BULLETIN OF EARTHQUAKE SCIENCE AND ENGINEERING
Year:2020 | Volume:17 | Issue:2|Papers Count:6

Structural response modification factor refers to a coefficient that indicates the energy absorption capacity of the structure in the inelastic region. By using it, the shear force of the structural design is reduced by several times. In this research, the response modification factor of frames equipped with eccentric buckling restrained braces were investigated based on performance levels under the records far from the fault. In this research, three 3, 6, and 10-story frames with eccentric buckling restrained braces were designed using ETABS software and then SeismoStruct software was applied in order to perform nonlinear static pushover analysis and incremental dynamic analysis (IDA). In IDA analysis, the intensity parameter corresponding to (PGA) and the damage index corresponding to the maximum relative displacement of the floors (MIDR) were considered and the two performance levels of life safety (LS) and collapse prevention (CP) were investigated. The results presented that the collapse prevention performance limit was about 30 to 35 percent higher than response modification factor corresponding to the life safety performance limit, which means that the response modification factor of safety life limit can be increased by 30 percent in order to be used for structural design at the functional level of collapse prevention.

Dampers and isolators are energy dissipating devices to control and reduce the seismic responses of structures under strong earthquakes. One of the most widely used of these devices is a special type of damper called TMD mass dampers and LRB lead core separators. Tuned Mass dampers (TMD) and Lead Rubber Bearings (LRB) are of the most widely used types of these devices. The scientific study of the seismic characteristics and nonlinear behavior of structural systems equipped with this type of dampers and isolators can significantly help to understand the behavior of these systems against the lateral forces imposed by strong earthquakes. In this research, a 12-story concrete structure with mass damper with various mass fractions, without isolator and with lead core isolator is designed. Then, applying time history analysis in SAP2000 software in isolated states without damper and with damper with different mass fractions, the responses of the structures against the mentioned earthquakes are obtained. The results of the analysis presented that the presence of mass damper is very effective due to the increased displacement in the isolated level. This effect is increased by increasing the mass fraction of the damper. The seismic isolator, on the other hand, can reduce the relative displacements of stories by 20 to 30% on average. The mass damper controls the large displacements of the isolator. The results in the studied structure revealed that in order to utilize the maximum capabilities of the isolator, it should be combined with mass damper.

Basically, among the various stages that are considered to design a building for earthquake resistance, the decision stage regarding the building configuration is of fundamental importance. One of the important factors that has a significant impact on the seismic performance of the structure is its regularity or irregularity in plan and height. Most buildings with irregular configurations are more damaged by the earthquakes. Therefore, determination of the criteria for detecting various types of irregularities in structures is of particular importance. In this study, in order to investigate the error rate of seismic analysis methods (static equivalent, spectral and time history) in mid-rise structures with stiffness irregularity, four 5-story steel samples with 4 states of stiffness irregularity in height including dual moment frame and bracing system are modeled in SAP software. The results revealed that the equivalent static analysis method always provides conservative and somewhat uneconomical values compared to other methods, so it can be said that for mid-rise buildings, equivalent static analysis is reliable. However, time history analysis is better than spectral analysis for structural behavior during an earthquake.

In general, reinforced concrete elements tend to be very crisp, and this phenomenon is most evident when shaken. One of the advantageous of concrete enclosure is the increased concrete strength and concrete ductility which, if the concrete is properly enclosed and the seismic forces cause the concrete to be drilled, the enclosed concrete will resist well. The best way to do this is to use spiral stirrup. In this study, the effect of spiral with regard to the step variables of the spiral and the angle of the bolts on the shear and torsional capacity of rectangular reinforced concrete beams is investigated. ABAQUS finite element software was applied to simulate the samples. The results showed that the angle of cracks created in the specimens with closed stirrup was greater than that of the corresponding specimen (equal distance between the bends and the closed stirrup) with the spiral, and the use of stirrups both in the closed and in the bends situation, have increased the shear and torsional capacity of reinforced concrete beams. This research presented that reinforced concrete beams containing spiral stirrups had greater shear and torsional capacity than those of equipped with the closed stirrups. The use of closed stirrup and spiral stirrup with a step spacing of 80 mm have increased the shear capacity of reinforced concrete beams by 52% and 59%, respectively, which is a significant difference, and the shear and torsional capacity of reinforced concrete beams decrease with increasing step distance. By increasing the distance between the turns from 80 mm to 150 mm, the shear capacity of reinforced concrete beams is reduced by 30%, which is a significant amount. According to the results, the optimum spiral angle that best fits the rectangular reinforced concrete beams is 80 degrees. In this case, the angles of the diagonal cracks are lower than the other cases, and its shear and torsional capacities are significantly greater than the other specimens with equal steps and different angles.

Frictional pendulum systems are one of the most widely used isolation systems. Frictional isolators are of the common type of the conventional isolators. Among this group, the frictional pendulum isolator has an important situation. Due to the advent of these systems in recent years, no significant research has been carried out regarding the design and analysis of this type of isolator. Therefore, in this study, an attempt has been made to fully study the behavior of frictional pendulum seismic isolators by examining this separator with variable curvature under near-field earthquake. In order to evaluate the seismic behavior of frictional pendulum separators with variable curvature, firstly, different geometric functions were selected for the non-spherical slip surface and the relationships required for their dynamic analysis were obtained. Then, a numerical model was presented to analyze the time history of the structure isolated by these isolators and the behavior of frictional pendulum isolators with variable curvature was simulated using MATLAB software. The results of this simulation for the three isolators including fourth order function (O4), sixth order function (O6) and FPS indicated that the frictional pendulum isolators perform better than FPS isolators. When structures are exposed to earthquakes with low intensity level (far-filed), the most suitable function to improve the optimal performance of an isolated structure is fourth order function and when the structure is subjected to severe earthquakes (near-filed), the sixth order function will perform well.

Recorded earthquakes on near fields have special characteristics different from other usual records. Buildings placed near faults should be designed for the effects of such faults. Because of existing such faults in some cities in Iran such as Tehran and Tabriz, the effect of them should be considered in seismic codes in Iran. In this paper a comparison among various seismic design codes has been carried out almost this effect leads to increase the base shear using a separate coefficient that implies to design spectrum. This coefficient is a combination and a function of distance and period being larger in more near distance and longer periods. This effect has been considered in seismic design codes in some countries such as Taiwan, New Zealand and United States. In this paper a brief review of some seismic design codes and the new version of the Iranian seismic code of practice is presented.