Iranian Journal of Science and Technology Transactions of Civil Engineering
Volume:39 Issue: 2, 2015

Changes in dynamic properties of structures indicate occurrence of damages in the structures. In this paper, a new two stage algorithm for damage detection of large structure is introduced. A modified Residual Force Method is utilized to locate the damage regions in structures, especially barrel vaults, and then the enhanced Charged System Search (CSS) Algorithm is used to quantify the amount of damage. The proposed method requires modeling the structure in its undamaged state to obtain the dynamic properties such as frequencies and mode shapes of the structure in its damaged state. The validation of the method is investigated by some numerical study on space structures. It is shown that if dynamic properties are not affected by noise, the method can still identify damaged elements, but when measurements are corrupted by noise, the exact determination of damaged region is not possible. To overcome this problem, it is proposed to repeat measuring modal properties several times, and then with utilizing residual force vector and statistical analysis, the damaged region is predicted accurately. In this paper, a modified vector is utilized for enhancing efficiency of the modal Residual Force method and suppressing effect of noise, and then a threshold is defined to distinguish DOFs associated with damaged elements. Then, using the CSS algorithm, severities of damaged elements are assessed.

This paper presents new sensitivity-based methods for detection of structural damage using incomplete noisy modal data. These methods are based on the first-order derivative of modal parameters. Changes of natural frequency do not usually provide spatial information on the structural damage. They are also not sensitive to the local damage. In this paper, a new sensitivity function is proposed using method of Lagrange multipliers in order to deal with these weaknesses when applying natural frequency in the sensitivity-based damage diagnosis. Mode shape is the other vibrational data which leads to better results in comparison with natural frequency. However, usually some mode shape’s sensitivities require all modes to obtain exact sensitivity functions. Thus, an improved sensitivity of mode shape is presented to constitute an applicable formulation based on using incomplete modes. To determine the damage quantity, a powerful iterative method named Least-Square Minimal Residual (LSMR) technique is proposed in the condition of incomplete modes. Subsequently, Regularized Least-Square Minimal Residual (RLSMR) method is presented to detect structural damage when the incomplete modal parameters are contaminated by noise. Applicability and effectiveness of the proposed methods are numerically verified using two practical examples consisting of a six-story shear building and a planner truss. Eventually, numerical results indicate that the LSMR and RLSMR are influential algorithms for precisely determining the damage severity. Furthermore, obtained results of damage diagnosis process in the free-noise data show that the proposed sensitivities of natural frequency and mode shape can provide reliable and accurate results for structural damage diagnosis.

Recently, two of the authors have introduced the collapse capacity spectrum methodology, which allows the assessment of the collapse capacity of highly inelastic P delta sensitive regular frame structures without performing non-linear time-history analyses. The main ingredient of this method is the collapse capacity spectrum, which represents the seismic collapse capacity of an inelastic non-deteriorating single-degree-of-freedom system vulnerable to the Pdelta effect as a function of its initial period, negative post-yield stiffness ratio, viscous damping coefficient, and the shape of the hysteretic loop. In the present study, multiple linear regression analyses are applied to provide enhanced analytical expressions of these spectra. The record-torecord uncertainty of the collapse capacity is captured through median, 16th and 84th percentile spectra. For several test systems analytical collapse fragility functions based on these spectra are set in contrast with the corresponding sorted individual collapse capacities. These examples prove the superiority of the proposed analytical expressions compared to its original formulation.

In this paper, the possibility of using recycled aggregate (RA) and ground granulated blast furnace slag (GGBFS) to produce a structural concrete was studied by compromising strength to a lesser extent. Hence an attempt has been made to study the strength and durability characteristics of the GGBFS-based recycled aggregate concrete. Four different groups of mixes were designed by replacing natural coarse aggregate (NA) 0, 25, 50 and 100% with recycled coarse aggregates. In each group the effect of RA was studied by replacing cement with GGBFS in different percentages from 20 to 50% with an increment of 10%. The effects of RA and GGBFS on fresh and hardened concrete properties were studied and the results were compared with natural aggregate concrete (NAC) and optimum replacement of RA and GGBFS was also found. Experimental results indicate that increase in concrete strength for all NA replacements with RA for GGBFS concrete upto 40% at the age of 90 days and further addition of GGBFS shows reverse trend.

