نشریه مهندسی سازه و ساخت
سال هفتم شماره 1 (پیاپی 30، بهار 1399)

Optimum implementation of nonlinear dynamic analysis (NDA) in progressive collapse is one of the most important issues that has not been addressed recently. Using detailed FE software like ABAQUS and ANSYS in progressive collapse analysis despite their accuracy is not appropriate because of consuming a lot of time. For this reason, many of researchers utilize the macro software such as OpenSees. In this study, authors tried to introduce an optimum and appropriate method for element removal in open program, OpenSees. Since nonlinear dynamic analysis shows the exact behavior of structure, this algorithm is developed for NDA. For this purpose, the developed algorithm has been compared with the other methods and it has been found that it has a high accuracy. Developed algorithm can remove one or more columns at the same or different time intervals and the effects of sequence column removals to their time durations can be seen. The application of the developed algorithm for nonlinear incremental dynamic analysis (NIDA) has been implemented and its application for an ultimate capacity of the structure has been shown. Analysis results showed that using of developed algorithm will lead to the speed and accuracy improvement of calculations in the sequence of columns removal and ultimate capacity of the structure using NIDA. At the end of this research, the application of this method has been shown for other element removal such as beams and infill walls.

ABSTRACTMost of the steel moment resisting frames, damage at the beam-to-column connections during seismic loads. Application of passive dampers in order to prevent damages to the main components of structure has been an interesting subject for many researchers recently. In this present study, performance of the metallic yield damper is slit steel damper (SSD) on the steel beam-to-column connection is carried out using finite element model. Thus, first this damper and its connection is simulated. After validation of results, the effect of various dimensional parameters including radius slot, of damper on the performance of damper and its connection was investigated. Then various forms of this damper simulated and their effect on the performance of damper and the steel beam-to-column connection is studied. Moment- rotation, Hysteresis curves and plastic strains for each size of radius slot and different shapes extracted from the analysis were compared. Furthermore base shear force for steel frames with and without slit dampers in term of energy absorption and deformation are compared together. Findings indicated seismic behavior fitted to such dampers. The results indicate that the slit damper shows a proper function in absorbing energy. Recommendation for appropriate size and shape of slit steel dampers proposed.

Selection, design and control of materials are very important for achievement of a compatible, durable and economical repair overlay. Suitable bonding of repair overlay to substrate and good resistance against fracture are basics for a durable overlay. Using Polymer Concretes (PC) and Polymer Modified Concretes (PMC) which present great performance and durability can be considered as a method of restoration in damaged structures. In this research, mechanical properties and strengths of bonding to substrate concrete of Polymer Concretes and Polymer Modified Concretes as repair overlays are investigated and compared. Strength of Bonding to substrate concrete was obtained by Pull-Off test method. Bonding strength to substrate concrete in Polymer Concretes mix designs samples was weaker than Polymer Modified Concretes mix designs samples in similar conditions. In comparison with conventional concrete repair overlays in Pull-Off test, the maximum increment of bonding strength to substrate concrete was observed in mix design containing %50 of water replacement with SBR-based polymer which was %30 of increment and after that in mix design containing %50 of water replacement with Acrylic-based polymer which was %28 of increment. In Polymer Modified Concretes, %5 replacement of cement with Silica Fume decreased the amount of shrinkage but in higher values of replacement the amounts of shrinkage were increased in samples.

The serviceability and safety of building and bridge structures may gradually degrade due to long term use, natural environmental deterioration, poor initial design and/or construction, increased design loads, extreme events such as earthquakes, and/or lack of maintenance. It is needed to use materials that are not susceptible to the same causes that trigged the deterioration in the first instance. Since the rebuilding of old infrastructure is very expensive and time consuming, cost-efficient and durable techniques of strengthening/rehabilitating are needed. There is a growing interest in the use of Fiber Reinforced Polymer (FRP) composite materials for the strengthening and retrofitting of concrete beams. Also it is too important to know the influence of effective parameters on the fractural behavior and strengthening of cracked concrete beams, so the present study is dedicated to numerical analysis of carbon fiber reinforced polymer (CFRP) sheet-strengthened notched concrete beams. The bearing capacity, CFRP debonding and crack mouth opening displacement (CMOD) of notched beams were evaluated with nonlinear finite element method. Numerical results have been compared with experimental results of other researches. After validation the numerical modeling of CFRP plated notched beams, parametric studies reveal quantitatively the effects of various factors such as; thickness of CFRP sheet, compressive strength of concrete, initial crack length and interfacial bond strength on the CMOD curve of mentioned beam, which is found to be characterized by two peak loads. The first and second peak loads increase with the increase in concrete strength, the thickness of CFRP sheet and the interfacial bond strength. It is also found that increasing the initial crack length decreases the first peak load but no exerts on the second peak load.

