Scientia Iranica
Volume:19 Issue: 1, 2012

In this paper, a semi-active control method for seismically excited nonlinear benchmark buildings equipped with controllable fluid viscous dampers is presented and evaluated. A Fuzzy Logic Controller (FLC) is designed to alter the damping coefficient of controllable viscous dampers during an earthquake. A Genetic Algorithm (GA) is employed to find optimum fuzzy controller rules, as well as adjusting the membership functions to minimize the overall damage index. This combination of a Genetic algorithm and a Fuzzy Logic Controller (GFLC) is also used to minimize the peak value of the top story displacement response, in order to verify the effectiveness of minimizing displacement on the damage index. The effectiveness of the FLC is verified and illustrated using the simulated response of 3-story and 20-story full-scale nonlinear benchmark buildings excited by several historical earthquake records, through a computer simulation on MATLAB. For comparison, a Linear Quadratic Gaussian (LQG) active controller is designed to minimize the peak value of the top story displacement response. Moreover, the results of a study that has used Self-Organizing Fuzzy Logic Controllers (SOFLC) with active actuators are used. The results demonstrate that the GFLCs are quite effective in overall damage reduction for a wide range of motions, compared with the SOFLC and LQG controllers.

Application of standard binary coded genetic algorithms for the solution of problems with continuous design variables requires discretization of the continuous decision variables. Coarse discretization of the design variables could adversely affect the final solution, while finer discretization would increasingly enlarge the scale of the problem, leading to higher computation cost. A rebirthing procedure is used in this paper as a remedy for the problem just outlined. The method is based on the idea of limiting the originally wide search space to a smaller one once a locally converged solution is obtained. The smaller search space is designed to contain the locally optimum solution at its center. The resulting search space is refined and a completely new search is conducted to find a better solution. The procedure is continued until no refinement is necessary or no improvement could be made by further refinement. The method is applied to a benchmark problem of a storm water network design, and the results are compared with those of the existing method. The method is shown to be very effective, efficient and insensitive to the population size of the genetic search and the search space size of the optimization problem.

This article deals with the mechanical properties of steel-reinforced concrete precast slab tracks on non-ballasted elasto-plastic foundations. To work out the spanning behavior of slab tracks, a FEM analysis was executed for discrete and continuous systems. At first, full-size slabs without foundation including solid and hollow-core specimens (with 30% weight reduction) were tested under centric static (monotonic) line loads, and load–deflection curves were extracted. Then, FEM results for zero foundation stiffness were verified with those of experiments, which were in good agreement. Original results include the effects of several parameters on the cracking load, ultimate load, and energy absorption of slabs placed on elasto-plastic foundations including the slab width, concrete tensile strength and load factor. Analyses revealed that mechanical properties in hollow-core sections are not so different from those in solid ones, and thus hollow-core sections are more efficient because of significant weight reduction.

Seismic performance of Steel Moment-Resisting Frames (SMRFs) with side-plate connections has been investigated, incorporating record-to-record uncertainties. Based on experimental and finite element results, a connection model was proposed and calibrated to represent the side-plate connection behavior. Three code-conforming special SMRFs, varying in height, were designed and were modeled using fully nonlinear assumptions and incorporating the connection model calibrated in the previous step. Subsequently, more than 1500 Nonlinear Dynamic Analyses (NDAs) were performed within the Incremental Dynamic Analysis (IDA) procedure and were used as input to the Performance-Based Earthquake Engineering (PBEE) methodology. The structures performance was then quantitatively assessed in terms of Limit State (LS) frequency and “seismic demand hazard curves”. The effect of neglecting some level of accuracy in the structural modeling process on the predicted performances was also assessed disregarding calibrated connection behavior and employing two alternate sets of building representatives. The performances predicted by inaccurate conventional models, through about 3500 additional NDAs, were then assessed qualitatively, and the significance of incorporating accurate structural models in the performance evaluation process was highlighted with reference to the obtained comparative results.

