فهرست مطالب

Rehabilitation in Civil Engineering - Volume:8 Issue: 4, 2020
  • Volume:8 Issue: 4, 2020
  • تاریخ انتشار: 1399/06/24
  • تعداد عناوین: 12
  • Atiye Farahani *, Hosein Taghaddos Pages 1-14
    Chloride-induced corrosion is a key factor in the premature corrosion of concrete structures exposed to a marine environment. Fick's second law of diffusion is the dominant equation to model diffusion of chloride ions. This equation is traditionally solved by Finite Element Method (FEM) and Finite Difference Method (FDM). Although these methods are robust and efficient, they may face some numerical issues due to discretization process. This study solves the Fick's equation using the Element-Free Galerkin (EFG) method as well as traditional FEM and FDM. The results of these numerical methods are compared together, and validated with the analytical solution in special cases. The results show that the EFG method predicts the service life of the concrete structures, more accurately than the other methods, and exhibits the lowest displacement error and energy error for a constant diffusion coefficient problem. FDM can be performed very efficiently for simple models, and the displacement errors produced by this method do not differ considerably from the EFG results. Therefore, FDM could compete with the EFG method in simple geometries. FEM can be used with a sufficient number of elements while the convergence of the results should be controlled. However, in complicated models, FEM and especially the EFG method are much more flexible than FDM.
    Keywords: Concrete, Diffusion, Element-Free Galerkin (EFG), Finite element method (FEM), Finite Difference Method (FDM)
  • Yasser Sharifi *, Mahmoud Hosainpoor Pages 15-27
    Artificial neural networks (ANNs) as a powerful approach have been widely utilized to demonstrate some of the engineering problems. A three-layer ANN including three neurons in the hidden layer is considered to produce a verified pattern for assessing the compressive strength of concrete incorporating metakaolin (MK). For this purpose, an extensive database including 469 experimental specimens was obtained from the literature. Next, novel equations utilizing the developed ANN approach were developed to measure the compressive strength of concrete mixtures incorporating MK. To examine the model accuracy a comparison between the proposed formulas based ANN and an empirical formula based nonlinear least-squares regression (NLSR) was carried out. The results show that the proposed formula based on the ANN yields a higher correlation coefficient and fewer errors compared to the NLSR method. An analysis based weights incorporating was utilized to show the significance of input variables. Accordingly, the ratio of fine aggregate to coarse aggregate exerts a dominant influence on the compressive strength of the concretes containing MK.
    Keywords: Artificial Neural Network, Compressive Strength of Concrete, Metakaolin, Garson’s algorithm, Nonlinear least squares regression
  • Abuzar Hamzenezhadi, Mohammad Sharbatdar * Pages 28-46
    The application of external post-tensioned steel bars as an effective way to strengthen an existing bridge has been so far used in many different countries. In recent decades, however, they have been replaced by bars made from Carbon Fiber Reinforced Polymer (CFRP), as a material with high tensile strength and corrosion resistance, to address several concerns with steel bars such as their application costs and difficulties, and also their durability.  Post-tensioning these sheets can be a new efficient method in strengthening the beams and utilizing the high strength of these material.This study has focused on the flexural behavior of beams reinforced by Post-tensioned non-bonded CFRP sheets. 15 beams were categorized in 3 groups of 5m-, 10m-, and 15m-span in order to evaluate the effect of some parameters such as level of post-tensioning, sheet length, and beam span on its load capacity, failure mode, ductility, and cracks behavior. The results indicate that even though the increase in post-tensioning levels improves the effectiveness of the method, but this capacity improvement is much more for small span beams especially when CFRP sheets are 90% of the beam span, compared to long span beams. There has been a noticeable capacity increase around 50% in the beams when decreasing the sheet length from 90% to 45% of the beam span and also causing 11-14% increase in ductility in various conditions.
