Scientia Iranica
Volume:21 Issue: 6, 2014

This paper presents a review of recent advances made in vibration-based Structural Health Monitoring (SHM), using the responses of the structure to an excitation. The review is divided into parameter and feature estimation, based on linear structural behavior, SHM taking into account the nonlinear structural behavior, sensor layout and data collection strategies and integration of SHM, with the vibration control of structures, wireless monitoring, and application of LIDAR.

Cantilever beams are widely used in reinforced concrete structures. Due to deflections in a cantilever، cracks occur. The formation of a crack is unfavourable and decreases the bearing capacity of a reinforced concrete member. The use of steel ber in reinforced concrete is widespread، since it prevents the formation of cracks and increases ductility. The signi cance of this study is an experimental report of the use of steel bers in a reinforced concrete cantilever beam، and their contribution to its behaviour. Therefore، reinforced concrete cantilever beam samples، with and without steel ber additions، were produced. As cyclic loads were applied، the behaviour of the samples was examined. As a result of the study، it is experimentally found that by addition of steel bers into reinforced concrete cantilever beams، crack widths decrease and their ductility increases. Sti ffness decreased less in reinforced concrete cantilever beams with steel ber addition، and، thus، it was concluded that use of steel bers is favourable for their behaviour.

This study investigates the anisotropy in the stress-strain-strength behavior of Firoozkuh sand، focusing on the e ect of the initial stress state and soil fabric. The experiments are performed using a hollow cylinder apparatus. Di erent initial conditions are considered by maintaining the principal stress ratio (i. e. Rc = 01 c=03c) and the inclination of initial principal stress (i. e. c) during consolidation. The specimens are prepared using two di erent methods to assess the influence of soil fabric. The direction of increments of principal stresses، ، is controlled to remain constant during the test. Generally، the results showed a decrease in the shear strength of sand by increasing . In specimens consolidated under the inclined anisotropic stresses، the trend of shear strength variation versus  di ered and depended on c. The vertical consolidated sands (Rc = 2، c = 0) were found to be highly in uenced by . The anisotropic behavior of medium dense sands was observed to be strongly dependent on the initial stress state. The results showed that the stress-strain behavior of specimens prepared by the wet tamping method is strongly in uenced by . The e ect of soil fabric and initial stress state on the shear modulus was more noticeable than that on the shear strength of sand.

Calcium carbide gas pressure (CCGP) method is a rapid measurement procedure for obtaining soil water content (SWC)، which relies on the chemical reaction of the calcium carbide reagent with the water in soil pores. Currently، the method is limited to soil samples of 20 g or larger. However، test equipment for much smaller sample size does exist، and there is a need in quantifying the reliability of SWC measurements of small samples. The research involved a total of 232 tests that were carried out on 22 different soil types containing particles no larger than No. 4 sieve size. Water contents of the soil specimens with varying wetness were measured with both the conventional oven-drying and the CCGP methods. The error distribution analysis indicated that the CCGP method، as used in this study، estimates the SWC for all soil types with a mean absolute discrepancy around 0. 3% and 0. 2% with general and soil specific calibrations، respectively. It was found that CCGP method is reliable for SWC determination of small specimens up to 20% of SWC if the manufacturer’s instructions are followed.

Bed forms are generated in alluvial streams due to mutual interaction between flow and erodible bed material. Among the variety of possible bed forms, dunes are the most important feature and much attention has been paid to them in the literature. In the present study, an experimental approach was carried out to investigate the geometry of dunes and their celerity in an erodible sand bed. The tests were conducted under live bed conditions in an experimental flume capable of re-circulating both water and sediment. Present experiments showed that the Shields number had a considerable eff ect on dune height and celerity, while the e ect of this parameter on the dune length was not signi cant. Furthermore, dimensional analysis is used to present the relationships between dune height and length, as well as celerity. These relationships were also compared with previous empirical equations and experimental data, which showed their acceptable accuracy. Bed roughness related to bed forms was also analyzed based on all available experimental and eld data. Results demonstrated that by increasing the Shields number, the ratio of Manning coecient, related to bed forms, to total Manning coecient increased with a logarithmic trend.

Semi-active devices are low-cost as well as small and by changing the properties of low-power intensive structures, the aims of control are accomplished. On the other hand, the limited control force which can be applied to the structure for each damper causes more dampers to be used in the structures compared to the larger and stronger control devices which are more costly. These dampers coupled with sensors and the structure themselves make a complex dynamic system which is best controlled by a decentralized method such as Market-Based Control (MBC). In MBC, the actuators and the supply energy are modeled as buyer and seller respectively in the market place. To define the demand function of the buyer and the supply function of the seller some weighting constants have to be chosen. The performance of the MBC correlates with prudent selection of the weighting constants. In this study, the novel method for designing MBC by using Genetic Algorithm (GA) is presented. The MBC approach is applied to three linear structures having five, ten and twenty floors and the resulting solutions show the merits of the new methods for tuning MBC as opposed to the solutions using a centralized linear quadratic regulator (LQR).

