نشریه سازه و فولاد
سال هجدهم شماره 45 (پاییز 1403)

In steel structures, beams in Moment-Resisting Frames play an essential role due to their ductile nature as structural members. The capacity of conventional steel beams is determined by the lower of the plastic moment capacity and the lateral-torsional buckling capacity. In order to attain the plastic moment capacity of the beams, lateral-torsional bracing is applied to postpone lateral-torsional buckling, allowing the plastic moment of the section to control the behavior. However, in some structures, it is a challenge to appropriately place lateral-torsional bracing, leading to difficulties in the execution of certain beams. A practical solution to this challenge is to use double channel sections instead of I-sections, along with intermediate connectors between the channels to improve the section's buckling capacity.  In this study, several intermediate connectors were considered for joining double channel beams, and among them, the connector with the best performance was identified as the most suitable intermediate connector for joining double channels. Using ABAQUS software, this research modeled double channel beams and compared them with I-shaped beams. To apply different loading conditions, the concentrated moment applied at one end of the beam was considered as a multiple of the moment applied at the other end. Additionally, in this study, four different beam lengths—4, 6, 8, and 10 m—were used to examine the effect of beam length. The research results revealed that, among the various spacing lengths considered for the intermediate connectors in the double channel sections, the restraining length used for the single channel section beams was found to be appropriate. Moreover, the study confirmed that the intermediate connector used to join the channels performed effectively, leading to a flexural capacity in double channel beams comparable to that of equivalent I-shaped beams.

The purpose of this research is to evaluate the life cycle cost of performance-based optimally designed concentrically braced frames. Today, the effect of earthquake on the design of a structure is considered with the aim of optimizing the weight and reducing the initial construction cost of the structure. Although such a structure will have the lowest construction cost, it is not possible to estimate the cost due to an earthquake during its operation. Life cycle cost analysis is a suitable method to examine the cost and performance of structures that should be in service for a long time. In the first step of this study, two three-span ten-story frames with a concentrically bracing system with the position of the bracing in the middle span and the side spans, using the center of mass meta-heuristic algorithm in the framework of the performance-based design method, and considering the weight of the structure as the objective function and the external penalty function method have been optimized. In this study, OpenSees software was used to perform nonlinear modeling and analysis, and MATLAB software was used to implement the optimization problem. In the second step, the life cycle of the frames resulting from the optimization process has been investigated using the Wen and Kang relationship. According to the obtained results, placing the brace in the side spans reduces the structural component life cycle by almost 10% compared to the brace frames with the placement of the brace in the middle span.

One of the methods to prevent the direct transmission of earthquake forces from the ground to a structure is seismic isolation. The main goal of seismic isolation is to reduce the amount of acceleration and forces transferred to the structure during an earthquake. This goal is achieved by installing the structure on separating layers that possess high horizontal flexibility. In this way, when the ground beneath the structure vibrates strongly during an earthquake, minimal movement is induced in the structure. In this research, a new index has been introduced to compare the performance of base isolation systems, which can lead to the design of base isolation systems with improved performance for the structure. By utilizing the proposed index, known as the Weighted Relative Performance Index (WRPI), the performance of both the structure and the base isolation system can be evaluated. This evaluation considers the amount of strain energy absorbed in the structure and the isolator, as well as the maximum acceleration value and the displacement created in both the structure and the isolator. To evaluate the effectiveness of the proposed index and compare it to previous indices, a 6-story steel moment frame with and without base isolation system was studied. Overall, the WRPI index demonstrates better efficiency and performance than previously used indices, particularly in structures sensitive to acceleration and inter-story drifts.

In this study, an innovative buckling-restrained brace (BRB) equipped with an elastomer was numerically analyzed. The analyses were performed using Abaqus software. The components of the BRB, including the casing, core, and elastomer, as well as their connections, were described in detail. The elastomer's connection method to other components was also detailed. Subsequently, the numerical modeling process was thoroughly explained. The force-displacement hysteresis of a BRB equipped with elastomer was compared to that of a BRB without elastomer and of the same dimensions. Additionally, stress distribution in critical areas of the core and casing was examined. The failure mode of this BRB was identified as buckling of the yielding portion of the core. It was observed that incorporating an elastomer into the BRB increased the post-yield stiffness and reduced residual displacements. A model with an elastomer stiffness twice that of the experimental specimen was also studied, demonstrating a 20% increase in post-yield stiffness. The BRB equipped with elastomer has a greater energy dissipation capacity compared to the BRB without elastomer. Compared to conventional BRBs, BRBs equipped with elastomers maintained structural stability even after core fractures, owing to the elastomers' role in critical regions. This characteristic highlights the high efficiency of this system in improving the seismic performance of structures.

Due to the increasing use of steel frames in buildings built in Iran and the increase in the price of profiles compared to raw sheets, the use of plate girders in the construction of steel frames has become significantly popular. Connections are one of the basic components in all structures, including steel structures, and are considered the main factor in the integrity of structural systems. The purpose of this research is to determine the optimal dimensions for the end plate connections to reduce the cost using particle swarm optimization algorithm, so that the mechanical limitations related to the bending moment and the resulting initial stiffness, as well as the safety and integrity of the connection, are not compromised. In the optimization model, six design variables including the dimensions and thickness of the end plate, the diameter and location of the bolts were selected to compare the obtained values with the results of the past literature. To calculate the bending moment and rotational stiffness of the connection, the "component method" found in Eurocode 3-part 1-8 was used, and the analysis and optimal design of beam-to-column connections for two-dimensional steel frames was done in the MATLAB computing environment. The output of this program shows completely satisfactory results compared to the results in the past literature, so that the results obtained by using the aforementioned optimization algorithm show the cost of connections A, B, C and D of the investigated frame, respectively cost reductions of 46.24%, 5.29%, 55.10% and 56.44% by compared to the research of Cabrero and Bayo in 2005. Also, the cost of connections A and C of the examined frame decreased by 8.07% and 0.22%, respectively, compared to Diaz et al.'s research in 2012.

Yielding dampers are widely regarded as an effective method for reducing the destructive effects of earthquakes due to their high energy absorption capability and enhancement of structural stability. Elliptical roller damper integrates rollers into the conventional elliptical damper that is a yielding type, enabling it to sustain substantial gravity and vertical earthquake loads, and alleviates the dead load imposed on other components within the gravity load-bearing system. In this study, the performance of the elliptical roller damper, as an optimized example of such dampers, was investigated. Using numerical modeling in Abaqus software, the effects of geometric parameters such as length, width, thickness, height, and the number of rollers on the damper's performance were analyzed. The results showed that increasing the width and thickness and reducing the height significantly improved the energy absorption capacity and lateral strength of the damper, while increasing the length and number of rollers had no effect on the hysteresis curve but contributed to the stability of the damper under gravity loads. Furthermore, the elliptical roller damper demonstrated stable hysteresis loops in various loading angles, indicating its capacity to withstand different directions of loading. Finally, an optimized design for using this damper in seismic isolators was proposed, which is expected to effectively reduce the transfer of seismic energy to the structure and enhance its overall stability.