Tarbiat Modares University
Modares Civil Engineering journal
2476-6763
16
4
2016
1
1
The Earthquake Magnitude Prediction Using Multilayer Perceptron Neural Network
1
8
FA
Mohammad Ali
Arjomand
Assistant Professor / Shahid Rajaee Teacher Training University
Jamal
Mahmoudi
M.Sc. of Earthquake Engineering/K.N.Toosi university of technology
Masoud
Rezaei
M.Sc. Student of Earthquake Engineering / Building and Housing Research Center
Mohammad Hossein
Mohammadi
M.Sc. Student / Kharazmi University
Theoretical and empirical equations developed for calculating the magnitude of earthquakes are affected by a lot of parameters. Most of these parameters need to be measured and entered in the equations accurately, while, in many areas, due to the lack of required equipment, these parameters mostly are measured approximately and with low precision or even sometimes assumed. Moreover, these equations usually are exclusive of a specific region or state, so they are not reliable enough for other new regions. On the other hand, neural networks have been proven to be one of the most practical effects in modelling and forecasting. There are three major advantages of neural networks. First, neural networks are able to learn any complex non-linear mapping. Second, they do not make a priori assumption about the distribution of data. Third, they are very flexible with respect to incomplete, missing and noise data (Vellido et al, 1999). Moreover, neural networks, regardless of the region and country, are a general solution in all areas. The aim of this paper is to use a kind of neural network system named Multilayer Perceptron (MLP), which is one of the most influential neural network models, to predict the magnitude of the earthquakes. This method consists of several layers of nodes. It includes an input layer, an output layer, and a hidden layer, each of which contain input node(s), output node(s), and hidden node(s), respectively.The input nodes are based on some variables. In the current research, six independent variables including three spatial variables, one time variable and two variable related to physical characteristics are defined. The output nodes of neural networks are the prediction outputs or labels. The seismic data that have been used in the research are got from the whole instrumentally recorded earthquakes occurred in Iran.From whole data, 85% are used for network training and 10% for network testing and revising. The remained 5% is dedicated to derive the final prediction of the magnitudes of earthquakes. Then, these predictions have been compared with exact values to assess the network prediction ability. In the hidden layer, as there is no method to decide the optimal number of hidden nodes directly, four different numbers of hidden nodes are chosen, including 8, 12, 16 and 20. Moreover, a well-known concern with neural networks is ‘‘overtraining”. To ease this problem, a set of four different learning epochs are used, including 1, 2, 4 and 8. Moreover, in training part, two different training methods, named Batch and Online, were applicable. In order to reach to more comprehensive results, both of these methods are applied. As a result, we setup 32 different groups of parameters and models. After all, the results of the study indicate that MLP network has a good capability for predicting the magnitude of earthquakes. The average correct prediction of the models is about 70%. To conclude, according to the results, the network is a functional device in predicting the magnitude of the earthquake of a region in an arbitrarily considered time.
Tarbiat Modares University
Modares Civil Engineering journal
2476-6763
16
4
2016
1
1
Development of Parametric Equations for Behavior Characteristics of Conventional Saddle-like Connections
9
22
FA
Hamidreza
Amiri
Ali akbar
Aghakouchack
Faculty ofTarbiat Modares University
faculty of Tarbiat Modares university
Saddle-like (Khorjeeni) connections are formed in steel frames, when the beams are not cut at their intersection with columns, and they by-pass the columns continuously and are connected to them using top and bottom angles. This type of connection provides some benefits, such as ease of construction and superior behavior under gravity loads, however it has some disadvantages that can cause significant damage to the structure when subjected to seismic loads. Past studies have shown that saddle-like connections can be classified as semi-rigid connections, but they do not have the ductility capacity observed in those types of semi-rigid connections, which have been recommended in various codes for seismic resistant design.In recent years several researchers have tried to propose details for improving the behavior of Khorjeeni connections. However, in the process of seismic evaluation and rehabilitation for existing structures, in which the conventional type of Khorjeeni connections are used, it is necessary to have typical behavior curves and acceptance criteria for different levels of structural performance. The aim of this paper is to develop parametric equations for behavior characteristics of the conventional saddle like connections.Experiments and finite element modeling have been conducted on fourteen different specimens