جستجوی مقالات مرتبط با کلیدواژه « seismic retrofit » در نشریات گروه « عمران »

The present article describes experimental procedures to investigate the seismic retrofitting techniques used for damaged unreinforced masonry (URM) walls. In total, three tests were performed on a full-scale wall under constant vertical load, along with lateral loading according to the loading protocol. The wall was built similarly to the walls of typical masonry buildings in Iran. The unreinforced wall was tested as a reference sample. Then, the damaged sample was retrofitted with two vertical concrete ties and bed rebar on one side and retested. Finally, the damaged sample was repaired with shotcrete on two sides and tested. The experimental results showed that strengthening the damaged unreinforced wall with the vertical concrete ties and bed rebar not only restored most of the original seismic characteristics of the wall but also significantly improved the seismic behavior of the wall. Moreover, improving seismic characteristics, the retrofitting by vertical concrete ties, bed rebar, and two-side shotcrete increased wall energy absorption capacity. The experimental results showed that by repairing the damaged wall with vertical concrete ties and bed rebar, the lateral resistance increased by 219%. and by repairing the damaged wall with vertical concrete ties, bed rebar, and both side shotcrete, the lateral resistance increased by 464%.

Unreinforced masonry buildings are quite popular in many countries such as Iran, even though they are prone to significant damage against even moderate seismic excitations. In 2003, 26000 people lost their lives and 30000 were injured in southeastern Iran during Bam earthquak. The regional investigations showed an almost total overall collapse of all adobe and masonry buildings. Even to this day, notable number of buildings in Iran is masonry buildings due to low costs, especially in rural areas. Hence, the popularity despite the poor performance has sought researchers to study efficient ways to improve the URM buildings. Generally speaking, masonry buildings are classifed into unreinforced and reinforced; the latter usually beneft from horizontal and vertical steel bars which signifcantly improve strength, ductility and energy dissipation capacity of such walls. Masonry buildings can also be categorized as confned and unconfned. Using horizontal and/or vertical ties usually in the form of lightly reinforced concrete members at the perimeter of walls, their intersection with perpendicular walls and if necessary, around large openings can considerably enhance their ductility and in some cases their strength. Unlike other masonry construction types, the behavior of Confned unreinforced Masonry (referred to CM hereafter) buildings has not yet been fully formulated. This is mainly because of more sophisticated behavioral characteristics of CM walls compared to unconfned Unreinforced Masonry (referred to URM hereafter) buildings. CM is the only masonry system that has been allowed practicing by the Iranian Code of Practice for Seismic Resistant Design of Buildings (Standard 2800) in seismic-prone areas. When it comes to seismic retrofitting of masonry buildings, there are a rather wide variety of available options, e.g. shotcrete, adding steel or FRP sheets on the wall, adding Reinforced Concrete (RC) ties, changing the arrangement and size of the openings, and using Near Surface Mounted (NSM) rods on the walls. Considering the significant number of two-story URM school ‎‎buildings in Iran, there is a need to provide instructions for ‎the ‎type of improvement plan of these buildings. In response ‎to this ‎need, the Organization of Renovation, Development ‎and ‎Equipping of Schools of Iran developed a guideline for ‎the ‎purpose of typifying the improvement plan of these ‎buildings ‎with the shotcrete method in 2021.‎ In this research, based on the results obtained from five two-‎‎story URM schools, the accuracy of this guideline is ‎evaluated. ‎For this purpose, the investigated buildings in the ‎two cases of ‎flexible roof without ties and rigid roof with ties ‎in ETABS ‎software using shell elements and spectral dynamic ‎analysis ‎method, once based on the requirements of the ‎guideline and ‎once based on the Code 360 was evaluated and ‎retrofitted using ‎Shotcrete.‎ The results of this research show that the guideline ‎in the case ‎‎of flexible roofs leads to ‎minimum vulnerability of the walls of ‎‎the studied buildings. In the case of a rigid roof, the ‎results ‎‎show that according to this guideline, some of improved and ‎‎unimproved walls are ‎vulnerable in both direction; In such a ‎‎way that the ‎average percentage of vulnerable walls in terms ‎of ‎‎relative length for the first floor is 63% in the ‎longitudinal ‎‎direction and 38% in the transverse ‎direction, and for the ‎‎ground floor in the ‎longitudinal direction is 56% and in the ‎‎transverse ‎direction is 17%.‎

