نشریه سازه و فولاد
سال شانزدهم شماره 37 (پاییز 1401)

Buckling restrained braces have been shown to exhibit favorable energy dissipating characteristics in steel structures during an earthquake and are therefore widely used in passive control of structures. However, they face the problem that they do not return to their original shape upon unloading, and consequently, the structure experiences large permanent deformations after the earthquake which usually makes the structure impossible or uneconomical to repair. In recent years, to solve this problem, researchers have used Iron-based shape memory alloys which have two essential properties of superelasticity and shape memory behavior. These alloys have been considered due to their high energy dissipation capacity, their ability to withstand large strains, recentering and not leaving permanent deformations upon unloading, and their much lower cost than other shape memory alloys. In this research, the seismic behavior of three-story structures braced with buckling restrained braces, iron-based shape memory alloy, and nitinol shape memory alloy, which is one of the most famous shape memory alloys, is investigated. Modeling and Nonlinear dynamic analysis for these structures have been performed in Seismostruct software. Maximum displacements, residual strains, and force-displacement diagrams of these structures have been studied due to the application of different accelerometers of large earthquakes of recent years with different intensities. The results of this study show that braced structures with Iron-based shape memory alloys experience more maximum displacements than Buckling restrained braces, although unlike BRBF they do not leave any permanent displacement. Also, these structures undergo fewer maximum displacements and permanent displacements compared to nitinol-braced structures, and in general, show better performance.

The numerical structural analysis schemes are extensively developed by progress of modern computer processing power. One of these approximate approaches is called Dynamic Relaxation (DR) method. This technique explicitly solves the simultaneous system of equations. For analyzing the static structures, the DR strategy transfers the governing equations to the dynamic space. By adding the fictitious damping and mass to the static equilibrium equations, the corresponding artificial dynamic system is achieved. The static equilibrium path is required in order to investigate the structural stability behavior. This path shows the relationship between the loads and the displacements. In this way, the critical points and buckling loads of the nonlinear structures can be obtained. The corresponding load to the first limit point is known as buckling limit load. For estimating the buckling load, the variable load factor is used in the DR process. A new procedure for finding the load factor is presented by minimizing external work and kinetic energy, simultaneously. The proposed formula only requires the fictitious parameters of the DR scheme. To prove the efficiency and robustness of the suggested algorithm, various geometric nonlinear analyses are performed. Several trusses, frames and shells structures with nonlinear geometrically behavior is analyzed. The obtained results demonstrate that the new method can successfully estimate the buckling limit load of structures.

The main purpose of this study was to evaluate the vulnerability of steel shear walls using brittleness curves. To achieve this goal, the amount of ductility and damage index of several sample frames reinforced by steel shear wall are evaluated by modeling and performing nonlinear static and dynamic nonlinear analyzes, and damage index control based on ductility is introduced as a performance acceptance criterion. Seismic performance was evaluated using IDA incremental dynamic analysis and fragility curves were obtained using displacement and ductility index. According to the results, a relationship between performance levels and damage index is presented, which indicates the probability of passing performance levels in each of the structures. Damage index for each model was calculated by extracting increasing curves, as a result of which other structural characteristics such as stiffness, ductility and yield point of the structure can be obtained. The advantage of this method and the introduction of fragility curves based on damage index is determining the ductility parameter based on the structural criteria of the structure. Based on this, the strength of each structure can be calculated according to the yield displacement of the structure.

Determination of the behaviour factor and displacement amplification factor is one of the most important issues in the seismic design of structures. Seismic codes use the behaviour factor to consider the inelastic behavior of structures. The inelastic behavior of structures is defined by ductility factor. This factor refers to the ratio of the maximum structural displacement to the displacement corresponding to the yield strength. So, the determination of the behaviour factor is directly dependent on the ductility factor so that a greater ductility factor leads to the greater behaviour factor. The behaviour factor is related to other factors such as the overstrength, degrees of redundancy and damping. The calculation of the real lateral displacement of structures including the elastic and inelastic parts is essential for design purposes. Seismic codes use displacement amplification factor for the determination of the maximum real lateral displacement of structures  The main objective of this study is to evaluate the effects of the height of the structure and the length of the link beam on the seismic response modification factors including the behaviour factor and displacement amplification factor in eccentrically braced frames (EBFs). Since the seismic behavior of eccentrically braced frames depends on the link beam length, several eccentrically braced building structures with different lengths of the link beam and various numbers of stories including 2, 5, 10 and 15 story buildings were considered. The ASCE 7-10 code was used for seismic loading and the AISC 360-10 and AISC 341-10 codes were used for the design of structural models. The nonlinear static and nonlinear time history analyses were used to obtain the behaviour factor and displacement amplification factor. The values obtained for the behaviour factor and displacement amplification factor were compared with those prescribed by the ASCE 7-10 code. The seismic codes recommend constant values for the behaviour factor and displacement amplification factor for eccentrically braced systems. The results of this study show that the behaviour factor and displacement amplification factor for eccentrically braced structures significantly depend on the height of the structure and the length of the link beam. So, considering constant values for the behaviour factor and displacement amplification factor for eccentrically braced frames in seismic codes is not reasonable. Then, the effects of the height of the structure and the length of the link beam should be considered in these factors. Finally, with regard to the values obtained in this study, some equations were proposed to calculate the mentioned factors based on the length of the link beam and the fundamental period of structures. The results indicate that the behaviour factor and the displacement amplification factor decrease with the increase in the height of the structure and also by the increase in the length of the link beam.

In recent years, the construction of high-rise structures in the world has increased significantly. One of the important challenges of structural engineering is the use of different materials, including steel, to build structures based on these characteristics. Among the available steel sections, cross-shaped sections have been investigated and analyzed in this research due to biaxial symmetry and the possibility of proper connection. Due to the improvement in the process of implementation, design and construction of cross-bearing members inside the country, the present laboratory and numerical studies and investigations have been carried out with the aim of showing the behavior and characteristics of such columns. In order to investigate and draw better conclusions about the use of these types of sections, in this research, a parametric study was first attempted on the behavior of such columns. Then, how the dimensions and geometrical characteristics and estimation of the bearing capacity of the column have been analyzed. After that, the numerical model of the sample was created in Abaqus finite element software, and a laboratory sample was made. Finally, the obtained results were presented in the form of load-displacement curves and force-strain ratio. Therefore, the results of the research show that the installation of adhesive inks in these samples can improve the performance of these columns in terms of the load capacity, increase in stiffness, and the way of yielding and buckling in these members.

Buckling-Restrained Braces (BRBs) are one of the new seismic resistant systems. The cross-sectional area and length of the BRB brace is one of the most important characteristics of these braces that directly affects the cost of BRB frames. Since beams, columns, and connections are designed for the maximum forces developed in BRB, the decrease in cross-sectional area of the BRBs decreases the steel consumption in the whole structure.The main purpose of this study is to optimize the weight of the structure, BRBs weight while uniforming the drift profile by changing the cross-sectional area and the length of the BRBs using genetic algorithms and other multi-objective optimization algorithms. Optimization is based on the results of nonlinear time history analysis under seven earthquake records using OpenSEES software. For this purpose, the objective function and constraints were defined in the genetic algorithm NSGA_II, MOPSO, MOEA_D, PESA_II, SPEA_II, and the initial population produced was entered as the initial cross-sectional area and length of the braces in the OpenSEES software. The optimization results show that for all three objective functions, the optimization values with high percentages of structural performance were optimized in such a way that the weight of BRB can be decreased up to about 50%.