of the connections. The details have been selected based on what are normally found in medium rise buildings in Iran. The beam height and the angle length vary in the specimens. Six specimens have been tested in laboratory and then modeled by finite elements. FE analysis has considered the crack initiation and propagation using a micromechanical model originally proposed for predicting crack initiation in ultra-low cycle fatigue, ULCF. ABAQUS multi-purpose software has been employed for this work. As the Finite Element modeling of tested saddle-like connections has proved to be successful in predicting the behavior of this type of connection, additional samples have been modeled and analyzed using FE models. Based on the results of experiments and FE modeling, backbone curves representing moment rotation behavior of the connections have been determined following the FEMA recommendations Characteristic parameters of the backbone curve have been identified as initial stiffness, yield moment, ultimate moment and ultimate rotation of the connection. Also the parameters affecting these characteristic values have been found to be beam depth, top angle size, top angle length, and bottom angle length . Finally, using the regression methods, some relationships have been proposed for each characteristic parameter of the backbone curves. A comparison of the experimental and numerical results and the results of parametric equations shows good accuracy. The differences in initial stiffness, yield moment and ultimate moment are less than 10 percent for majority of the specimens. The differences in ultimate rotation are also less than 15 percent in majority of the cases. The proposed equations in conjunction with FEMA recommendations for acceptance criteria can be used in seismic evaluation and rehabilitation of steel structures with saddle like connections.
Tarbiat Modares University
Modares Civil Engineering journal
2476-6763
16
4
2016
1
1
Performance Evaluation of Passive Tuned Mass Dampers
For Reduction of Bridges Traffic Vibrations
23
34
FA
1
1
1
nowadays, bridges play important roles in transportation. Due to their structural shape, bridges often have a wide and thin deck and so, they are prone to vibration in the vertical direction. Vertical vibrations of bridge deck resulting from passing vehicles, can affect the security and service of these structures. Using of control systems is one of the main strategies to reduce the vertical vibrations in bridges. In general, control systems can be classified into three categories: passive, active and semi active control systems. In this study, the reduction of vertical vibrations in bridges are investigated using the passive tuned mass dampers control systems. Passive tuned mass dampers contain a relatively small mass (compared to the mass of the bridge), a spring and a damper which are installed and operated to reduce the dynamic response of the bridge deck and their Performance principally depends on the energy dissipation of the deck vibrating motion by the oscillating motion of the mass damper. Thus, tuned mass damper frequency is proportional to the frequency of the dominant vibration modes of the bridge structure (usually the first mode) and when this frequency is excited, the tuned mass damper begins to vibrate in the opposite direction of the bridge vibration and the bridge energy is dissipated by the force of inertia inserted to the bridge by damper. Therefore, the inertia force of the passive tuned mass damper is the main cause of energy dissipation in the deck. Dynamic characteristics of tuned mass damper (including mass, damping percentage and frequency settings), installation location of tuned mass damper, speeds of the passing vehicles, are effective parameters influencing tuned mass damper performance in reducing vertical vibrations of bridge deck under traffic loads. In the first part of this study, the effect of mass on the performance of tuned mass damper is investigated by assuming other parameters to be fixed. Then, the effect of damping percent and setting frequency on reducing vibrations of different sections of bridges and different places where tuned mass damper is installed is investigated. In the second part, tuned mass damper performance in different speeds of vehicles is investigated. The results show that TMD mass is more effective in reducing vertical vibrations in comparison with the damping percentage. Also, it is found that TMD is extremely sensitive to its regulatory frequency, in such a manner that with a little deviation from set frequency, its performance decreases. The effect of TMD is positive and considerable in certain speeds and certain TMD placement locations. Furthermore, the results reveal that the most important effect of TMD on reduction of the vibration response of the bridge deck structure occurs in the free vibration response. For the mentioned bridge with TMD, the maximum reduction of 24% and 59% in the dynamic response of the bridge deck occur for forced and free acceleration amplitude respectively, and the maximum reduction of 13% is obtained in the maximum displacement of the bridge deck, that these results are related to the occurrence of resonance in the bridge deck.
