Iranian Journal of Science and Technology Transactions of Civil Engineering
Volume:39 Issue: 1, 2015

The present approach is a combination of the force method and displacement approach to achieve the analysis using the substructuring technique. In this method, the inverse ofthe stiffness matrices of the substructures are constructed for the formation of the flexibility matrices. This part of the solution is equivalent to the stiffness approach. In the subsequent stage, the results of the analysis are assembled using the singular value decomposition (SVD) and the solution for the entire structure is obtained. In fact, for assembling the structure, we need the flexibility matrices of the substructures which are obtained by the stiffness method. In this paper, a mixed force-displacement method is applied to finite element models for increasing the speed of their solution. Each substructure is analyzed independently by singular value decomposition of the corresponding equilibrium matrix. Methods are then utilized for transforming the substructures into regular forms whenever it is possible. The application of this method in finite element models with different substructures improves the process of analysis, and makes the use of the existing solution techniques possible for regular systems.

In this paper, optimum parameters of Tuned Mass Dampers (TMD) are determined to minimize the dynamic response of multi-story building systems under seismic excitations. Charged System Search (CSS), as an efficient optimization algorithm, is revised and applied for tuning passive mass dampers. A MATLAB program is developed for numerical optimization and time domain simulation. Optimization criteria are the peak values of the first story displacement with and without TMD, and the transfer function from input ground acceleration to the first story acceleration response. An alternative formulation is also presented for solving state space equations. Compared to other population-based meta-heuristics, the charged system search has a number of advantages distinguishing this algorithm from the others. However, for improving exploitation (the fine search around a local optimum), it is hybridized with HS that utilizes charged memory (CM) to speed up its convergence. To ensure good performance of this approach, some numerical considerations are conducted to verify the effectiveness and feasibility of the presented approach.

Deficiencies of RC structures can be overcome by strengthening/retrofitting using different strengthening methodologies. This paper emphasises the effectiveness of externally applied U-shaped CFRP anchorages/strips on the performance of RC beams of relatively low compressive strengths (approximately 21 MPa). Three types of the beams were cast based on the shear reinforcement detailed as containing “no shear reinforcement”, “minimum shear reinforcement” and “adequate shear reinforcements” as suggested in ACI 318-08. All beams were provided adequate flexural reinforcement as recommended by ACI 318 to fail the beams in flexure. U-shaped CFRP anchorages and strips were bonded to the beams in the predominant shear and flexural loading regions and tested under four-point bending condition by varying shear spanto- depth ratio (a/d) as 2.46 and 3.38. Different strengthening schemes (including CFRP anchorage alone and in combination with CFRP strips) as well as the effect of U-shaped CFRP anchorages applied over full and partial applied beam depth was also the parameter of investigation in the current study. Results showed that externally bonded U-shaped anchorages applied along the beam span and at the ends together with CFRP strips improved the deformability, strength and performance of RC beams by transforming failure manner from brittle to ductile. Moreover, use of partial depth anchorage is beneficial to attain higher load in comparison to the full depth anchorages, particularly anchorage height equal to 3/4 of the beam depth is found to be most suitable.

Shear capacity of concrete (Vc) in reinforced concrete members depends on a number of influencing parameters including compressive strength of concrete (ƒ'c), ratio of tension reinforcement (p), shear span to depth ratio (a/d), size effect or depth factor (ξ), size of the aggregate in relation to the minimum size of the member (aggregate interlock aspects). Over the last several decades, researchers have tested reinforced concrete beams (without web reinforcement) to study these parameters over a range limited by the breadth and depth of their experimental investigations and, on the basis of their experimental results, proposed empirical equations for predicting the shear capacity of concrete in reinforced concrete beams. In this paper a relational database using ACCESS software is developed. The database contains experimental results of 2145 shear critical reinforced concrete beams without web reinforcement. Using the ACCESS shear database developed in this study, an evaluation was conducted to assess the predictive accuracy of shear design equation of Euro Code EC2. The results indicate that the Euro Code EC2 design equations are found to be adequately conservative to predict the shear capacity of reinforced concrete beams over the range of variables considered in this study.