    Keywords: carbon composite, Finite Element methods, post-tensioning, Unbounded, External reinforcing
  • Shahin Lale Arefi, Amin Gholizad *, Seyed Mohammad Seyedpoor Pages 47-60
    The subject of structural health monitoring and damage identification of structures at the earliest possible stage has been a noteworthy topic for researchers in the last years. Modal strain energy (MSE) based index is one of the efficient methods which are commonly used for detecting damage in structures. It is also more effective and economical to employ some methods for reducing the degrees of freedom in large-scale structures having a large number of degrees of freedom. The purpose of this study is to identify structural damage via an index based on MSE and reconstructed mode shapes. The Guyan reduction method (GRM) is utilized here to reconstruct the mode shapes. Therefore, in the first step by employing GRM, mode shapes in slave degrees of freedom are estimated by those of master degrees of freedom. In the second step, the modal strain energy based index (MSEBI) is used to find the location of damaged elements. In order to assess the efficiency of the method, two standard examples are considered. Damage is identified with considering complete mode shapes and reconstructed mode shapes, and the results are compared together. The outcomes show that the combination of MSE and GRM can be useful for the structural damage detection, when considering the noise.
    Keywords: Damage Detection, Mode shapes, Modal strain energy, Guyan method, Structures
  • Afshin Hossein Sharifzadeh, Saeed Tariverdilo * Pages 61-72
    Rebar fracture in boundary elements of lightly reinforced shear walls in recent earthquakes motivated research on the minimum longitudinal reinforcement of shear walls. These researches lead to change in the ACI 318-19 requirement for minimum longitudinal reinforcement of boundary elements. New ACI 318 requirement increases minimum longitudinal reinforcement ratio for boundary elements of shear walls with low demand, that could have economic burden. This study experimentally investigates is it possible to avoid this increase in minimum rebar by debonding rebars in critical region of boundary elements in lightly reinforced shear walls. Tests includes specimens with bonded and debonded rebars, which are tested under monotonic and cyclic loading. Load protocol to account for failure types of low reinforcement shear walls is asymmetric.  Test results show that out of plane buckling of specimens with debonded rebars initiates at lower axial strains that could be attributed to reduction in element lateral stiffness due to use of debonding. On the other hand debonding resulted in reduction of local strain demand on rebar. It could be concluded that larger minimum dimension for boundary elements will be required when debonding is employed.
    Keywords: Boundary element, Shear wall, Bonded rebar, Debonded rebar, Rebar fracture
  • Ahmad Dalvand *, Ebrahim Sharififard, Fereydoon Omidinasab Pages 73-89
    Cementitious composites are one of the most consumed construction materials in the world. The use of cementitious composites is increasing due to their special characteristics. The behavior of high strength cementitious composites is improved by increasing the fiber percentage. In the present paper, the effects of steel microfibers and polypropylene fibers on mechanical properties and impact resistance of high strength cementitious composites are investigated. The percentage of fibers used in the study was 0, 0.5, and 1.5% in seven separate and three combined mix designs. Experiments were carried out on 120 specimens in 10 mix designs. Compressive strength, tensile strength, flexural strength, and dynamic impact tests were carried out on 10 mix designs manufactured in this research. The dynamic impact strength of the disc specimen was investigated by a drop hammer test machine with a capacity of 7500J. After testing the samples, it was shown that using a high percentage of steel and polypropylene fibers reduces the compressive strength and increases tensile strength, flexural strength, and impact strength. The effects of steel microfibers on the reduction of the crush displacement resulting from the dynamic impact were higher than that of polypropylene fibers.