Concrete has a brittle behavior as one of the most frequently used construction materials. Adding fibers to concrete can enhance its ductility and some of its mechanical properties. For this purpose, a laboratory study was conducted in order to present an experimental model to investigate the effect of size of micro-synthetic-fiber-reinforced concrete on fracture energy changes. In this study, concrete beam specimens were manufactured and evaluated with three different thicknesses and widths. Results indicated that by increasing the thickness of the specimens, fracture energy was respectively increased by 15.3-63.1%. Moreover, by increasing the width of the specimen, fracture energy was respectively increased by 42.04-50.72 %.Fiber-reinforced concrete is a type of concrete that is mixed with fiber. Various types of fibers are used to produce fiber-reinforced concrete, which include glass, polymer, carbon and steel .In the present research, macro-synthetic polymer fibers were used. Some of the consequences of applying macro-synthetic fibers in concrete include reduced shrinkage of fresh and hardened concrete, increased ductility, vulnerability and hardness of concrete, increased strength against fatigue stresses, increased durability and lifetime of concrete, improved concrete mechanical properties (tensile strength, flexural strength, etc.), control of secondary/thermal cracks of concrete, preventing the in-depth propagation of cracks, post-cracking chargeability and reduced permeability against chloride and sulfate ions . To date, numerous studies have been conducted on fiber-reinforced concrete, most of which have been focused on the evaluation of fiber-reinforced concrete using steel and plastic fibers or their combination.

Fiber reinforced polymers (FRP) are the safe alternatives to steel bars in reinforced concrete members in a humid environment. They have a very high durability against corrosion, especially in the extreme weather conditions. As a result of the low modulus of elasticity of these bars compared to the steel bars, the deflection and crack width are high in the beams reinforced with these bars. In this paper, the flexural behavior and cracking of concrete beams reinforced with GFRP bars are investigated. Twelve reinforced concrete beams with the cross-section of 250 × 250 mm and length of 2200 mm were manufactured and tested in a four-point bending setup. Eight beams were made with low reinforcement ratio, 4 of which were made of high-strength concrete and 4 were made of normal-strength concrete. In addition, 4 other beams were made with high reinforcement ratio made of normal-strength concrete. The parameters studied in this research are concrete strength, tensile bars arrangement and reinforcement ratio. The results show that the change in the strength of concrete as well as the change in the arrangement of tensile bars and reinforcement ratio lead to a change in the flexural behavior of the beams and also affect the cracking. Moreover, the experimental results are compared with the values predicted by ACI440.1R code.

Designing and studying the energy absorption behaviour is one of the important subjects in the industries. The thin-walled metal tube structures are one of the most widely used types of energy absorbers that have always been considered by researchers for predicting how to crushing behaviour and determine the amount of energy absorbed by them. A numerical study of crashworthiness design using to investigation of axial crushing behaviour of axisymmetric tubes is presented in this paper. First, a Finite Element (FE) model for material properties was constructed and validated by experimental data. Next, a model for the thin-walled tube under axial loading was adopted. The comparison between the FE results as well as the experimental results showed good agreements. The validated FE model was then used to investigate axisymmetric thin-walled tubes with different cross-section and parametric studies of best geometry under axial loading. Making a totally 7 different geometries and the collapse behaviour of the absorbent geometries then has been simulated. A comparison of energy absorption results and maximum crushing load between the different geometries show that conical tubes with shallow spherical caps are better than others. So, the parametric studies were carried out to determine the effect of different parameters such as the spherical region radius, semi-apical angle, thickness and foam density on crushing behaviour of this geometry. The outcomes of the study are new research information which will facilitate the design of axisymmetric thin-walled tubes as energy absorbers in crushing applications.

The dual steel resistant skeleton is one of the functional structural systems for resisting against lateral forces induced by earthquakes that in addition to the enhanced ductility possesses high stiffness, dynamic stability, and redundancy. Behavioural pattern of dual resisting systems is based on shear and bending performance modes. This research includes a study on the nonlinear seismic behaviour of dual eccentrically braced resisting frames with knee rods. The extent of the effectiveness of knee bracing has been assessed based on nonlinear time history analyses of the structure subjected to severe three-component earthquake records. A strong near-field ground motion possesses particular characteristics. The existence of high amplitude and long period pulses in recorded strong ground motions along with the severe accumulation of kinetic energy and its short release time, are of noticeable differences between near-field and far-field earthquake records. The particular objective of this research is to evaluate the amplitude of variations in maximum responses and to determine the time history of absolute acceleration, relative displacement, and relative velocity of floors along with the base shear of the studied models. Analysing the results indicate an observable appearance of the effects of large coherent velocity pulses presented in the time history of near-field earthquake records, on the relative lateral displacement of floors and on the rapid propagation of plastic hinge mechanism throughout the primary elements of dual resistant skeleton.

