Scientia Iranica
Volume:25 Issue: 1, Jan - Feb 2018

The dependence of Conditional Mean Spectrum (CMS) and the corresponding standard deviation on different target period values has been investigated in this paper by means of two types of target spectra, i.e. based on Epsilon and Eta indicators. The structural collapse capacities, as well as the mean annual frequency (MAF) of exceeding a limit state, are taken into consideration. The results show that the dependence of Eta-based CMS (ECMS) on the choice of target period is insensitive to the target period in the case of MAF calculation. However, this dependence is meaningfully less in the case of ECMS when compared to CMS in Intensity based ground motion selection. The Sum of the Squared Error (SSE) is utilized to compare different CMS cases. SSE is less in the case of shorter return periods, e.g. 75 years, in comparison with longer return periods, e.g. 2475 years. This dependence is also a function of choosing attenuation relationships. Therefore, four Next Generation Attenuation (NGA) relationships have been employed in this study. In general, ECMS has shown less dependence in all cases when compared with the conventional CMS.

Reinforced concrete (RC) structures located on ocean coasts are damaged by salt water so that the concrete deteriorates. Due to this deterioration of the concrete, the metal rebar inside starts to corrode because there is no longer concrete around it to prevent exposure to the outside environment. Such corrosion causes serious degradation of the cohesion between the steel and the concrete, and the performance capacity of the bonding between concrete and rebar. In this study, the performance of RC beam-column joints was evaluated. The interface between concrete and reinforcing bars in the RC beam-column joint specimens were partially un-bonded to simulate corrosion. The mechanical behavior and energy dissipation capacity of un-bonded RC beam-column joints were evaluated by experiment. Numerical studies were also performed with the help of finite element methods. Results from the experimental tests and numerical studies of RC beam-column joints at bonded and un-bonded interfaces between concrete and reinforcing bars, are compared and discussed in terms of the energy dissipation capacity, strength, and crack distribution.

Portland cement can be mixed with sand to improve its mechanical characteristics. Considering the inherent anisotropy of most sands, it is not clear whether the added cement shall contribute to equal increases of strength and stiffness in vertical and horizontal directions or not. Literature review reveals that all the previous experimental studies on the cement-treated soil topics are limited to conventional triaxial device test results. The effect of cement addition to clean sand in more sophisticated stress-path controlled devices like "Simple Shear Apparatus" (SSA) and "Hollow Cylinder Apparatus" (HCA) simulating more realistic loading conditions, such as earthquake, have not been investigated yet. This paper presents innovative methods for testing artificially cemented sands through making modifications to the loading platens, specimen molds and specimen preparation methods of SSA and HCA. Undrained tests under different monotonic stress paths were carried out on different mixtures of Portland cement with clean sand to investigate the effect of principal stress rotations. The test results revealed that the cement mixture reduces the anisotropy, while it improves the mixture mechanical properties compared to the compacted uncemented sand.

Various durations of Endurance Time Acceleration Functions (ETAFs) associated with different seismic hazard levels are presented to enable Endurance Time (ET) method for use in probabilistic seismic demand assessment studies. Various Intensity Measures (IMs) were, first, regarded for establishing multiple “IM-duration” relationships. A set of 30 RC moment resisting frames were, then, subjected to IDA analysis using 44 ground motion records and the median IM values corresponding to different structural response levels were extracted. These values were compared against the ET-derived IMs by computing the errors corresponding to various demand levels and summating these errors over a complete range of response levels to derive an overall error index. The error indices were then averaged over all structural models and were compared for different IMs revealing that maximum compatibility with the ETAF generation method dominates selection of the best IM.

