Iranian Journal of Science and Technology Transactions of Civil Engineering
Volume:37 Issue: 2, 2013

This paper investigates the performance of a multiple-deme genetic algorithm (GA) with modified reproduction operators, in optimal design of planar steel frames according to the AISC-LRFD specification. The design objective is to minimise the weight of frame subject to strength, displacement and constructability constraints. A number of new crossover and mutation operators, used alongside the standard operators are utilised in optimum design of a number of steel frames subjected to the constraints of the AISC-LRFD specification, with and without considering the second order effects, as set out by the code requirements. This modified GA (MGA) is shown to have a very fast convergence and to produce relatively high-quality designs. This paper also utilizes the concept of multiple-deme in the GA, as it has been used successfully for other metaheuristic population-based methods. The multiple-deme GA is used alongside the modified GA operators and the algorithm is named the modified multiple-deme GA (MMDGA). The modified GA (MGA) and modified multiple-deme GA (MMDGA) are applied to three benchmark problems and the results are compared to those obtained by other metaheuristic methods. In the majority of cases, the results of comparisons suggest the superiority of the MMDGA in terms of the quality of final design and the total number of performed finite elements analyses.

In this paper, structures transformable to regular forms are studied. Here, two cases are investigated. In the first case, the effect of different boundary conditions on these structures are explored, and in the second case the effect of adding or removing members and nodes are studied. In some structures the graph model is regular and different boundary conditions change the corresponding block matrices into non-regular ones. In some other structures the addition or removal of nodes and/or members changes the structure into a regular one. Here an efficient method is presented for dealing with the above-mentioned irregularities. The main idea steams from the fact that on the one hand there exist simple relationships for finding the inverse of some block matrices related to regular structures, and on the other hand we want to find out how to obtain the inverse of matrices corresponding to structures which become regular by the addition or removal of some members and/or nodes. One of the applications of the present method is related to the finite difference (FD) method for the analysis of plates with some irregularities in their boundary or having different support conditions.

Ground motions in the near field of a rupturing fault differ from ordinary groun motions, as they contain a large energy, or “directivity” pulse. This pulse can cause considerable damage during an earthquake. Failures of modern engineered structures observed within the nearfault region in the recent earthquakes have revealed the vulnerability of existing RC buildings against pulse-type ground motions. This may be due to the fact that these modern structures had been designed primarily using the design spectra of available standards which was developed using stochastic processes with relatively long duration that characterizes more distant ground motions. Many recently designed and constructed buildings may therefore require strengthening in order to perform well when subjected to near-fault ground motions. This paper presents the results of a study of the response of typical existing RC buildings to near-fault ground motions and the potential improvements achievable after FRP retrofitting of the buildings. Results show that in case of near-fault records, they impose higher demands in comparison to far-fault records, though the maximum drift is generally concentrated at the middle story levels. It is demonstrated that strengthening with FRP is very effective in reducing drift demands for structures for a wide range of natural periods. The rehabilitated buildings possess an elastic stiffness 1.4 times that of the original buildings and have a total shear force capacity, 1.5 times that of the original buildings. The cumulative energy dissipation for rehabilitated specimens is 2.3 times that of the original building, on average.

A major obstacle in using FRP sheets for flexural and shear strengthening has been debonding failure, which leads to premature and noneconomic failure of the strengthened element. Surface preparation of concrete member has been widely used to provide good bonding of the composite sheet onto concrete surface; however, it is found to be only partially effective in delaying debonding. Recently, Grooving Method (GM) has been introduced as a novel substitute for conventional surface preparation in Externally Bonded Reinforcement (EBR) strengthening of concrete beams. Although the previous experiments have shown that grooving method can postpone or even eliminate the debonding of FRP sheets from concrete substrate, the method is still in its early stages of development and awaits further research for maturation. The present study is an attempt to examine the effectiveness of grooving method for beams with different concrete compressive strength; also, to investigate the effects of groove depth and width on controlling FRP debonding for concrete specimens with different compression strengths. The experimental specimens included 44 concrete beams, which with three samples per beam resulted in a total of 132 specimens. Initially, two grooves of varying widths and depths were made in each specimen. The grooves were then filled with an appropriate epoxy resin and the FRP sheets were bonded onto the concrete surface. After the resin had hardened, the specimens were subjected to the four-point flexural test. The experimental results showed the superiority of grooving method on conventional surface preparation for all four experimental categories with different compressive strength of concrete. The results also showed that a depth of 10 mm may be the optimum groove depth for all four experimental categories.