Iran holds the first and third rank worldwide in gas and oil reservoirs, respectively; therefore, projects executed in this noble industry requires accurate contracts.In order to optimize accomplishment of industrial design-build projects, designing new coordinates of EPC identical documents with the least potential problems is inevitable, In a way that, with certifying the time and fee stated in the contract to the most possible extent, with the highest quality and fewest claims, the desired results be obtained and the project be fulfilled with the best quality. To this end, with the pursuit of library based studies, investigating international documents particularly FIDIC silver book, reviewing articles, theses, rules and regulations, existing problems and deficiencies have been identified through designing, distributing, and collecting a survey from the elites after having the obtained results analyzed and thus this conclusion has been drawn that, in addition to reforming the structure and conceptual model of the document, it is also required that the obligations and liabilities of both parties of the contract be minutely specified and the logical distribution of the risk be conducted. Especially, in the area of delay in payment to the contractors, the mechanism required for compensating the loss needs to be anticipated. moreover, the articles related to contract price, commencement date conditions, basic design, assurance, guaranty letters, contractors facilities, suspension, provisional taking over, termination, waiver and other cases need to be corrected and the rules that are required to be expressly mentioned in the context of general conditions of the contract be stated.

Recently, there has been an increasing trend toward the use of sustainable materials. Sustainability helps the environment by reducing the consumption of non-renewable natural resources. Concrete – the second most consumed material in the world after water – uses a significant amount of non-renewable resources. As a result, an experimental investigation was conducted to study the hardened properties of concrete constructed with 20% and 50% recycled concrete aggregate (RCA) (coarse) as well as 6% silica fume. This experimental program consisted of six mix designs. Experimental results showed that the compressive and tensile strength of concrete improve by using recycled concrete aggregate (RCA) (coarse) and silica fume in the mix design. For example, the compressive strength of concrete containing 6% silica fume and 20% recycled concrete aggregate at age of 56 days was 14% greater than reference concrete mix, while with increasing percentage of recovered concrete aggregate by 50%, this parameter increased by only 4% which shows the inverse effect of increasing the amount of recovered concrete aggregate on compressive strength. The hardened properties of concrete were compared with the provisions of the international design codes (U.S., Australia, Canada, Europe, and Japan) as well as a database of conventional concrete. Results showed that the RCA has superior hardened properties compared with the reference concrete mix.

Selection of appropriate accelerograms for nonlinear dynamic analysis is one of the most important challenges due to their significant influence on the interpretation of analysis results. A rational method for initial refinement of ground motion records with a minimum computational cost is application of refining process based on an optimum intensity measure (IM) which is able to predict the structural responses in a reliable manner. In this research, a list of intensity measures is considered to determine most effective ones from the point of view of effective relation with the dynamic response of the structure. Selected models in this paper includes four two-dimensional (2D) reinforced concrete (RC) structures of 2, 4, 8, 12 and 20 story with special moment-resisting frame (SMRF) system, which are modeled nonlinearly. Collapse capacity of the structures was obtained using the intensity measure spectral acceleration at the first-mode period of structure Sa(T1) and the Incremental Dynamic Analysis (IDA) under the influence of 40 ground motion records selected from the refined data set to observe the gradual behavior of the structure from the linear stage to collapse. Finally, by establishing a statistical relationship between the dynamic response parameters and 27 intensity measures with the use of Pearson correlation index, the most effective intensity measure that All have an integral form and independent from the period of structure, for each of the selected structures was introduced. By examining the results, it can be said that for the 4 story low-rise structure, the Response Spectrum [Housner] intensity measure and for the 8 and 12 story mid-rise structures, the Housner intensity measure and finally, for the 20 story high-rise structure, the Displacement Response Spectrum intensity measure has the highest correlation among the intensity measures studied.