Retrofit actions are amongst the most commonly used measures for reducing the seismic vulnerability of buildings. For any given building, multiple seismic retrofit options are often available. Each option has specific requirements, cost, and performance. Estimating the cost of each candidate action is essential to the selection, planning, and implementation of seismic retrofit initiatives. Primary cost estimation plays a vital role in allocating budget for retrofit projects. Past studies used a variety of methods to develop cost estimation models. This research harnesses the capabilities of various regression models via modern machine learning methods for cost estimation. A dataset from 167 retrofit projects for masonry school buildings in Iran was used to develop models. Three main retrofit actions were implemented in the projects, namely Shotcrete, Steel belt, and Fiber reinforced polymer. Several regression methods including multiple linear regression, ridge regression, lasso regression, and also elastic net regression were applied to the dataset. The proposed framework comprised 12 models, which were attained by four regression methods on three retrofit actions. The cross-validation method was used for model evaluation in order to use all available data for training and testing. The model at the beginning of the development process contained all the probable effective parameters. Next, to increase the simplicity and accuracy of the models, a simple model reduction method was implemented. This model reduction method eliminated almost two-thirds of the parameters in the majority of basic models. Then, the candidate models were evaluated in terms of quantity and quality of prediction, heteroscedasticity, autocorrelation of residuals, and non-normality. This paper identifies the height of the building as the most influential parameter governing retrofit cost. Furthermore, lateral area of walls, footprint area, and added lateral strength are influential in the mentioned retrofit actions. This research contributes to enhancing the understanding of the factors, the effects, and the costs of the retrofit actions.

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.

Today, there is an increasing interest in use of seismic isolation to protect the structures against earthquakes. The most common type of seismic isolation is called base isolation, in which the isolation layer is installed under foundations to separate the structures from the ground and reduce the earthquake forces. However, the use of this type of seismic isolation faces some difficulties such as construction in congested urban areas or construction of near sea structures. Furthermore, for seismic retrofitting of existing buildings, the installation of the isolation under foundations is difficult or even impossible; so, it needs to be located on the middle floors of the buildings. The method of isolation design is called floor or middle story isolation. Despite the advantages of seismic isolations, they have some limitations such as instability in large deformations, residual displacement, and the need for replacement after severe earthquakes. The use of Shape Memory Alloys (SMA) regarding their unique properties is considered as an appropriate solution to overcome the above problems. These smart materials show high strength and strain capacity, high recentering ability, and high resistance to corrosion and to fatigue. The purpose of this study is to investigate the effect of combination of middle story isolation utilized by Natural Rubber Bearing (NRB) and iron-based shape memory alloys in steel structures and to compare the performance of such structures with and without the presence of the shape memory alloy in the middle-story isolation system. Then, a three-story steel structure has been modeled and evaluated. For this purpose, a structure with floor isolation and iron-based shape memory alloy was modeled in OpenSees computer program. The structures were then subjected to seismic loading. The results were presented in the form of story drift, floor acceleration, floor shear forces, and base shear. The outcome of this research showed that the use of these alloys in the middle-story isolation reduced the overall base shear and floor shear forces. The overall story drift, floor acceleration and displacement are reduced; with the exception at the isolation level. Thus, utilizing the natural rubber bearing isolator along with the iron-based shape memory alloy can be considered as a desirable system for the seismic protective design of buildings.

In recent earthquakes, masonry structures have not been resistant to earthquake due to high weight and low ductility and low shear resistance, and they have experienced a lot of damage. The improvement and strengthening of masonry structures is a serious discussion in urban construction. The walls of masonry are not resistant to the earthquake and their basic weakness is in ductility; its ductility is reduced when the length ratio increases to the wall height, in addition to the wall resistance. The purpose of this study is to investigate the seismic performance of masonry structures with two aspect ratio of 0.5 and 0.7 with scale 1: 2 and strengthening of masonry walls using different layouts of steel tie. The analytical evaluation of seismic behavior of reinforced masonry structures has been done using ABAQUS finite element software. In the FEMA356 method, the displacement curve of all samples is bilinear. Analytical results were compared in dominant resistance, effective stiffness and ductility. The ultimate strength, effective stiffness and ductility of strengthened specimens using steel tie increased in comparison to the control sample.