In this paper the traditional covering plaster of masonry buildings is supplied with Polypropylene and steel fiber to enhance their seismic behavior. The plaster mix proportion is determined by some initial mortar tests. Also, one story, single span masonry building specimen plastered with different mortars is tested on a shaking table 8 times under a seismic input and the performance of the specimens with the above types of mortar is evaluated. The specimen plastered with a traditional plaster was regarded as control and its earthquake behavior was compared to that reinforced by fiber plaster. Steel fiber or polypropylene addition significantly increased stiffness, displacement ability and energy consumption ability of specimens as compared to control. The suggested reinforcement method was proven to strengthen masonry buildings in a fast, reliable and economical way. Moreover, it can easily be adapted to any masonry building without causing any negative impact. The suggested method is fire and corrosion resistant.

In this paper, the consequences of well-known characteristics of near-fault ground motions, forward directivity and fling step, on the seismic response control is investigated. An integrated fuzzy rule-based control strategy for building structures incorporated with semi active friction damping system with amplifying braces (FDSAB) is developed. The membership functions and fuzzy rules of fuzzy controller were optimized by Genetic Algorithm (GA). The main purpose of employing a GA is to determine appropriate fuzzy control rules as well to adjust parameters of the membership functions. Numerical study is performed to assess the effects of near-fault ground motions on a building that is equipped with FDSABs. To demonstrate the effectiveness of the fuzzy logic algorithm, it is compared with that of a conventional linear quadratic regulator (LQR) controller, while the uncontrolled system response is used as the base line. Results reveal that the fuzzy logic controller with FDSAB is capable of improving the structural responses and is promising for reducing seismic responses during near-fault earthquakes. It is also shown that, the near-fault earthquakes require much more control force than the far-field earthquakes and result in less response mitigation.

The effect of underground cavities in site seismic response was studied over the subway tunnels that are under construction in the city of Karaj, using ambient noise measurements as well as numerical modeling. The idea for this research comes from the observation of differences in experimental site transfer function, calculated on more than 100 locations of the city, for the areas near the under construction subway tunnel, compared with other parts of the city. In the present study a series of multi stations ambient noise measurements at 11 test sites across the tunnel were performed to evaluate the effect of the tunnel on seismic site response. This paper shows the results of these measurements as well as the result of a numerical modeling for one of the locations. The results show that the site effect in areas near the tunnel are affected by tunnel and vary based on dimension of excavation and distance from tunnel axis.

Expansive soils may cause disaster if not adequately taken care of. Lime continues to be commonly used for modification of these types of soils although it may have limited success in some applications. Thus, the present study was performed to address the viability of using silica fume (SF) as industrial waste to modify the behavior of expansive soils. This achieves the double objectives of overcoming the restrictions associated with lime treatment, and also of providing reliable data for using SF in the field of geotechnics to reduce its environmental cost. The additives including lime, SF, and lime-silica fume (LSF) mixture were separately added to the expandable smectite clay at wide ranges from 2% to 30% by mass, respectively. A set of laboratory tests including Atterberg limits, swelling, unconfined compression strength, permeability, electrical conductivity, and pH measurement were carried out at various curing periods to evaluate different influences of the additive types on the soil performance. The SSA and sedimentation analyses were conducted to assess the soil microstructure changes. The micro level structures of natural and modified clayey soil samples and their chemical composition were also studied using scanning electron microscope (SEM) equipped with Energy Dispersive X-ray (EDX) microanalysis. The results show that the geo-mechanical properties of highly expansive soil can be modified by the large content of lime. Besides, with the addition of lime and inadequate curing, where mainly flocculation occurs, the permeability of soil is negatively affected and slight increase in strength is observed. On the other hand, the addition of SF alone, even up to 30%, has less effect on the swelling power and produces a negligible change in the soil strength, regardless of the curing periods. It is found that the defects arising from the lime treatment can be greatly enhanced by the use of lime-silica fume mixture. Based on the results of macro and microstructure tests, the LSF blend improves the engineering parameters of smectite with a lower amount of lime and shorter curing time as compared with lime treated samples. This can occur due to extending the synthesis of the new pozzolanic compounds. It also significantly decreases the soil permeability through physicochemical interactions and induces a sharper decline in free lime, which results in the decrease of post instability problems in chemically modified soil. The study concludes that the combination of silica fume and lime can be successfully utilized as an additive to increase the efficiency of soil stabilization from economic, technical and environmental point of views.

