جستجوی مقالات مرتبط با کلیدواژه "shape memory alloys" در نشریات گروه "عمران"

Regarding the high cost of retrofitting and replacing the isolation system after an earthquake, re-centering seismic isolation systems have become one of the most popular research fields in the past decades. A new generation of seismic isolation is based on equipment with improved performance in terms of damping and re-centering capability. Since lead-rubber base isolation (LRB) is one of the most common seismic isolation systems, this paper focused on suggesting a re-centering LRB system with high damping capacity. The LRB isolation is based on an experimental study under a constant vertical load equal to 650 kN. The proposed combined system of LRB and friction yields a high damping capacity. A spherical shell is placed on the base isolation system, and the friction reaction emanates from this shell's sliding on the provided foundation. The primary purpose of this spherical shell is to increase the damping capacity of the isolation system. An adjustable friction reaction is reachable depending on the connection details of the spherical steel shell with the system's upper steel cap plate. The higher stiffness of this connection is provided greater friction force and, as a result, higher damping capacity. In short, this connection was performed as adjustable friction and can increase energy dissipation and maximum shear force, respectively, in the range of 12% to 80% and 6% to 230%. Increasing re-centering capacity is another goal of this study. It is of great interest to implement shape memory alloy (SMA) in the form of wires because of their re-centering capabilities. The axisymmetric design is implemented to reduce this hybrid system's vulnerability to earthquakes of arbitrary direction. The combined action of wires and the spherical steel shell provides the re-centering capacity of the system.The final system, which is a combination of LRB, friction mechanism, and vertical load transfer through SMA wires (MDLRB-SMA), exhibits enhanced properties such as reduced residual deformation, controllable shear reaction at each stage of the deformation, and increased damping capacity. The energy dissipation capacity of the MDLRB-SMA is shown to be increased by 50%, with a decreased residual deformation of up to 45% compared to the LRB. A set of parametric studies are also performed to investigate the influence of frequency, displacement amplitude, loading rate, the wires' configuration and properties, and the aspect ratio of the system's performance.

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.

Seismic loading in seismic prone countries is very important and the lack of enough attention can make a irreparable damages to structures and non-structural elements. Dissipating earthquake energy just by using elastic capacity of structure, will increase the dimension and weight of structural elements like column, beam, walls and the cost of building will increases. Incoming seismic energy should be dissipated in plastic process and in this process, structure must remain stable. Shearwall is using widely because of its suitable behavior against seismic loading and good ability of energy dissipation. Sometimes it is inevitable to avoid openings in shear walls due to architectural considerations. Providing enough ductility in these shear walls is a difficult job because of stress concentration around of voids. The other problem of using shear walls is plastic deformations that make the structures useless. One way of improving ductility and self-centering ability of shear walls is using smart materials. Shape memory alloys are one of the newest smart materials that have two important behaviors, called shape memory effect and superelasticity. Removing the residual deflection after unloading of elements made by shape memory alloys by heating called shape memory effect. Superelasticity in SMAs is returning the elements to their initial shape after unloading by them. Good corrosion resistance, good fatigue behavior and weldability are the other positive behaviors of shape memory alloys. In this paper the improvement of the shear walls ductility and self-centering ability with using shape memory alloys in superelastic phase is investigated. Shape memory alloys can withstand up to 7 percent of strain without any residual deformation. In this paper shear walls modeled by shear-flexure interaction multi-vertical-line-element-model (SFI-MVLEM). This model was implemented in the Open System for Earthquake Engineering software (OpenSees). Considering interaction between shear and flexural response in shear walls has made this element superior. Superelastic reinforcement bars were embedded in plastic hinge of boundary elements, coupled beam and walls web separately. Shear wall modeled by using SMA in boundary element, coupled beam and walls web called SMAB, SMAC and SMAW respectively. To examine the effects of these alloys on energy dissipation capacity and self-centering ability of the shear walls, structure were evaluated in cyclic analysis. Place of using SMAs on shear wall in very important and can influence widely on shear wall so most optimized place of using SMAs in shear wall should recognized. Two case of optimization considered in this analyze. In first one, best place is a place that using SMAs on it causes minimum energy dissipation reduction and maximum residual displacement removing. In second one, best place is the place that causes maximum residual displacement removing with minimum usage of SMAs. In order to find the influence of SMAs on ductility of shear wall, pushover analyse was used. Using SMAs in boundary elements of shear walls made maximum increasing in ductility of shear wall but the best place for using SMAs for optimization of usage of SMAs is walls web. Based on the results, with using shape memory alloys, ductility of shear walls was increased and its residual deformation and energy dissipation capacity was decreased. The best place of using SMAs in shear wall is coupled beam for optimization of energy dissipation and residual displacement and for optimization SMA usage is walls web.