Tarbiat Modares University
Modares Civil Engineering journal
2476-6763
16
4
2016
1
1
Effective stress relationships validation of unsaturated soils based on the laboratory stress paths
35
44
FA
Unsaturated soils are often encountered in civil engineering practice, such as compaction works in construction of roads, dams and other types of embankment. In unsaturated soil stress-strain behavior is complex and is influenced by many factors including externally applied stresses, soil type, structure, density, and suction. Therefore, constitutive models for soils should ideally represent the soil behavior over entire ranges of possible pore pressure and stress values. In the literature, there are different approaches differing upon the choice of the set of suitable variables describing the material behavior. This choice is a key point in unsaturated soils modeling. Possible pairs of suitable stress variables for use in unsaturated soil mechanics presented by Fredlund and Morgenstern (1977) with Three combinations Consist of and , and , and . The stress-state variables employed in early models are the net stress and the suction. It is the simplest and most practical choice in terms of stress path representation, but, it poses difficulties when trying to incorporate hydraulic hysteresis effects. This combination could fail to provide a straightforward transition between saturated and unsaturated states, i.e. for a null suction. On the other hand Terzaghi’s effective stress cannot be exactly recovered. Also these stress variables were also difficult to implement in existing finite element codes for saturated soils because most saturated soil relations are described in term of effective stress. The effective stress principle is probably the single most fundamental contribution in the field of soil mechanics. In the other hand, it is true to say that the principle of effective stress lies at the foundation of most modem soil mechanics theory and practice. Therefore, the choice of appropriate stress variables for unsaturated soils has often been an intensively debated issue, often in connection with the possibility of defining an effective stress measure. So far, many efforts have been made to the development of effective stress for unsaturated soils in the literature. This paper presents a comparison between the values of effective stress parameter of unsaturated granular soils obtained from relations proposed by Schrefler, Aitchison, Kohgo, Khalili-Khabbaz and Dangla-Coussy with special regard into the critical state concept. The principle is to re-plot experimental data obtained of both consolidated drained and constant water content tests in conventional stress–strain coordinates, stress variables being usually net stress and suction, into new effective stress coordinates. The critical state lines (CSL) at different suction values tend to converge remarkably towards a unique saturated line in the deviatoric stress versus mean effective stress coordinates. Therefor since critical state lines presented as independent of suction, comparison of soil properties in many cases becomes easier. The comparison results show that effective stress values determined from the relations proposed by Khalili-Khabbaz well best agrees with the values from the both type test results
Tarbiat Modares University
Modares Civil Engineering journal
2476-6763
16
4
2016
1
1
Evaluation of non-homogeneous stratification pattern on undrained shear strength of sand-gravel mixtures
45
56
FA
Yaser
Jafarian
The Caspian Sea is regularly visited by many tourists and the surrounding lands are densely populated. Accordingly, many tall buildings and heavy structures are constructing over this coastal area. The region is overlaid by poorly graded clean sand which seems to be susceptible to liquefaction occurrence. However, there is no documentation and field observation to ensure liquefaction triggering in the uniform rounded- shape deposits of the region during an actual earthquake. Hence, lots of research works are expected to be carried out to recognize cyclic behavior of these coastal deposits before the probable scenario earthquake and the consequent disaster happen. The undrained shear strength of granular soils is one of the most important parameters to evaluate flow liquefaction, which is prone to produce beneath the shallow foundations bearing considerable levels of static shear stress. Mechanism of flow liquefaction is commonly studied using triaxial apparatus to obtain a better understanding of the parameters controlling the phenomenon. Soils experiencing the flow type of liquefaction commonly undergo large deformations because the driving stresses tolerated by the soil exceed the shear strength reduced by generation of excess pore pressure in earthquake condition. There exist numerous studies for evaluation of undrained behavior of mixed soils comprising of gravels, sands, silts, and clays. Majority of the previous studies have focused on the homogeneous mixtures of soils while natural soils are generally found in the nature with levels of non-homogeneity. In this study, mechanical behaviors of homogeneous and heterogeneous mixtures of the Babolsar sand and gravels are compared under different conditions such as initial effective stress, relative density, and heterogeneity. Several triaxial experiments were conducted to evaluate effects of stratified sedimentation and increase of heterogeneity on undrained shear strength and pore pressure development of sand-gravel mixtures in various relative densities and confining pressure. A simple formula has been defined to specify level of heterogeneity of the sample, which varies from zero to one percent. The results demonstrate that shear strength, the potential of pore water pressure, and internal friction angle of homogeneous samples are quite different from heterogeneous samples. The undrained shear strength of sand-gravel mixtures increases with increasing degree of heterogeneity even in identical gravels content. Excess pore water pressure in sand - gravel mixed samples with higher degrees of heterogeneity reduces and tends towards dilative behavior, with respect to the more homogeneous samples. The undrained shear strength of the samples is proportional to initial relative density and effective confining pressure, as expected and observed in other granular materials. It is found from the results of the tests that the type of mixed sedimentation has significant impact on the soil's friction angle. In fact, increase of heterogeneity level increases the internal friction angle of the mixed soils while the gravels content is kept identical.