This paper describes a laboratory investigation of the resistance to freezing and thawing of Expanded Perlite Aggregate (EPA) concrete, compared with that of natural aggregate concrete. The effects of EPA ratios on High Strength Concrete (HSC) properties were studied for 28 days. EPA replacements of fine aggregate (0–2 mm) were used: 10%, 20% and 30%. The properties examined included compressive strength, Ultrasound Pulse Velocity (UPV), porosity, microstructure and the Relative Dynamic Modulus of Elasticity (RDME) of HSC. Results showed that the compressive strength, UPV and RDME of samples were decreased with an increase in EPA ratios. Test results revealed that HSC was still durable after 100, 200 and 300 cycles of freezing and thawing in accordance with the ASTM C666. After 300 cycles, reduction in compressive strength and RDME ranged from 7% to 29% and 5% to 21%, respectively. In this paper, feed-forward Artificial Neural Network (ANNs) techniques were used to model the relative change in compressive strength and UPV in cyclic thermal loading. Genetic algorithms were applied in order to determine optimum mix proportions subjected to 300 thermal cycling. The best performance was obtained from HSC with about 10% EPA.

A new computational procedure is developed to predict the full torsional response of reinforced concrete beams strengthened with Fiber Reinforced Plastics (FRPs), based on the Softened Membrane Model for Torsion (SMMT). For validating the proposed analytical model, torque-twist curves obtained from current theoretical approaches are compared with experimental ones for both solid and hollow rectangular sections. The good agreement results of this comparison show that the proposed analytical model is reliable for predicting the torsional behavior of FRP-strengthened reinforced concrete beams before and after cracking. By means of the developed approach, the power of the SMMT method, in extending to FRP-strengthened reinforced concrete beams, is demonstrated in this paper. Moreover, the contribution of FRP fabrics to the torsional response, as an external bonded reinforcement, is studied in various practical strengthening configurations. Therefore, the efficiency of each configuration is illustrated as well.

The seismic behavior of two conglomerate rockfill materials used in two high rockfill dams, subjected to earthquake loads, has been studied. In order to evaluate the dynamic behavior, an extensive series of monotonic and cyclic tests were carried out on large cylindrical specimens of conglomerate rockfill materials using triaxial tests. This paper first studies the effects of drainage (dry, saturation) conditions on the mechanical behavior of conglomerate rockfills at two compaction levels, and grain size distribution subjected to monotonic loading. Loading frequency and drainage condition effects on specimen behavior have been studied under cyclic loads under both isotropic and anisotropic initial stress conditions. To study any sign of material degradation due to cyclic loading, the post-cyclic monotonic stress–strain curves were compared under “dry” and “saturated” conditions. Geotechnical parameters to be used in static and dynamic numerical analyses models have also been presented.

Concrete is the most practical construction material, with widespread areas of utilization, which increase every passing day, parallel to developments in concrete production technology. A most important invention in concrete technology, i.e., the use of curing materials during concrete production, has favorable effects on concrete strength and provides better curing conditions for concrete. In this study, some chemical curing materials recently used in concrete production, to provide and facilitate the curing conditions of concrete, were investigated experimentally, in terms of their effects on the compressive strength values of the concrete specimens produced with more than one chemical admixture. The results were compared with those of the specimens produced under the same conditions, but cured with water in order to determine which curing materials gave which sort of results for which kinds of concrete.

The reliability of structures is dependent on the variability in random characteristics of strong ground motion. In this paper, the influence of the variability of ground motion variables on the variability of structural response has been accessed by sensitivity analysis. The Variance-Based Sensitivity Analysis (VBSA) is considered using the Monte Carlo based procedure, for computing the set of first order sensitivity indices of variables to estimate the contribution that each uncertain input makes to variance in the model output. The variables that have a dominant influence on the uncertainty of structural response are identified. The analytical results show that the formulation of dynamic structural response in the frequency domain, based on the seismological information of excitation, can be used in some countries, such as Iran, with lack of recorded strong motion.