Changes in the dynamic properties of structures indicate an occurrence of damages in them. In this paper، the dynamic properties of damaged structures under diff erent scenarios are obtained using suitable nite element modeling. An optimization technique is then used to minimize the di erence between experimental and modeled data، i. e. to minimize the value of the objective function. In this paper، the Charged System Search Algorithm (CSS) is used for the optimization process. For enhancing the search algorithm، the input data is modi ed in a recursive process utilizing the results of previous iterations. Natural frequency and mode shapes are used in the objective function. A beam and two frames are used to evaluate the performance of the algorithm. Results show that this algorithm is e ective in damage detection and its performance is better when a curvature mode shape is used in place of a displacement mode shape. Increasing the element or the number of damaged elements does not a ect the eciency of the algorithm، and only increases the number of iterations.

Using the concept of strain energy change, due to damage at the element level, a new structural damage detection method is developed. The proposed method employs either static or dynamic response of a structure and simultaneously localizes and quantifies multiple damages. It requires only the stiffness and mass matrices of the baseline structure and a few measured responses of the current structure to find the exact location and severity of damage. A numerical example was used to investigate the behavior of the algorithm. It is shown that those mode shapes and static loadings which yield a uniform distribution of strain energy in elements predict the location and magnitude of damage with more accuracy. Some higher mode shapes which might induce significant levels of strain energy in some elements are not always reliable. Finally, it is shown that in order to obtain reliable results, the number of equations (mode shapes or static loadings) must be greater than the number of predicted damaged elements.

To identify structural damages many different methods have already been developed. Evaluating the performance of these methods is not a convenient task because they have been applied to different structures or constructed for specific purposes. Most of these methods use model-updating techniques، as a tool، to detect and assess damage. On the other hand some methods، including Energy Index Method، use the concept of strain energy to detect damage. This paper tries to compare the performance of Energy Index method with the performance of a model-updating-based model. In order to facilitate the comparison of various damage identification methods a structure proposed by the IASC-ASCE Task Group on Structural Health Monitoring is considered as the benchmark structure. Finally، the effects of measurement noise and incompleteness of data on the performance of the proposed algorithm are investigated.

In this paper، an elastoplastic model is proposed to analyze the circular tunnels below the groundwater table under axial-symmetric condition، considering the effects of seepage and gravitational loads. In the proposed method، the strain-softening behavior model and Hoek-Brown failure criterion are used. To evaluate the effect of gravitational loads and variations of pore pressure، the equations concerning different directions around the tunnel (crown، wall، and floor) are derived. Since، the derived differential equations do not have closed-form solution in the plastic zone، the numerical finite difference method is applied. Considering the strain-softening behavior of the rock mass، the problem in the plastic zone is solved through a stepwise method، where the strength parameters of the rock mass varies step-by-step from their maximum values to the constant values. Besides، the stresses، strains، and deformations of the rock mass also vary step-by-step from elastoplastic boundary to tunnel boundary values. Furthermore، the closed-form analytical solutions are obtained for the elastic zone. The accuracy and application of the proposed method is demonstrated by a number of examples. The results well exhibited the effects of dilatancy angle and increment of elastic strain in the plastic zone. Based on the results obtained، ignoring the effects of gravitational loads and seepage will definitely produce computational errors.

In this paper a time varying optimal control algorithm (β-Method) is proposed to control building responses, against environmental earthquake excitations. The proposed method is presented through defining a rational relation between the state variables of a structure with active and passive control systems with identical mechanisms. This procedure results in a time varying gain matrix with adaptable ability to external excitation in order to decrease the extra need for maximum and/or total control force. Performance of the proposed method is examined by applying it to an eight-story shear type building subjected to various ground accelerations. Numerical results indicate that the proposed algorithm in some cases reduces the power consumption demand significantly without any reduction in control system performances in comparison with the classical closed loop optimal control method and acts similar in the worst case.

Many industrial problems are concerned with optimization of large and complex systems involving many criteria. Indeed, optimization problems encountered in practice are seldom mono-objective. In general, there are many conflicting objectives to handle. This study introduces a new method for the solution of multi-objective optimization problems. Multi-objective optimization is utilized to nd the most suitable solution, which covers the requirements and demands of decision makers. The main goal of the resolution of a multi-objective problem is to obtain a Pareto optimal set and, consequently, the Pareto front. This method is based on the Charged System Search (CSS) algorithm, which is inspired by the Coulomb and Gauss laws of electrostatics in physics. In order to illustrate the eciency of the proposed method, numerical examples are solved and results are compared to show the ability of the CSS in nding optimal solutions.