for validation. There is an acceptable agreement between the present results and previous studies.

Diaphragms, beyond supporting gravity loads and transferring them to vertical structural elements, play a crucial role in collecting lateral forces and distributing them among lateral resistant systems. In some cases, due to architectural considerations in the design of braced steel building frames, structural designers are compelled to shift the braces in the structure's plan, classifying them under structures with out-of-plane offset irregularity. In such structures, the horizontal diaphragm between the shifted braces must effectively transfer the shear forces induced by earthquakes. Therefore, investigating the behavior of these diaphragms is imperative for the analysis and design of these structures. The objective of this research is to investigate the seismic behavior, drift demands, over-strength factor, and the degree of vulnerability of discontinuous concentrically braced frames (CBFs) when subjected to earthquakes. To achieve this goal, two three-dimensional CBFs of 3 and 6 stories, both with and without out-of-plane offset irregularity, incorporating inelastic diaphragm behavior, were taken into account through pushover and nonlinear time-history dynamic analyses using ABAQUS. The examination of the results indicated that the concrete diaphragm existing in the discontinuity region, with a thickness of five centimeters, has been damaged at a 45-degree angle. It lacks the ability to transfer the shear force resulting from the lateral load of the braces. This is in contrast to thicknesses of 10 and 20 centimeters, where the diaphragm in the mentioned area remains undamaged. Given the information provided, it seems that, with the minimum thickness specified in Iranian National Building Code (part 9), was vulnerable to the transferred shear forces. Additionally, it was found that the inter-story drift demands of the frames exceed the estimations proposed by the building code.