نشریه سازه و فولاد
سال پانزدهم شماره 32 (تابستان 1400)

Provisions and recommendations of seismic codes are generally based on the determination of design earthquake and the effects of aftershocks is neglected. Moreover, in regions with high seismicity risk, aftershocks with different magnitude and occurrence time would take place following the main-shock. The main purpose of this paper is to evaluate seismic response parameters of a twenty-story bundled tube frame subjected to three-components near-field ground motions considering the effects of aftershocks. Spectral and physical characteristics of aftershocks encompass a wide range of magnitudes. Hence, the ground shaking modeling procedure is conducted via different ratios of aftershock peak ground acceleration divided by that of the proposed main-shock. The main characteristic of strong ground motion records used in this research is the existence of consecutive long duration, high amplitude velocity pulses. Occurrence of successive strong aftershocks leads to accumulation of permanent displacement as well as residual drift in the structures. In order to generate aftershocks, the randomized approach was applied in this study. The response parameters studied in the current paper include maximum story drift envelope, story residual drift, plastic hinge mechanisms and induced axial forces due to shear lag effect. The results obtained from analyses reveal that the amount of story drift and its variations, are directly proportional to the presence of rupture directivity effects in earthquake record as well as the ratio of aftershock peak ground acceleration to that of the main-shock, i.e. PGAas/PGAms. Moreover, residual drift under earthquake tremors containing successive aftershocks, either free field records (related to the actual dynamic behavior viewpoint) or scaled ones (related to the performance based design viewpoint) represents considerable variations.

As yet, various systems have been proposed to withstand lateral loads caused by wind and earthquakes, which can be called bending frames, types of braces, concrete shear walls, types of energy consuming systems. The most important parameters that are considered in choosing a system resistant to lateral loads are: hardness, strength, ductility. Steel shear walls, which are used in both smooth and reinforced forms, are a relatively new system for lateral wind resistance, and studies by researchers over the past four decades have shown that thin shear walls are a suitable and economical bracing system. Due to their geometric shape, corrugated sheets have a higher hardness and ductility than flat sheets. In this laboratory study, the stiffness, strength and shear base of a smooth and corrugated shear wall of vertical trapezoids with a wave angle of 45 ° under cyclic loading have been investigated.

During the recent two decades, many structures have become in need for seismic retrofit due to changes in either requirements of relevant seismic codes or the building usage. Viscous dampers are increasingly employed for seismically retrofitting various structures thanks to their easy installation and minimum intervention into the existing structure while significantly improving the seismic performance of the building. The main objective of this paper is to present a new design procedure for the evaluation of probabilistic seismicity of steel structures equipped with nonlinear viscous dampers. For this purpose, numerical investigation was conducted on, as benchmark structures, three steel-made moment-resistant frames with 3, 9, and 20 stories. Due to significant differences in their dynamic characteristics, these three structures represented low-rise, mid-rise, and high-rise buildings, respectively. Nonlinear time history analysis was performed using two near-field and two far-field accelerograms at various intensities (making up a total of 10 accelerograms). Moreover, nondimensional analysis was done to evaluate the uncertainties associated with velocity exponent of the nonlinear viscous dampers. Next, probabilistic estimation of damages occurred to the benchmark structures was performed and the effect of the uncertainty associated with the dimensionless power intensity of the damper on the seismic response of the structure. Investigating the results of the seismic analysis for viscous dampers at power intensities of 0.2, 0.3, and 0.4, it was found that although the seismic retrofit had significantly improved the seismic performance in all model structures but the proposed design procedure was more effective in the retrofitted high-rise structures at the power intensity of 0.4.

The building codes and seismic design specifications restricted using high strength steel in lateral load resisting systems because of its less ductility. However, it is possible to use lower grade steel in design of lateral load resisting systems, including X-Braces. In addition, the seismic design specifications generally do not consider significant difference in seismic design parameters of steel with different grades. In this research, effects of using steel with different grades in seismic behavior of X-Bracing system studied using nonlinear dynamic analyses. The frames with 4, 8 and 12 stories in high seismic zone designed with conventional design methods and regarding requirements of building codes. Two different steel grades considered in design of diagonals in X-Braced frames.According to nonlinear static and dynamic analyses on different frames, it is found that the seismic performance of frames designed with lower grade steel is higher under near and far field earthquakes. In addition, the analysis of both type of frames under near-field ground motions showed more sever responses and bigger displacements, especially in progressive-direction faults.

The bracing elements experience both tensile and compressive forces during an earthquake, however buckling is critical in bracing members. Buckling-Restrained brace (BRB) - in which the buckling is prohibited - have long been used in developed countries due to their high ductility and similar behavior in tension and compression. One of the improvements on this system is the use of BRB with reduced length rather than the whole brace as in conventional BRBs. The most unique advantage of this innovation is the independent selection of two parameters of stiffness and strength. Other advantages of reducing the length of buckling-restrained are reduced construction cost, flexibility in installation position, and increasing stiffness after yielding.In this study the proposed reduced length BRB system is presented. Afterwards the advantages of reducing the length of buckling-restrained brace are introduced. The basics of seismic design of reduced length BRB frames are expressed. The comparative results of seismic design and time history analysis for BRB and reduced length Buckling-Restrained brace (RLBRB) are discusses. The results of the nonlinear analysis show better seismic performance of RLBRB in comparison to conventional BRB.

In this study, a novel hybrid self-centering system is introduced. This system consists of self-centering systems and Pall friction dampers. The design methodology of hybrid self-centering systems is developed and some examples of the designed hybrid self-centering are presented. Hybrid self-centering systems not only have the capability of dissipating energy but can also remove or arrange residual drifts of structures. In fact, hybrid self-centering systems can maintain design target residual drifts (zero or any) during lateral loading similar to the pure self-centering system and have the benefits of high capacity energy dissipation of Pall friction dampers. The hybrid self-centering system is developed for seismic resilience in terms of economic benefits, ready-to-use after the earthquake event, and practical fabrication. To evaluate the proposed design method, different low- to high-rise (3-, 6-, 9- and 12-story) buildings are selected from the literature, redesigned based on the proposed method, and then subjected to cyclic loading. Results are presented in terms of the cyclic response, residual drift and energy dissipation capacity of structures. Results indicate that the proposed design equations precisely meet the target structural performance (target residual drift) of the hybrid self-centering system and this system is obviously a superior system compared to the pure self-centering or Pall system.