    Keywords: Cementitious Composites, Mechanical properties, Impact Resistance, High Strength
  • Mohammad Malekshahi, AmirHoshang Ahakhaveissy * Pages 90-105

    Given the sophisticated nature of the blast phenomenon in relation to structures, it is of significance to accurately investigate the structure behavior under blast loads. Due to its rapid and transient nature, blast loading is one of the most important dynamic loadings on the structures. Since masonry materials are widely used as the partition and bearing walls in the existing and newly-built structures, the current research aims to investigate the buried blast effects on unreinforced masonry structures. In order to apply the blast load on a crater as time history, it is required to determine the maximum free field pressure caused by the blast. Accordingly, Finite Element Model Updating (FEMU) was used to calculate the maximum free-field pressure. Thus, for a non-linear dynamic analysis of a blast-loaded structure, a code written in FORTRAN was used. Mohr-Coulomb yield surface with tensile and compression cap and classic Mohr-Coulomb yield surface were used for the structure and the soil modeling, respectively. The comparison of the numerical analysis results in FEMU to field data shows a good consistency between the numerical results and the field data.

    Keywords: finite element model updating, close-in explosion, underground explosion, Unreinforced masonry structure
  • Farid Mahmoudi, Payam Tehrani * Pages 106-117
    Moment resisting frames (MRF) as one of the conventional lateral load resisting systems in buildings suffer from some limitations including code limitations on minimum span-to-depth ratio to warrant the formation of plastic hinges with adequate length at the ends of the beam. According to seismic codes, in ordinary steel MRFs the span-to-depth ratios should be larger than 5 and in special steel MRFs this ratio should not be less than 7, which is typically difficult to achieve in some cases. For instance, framed-tube structures typically have MRFs with span-to-depth ratios less than the above mentioned ranges. Therefore, existing buildings with small span-to-depth ratios may exhibit poor seismic performance when subjected to seismic excitation. In this paper, a method is presented to rehabilitate such MRFs. Although the idea of using shear link for design of new buildings has been investigated in recent years, this idea can also be used to rehabilitate existing MRFs. Moreover, the novelty of this proposed rehabilitation method in this paper is that it can be used for damaged MRFs after earthquakes to enhance their remaining strength and ductility capacity. While most of the available rehabilitation methods focus on improving the system strength and stiffness, the proposed rehabilitation in this paper is based on the weakening of the beam mid-span that causes the formation of the shear plastic hinge in middle of the beam instead of the two beam ends. Numerical evaluation is conducted to show the efficacy of this method, and the results show that the use of the proposed rehabilitation method considerably increases the ductility capacity of the system during subsequent earthquakes.
    Keywords: Rehabilitation, Steel moment resisting frame (MRF), Shear link, Short span frame
  • Erfan Naderi, Adel Asakereh *, Masoud Dehghani Pages 118-136
    There can be many reasons for engineers to place the footings near a slope such as leakage of suitable sites or architectural considerations. One of the approaches to increase the amount of bearing capacity, especially in soft soils, is adding stone columns to the soil. In this research, the behavior of a strip footing placed near a stone column reinforced clayey slope was investigated. For this purpose, some small-scale model tests were performed on a clayey slope reinforced with stone columns. The effects of the length of the stone column and the length of encasement on the footing were studied. Additionally, vertical encased stone columns in a group arrangement were investigated. Some numerical analyses were also performed using the Midas GTS NX finite element software, and the factor of safety was studied. Results show that the optimum length was equal to four times the diameter of stone columns. It was observed that by increasing the length of encasement, the bearing capacity of strip footing was also increased. The safety factor of slope showed an increase when stone columns were added to the slope, but the maximum influence on the factor of safety appeared when the stone column was in the upper middle of the slope.