This paper presents a procedure based on artificial neural networks that is capable of generating spatially varying accelerograms from a single response spectrum. Non-uniform ground motions may have influence on the seismic response of structures that have spatially extended foundations. Artificial spatially varying earthquakes will be required for more nonlinear analyses to consider this effect. Learning capabilities of two types of Artificial Neural Networks (ANNs) are used to develop a method for generating spatially varying accelerograms from response spectra and distance parameter. Recorded accelerograms from SMART-1 array in Loting, Taiwan were used to train neural network. Generated artificial spatially varying earthquakes can be used for time history analysis of large structures such as bridges and dams.

This paper focuses on examining the combined effects of frequency content and spatial variability of the ground motion on the response of a large embankment dam. A series of nonuniform ground motion time histories are generated using spectral representation method. The influence of frequency content of seismic excitation is taken into account by using three different types of target response spectra selected from Uniform Building Code (UBC 1994). It is found that the use of identical ground motions compatible with the Types 1 and 2 response spectra overestimates the acceleration responses by up to 15% and 30% higher than those of multi support excitations، respectively. The above trend is qualitatively valid for Type 3 input motions at lower elevations of the dam، but not at the crest of the dam where the uniform excitation yields slightly lower acceleration response. By comparing the displacement responses، it is demonstrated that the uniform input motions compatible with Types 1، 2 and 3 response spectra result in 10%، 35% and 40% larger crest settlements، respectively، than those predicted under multi-support excitations.

The present paper reveals the results of an analytical method to calculate the bearing capacity of a footing supported on one or two rows of piles stabilizing slope. The varied parameters here include pile diameter, pile length, location of pile rows, location of footing relative to the slope crest, foundation width, center to center spacing of piles in a row, and fixity of the pile head. Passive pile resistance is determined based on normal and shear resistance of the soil surrounding the pile considering plastic deformation of soil between piles. The Pile resistance obtained through the present method is compared with other analytical as well as 3D numerical ones. The results indicate acceptable agreement. The footing bearing capacity is calculated according to both the limit equilibrium slope stability analysis and soil stability beneath the footing. The predicted results were compared with those reported from other experiments and indicated an acceptable agreement with increasing pile spacing. The results indicate that stabilizing the earth slope with rows of piles has a significant effect on the footing of bearing capacity improvement.

Hydrodynamics of open channel confluences is very complicated. Simulation of rightangled channel confluences using a recently-developed 2D high-resolution model is presented in this paper. Finite-volume discretization of governing equations is adopted and fourth-order Runge- Kutta time integration is used. The effects of three different and most influential parameters, including discharge ratio, width ratio and downstream Froude number on hydrodynamics of rightangled confluences are studied. Standard Cartesian grids are employed and a single-block strategy is used for modeling the confluence. Turbulence shear stresses are included within the numerical model based on an eddy-viscosity approach. The results are compared with experimental data and show satisfactory agreement, particularly in cases when 3D features of flow are negligible. Despite having 3D characteristics in most cases, the results of this study show that a much simpler and computationally effective 2D numerical model is also capable of detecting most important features of flow in confluences, including separation zone and zone of high-velocity.

In this paper, by using depth-averaged equations, three different dispersion models for simulating flow at channel bends are presented and compared. Two of these models employ power law velocity distributions for longitudinal velocity components and linear distributions for transverse component, and the last model employs logarithmic velocity distributions in transverse and longitudinal directions. The first two models differ in how the effect of secondary flow is evaluated. Boundary-fitted curvilinear coordinates in conjunction with finite-volume method have been used for discretization of the governing equations. The numerical results are compared with available experimental data of a 270o bend. The study shows that a power law distribution for streamwise velocity with a suitable estimation of secondary flow intensity gives the best results for simulation of two-dimensional depth-averaged flow in open channel bends among the models studied. This model can successfully predict the most important characteristics of flow in curved channels.