This study presents optimal distributions of steel materials in steel thin plate structures determined by using a classical element-wise and the present node-wise topology optimization design methods for a dynamic problem. More specifically, the present article describes an application of a node-wise topology optimization technique to the problem of maximizing fundamental frequency for plane structure. The terms element-and node-wise indicate the use of element and node densities, respectively, as design parameters on a given design space. For a dynamic free vibration problem, the objective function in general is to achieve maximum eigenfrequency with first-order eigenmode subject to a given limited material, since structures with a high fundamental frequency have a tendency to be reasonably stiff. For both static and dynamic problems SIMP (Solid Isotropic Microstructure with Penalization for Intermediate Density) material artificially penalizing the relation between density and stiffness is used in this study, and an implemented optimization technique is the method of moving asymptotes usually used for topology optimization. Numerical applications topologically maximizing the first order eigenfrequency and depending on element or node densities as design parameters and varied boundary conditions to verify the present optimization design method provide appropriate manufacturing information for optimally form-finding of steel materials with Poisson’s ratio of 0.3 into thin plates.

There are many reasons for rehabilitation of existing buildings. Adding stories is one of the most common reasons. When a steel building is retrofitted by concrete jacketing for adding stories, this system contains several structural systems. These systems are composite concrete and steel systems in initial stories, welded steel system in middle stories and cold-formed steel frames in upper stories. Dynamic analysis of hybrid structures is usually a complex procedure due to various dynamic characteristics of each part, i.e. stiffness, mass and especially damping. Availability of different damping factors causes a higher degree of complication for evaluating seismic responses of hybrid systems. Due to using several structural systems, an existing building is changed to hybrid system. Damping matrix of these structures is non-classical. Also, the nonlinear software is not able to analyze these structures precisely. In this study, a method and graphs have been proposed to determine the equivalent modal damping ratios for rehabilitated existing steel buildings for adding stories.

This paper investigated the shear, impact and fracture strengths of high-strength concrete reinforced with two different industrial waste fibres. Locally available steel lathe waste and nylon waste were used at different volume fractions as fibre cocktails in concrete. Steel lathe wastes were used as-received lengths and nylon fibres were chopped into 40 mm lengths in this investigation. In total, 12 hybrid mixes were casted and tested at four different volume fractions (0.5%, 1.0%, 1.5% and 2.0%). The experimental programme was used the slump test and the air content test on the fresh concrete. The hardened concrete was tested for its shear and impact strength. A flexural test on notched beams under three-point bending was also carried out according to the RILEM 50-FMC committee recommendations. Load vs. mid-span deflection and load vs. crack mouth opening displacement were obtained and the fracture energy was evaluated.The best performance was obtained in hybrid which was enhanced due to the hybrid nature of the fibre cocktails of all the mixes, 2% volume fraction with a combination of steel ½ + nylon ½ fibres gives the best performance. The steel lathe waste fibres mainly contributed to limiting the crack initiation and lightweight non-metallic nylon fibres restricted the crack propagation. The combined advantages of these fibres provide high mechanical and fracture strength. Hence this hybrid fibre reinforced concrete with industrial waste fibres is doubly advantageous as it provides a superior performance without increasing the cost of the concrete.

A new, FRP flange-bonded scheme, with practical application to 3D RC frames and with the aim of relocating plastic hinges away from the joints is presented and its performance is compared with that of the web-bonded scheme. For this purpose, nonlinear pushover analyses of detailed Finite Element (FE) models of retrofitted joints of a benchmark RC frame are carried out. The optimal thicknesses of the Fibre Reinforced Polymer (FRP) sheets for relocating the plastic hinges are first determined. The moment-rotation curves of the joints are then utilised to create a representing model for the RC frame. Further, nonlinear pushover analyses are carried out on the retrofitted and the original frame to evaluate their capacity curves and seismic performance parameters such as ductility, behaviour factor and performance points in relation to a specified demand earthquake. The results are then compared with those of the same frame when retrofitted with the web-bonded scheme as well as with steel bracing. Results point to the marked superiority of the flange-bonded scheme compared to the web-bonded scheme in different aspects including, capacity, ductility and the performance level and cost. The performance of the FRP flange-bonded scheme also compares well with that of the steel bracing method.