History of the earthquakes occurred in Iran indicates that almost every 10 years, a destructive earthquake inflicts damages to the buildings leading to broad life and financial losses. Indeed, seismic retrofit is the process of retrospectively adding elements into the existing buildings in seismically active areas in order to enhance their resistance to the shakings during earthquake events. In doing so, one of the efficient methods is to utilize steel bracing system. To better understand the seismic performance of such systems, in this paper, RC frames retrofitted by CBF and EBF systems were subjected to 7 near-field earthquakes with varying intensities. To this end, two 10-story frames were first designed in compliance with code-based provisions and then, an IDA was conducted on them. The braces are placed in the first and fifth bays of the frame. Based on the results, retrofit of the RC frames by means of concentric and eccentric braces manages to enhance the yield capacity of the frame by 2.3 and 2times, respectively. Moreover, it was found that application of eccentric braces in RC frames, is capable of reducing the base shear by 7times, compared to the frame benefitting from concentric braces. Concerning the impact of this retrofit system on displacement demands, it was observed that when eccentric braces are employed, the roof drifts are less than those of the CBF. Lastly, it was concluded that more flexible behavior of EBF against earthquake records, increases safety and performance level of the structure in Life Safety performance level.

Reinforcement of weak concrete structures against lateral loads is essential. Rehabilitation of existing structures is one of the important issues of the day in the construction industry in Iran and even in other countries. Due to the fact that many structures are almost impossible to use due to reasons, type of use, the presence of important equipment, the importance of building and many other things, the need for retrofitting is felt more and more. Today, there are many varied methods in rehabilitating structures around the world, which is used in the field of knowledge and technology and the needs of the region. In our country, some methods are used, such as the use of concrete armored and steel armored vehicles, the use of polymer fibers, seismic separators, the addition of concrete and steel shear walls and the addition of braces and so on. Each of the above methods may be suggested for reasons such as structural weakness, time and economic efficiency, user continuity during retrofitting, maintaining architecture, and most importantly, answering technical and computational design. Each of the above methods may be suggested for reasons such as structural weakness, time and economic efficiency, user continuity during retrofitting, maintaining architecture, and most importantly, answering technical and computational design. Among the existing methods, the use of unbreakable steel bracelets is one of the main recommendations. In this study, 6 samples of braided curtain braced frame were made by a bulking resistant brace and analyzed using ABAQUS software.

The presence of concrete slab in steel framing frame structures always makes retrofitting problems difficult. On the other hand, most of damages reported from Northridge earthquake (1994) has focused on the bottom flange of the beam. Therefore, in order to reduce the cost of retrofit, this operation can be done only at the bottom flange of the beam. In other words, by defining and applying a ductile fuse in the beam, by weakening the bottom flange of the bottom, it is possible to reduce the concentration of tension in the near-weld regions in connection and fracture failure in these areas. In this paper, a steel frame connection was analyzed numerically and after ensuring the modeling accuracy, it was under rehabilitation. Retrofit techniques have been made on the bottom flange of the beam. Two general "cut-off" and "heat" methods were considered to weaken the beam. In this research, incomplete penetration of the weld was placed in the bottom flange connection to the column as a deficiency in the joint. The modeling results showed that the connection with incomplete weld, underwent 4/4%, suffering a deflection in the weld area in the flange. In the specimens which were retrofitted asymmetrically, the possibility of brittle fracture in the weld region was decreased due to relocation of the plastic hinge in the weakened area of the beam. In the method of reducing the bottom flange, lateral buckling led to sudden decrease in flexural strength at 2.5 % inter story drift. In the heat-induction method accompanied with steel annealing, the out of buckling occurred at 5% drift. The retrofitted connection with this method showed a post elastic displacements of 3% and could be expected to be suitable for special moment frames.