    Keywords: Bearing capacity, Stone column, Strip footing, Safety factor, slope
  • Behnam Behzadfar, Ahmad Maleki *, MohammadAli Lotfollahi Yaghin Pages 137-155

    Spacious experimental and numerical investigation has been conducted by researchers to increase the ductility and energy dissipation of concentrically braced frames. One of the most widely used strategies for increasing ductility and energy dissiption, is the use of energy-absorbing systems. In this regard, the cyclic behavior of a chevron bracing frame system equipped with multi-pipe dampers (CBF-MPD) was investigated through finite element method. The purpose of this study was to evaluate and improve the behavior of the chevron brace frame using multi-pipe dampers. Three-dimensional models of the chevron brace frame were developed via nonlinear finite element method using ABAQUS software. Finite element models included the chevron brace frame and the chevron brace frame equipped with multi-pipe dampers. The chevron brace frame model was selected as the base model for comparing and evaluating the effects of multi-tube dampers. Finite element models were then analyzed under cyclic loading and nonlinear static methods. Validation of the results of the finite element method was performed against the test results. In parametric studies, the influence of the diameter parameter to the thickness (D/t) ratio of the pipe dampers was investigated. The results indicated that the shear capacity of the pipe damper has a significant influence on determining the bracing behavior. Also, the results show that the corresponding displacement with the maximum force in the CBF-MPD compared to the CBF, increased by an average of 2.72 equal. Also, the proper choice for the dimensions of the pipe dampers increased the ductility and energy absorption of the chevron brace frame.

    Keywords: Chevron brace frames (CBF), Multi-pipe dampers (MPD), Nonlinear finite element method, energy absorption, Cyclic behavior
  • Mahsa Karimian Sichani *, Abolghassem Keramati, Farshad Behzadinia Pages 156-172
    The potential of buckling in compressive members has been considered as a disadvantage when using steel members in the construction industry. In spite of the progress made in this regard, buckling is still considered as a challenge in the analysis and design of compressive steel structural members. Such a challenging phenomenon can be controlled by strengthening of compressive members. Stiffened compressive members can control the weakness of steel members in the global buckling. In this paper, elastic buckling behavior of three-segment symmetric steel members with pinned ends is investigated. The differential stability equation for non-prismatic three-segment members is solved numerically. Critical load parameter for stiffened members is calculated considering different stiffened length and moment of inertia ratios. Based on a wide range of the calculated data, the buckling load could be accounted as a safe measure to be used in the design formulas. Evaluation of the effects of various parameters on the buckling load shows that the desired buckling load value can be achieved by a partially stiffened member. By constant increase of a member’s weight, the shorter the length of the variation in the cross-section, the higher moment of inertia is essential in the stiffened segment; and the maximum critical load parameter is achieved by a stiffened length ratio between 0.4 and 0.6.
    Keywords: Stiffener length, critical load, Moment of inertia, Variable cross section, Buckling
  • Aziz Hosseinnezhad, Amin Gholizad * Pages 173-196
    Long-span bridges, as vital structures, play an important role in economic development. Previous studies revealed that the seismic responses of such structures, under non-uniform excitations, are different from the same result due to uniform excitations. Furthermore, the results of several earthquake-damaged bridges showed that their seismic behavior was different from that predicted under uniform excitation and, in some cases; the responses were more than predicted results. Therefore, the damaged bridges under non-uniform excitations were re-analyzed and the obtained results were in good agreement with the recorded outcomes. Considering current bridge designing codes it is clear that almost all of them ignored it and just the Euro Code 2008 prepared some recommendations. It is found that the main reason for the differences in results from the uniform and non-uniform excitations is the spatial variation of earthquake ground motions. Based on the papers three phenomena were introduced for spatial variability of ground motion: the wave-passage, the incoherence, and also the site-response effects. The responses of structures under non-uniform excitations obtained from the superposition of dynamic and pseudo-static components. This paper investigated the seismic behavior of a long-span structure under non-uniform movements to evaluate the most undesirable conditions. So, different soils and load combinations considered and soil-structure effects included. The effect of wave-passage, incoherence, and site-response on the structure was measured and the results were compared with the uniform excitation. The results indicate that the variation in soil condition significantly affects the seismic responses under non-uniform excitations. Also, it is found that the results from uniform excitations with considering soil-structure interactions are remarkably increased. Moreover, the outcomes of analysis under-considered load cases and soil conditions showed that ignoring the spatially varying ground motions may lead to a non-conservative design.
    Keywords: Long-Span Bridges, SVGM, Pseudo-Static Response, SSI