جستجوی مقالات مرتبط با کلیدواژه "پایش سلامت" در نشریات گروه "عمران"

Introduction     Considering the importance and high cost of construction and maintenance of offshore platforms for the purpose of oil and gas extraction, and considering the fact that in case of failure, they can cause many environmental disasters, technical inspection of their structural condition is of serious importance. In addition to the high cost, this does not cover all aspects and it is very difficult to detect the failure in this case. Due to the repetitive nature of most of the environmental loads in the seas, these structures are constantly exposed to multiple and repetitive loadings. The phenomenon of fatigue is one of the effective factors in the health of these types of structures, so that during the past decades, the offshore industry has witnessed unfortunate events that often occurred due to the phenomenon of fatigue. One of the unpleasant cases can be mentioned the disaster of the Norwegian semi-submerged oil platform in the North Sea, named as the Kyland Alexandria, in which 123 people of the platform's crew lost their lives. One of the main braces connected to the base of the pontoon was completely broken and separated from the platform, causing the platform to completely overturn. The semi-submersible rig Sedo 135, which began operating in the Gulf of Mexico in 1965, suffered a fatigue failure in one of its rigs in 1967 after two years. One of the most widely used platforms in the Persian Gulf is the fixed platform of the stencil or jacket type, which is a steel structure that has braces and a deck, and foundations that are fixed on the sea floor by numerous piles. Fatigue cracks are the main failure factor in fixed jacket platforms (Ibrion et al., 2020).

Due to the increase in the age of bridges, the explanation and use of effective methods for detecting partial damage with the help of health monitoring systems are crucial since they provide the conditions for repairing the damage before the damage increases. One of the primary goals of this article is to provide an effective and efficient method for health monitoring and detect partial damage in the pier of a straight bridge using fragility curves of harmonic forces, and provide a criterion to show the correlation of fragility curves in healthy and damaged states. To achieve these goals, a complete model of a straight bridge that includes 8 piers, each 10 meters high and its deck includes 5 openings, the length of each is 15 meters, and the total length of the deck of 75 meters was modeled in the CSI Bridge software, and then, was validated. To detect the most vulnerable column for damage modeling for health monitoring operations, three earthquake records corresponding to the soil of the construction site were successively applied to the model. By examining the decrease in the slope of the moment-rotation graphs, the level of damage that occurred at the bottom of the column was detected. Finally, it was found that the middle piers were more prone to seismic damage and health monitoring operations were performed on one of the middle columns. To draw the fragility curves of harmonic forces, harmonic forces were first applied to the top of the column, and then, the rotations created at the bottom of the column were measured. Based on the obtained data and using the relevant relations, the fragility curves were drawn. The index of the maximum intensity of force, the index of rotation damage that occurred at the bottom of the column, and the defined performance level of exceedance from the linear state are the characteristics of these fragility curves. To draw the fragility curve in the damaged state, the desired earthquake (seismic) force was first applied to the bridge, and then, harmonic forces were applied to the top of the desired column. The results of this study revealed that the fragility curves of harmonic forces have been drawn well in healthy and partially damaged states according to the defined characteristics, and partial damages with different intensities have been correctly detected. To examine the correlation of the fragility curves of harmonic forces in both damaged and healthy states, two new criteria PCOMAC and PDI were introduced. The results of this article indicate that the PCOMAC criterion is very suitable for examining the correlation of fragility curves of harmonic forces.

In recent years, vibration-based damage detection has been used to assess damage to structures. In this case, damage detection is based on changing the dynamic response before and after damage. In this research, the flexibity matrix method has been used to identify damage in segmental bridge (Imam Khomeini intersection bridge). The advantage of this method is its accuracy and sensitivity to a small number of modes for large structures. The bridge was simulated in Abacus software and after applying weight and prestressing force to the cables, the frequency and modal shapes were extracted for healthy and damaged conditions. Damaged conditions included three scenarios, single (in one place) and multiple (in several places and simultaneously) on the bridge deck as well as damage to the column. Each of the above damages involved a 30% reduction in stiffness. The results showed that this method was able to detect damages in the deck. But more investigation was needed to monitor the health of the column. The accuracy of health monitoring was also assessed using two modes, which had acceptable accuracy compared to the use of 6 modes.