In this paper, the coupling responses of water and high bent-type aqueduct under wind loads were studied by the Arbitrary Lagrangian-Eulerian (ALE) method. The natural vibration characteristics and transversal displacement, stress, overturning force, overturning moment and hydrodynamic pressure of water-aqueduct coupling system were comparatively analyzed in different conditions of water-depth, cross-section shape of aqueduct tank and support bearing. The research results show that the isolated support changes the dynamic characteristics and reduces wind resistance performance of U-shape aqueduct and the dynamic performance is better under the effect of the fluctuating wind but smaller stiffness for U-shape aqueduct than that for the rectangular aqueduct with the same water flux.

This paper presents numerical solution of Richard's equation for water flow through unsaturated porous media. Differential Quadrature Method (DQM) is employed for the first time to solve the governing equations in two space dimensions. The moisture content based of Richard's equation is considered. This equation is known as a highly nonlinear partial differential equation due to strong nonlinearity between hydraulic conductivity (and diffusivity) and moisture content. In order to investigate the robustness of DQM in dealing with such strong nonlinearities, two popular constitutive models i.e. White and Broadbrige (1988), and Van Genuchten (1980) models are investigated for the 2D case. Analytical solution based on Brooks and Coley model in a special 1D case is used to compare the results with those of DQM. For the 2D case, the study also demonstrates that DQM with considerably smaller number of grid points gives excellent results which are in close agreement with other numerical techniques such as multigrid approach reported in the literature.

Delays and disruptions are extremely challenging issues to deal with in project management. In this article, a novel optimization approach to the buffer sizing method is introduced aimed at maximizing the robustness of the buffered schedule generated. The measures affecting the buffer sizing include the network complexity, flexibility, criticality, and robustness. The methodology presented is based on the critical chain project management concept, yet novel metrics are introduced to cover the uncertainties connected with the critical and non-critical chains. The overall purpose of the approach is to investigate the necessity and design of a decision support system to improve the process of critical chain project management. Utilizing a robust and flexible framework, this study tries to efficiently determine the size of feeding and project buffers. The weaknesses of the current critical chain project management approaches were overcome in the critical chain project management, and a new method was developed based on the integration of simulation and optimization techniques. In order to verify the efficiency of the method proposed, a case study is conducted. The outcomes indicate that the robust buffer allocation method proposed yields more stable project schedules, as against the traditional buffer sizing methods.

Construction projects play an important role in the economic development of every country. Nevertheless, review of projects documents indicates that in most cases, the projects are not finished on schedule and on assigned budget, such that they sometimes loss their economic justification, and simply fail. Consequently, devising suitable solutions is essential to prevention of such failures. This is impossible without identifying the foremost causes of failure. In this study, first, all factors of failure are identified using Fault Tree Analysis (FTA). FTA as a diagnostic tool allows us to efficiently isolate root causes of failure. To rank these factors, dedicated specialists are requested to assess the risk of each cause using linguistic terms; thereby relevant calculations are carried out using the Linguistic Weighted Average (LWA). Undeniably, considering the complexity of construction projects and incomplete expert knowledge, judgments must not be made using crisp value conception. Hence, fuzzy theory is utilized to achieve more accurate results. Results indicate that the majority of problems in projects stem from financial concerns and shortcoming of bidding process. In the last section, an actual case study is used to validate our results.

The construction industry is recognized as one of the most hazardous industries. Visibility-related accidents are the cause of a significant portion of the total fatalities within the construction industry. Limited visibility due to construction equipment blind spots and a lack of visibility in construction areas such as slab openings are among the main causes of these fatalities. This study introduces the Virtual Blind Spot Identification System (VIBSIM), which was designed to identify and examine blind spots in construction equipment and construction sites. The VIBSIM consists of a three-dimensional point cloud model that uses 3ds Max software in conjunction with a V-Ray rendering motor. A real case is used to validate the applicability of the proposed system. The results indicate that the system can significantly improve safety applications for construction projects. Moreover, the system can aid construction managers in making decisions regarding the better management of such safety applications for construction projects.