In recent decades, composite material has been used in order to increase the effectiveness of reinforced concrete structures. The difficulty of numerical modeling of beams in finite element method is a problem encountered during such a study. Destruction of structures such as buildings and bridges has been reported due to earthquake and time-dependent erosion. Highlighting the need for reinforcing the structures, many researchers have tried to estimate the efficiency limit and strength of reinforced concrete structures.In order to retrofit concrete structures polymer coating was first developed in Europe and Japan in 1980. In Europe, FRP systems have been used to replace steel sheets, that as a common resisting method, their connection to the tensile concrete parts of the members with different adhesives was popular to increase the flexural strength of the members. Since steel sheets deform under operation, they would destroy the connection of sheets and concrete. On the other hand, their installation is generally difficult and needs heavy machinery, so researchers sought a replacement of steel coatings with FRP. FRP with 20% weight of steel jackets is approximately 8 to 10 times stronger than steel. One of the problems of numerical studies in these systems is their computer modeling in the form of finite element analysis (FEM). On the other hand due to a variety of effective parameters on their behavior, applying lab methods causes problems as they are time-consuming and expensive.In this research, retrofitting reinforced concrete beams using FRP is studied. At first in order to validate the accuracy of modeling, the analysis results obtained by ABAQUS software are compared with those of experimental studies. Then, beams that need strengthening are retrofitted and the effect of FRP on the curvature and rotational capacity of these beams is investigated. Finally, ductility proportion to the performance levels and the acceptance criteria for retrofitted beams regarding force or displacement control scenarios are obtained. Based on the results, the behavior of beams changes after retrofit, and since the displacement demand to capacity ratio (DCR) is less than 2, force control scenario must be considered regarding their acceptance criteria.

In this paper، the structural performance of steel rigid frame with pall friction damper is compared with a braced moment resisting frame as existing structure. The existing structure is a 5-story steel frame which is designed based on the 2nd edition of the No. 2800 standard and the performance evaluation is based on seismic retrofit guideline. It is observed that when the slip force reaches to 20 tons، about 100 percent of the total dissipated energy in the structure is dissipated by damper devices، which makes 20 tons be the optimum slip force. Also، under the design basic earthquake، the acceptance criteria for immediate occupancy performance level are satisfied in both frames. Investigating the demand to capacity ratio، DCR، in deformation - controlled actions reveals that in the existing frame about 20 percent of the elements reached nonlinear region and the acceptance criteria for force – controlled actions are hardly satisfied، while all members of the frame with damper remained in elastic behavior and the acceptance criteria for these actions are satisfied with 16 percent safety margin. Similar desirable results are obtained under an earthquake stronger than the basic design earthquake.

In this paper, non-linear behaviors of structures with special moment resistant frame were studied and possibility of using knee bracing for retrofitting of those structures was examined. For this aim, the structures that have been designed according to the second edition of Iranian seismic code (2800), were selected. Those models have 3, 5 and 7 stories. To evaluating and retrofitting of models, non-linear static analysis and capacity spectra (ATC-40) method were applied and providing of the expected performance levels in FEMA356 was studied. Results showed that the response of structures that designed according to the previous edition of code (second edition) is not compatible with the criteria of third edition. The knee bracing system was used to seismic retrofit of those structures. It would be concluded that the using of knee bracing increases lateral stiffness and improves their energy dissipating capacity. Furthermore, the results showed that the shorter knees have the highest stiffness and ductility in the structures.

Shear Panel system is one of the most useful passive control methods, in which shear panel devices are installed between node of chevron braces and flange of the floor beam. In this paper, the effect of using shear panels or vertical link beams on seismic energy dissipation is numerically investigated. Using shear panel system, the seismic behavior of 2D, 5 and 8-story frames was improved. Analyses were conducted by Seismo-Struct software. In this way, base shear, top story displacement and maximum acceleration of the floors in 5-story frames were reduced respectively 29, 70 and 62%, while in 8-story frames these parameters were reduced respectively 43, 27 and 40%. Story drifts were mostly reduced and became uniform. In all parameters, effectiveness of using SPS, Shear Panel System, in 5-story frame was more than that in 8-story frame. In fact by using SPS, in addition to increasing the ductility of the structures with decreasing seismic demand, seismic behavior of structures was improved. Finally, using shear panel is highly recommended as an easy to use and efficient way consistent with construction condition in the country for seismic design of new steel building structures and also for seismic retrofit of existing steel buildings.