The problem of determining the prestressing force in the tendons of prestressed concrete structures and monitoring the non-exceedance of prestressing drops is an issue that has been addressed by many researchers over the past decades and has provided methods in this field. Today, pre-installation sensors are installed in important prestressed concrete structures to monitor prestressing loss. However, due to the unpredictability of such equipment in older structures, monitoring of these forces requires destructive or non-destructive testing but is inaccurate. Therefore, in this paper, a method is presented that without the need for these sensors and destructive tests, only by measuring static displacement, is able to detect the amount of prestressing loss in the cross-sectional tendons of a prestressed concrete beam. In this regard, an algorithm in the Python program environment based on genetic algorithm as well as modeling in the finite element analysis program is provided. The numerical example presented in this research shows that the proposed algorithm detects the values ​​of prestressing loss with good accuracy even in spite of 10% of the intentional error due to measurement. In recent years, the use of prestressing methods has become much simpler and more effective, and its materials have been optimized. Today, a high percentage of structures under construction worldwide are built using this technology, and the advance has found wide applications in the construction of office buildings, residential, commercial, parking lots, sports stadiums, concrete tanks and special structures such as piers. Therefore, in recent years, for long-term monitoring of prefabricated structures, equipment and sensors sensitive to force drop, such as fiber optic sensors and FBG sensors in the construction phase are predicted and installed in the desired locations. [13] However, since the above equipment requires a lot of money and it is not possible to use them in old structures, the need for a technique that shows the amount and location of force reduction in all tendons without using them remains. Therefore, in this paper, a method is presented that, while using the simplest tools, provides the most accurate results only by measuring static displacements under the effect of various loading scenarios and using an artificial intelligence algorithm based on genetic algorithm. The proposed method is based on computer analysis and comparison of the results of two prestressed concrete beams with the same geometry, loading and arrangement of tendons. First, a specific prestressing beam is modeled in the SAP2000 analysis program and the desired prestressing forces are applied to it, and then these forces are reduced in some of the studied tendons. This deliberate change in prestressing values ​​is considered as failure and the technique presented in this mapping tries to discover the extent and location of failure of this beam. In other words, this paper is the determination of the amount of prestressing force in prestressed concrete beams in which force measuring sensors are not predicted without the need for destructive testing and only by measuring the static displacement under load. In the form of a numerical example on a prestressed concrete beam consisting of 6 steel tendons and using a genetic algorithm, it was shown that the displacement is a function of the amount of prestressing and its location and amount of reduction by the technique used. It was correctly detected with 93% accuracy when 10% of the deliberate error due to displacement field measurement was applied. As a suggestion for future work, this research will be able to be developed in the simultaneous diagnosis of prestressing reduction and beam concrete failure.

Dams are among the most important and largest engineering structures used to supply potable and agricultural water and hydro-power as well as prevent seasonal floods. Human experience has shown that when damage occurs in these megastructures, there are irreversible financial, environmental, and fatal losses. Therefore, the issue of safety and proper performance of dams is of vital importance both in terms of designing new structures or health monitoring of the old structures. This subject is of more particular importance in the case of concrete arch dams, since in the absence of identification of small and minor damages, with the growth and spread of structural damages, a general failure may occur in these infrastructures with considerable human and financial losses. Nevertheless, concrete arch dams have been less broadly considered in the investigation, and most of the researches have included simple structures with a low-degree-of-freedom. Therefore, the main objective of this paper is to study the effects of structural damages (including the intensity and location) on the modal properties of these engineering systems. For this purpose, the modal characteristics of the structure, such as the natural frequencies and the shape-modes, are initially computed using finite element model on a concrete arch dam. Considering different scenarios for damage including, nine different locations (on different vertical and horizontal locations) are chosen. The modal characteristics of the system would be then determined before and after the occurrence of damage. In the presented research, the effects of damages at three levels of arch dam body on the modal parameters of the system are investigated. The comparison of the structural modal parameters of the intact and damaged states shows that first, the natural frequencies of the system have lower sensitivity than mode-shapes of the system, and also the damages to the top and middle are more effective than the corner zones on the modal features of the arc-dam body.

The safety of bridges as the vital arteries of cities is of great importance. There are several factors that raise concerns about the safe performance of bridges, and resolving these concerns are dependent on the appropriate decision-making of urban managers throughout the service life of bridge. These factors can be divided into two major types. The first type are factors affecting the safety of a part or whole structure of bridge and producing concerns about the bridge collapse. For easing the danger of collapse, totally or partially, a proper decision should be made to improve the behavior of a specific part of bridge at a certain time during its service life. The second type are factors influencing the safety of one or more bridge elements and increasing the life cycle costs of bridge. To prevent the growth of bridge costs due to deterioration, an appropriate plan for repair and maintenance should be implemented in order to enhance the condition of one or several elements.  In order to make the right decision, it is necessary to obtain accurate information on the condition of bridges. One of the best ways to get this information is to use bridge health monitoring. Health monitoring is the process of information acquisition from structure by installing sensors on its components and analyzing the data obtained from implemented sensors. By bridge structural health monitoring and interpreting the gathered data, the access to accurate and timely information, which is consistent with the reality of the bridge structure, is provided. Having the correct information about the bridge, the managers can decide at a lower level of risk. However, choosing specific monitoring strategy among different health monitoring systems for a bridge is a challenge that should be solved. A Quantitative index is needed to find the best technically and economically monitoring system.  The value of information (VoI) analysis is used for determining the effectiveness of monitoring information in decision-making. VoI is a method which quantifies the price of information and specifies the cost-effectiveness of decisions made on the basis of monitoring. This analysis also makes it possible to choose the most economical monitoring strategy among several alternatives. In the VoI calculation, all the uncertainties involved in the performance of a Health monitoring and probabilities of any anticipated event are considered. Thus, the decision making based on VoI is risk-based and reliable especially for important structures like bridges. In this paper, after investigating the worries and solutions for eliminating worries about the bridges in detail and introducing the applications of structural health monitoring (SHM) systems for bridges, the equations governing the VoI analysis is presented and the procedures for determining the VoI is discussed, and as an example, the VoI analysis of a bridge is discussed. According to the results of this analysis, implementing a specific amount of strain gauges with specific accuracy can provide worthy information about the bridge safety for the manager. Moreover, by the VoI analysis, the best approach for sensing system of SHM in the bridge is determined.