Estimation of a proper fundamental vibration period is an important issue in the process of the design and/or evaluation of a building. Mostly, infill walls are assumed as nonstructural element and ignored in the estimation of the vibration period of a building in terms of stiffness. However, studies showed that infill wall has significant effect on vibration period and should be considered in the estimation of period. Even some proposed equations take into account infill walls in the estimation of period, they do not consider changing of the stiffness of infill walls. In this study, an empirical equation is proposed as a function of building height, elasticity modulus of infill wall and infill wall thickness. For this purpose, building periods were determined with considering different infill wall elasticity modulus, infill wall thicknesses (thus different infill wall stiffness) and building heights. Nonlinear regression analyses were conducted with a comprehensive statistical study. Effect of infill wall elasticity modulus and thickness on vibration period was investigated. Finally, comparisons of proposed equation of this study and previous studies were conducted.

In this research, a new three-dimensional limit equilibrium method was developed. In the proposed method, the slip surface was assumed to be spherical and slices were assumed along the slip surface radius converging to a point. Moreover, force and moment equilibrium equations were used. In order to calculate factor of safety (FS) with the proposed method, a code was developed for two- and three-dimensional state. The model uses the iteration (trial and error) method to simultaneously satisfy force and moment equilibrium equations. In order to ensure convergence of the numerical model developed in the three dimensional state, the under relaxation method used in the trial and error operations. In examples, the proposed method was obtained the value of FS, 2.5 to 12 percent higher than other methods. In order to validate the proposed method and assess the functionality of the three dimensional numerical model a few examples were solved under different circumstances and the results were properly compliant with the results of other methods.

This paper presents a numerical investigation on the static behavior of stepped soil nail walls constructed in dry sandy soils. The finite element method was used to study influence of wall geometry and soil parameters on behavior of the walls. We analyzed walls with heights of 10, 15, and 20 m .This study shows that the wall deformation and the nail tensile forces of the stepped soil nail wall are smaller than those of the typical soil nail wall. If properties of soils such as friction angle, cohesion, and elastic modulus decrease, these will result in more decrease in the wall lateral displacement and nail tensile forces of a stepped soil nail wall. Therefore, stepped soil nail walls are more effective in soft soils than hard soils. When a step is located at the middle of the wall height, the wall lateral displacement and nail tensile forces are minimized; hence, the ratio of the optimal step depth to the wall height is 0.5. As the step width increases, the wall deformation and nail forces decrease. Numerical analysis demonstrates that the minimum step width is approximately 0.1 times of the wall height.

The determination of erosion and deposition patterns in channels requires detailed knowledge and estimations of the bed shear stress. In this investigation, the application of a Genetic Algorithm based Artificial (GAA) neural network and genetic programming (GP) for predicting bed shear stress in a rectangular channel with rough boundaries. Several input combinations, fitness functions and transfer functions were investigated to determine the best GAA model. Also the effect of various GP operators on estimating bed shear stress was studied. The comparison between the GAA and GP technique abilities in predicting bed shear stress were investigated. The results revealed that the GAA model performs better in predicting the bed shear stress (RMSE = 0.0774), as compared to the GP model (RMSE = 0.0835).

Castellated beams and composite action of beams are widely applicable methods to increase the capacity of the beams. Semi-rigid connections can also redistribute internal moments in order to attain a better distribution. Combination of these methods helps to optimize the cost of the beam. In this study, some meta-heuristic algorithms consisting of the particle swarm optimization, colliding bodies optimization, and enhanced colliding bodies optimization are used for optimization of semi-rigid jointed composite castellated beams. Profile section, cutting depth, cutting angle, holes spacing, number of filled end holes of the castellated beams and rigidity of connection are considered as the optimization variables. Constraints include the construction, moment, shear, deflection and vibration limitations. Effect of partial fixity and commercial cutting shape of a castellated beam for a practical range of beam spans and loading types are studied through three numerical examples. The efficiency of three meta-heuristic algorithms is compared.