Re-centering is an exclusive characteristic of superelastic Shape Memory Alloys (SMAs) which can be used in manufacturing and retrofitting of reinforced concrete elements. Reinforced concrete beams retrofitted with SMA bars have more ductility and higher energy dissipation compared to conventional RC beams. Furthermore, these beams experience less damage in consecutive loading-unloading cycles. The current research aims to investigate the behavior of reinforced concrete beams retrofitted with SMA bars using Near-Surface Mounted (NSM) flexural retrofitting method. Eleven RC beam specimens with the cross section of 200*150 mm and length of 1150 mm were cast. Three of the specimens had no external strengthening, four of them were retrofitted with SMA bars and other four beams were retrofitted with GFRP reinforcements. The specimens were subjected to three-point bending test under either monastic or loading-unloading. Different parameters including load-carrying capacity, energy dissipation, deformation recovery and reduction capability of crack width were investigated. The results showed that RC beams retrofitted with SMA bars had more mid-span deflection and higher energy dissipation compared to other specimens under monotonic loading. Moreover, under loading-unloading, RC beams retrofitted with SMA bars method experienced less damage.

Shape Memory Alloys (SMA) are a kind of smart materials that their unique mechanical and thermal performances such as the ability to return to its original shape (Shape Memory Effect) and the ability to recover large strain (Super Elasticity), caused their wide application in various industries such as civil engineering. Due to the superplastic behavior of theses alloys, undergoing cycles of loading and unloading, results very little residual strain, even after exceeding the yield strain. This property causes recover forces in the structure so that they can close the cracks in the tensile zone of RC members and therefore, it is significant for structural members located in earthquake zones. In this paper the F.E modeling of reinforced concrete (RC) beams utilizing of shape memory alloys is produced and results of different loads response of modeling is verified with the available experimental results. For this purpose, firstly the finite element model of three RC beams by adding Nitinol wires in the tensile zone was made by ANSYS software. Then the beams were subjected to cyclic loading and their hysteresis curves were compared with the similar available experimental beams. The numerical results revealed, utilization of Nitinol wires in tensile zone of the beams resulted, increase in both loading capacity and ductility and decrease in residual displacement.

Shape Memory Alloys (SMA) are smart and novel materials that exhibit variable stiffness and strength associated with their different polycrystalline phases. The Shape Memory Effect (SME) and Superelastic Effect (SE) are two distinct properties that make SMA a smart material. Shape memory effect (free recovery effect) means that a large (pseudo-) plastic deformation can be reversed by heating. If the going back is prevented by, e.g., concrete, a stress in the SMA results (constrained recovery effect) [1]. A Superelastic SMA can restore its initial shape spontaneously even from its inelastic range upon unloading.
The specific objective of this study is to investigate the effect of ordinary and pretension SMAs with memory effect behavior in concrete shear walls separately. For this purpose, we analyzed concrete shear walls with different percent of SMAs and steels under monotonic loading in ABAQUS Finite element. Two different concrete shear walls, one reinforced with ordinary SMA together with steel rebars and the other with pretension SMA with steel rebars, have been analyzed. The seismic behavior of the two concrete structure models has been compared in terms of their load against displacement.

According to current seismic design codes, the serviceability of the building is not necessary after the sever earthquakes but the life safety is important. It is caused to reduce the cost of construction. In recent years, the researchers have proposed the new methods and materials to improve the purposes of seismic design and to reduce the cost of construction. For example, the use of materials such as shape memory alloys (SMA) with high elastic property and low residual strain was suggested. In this study the use of these alloys is evaluated for steel knee bracing. The result showed that the structures with SMA remain survival during the sever earthquakes with low retrofitting.

Most energy dissipation system have some limitations and drawbacks such as difficulties related to ageing, durability, complexity of the installation, maintenance and placing permanent displacements after strong earthquakes. Characteristics such as large strain range without any residual deformation, high damping capacity, excellent re-centering, high resistance to fatigue and corrosion and durability have made shape memory alloy an effective damping device or part of base isolators. This paper investigates the seismic performance of an elastomeric bearing type base isolation system utilizing the shape memory alloys. To provide this investigation, a nonlinear structural model has been developed. The results of dynamic time history analyses reveal the significant characteristics of the proposed structural isolation system. This smart base isolation utilizes the different responses of shape memory alloys at several levels of strain to control the displacements of the rubber bearing and base shear at excitation level. Furthermore the proposed based isolation systems has enhanced performance in terms of response reduction and re-centering capacity.

