Sharif Journal of Civil Engineering
Year:2009 | Volume:24 | Issue:46.1|Papers Count:13

Iran is located in a high-risk seismic region. The severe damage caused by recent major earthquakes has increased the importance of seismic evaluation methods and design procedures. To enhance seismic evaluation methods, it is necessary to provide acceleration records which are compatible with a design spectrum. In this paper, a new method is proposed to generate compatible records with a design spectrum. The method is based on dividing the response spectrum of a record into several period ranges and finding the scale factor for each range. The proposed method is used for seven real acceleration records to generate seven synthesized acceleration records, with a response spectrum compatible with the design spectrum of Standard 2800 (3rd edition) for soil type 3. Besides, seven artificial acceleration records are generated using SIMQKE I. The nonlinear dynamic behavior of two 12-story RC buildings, which were designed by a direct displacement based (DDB) method and standard-2800, are used for comparison between the two groups of records. Results state that the proposed method is suitable for analyzing and designing structures without a noticeable increase in the values of the input energy of the real records.

The liquefaction phenomenon is the cause of many effects on underground structures constructed at shallow depth. The shield tunnels of subways and underground routes are the most common types of these structures. One of the main effects of soil liquefaction around these kinds of structure is the variation of confinement, which occurs during earthquake loadings. In this paper, the changes in the circumferential condition of a shield tunnel, due to liquefaction, are presented. In this study, the influence of changes in shield forces and the uplifting of the tunnel due to liquefaction are investigated, using an appropriate model for soil liquefaction. For this reason, the FLAC package, which is capable of undertaking this type of modeling, is used. These are two main parts in this paper. In the first part, a reference model is defined and the ways of evaluating the influences of soil liquefaction are discussed. In the second part, the induced forces in the tunnel shield, due to liquefaction, are estimated, considering the appropriate changes in the circumferential condition of the tunnel. Research has been carried out on the assessment of the effect of different parameters on the variation of forces and the deformation of the lining.

Steel and composite shear walls are some effective lateral resisting systems with a steel core. The main advantage of these systems is their deformability, due to the buckling behavior of the steel sheet. Two general methods for benefiting this advantage are using steel-stiffening elements or concrete layers connected to the steel plate by shears connectors. In this research, by studying different behaviors, including the influence of middle beam rigidity, the type of beam to column connection, changes in shear connector's distance, the effect of concrete layer thickness on a composite shear wall, with and without stiffeners and a general behavior of this system against lateral loads, are investigated. In this regard, various models of such walls are analyzed using the Finite Element method.The analytical results show that the stiffened steel shear wall behavior is independent of the middle beam rigidity and the beam column connection type, but, in a steel shear wall without stiffeners, the increase of middle beam rigidity causes a better monotony of stress distribution in the steel sheet. Also, the shear resistance of a composite shear wall depends on the concrete layer thickness and the distance between shears connectors. So, by studying the analytical results, an optimum concrete thickness and, also, a suitable distance between shears connectors, are proposed.

In this paper, the effect of the deterioration behavior of hysteretic cycles in nonlinear static analysis (pushover) has been studied. One of the deficiencies of this method is that the deterioration of hysteretic cycles for structural elements is approximately considered in this analysis. To evaluate this effect, nine special moment resisting frames have been used and loaded, based on the Iranian Code of Practice for Seismic Resistant Design of Buildings (Standard No.2800, 3rd Edition). These frames have been analyzed and designed by ETABS software. The modified Takeda and non-degrading Clough models have been used in inelastic dynamic analysis. The two latter models have been successfully used in the past to analytically reproduce the hysteretic behavior of well-detailed flexural-controlled reinforced concrete elements. The inelastic dynamic analysis of the reinforced concrete structures program, IDARC, is used for nonlinear static and dynamic analysis, which considers damage analysis by using the Park & Ang damage model. The maximum displacement amounts obtained from nonlinear dynamic analysis are compared with those obtained from nonlinear static analysis. Results show that the average amounts of maximum displacement increase 4% and 9%, respectively; with considering deterioration effects at different seismic hazard levels (0.2g and 0.25g), rather than target displacements of a pushover analysis. In addition, in the modified Takeda model that considers deterioration effects, the damage index for a seismic hazard level of 0.2g, increases by 53% more than that of the bilinear model. This ratio is 81% for a seismic hazard level of 0.25g.

In recent decades numerical modeling of self-weight consolidation of very soft soils which has a number application in the analysis of consolidation of dredged materials and deposited materials behind tailing dams, has attracted special attention.In this paper, by considering large strain effects and variation of permeability and compressibility of soft soils during consolidation, and employing a new technique for discretization of spatial domain during stage filling, a finite difference based computer program has been developed that is capable of modeling different scenarios of self-weight consolidation. Then the performance of the model for analyzing of practical applications has been evaluated via comparing the model results with measured values in the site as well as computer programs results that have been verified previously.