Reliability analysis of rock slope stability has received considerable attention in the literature. It has been used as an effective tool to evaluate uncertainty so prevalent in variables. In this research the application of the jointly distributed random variables method for probabilistic analysis and reliability assessment of rock slope stability with plane sliding is investigated. In a recently published paper, the authors showed the dependency of the numerator and denominator of the safety factor relationship and argued that, as a result of this dependency, the method could not assess the reliability correctly. In the current research the authors present a new approach to solve this problem. In this approach, using the basic relations in this method, the safety factor relationship is obtained directly without separation of its numerator and denominator. Furthermore, in addition to friction angle of sliding surface, apparent cohesion, depth of water in tension crack, and earthquake acceleration ratio, in the present work the unit weight of rock is also considered as a stochastic parameter. The results are compared with the Monte Carlo simulation. Comparison of the results indicates good performance of the proposed approach for assessment of reliability. The new results of parametric analysis using the jointly distributed random variables method show that the friction angle of sliding surface is the most effective parameter in rock slope stability with plane sliding.

Semiarid regions with their exceptional weather conditions, low precipitation, and high evapotranspiration pose a great challenge to water resources managers. One possible way to face this challenge is the conjunctive use of both surface water and groundwater resources in these regions. This paper proposes a conjunctive use model which has been implemented in Najafabad plain in central Iran. The model is one of simulation-optimization in which the simulation portion combines the Fuzzy inference system and Neural Networks (FNN) in order to take the climate conditions and the uncertainty in the relevant data into consideration while the optimization portion consists of a multi-objective Genetic Algorithm (GA). The objectives of the optimization model include not only minimizing water shortages in meeting the irrigation demands by the three irrigation systems operating in the region but also minimizing groundwater drawdown in order to control groundwater extraction in the aquifer. These objectives are subject to constraints on the maximum amount of surface and groundwater allocated to the irrigation zones and the maximum capacity of surface irrigation systems and also maximum and minimum allowable cumulative drawdown in the planning horizon. The results of the proposed FNN-GA model demonstrate the importance of the interactions between surface water and groundwater resources considered in a conjunctive use model for the planning and management of water resources in semiarid regions.

In recent decades, attention to contamination propagation into soils and underground water has increased, which has led to a rise in the studies on soil contamination problems and methods of in-situ remediation. In this research, effective parameters on oil contamination propagation in soils and underground water have been investigated using FEM in order to determine propagation patterns. The main parameters are soil permeability, relative density of contamination and ground water table depth. Furthermore, the efficiency of pumping method as an in-situ remediation approach for contaminated soils has been examined. The results show there are different propagation patterns in coarse and fine grain soils and the higher efficiency of dual symmetric pumping approach for in-situ remediation.

Concrete shear walls are the most prevalent structural systems resisting lateral loads due to earthquakes in high-rise buildings. Very large in-plane stiffness of shear walls provides an excellent drift control in the structure. However, structural damages and early code shortcomings threaten the efficiency of existing structural walls against earthquake. Recently, fiber reinforced polymer (FRP) materials have been used considerably in strengthening and retrofitting of structural elements. High tensile strength and excellent tensile modulus along with other unique features of FRP materials make them the first alternative in the strengthening projects. However, the literature shows that few analytical and/or experimental studies have been conducted on the strengthening of slender reinforced concrete (RC) shear walls with FRP materials so far. In this paper, the effect of strengthening of boundary elements in slender RC shear walls with FRP on the overall behavior of shear walls is investigated. Nonlinear finite element is used to analyse the RC walls, using damage plasticity model and tension stiffening effects. Results of the current study show that applying FRP sheets vertically on the lateral faces of the boundary elements causes the load-displacement curves of the strengthened walls to have a larger load carrying capacity up to 20% compared to that of wall specimens without FRP strengthening. Furthermore, applying the FRP sheets on the boundary elements only in the plastic hinge region of wall can improve the wall load carrying significantly.