1. Shape memory alloys (SMA) have found various applications in structural engineering such as active, semi active and passive control due to its characteristics such as high damping capacity, durability, resistance to fatigue and corrosion and its unique characteristics such as shape memory and superelasticity [1]. One of the most important and effective applications of SMA in civil engineering is using these materials as braces, because of the superelastic and shape memory properties of SMA. They have the ability to re-center the original state and to provide high energy dissipation. However, most of the researches about the use of shape memory alloys in structural engineering are in theory level and few of them are experimental.2. For the comparison of models, a three-story structure has been used which was proposed by Sabelli. Assuming symmetry in the plan, only a two-dimensional frame of the structure has been analyzed. The height of each floor is 3.96 m and the construction plan is 9.14 in 9.14 m. Ceilings of the floors are composite that the height of the steel part is 76 mm and the height of concrete cover is 50 mm. It is noteworthy that the axial deformation of the beam is ignored [2].The models used for this study are in such a way that bracings, a combination of steel and SMA, have the consumption of 0, 20, 40, 60, 80 and 100 percent respectively in which the first model is the model of steel brace and the last one is the model of SMA brace. A schematic of the bracing is shown in Fig. 1. 3. There are two major differences between the behaviors of steel and SMA both of which are introduced as the special properties of shape memory alloys. One of the properties is that this alloy is able to return to its primary state after bearing a large strain; in other words, to minimize the horizontal displacement of the structure; and the other one is that it increases the strain energy in the structure. This section has attempted to provide the most optimum brace through comparing the graphs of the history of horizontal displacement and diagram of structural strain energy in all models, both in terms of SMA consumption and seismic behavior.Table 1. The comparison of the residual displacement of structure in the used bracing systems under the maximum acceleration of 0.6gResidual displacement of structure (mm) Considered model5.31 Bracing system SMA 0% and steel 100%1.46 Bracing system SMA 20% and steel 80%1.10 Bracing system SMA 40% and steel 60%0.93 Bracing system SMA 60% and steel 40%0.85 Bracing system SMA 80% and steel 20%0.84 Bracing system SMA 100% and steel 0%The use of SMA, resistant to earthquakes which has a maximum acceleration of 0.6g, has not a significant difference with steel and utilizing them is not economically justifiable since the entire capacity of the SMA is not properly used. To have a better understanding, hysteresis graph of steel and SMA bracings under maximum acceleration of 0.6g is shown in figs. 3 and 4, respectively. According to fig. (5), it is observable that in the presented models, an increase in SMA consumption increases the value of energy absorption and an increase in the intensity of earthquakes (from 0.6g to 0.9g) indicates this result more clearly. The main reason is due to the special properties of SMA. It means that the return of the structure to its primary state after unloading and resistance against fatigue both cause the hysteresis loops of SMA to be bigger than that of steel and it shows the higher ability of SMA in structure controlling and better energy dampering.To have a better comparison, figs. 6 and 7 show hysteresis graphs of SMA braces and steel braces under maximum acceleration of 0.9g. 4. The performances of bracing containing 20, 40, 60, 80 and 100% of shape memory alloy and steel bracing have been studied. The seismic response of the braced frames are determined and compared to the braced frame with steel brace. The results indicate that the most suitable location for putting shape memory alloy is the ends of bracing. Also, the secondary moment which is produced in bracing at the connection with gusset plate has transmitted better by shape memory alloy than steel. The total capacity of these alloys is not used in records with maximum acceleration of 0.6g and there is no difference between the behavior of steel brace and alloyed brace. Therefore, it is not necessary to use alloyed brace in low-intensity of earthquake due to economic considerations. It is observed that using 20% SMA in the alloyed brace improves 54% in seismic performance by 54% while by using 100% SMA, the amount of improvement is 94% It is concluded that the application of bracing which contains 20% SMA, leads to the improvement of seismic performance without considerable increase in constructional cost.

Shape memory alloys (SMAs) are novel materials that found more applications in many fields of science and engineering. Recently, SMAs have been used in structural engineering due to their re-centering, high damping capacity, durability and fatigue resistance. Aforementioned characteristics of SMAs rely on two unique properties: Shape Memory Effect (SME) and super elasticity. One of the applications of this material in structural engineering is to use SMAs as bracing elements. The results of investigations that carried out on SMAbraced frames represent the functionality and viable performance of this bracing system. In this paper, SMA wire braces were installed diagonally in the frame structures as seismic resisting members. Several SMA-braced frame models were designed to investigate the behavior factor (R) of these types of frames. The behavior factor and its components including ductility reduction factor and over strength factor were evaluated by performing pushover analysis of the models.