In traffic assignment problem (TAP) literature, methods like Frank-Wolfe have sufficient efficiency in solving the problem, assuming infinite capacities for the links, because, in these methods, each linearized sub-problem is equivalent to a shortest path problem between an origin and a destination (OD) pair. However, in reality, links have limited capacities and obtaining more realistic volumes needs those capacities to be taken into account. Explicit consideration of this type of constraint in TAP causes each linearized sub-problem to be converted to a minimal cost, multi-commodity flow problem that is difficult to solve. Another method is to consider the constraints implicitly and use some penalty functions that are sensitive to capacities, so that, by adding them to travel time functions, the capacity constraints lead to being satisfied. In this paper, a suitable penalty function is suggested and its usefulness is examined via some numerical examples. The results will be compared with the other methods of interest, such as: the inner penalty function (IPF) and the augmented Lagrangian multiplier (ALM). Also, the results are presented by applying the suggested method to the network of Mashhad city, asa real case example, where the links approaches to the signalized intersections are assumed capacitated.

The development of experimental science and the amazing improvement in computational tools have caused scientists and researchers to provide the infrastructure for the development of new methods, which are more appropriate and rational, in comparison with traditional methods, in structural engineering. Performance based design methods and increasing static and dynamic load methods are some examples. The behavior of liquid storage tanks under seismic loading has been considered by engineers and researchers because of their importance and soil-structure interaction. The initial analytical models for the dynamic analysis of liquid storage tanks were based on simple assumptions, such as wall rigidity, that have been gradually modified. In this paper, a new method for performance based earthquake analysis (seismic analysis) and the evaluation of structures, which is called the Time Endurance method, is introduced. In this method, the structure is subjected to accelerograms that impose increasing dynamic demand on the structure with time. The purpose of this projects investigating the application of an ET method in the seismic design of ground-supported steel storage tanks and comparing them with the seismic design codes of steel tanks (such as API and AWWA). In addition, the obtained results of the ET method are compared with the obtained results of the seismic design codes. Then, the compatibility of the ET method in seismic design is investigated, in comparison with the codes. First, an extensive literature survey was carried out, regarding the mechanical models introduced for the seismic analysis of tanks and related design codes. Then, the dynamic response of the flexible storage tanks under different input ground motions has been investigated, using the finite element method. For this purpose, the container and their fluids have been modeled with displacement based elements, using the general-purpose finite element code ANSYS. In an axisymmetric harmonic and 3D modeling, the specifications of one of the existing cylindrical tanks have been adopted. The compressibility and viscosity of the container's fluid have been also taken into account.

Transmission towers are the most important part of the systems for transferring electrical power and for its distribution. These structures are designed to support the conductors and ground wires of electrical power lines and withstand mechanical forces in the worst weather and under the worst atmospheric conditions. A linear analysis is generally sufficient for the analysis of a transmission tower. Since these structures contain a high number of elements, and due to their high cost, their optimization is of great importance from both a material and a construction point of view. Transmission towers are often subjected to many combinations of loading and, in the process of optimization; one is faced with many analyses. In this paper, a genetic algorithm is employed for optimization. One of the main difficulties in using the genetic algorithm for optimization is that, for each chromosome in each generation, one analysis should be performed. This requires the inversion of matrices and, with an increase in the number of elements and in geometric optimization, the convergence becomes very slow. In order to overcome this difficulty, neural networks are trained as analyzers to take on part of the computational load. Using neural networks increases the rate of convergence of the optimization. A multi-population genetic algorithm is used in this article.

The optimal operation of a reservoir is one of the most important optimization problems in water resource engineering. Many different methods have been devised and used for optimal operation of reservoir. Nowadays, meta-heuristic algorithms are being used more and more to solve optimization problems in water resources engineering and, in particular, reservoir operation problems. In this paper, the problems of a simple and hydropower operation of reservoirs is solved with a new version of the Ant Algorithm, called Max-Min Ant System (MMAS). Both storage volumes and water releases are considered as decision variables of the problem and the performance of the corresponding models are compared. The efficiency and effectiveness of the MMAS for solving a benchmark example, the optimal operation of the "Dez" reservoir, are tested and compared with those of other heuristic algorithms and that of a mathematical programming based optimization package (Lingo). Comparing the results of MMAS with those of other ant colony algorithms and Lingo has shown that the proposed method is capable of producing superior results for the example considered.

Two-way slab structures, owing to their economical and architectural advantages, are used extensively in civil engineering. In addition, these systems not only resist gravity loads and transfer them to the columns, but, also, sometimes in combination with other structural members, make a lateral resisting system. The elastic analysis of an equivalent frame method is a precise and comprehensive method that is widely used for the analysis of two-way slab structures. Because of the nonprismatic sections of slab-beams and columns, carryover and stiffness factors and the fixed end moments of members with a constant moment of inertia could not be used. Some tables have presented these values for different types of slab-beam, column and torsional member. Due to the low precision of these tables and the complicated and time-consuming procedure of the equivalent frame method, the precise analysis of two-way slab systems, using an equivalent frame method, including stiffness variation in all members, using slope deflection equations, in the form of developed software, is pursued in this research. Two analyses of example structures were performed and the efficiency and accuracy of the program were verified by comparing the hand-calculated results, including a finite element approach and the developed computer program results, which show some differences in support moments

In structural elements, with the variation angle of horizontal orthogonal accelerogram components, the response to the relation between the principal axes of the building will vary. In this paper, a steel building that is regular in plan and height, using seven acceleration records, is analyzed by nonlinear dynamic analysis and the critical angle and relation variation responses are calculated. Results indicated the critical angle is different with varying acceleration records. In this study, the increase in the intensity ground motion and nonlinear behavior in the structural elements is also investigated. Results indicated that critical angle and increase relation responses vary with an increase in the intensity ground motion.