In this paper, three different soil constitutive models for granular soils were implemented in the numerical simulation of a full-scale reinforced soil segmental wall in order to predict the wall response during construction. The soil constitutive models in the order of complexity are: linear elastic-perfectly plastic Mohr-Coulomb, Duncan-Chang hyperbolic, and a nonlinear elastic-plastic hardening model. The latter, which can be regarded as a modified version of the Mohr-Coulomb model, captures the nonlinear stress-dependent soil response. The nonlinear model can consider soil dilative behavior. In this regard, it keeps the simplicity in the formulation together with the accuracy in the prediction of soil response. By comparing the results, in general, there is good and acceptable accordance between numerical simulations and field measurements. It is seen that using a simple soil model can acceptably predict the performance of reinforced walls. However, the disadvantage relates to poorness in the prediction of wall facing displacement, which is sensitive to proper consideration of deformation parameters in a soil model. The accuracy of the prediction can be augmented by adopting reasonable functions for elastic (stiffness) and plastic (dilatancy) parameters with respect to the stress condition within the soil backfill.

Soil–Water Characteristic Curve (SWCC) is one of the most important parts of any model that describes unsaturated soil behavior as it explains the variation of soil suction with changes in water content. In this research, Gene Expression Programming (GEP) is employed as an artificial intelligence method for modelling of this curve. The principal advantage of the GEP approach is its ability to generate powerful predictive equations without any prior assumption on the possible form of the functional relationship. GEP can operate on large quantities of data in order to capture nonlinear and complex relationships between variables of the system. The selected inputs for modelling are the initial void ratio, initial gravimetric water content, logarithm of suction normalized with respect to atmospheric air pressure, clay content, and silt content. The model output is the gravimetric water content corresponding to the assigned input suction. Sensitivity and parametric analyses are conducted to verify the results. It is also shown that clay content is the most influential parameter in the soil–water characteristic curve. The results illustrate that the advantages of the proposed approach are highlighted.

The bearing capacity of foundations resting on slopes is commonly calculated using empirical equations. In recent years, it has been demonstrated that geosynthetic reinforced soil can enhance the foundation bearing capacity. In this paper, an analytical method for determination of the ultimate bearing capacity of surface strip footing on a sand slope reinforced with geogrid layers is presented. The angle of the slope with the horizontal direction is varied within 20o to 40o. In this study, the Coulomb-type lateral earth pressure theory has been used to compute the foundation bearing capacity. It is assumed that geogrid layers act such that the active earth pressures are reduced. The results obtained from the proposed method are compared with experimental and numerical results. Parametric studies have also been performed to show the effects of contributing parameters such as number of geogrid layers, locations of reinforcement layers, soil properties and the slope angle on the bearing capacity of strip foundations resting on the reinforced sand. The results indicate that the magnitude of bearing capacity of strip footings on the sand slope can be significantly increased by using geogrid layers.

To study the scale effect on the density of rockfill materials, the relative density tests were carried out by physical tests and numerical tests. Fractal theory was drawn into the grading of rockfill materials. Then, the fractal properties of scale effect on the density were studied by physical tests and numerical tests. There are close relations between fractal dimension D and densities of rockfill materials. The densities are largest when D is the critical value Dc. Further, Dc is independent of the relative density Dr and the maximum diameter dmax. Truncation error is one of the main factors of scale effect of densities of rockfill materials. The existing four scale methods in the standard can all be explained with fractal theory, and a unified formula was suggested. The achievements in the paper lay a good foundation for further studying scale effect of rockfill materials with fractal theory.

This study focused on the process of bacterial calcium carbonate (CaCO3) precipitation (BCCP) in organic soil. Two samples, organic soil and sand, in glass boxes having dimensions 6×6×2 cm were immersed in bacterial medium (Bacillus pasteurii, urea, and CaCl2) for 4 days. During the treatment period, the samples were treated with urea medium and CaCl2 every 6 h. Changes in pH values were monitored at different time intervals. At the end of the treatment period, the amount of CaCO3 was determined with a calcimeter test. The test results showed that the pH values fluctuated between 9 and 9.4 during the treatment period. This range of pH values indicates that the treatment medium is appropriate for BCCP. The amount of precipitated CaCO3 in the organic soil sample increased about 8% compared with the untreated sample. Calcium carbonate precipitation in sand is found to be higher than the organic soil. The results were supported by Scanning Electron Microscopy (SEM) analysis and Energy-Dispersive X-ray (EDX) analysis.

For dynamic analysis of structure, the calculation of eigenvalues and eigenvectors is necessary. When the structural models are symmetric, such calculation can be simplified using some of the concepts of graph theory. In this paper, two methods are presented for eigensolution of space Truss. The first method uses a graph model and employs a decomposition and healing process for factorization of the graph model and calculating the eigenvalues of graph. The second approach uses the canonical forms for the construction of submatrices, from which, the eigenvalues can be obtained. Both methods lead to identical results.