Rapid assessment of structural safety and performance following the occurrence of important events such as moderate to severe earthquakes is so significant and vital and reveals the need for developing online and pseudo-online health monitoring methods. Online monitoring methods can be implemented without the need for in-situ testing and expert staff to analyze the recorded data. In other words, these methods provide comprehensive information on the condition of the structure only by using the vibration data recorded by embedded sensors as well as the preset health monitoring algorithms. On the other hand, most civil structures exhibit a nonlinear response after severe incidents like earthquakes. In many cases, this nonlinear hysteretic behavior is along with stiffness deterioration, strength degradation, pinching effect, permanent plastic deformation, or a combination of them. Therefore, considering a comprehensive definition of damage that takes into account the nonlinear behavior of structures according to their type is one of the most important steps in the process of structural identification and evaluation. Various methods have been introduced in the literature for online estimation of states and parameters of nonlinear structures. However, the challenging part in most of these methods is the determination of parameters noise covariance matrix which becomes particularly important due to the increasing number of structural floors, thus increasing the number of unknown parameters. In this study, an effective method for online jointly estimation of state and parameters of nonlinear hysteretic structures with consideration of degradation and pinching phenomena is proposed. Simultaneous estimation of states and parameters is conducted using a combination of Unscented Kalman Filter as an effective estimator and Robbins-Monroe stochastic approximation technique as the parameters noise covariance matrix regulator. The abovementioned method was applied to two one-story and one three-story shear buildings and the results of the identification process were presented with emphasis on the effects of measurement noise, modeling error, and use of the Monte-Carlo random simulation method. Simulation results demonstrated the accuracy and efficiency of the proposed method in online jointly estimation applications as well as the desirable capability to track hysteretic curves of each structural floor.

According to the vital role that bridges play in transportation system and also communications of a society, monitoring their structural safety and keeping theme in service is crucial. Numerous methods have been proposed for detecting probable damages in bridges. Unfortunately most of them are based on comparison between the response of bridge in an intact and damaged state. Therefore intact state response must be known. However, not always it’s true in practice. So proposing a method which can determine and localize damages without prior knowledge of intact state is necessary. Such a method which was proposed by Sun et al. is studied carefully. Through the aforementioned method, the dynamic displacement response of a simply supported beam was decomposed into a dynamic component and a quasi-static component. Using Maxwell-Betti law of reciprocal deflection, the quasi-static component was attributed to the static deflection of the beam. Later damage which is defined by loss of stiffness, could be localized based on the abrupt changes in the static deflection curvature as it is related to bending moment and flexural stiffness of a beam. It is found out that the decomposition approach proposed by Sun et al. is restricted to fact that only one mode of oscillation must be dominant and also the natural frequency of motion must be determined through experimental measuring. Another limitation is that the abrupt changes in the curvature diagram cannot be related to damage essentially as curvature is also affected by the bending moment. In this study two modifications were proposed to get more accurate results in localizing the imposed damages. The first modification is the use of EMD method in order to decompose the displacement response into its intrinsic mode functions. Hence the aforementioned method could be used in real bridge displacement responses as higher modes corporations can also be determined and extracted through EMD process and finally the quasi-static component is determined as the residue of EMD algorithm. Also the ambient noise may be decomposed from the original signal, improving the method to work in real situations. The second modification is creating an imaginary constant moment length in the beam by the use of super position principle. So sudden increase in the curvature diagram is essentially a damage. Different scenarios of damage were studied and both methods have been used to detect damage in each scenario. Results show a great improvement in detection and localization of damage using the improved algorithm rather than the original proposed method. Eventually a five span real bridge model was taken into study. The improved damaged detection method could clearly determine the longitudinal position of the damage.