To investigate the seismic responses of dams due to ground non uniform displacements, the records of earthquake displacements are required. Such data is seldom recorded during a specific earthquake and in the most cases the recorded data in not sufficient for non-uniform analysis of a dam. In this study non uniform accelerations are generated using Kanai-Tajimi spectrum with considering wave passage, incoherent effects and coherence spectrum model. A 3-d finite element model of dam-foundation of Karun 4 arch dam as the highest dam in Iran is modeled. The model is analyzed under uniform and non-uniform generated accelerations and its responses are investigated. The results show that, non-uniform excitation leads to differences in seismic response than uniform excitation on dam. It is indicated that considering non-uniform excitation in seismic analysis increases the stresses and displacements on the dam body. To investigate the seismic responses of dams due to ground non uniform displacements, the records of earthquake displacements are required. Such data is seldom recorded during a specific earthquake and in the most cases the recorded data in not sufficient for non-uniform analysis of a dam. In this study non uniform accelerations are generated using Kanai-Tajimi spectrum with considering wave passage, incoherent effects and coherence spectrum model. A 3-d finite element model of dam-foundation of Karun 4 arch dam as the highest dam in Iran is modeled. The model is analyzed under uniform and non-uniform generated accelerations and its responses are investigated. The results show that, non-uniform excitation leads to differences in seismic response than uniform excitation on dam. It is indicated that considering non-uniform excitation in seismic analysis increases the stresses and displacements on the dam body.

The concentrically braced system is one of the most common lateral loadbearing systems among the steel structures. This system has various apparent forms where the main characteristic of them all is their significant stiffness and lateral strength. The weakness of these systems is buckling in compression. This issue causes that concentric bracings have low compressive loadbearing capacity together with undesirable ductility and limited energy dissipation capacity. In this study to solve this problem use has been made of a heuristic method. In this method a local fuse has been used in the middle of bracing where its periphery and inner circumference have been covered with an auxiliary casing within a casing. The local fuse is designed in a way that after yielding, the bracing undergoes local buckling at this area. But presence of an auxiliary element placed around the fuse prevents this local buckling and thus the bracing would exhibit almost a symmetric behavior during compressive and tensile loadings. The results of experimental and numerical researches of this study indicate the better performance of this structural system in terms of ductility and energy dissipation capacity compared to the common concentric braces. The suggested system can be utilized to reduce the capacity of bracing connection which results in reduced costs of the project.

In this research, the seismic performance of 5story buildings having dual systems of moment frame and chevron bracings, chevron panels along with zipper columns, inverted chevron panels, inverted chevron panels along with zipper columns is evaluated based on conducting nonlinear time history analyses under an ensemble of 3-component far and near-field records. Behavioral features of resistant skeletons having chevron braced panels are controlled through a near-buckling performance of diagonal limbs under pressure. It has been observed that when intensive earthquakes occur, this type of resistant skeleton reveals a gradual loss in resistant features. The appearance of the effects of the two phenomena of stiffness deterioration and strength degradation would be remarkable. A desirable solution in dealing with this weakness is adding a zipper column to the braced panel. The major criterion in selecting near-field records has been the existence of distinct or compound pulses with long period and large amplitude in the ground velocity time history. It has been noticed that this feature affects the seismic response parameters of the building to a great extent. All structures have been designed with a regular plan and according to seismic provisions in the code 2800 and also the 6th and 10th issues of the Iranian National Building Regulations. The specific outlook of this research lies in assessing the changes in maximum displacement responses, relative velocity, absolute acceleration of the floors, plastic hinge formation mechanism and the axial force of the columns. Results show a moderate superiority in the seismic performance of dual skeleton having zipper elements compared to the same resistant system with a lack of zippers.

The purpose of this paper is to reinforce the pressurized pipe containing a fully circumferential crack (internal and external) using composite patches due to their remarkable properties. In the research procedure, in order to validate the proposed models, firstly, a cylindrical pipe with a circumferential crack subjected to the uniform tension was modeled and the results of stress intensity factors were compared with an existing paper. Then, in order to examine the modeling process of composite patch and adhesive, a damaged plate with a central crack was considered and the results of stress intensity factors were investigated and evaluated. Stress intensity factor at the crack tip was calculated using the J-integral. In this regard, using the 3D finite element method, the stress intensity factors in the crack before and after the pipe repair were evaluated with boron/epoxy, carbon/epoxy and glass/epoxy composite patches. The crack geometries correspond to different relative depths from 0.4 to 0.8. The results showed that the use of composite patches significantly decreased the stress intensity factors, and this reduction in the external circumferential cracks is more than the internal ones. Also, in the research process, the effect of the patch thickness, the angle of the fiber and the adhesive material on the stress intensity factors has been investigated.