Doroodzan dam is a 57m high earth dam with rip-rap cover constructed on Kor River in Bakhtegan Basin. The dam is located at 30° 12'' 2'' north latitude and 52°25'' 5'' east longitude ~70km north of Shiraz, Iran. The dam reservoir supports a large amount of agricultural, industrial, and urban demands in the region. The reservoir volume is 993 M.C.M at the normal pool level and the dam crest length is ~700m. In this paper, a 3-D finite element model of the dam was constructed and analyzed for steady and transient conditions. Transient pore water pressure fluctuations were predicted at different piezometer locations for a 21-day rapid drawdown of 23.9m. It was found that seepage through the dam is not sensitive to hydraulic conductivity of downstream dam body, apparently due to the effective hydraulic behavior of the chimney drainage there. Under rapid drown down conditions, a maximum of 11.8m excess pore water pressure on upstream part of the dam was observed (compared to the steady state conditions) while no significant excess pressure was seen at the downstream part of the dam. Dynamics of the phreatic line location during the 21-day rapid drawdown was monitored in four 5.25-day time steps. A gradual phreatic line change at time steps ending at the 21-day period was predicted. Phreatic line at the upstream face of the dam closely followed the reservoir level rapid drawdown. However, phreatic line at the interior sections of the dam did not drop as fast. As a result, a gradient towards upstream face of the dam was developed after ~10 days which might jeopardize slope stability there. It is recommended that the excess pore water pressure be carefully considered in dam analysis researches, especially during the transient periods. In general, rapid drawdown should be cautiously analyzed in dams, especially those with short emptying times, as it may reverse the seepage direction, endanger the slope stability, and not allow excess pore water pressure to dissipate in an acceptable manner.

Two-dimensional diffusive and advective-diffusive chloride transport through clay and silt, respectively, was investigated using laboratory models. The observed chloride concentration plumes in the soil samples as well as the chloride concentration-versus-time profiles in the source reservoirs were predicted using the computer code MIGRATEv9. Equivalent horizontal and vertical chloride diffusion coefficients reasonably predicted the observed concentration plumes and concentration-versus-time indicating that soils were homogeneous and isotropic. The predicted diffusion coefficients were in the range of the reported values for similar soils. The effect of downward Darcy flux in the advection-diffusion tests on the chloride concentration plumes was assessed by comparing the shapes of the plumes with those in the pure diffusion tests. The downward Darcy flux caused the concentration plumes in the advection-diffusion tests to migrate further in the vertical direction compared to that in the horizontal direction. All plumes spread symmetrically with respect to the vertical axis of symmetry of the silt samples, and the applied horizontal base flow had no significant effect on the symmetrical shape of the plumes in the advection-diffusion tests. Overall, the existing two-dimensional advective-diffusive-dispersive contaminant transport theory as implemented in the computer code MIGRATEv9 reasonably predicted the chloride migration through silt and clay.

In this study, an artificial neural network (ANN) was applied to predict the performance of two rotating biological contactor (RBC) systems in removal of hydroquinone (a toxic aromatic compound). The first system was a two-staged conventional RBC and the second one was a onestaged packed-cage RBC with bee-cell 2000 biofilm carriers. Both systems had a total area of about 2 m2 for biofilm attachment. The main aim is to predict COD removal efficiencies in both systems using ANN. Efficiency evaluation of the reactors was obtained at different influent COD from 200 to 5000 mg/L. Exploratory data analysis was used to detect relationships between the data and the evaluated dependents. The appropriate architecture of the neural network models was determined using several steps of training and testing the models. The modeling results showed that there is a good agreement between the experimental data and the predicted values with a correlation coefficient (R2) of 0.998 and 0.997 for RBC with rotating disks and packed-cage RBC, respectively.

In this paper, the behavior of a fault is studied using a constant strain joint model. A combination of slider and spring is used to simulate the shear behavior of faults in the plastic region in contrast to the previous models that have only used an elastic shear spring. Furthermore, the proposed joint element was used to study the behavior of a fault crossing a tunnel regarding the represented shear plastic and dilation behavior of this joint element, and for this purpose a Matlab based program called FEAFB (Finite element analysis of fault behavior) has been developed. The corresponding normal and shear stresses, shear strengths and the factors of safety, for different horizontal to vertical stress ratios and shear stiffness are analyzed and compared to the results of a similar modeling in UDEC program. The analysis indicates that the normal and shear stresses, and the shear strength are increased in the fault elements near the tunnel, and they are decreased in the elements becoming far from the tunnel surface. However, the safety factor can either increase or decrease as it becomes closer to the tunnel surface, depending on the horizontal to vertical stress ratio. Moreover, it is also shown that safety factor depends upon the shear stiffness, i.e., as shear stiffness increases, shear stress increases, and as a result, the safety factor decreases.

The main objective of this research is to develop a practical damage criterion based on pushover analysis. For this purpose, damage analysis is performed on several Reinforced Concrete Moment Resisting Frames (RCMRFs). In the static method, performance point of structures is firstly determined using capacity spectrum method and then values of several different damage indices are calculated at these points. By comparing the results of two methods and evaluating correlation between two sets, explicit damage relations are derived based on the static results.