Tarbiat Modares University
Modares Civil Engineering journal
2476-6763
16
4
2016
1
1
Numerical Evaluation of Stress Distribution on the Cantilever Sheet Pile Wall Buried in Earthquake Conditions
57
68
FA
Mohammad Hossein
Jahangir
Assistant of Professor, Faculty of New Sciences & Technologies, University of Tehran
Zahra
Khosravi
Student of Master, Faculty of New Sciences & Technologies, University of Tehran
Analysis and design cantilever sheet pile walls with general Rankin and Coloumb Methods is done, until now. During recent years offer Wang theory’s for retaining wall with active backfill, researchers attempt to about type passive backfill its and scrutiny seismic condition this theory for retaining wall. In this paper is tried Wang theory’s toward the searching distribution of net lateral earth pressure on cantilever sheet pile wall, also is employed. Therefore within this theory it is considered stable coefficient and calculation was established on the basis of the trapezoid - shaped wedges is right around the cantilever sheet pile, However, the resultant effect of lateral forces acting on sheet pile is moved by changing the pressure distribution in comparison, but the outcome is the same and constant like Coulomb's theory. Therefore, it can be argued that Wang's theory provided the pressure distribution on sheet pile walls is well answered and can be used as a new method of calculation in the analysis and design of this retaining structure.Analysis and design cantilever sheet pile walls with general Rankin and Coloumb Methods is done, until now. During recent years offer Wang theory’s for retaining wall with active backfill, researchers attempt to about type passive backfill its and scrutiny seismic condition this theory for retaining wall. In this paper is tried Wang theory’s toward the searching distribution of net lateral earth pressure on cantilever sheet pile wall, also is employed. Therefore within this theory it is considered stable coefficient and calculation was established on the basis of the trapezoid - shaped wedges is right around the cantilever sheet pile, However, the resultant effect of lateral forces acting on sheet pile is moved by changing the pressure distribution in comparison, but the outcome is the same and constant like Coulomb's theory. Therefore, it can be argued that Wang's theory provided the pressure distribution on sheet pile walls is well answered and can be used as a new method of calculation in the analysis and design of this retaining structure. Analysis and design cantilever sheet pile walls with general Rankin and Coloumb Methods is done, until now. During recent years offer Wang theory’s for retaining wall with active backfill, researchers attempt to about type passive backfill its and scrutiny seismic condition this theory for retaining wall. In this paper is tried Wang theory’s toward the searching distribution of net lateral earth pressure on cantilever sheet pile wall, also is employed. Therefore within this theory it is considered stable coefficient and calculation was established on the basis of the trapezoid - shaped wedges is right around the cantilever sheet pile, However, the resultant effect of lateral forces acting on sheet pile is moved by changing the pressure distribution in comparison, but the outcome is the same and constant like Coulomb's theory. Therefore, it can be argued that Wang's theory provided the pressure distribution on sheet pile walls is well answered and can be used as a new method of calculation in the analysis and design
Tarbiat Modares University
Modares Civil Engineering journal
2476-6763
16
4
2016
1
1
Effects of Different Turbulence Models on Flow over a Triangular Flip-Bucket
69
81
FA
seyed hami
hojjati
Spillways have long been of practical importance to safety of dams, therefore these structures have to be built strong, reliable and highly efficient. Ski jump dissipator is one the flow energy dissipators which is applicable downstream of spillway chutes with velocity over 20 m/s. Flow over a flip-bucket is a two-phase and strongly turbulent flow. Turbulence modeling is one of the most limiting factors in accurate computer simulation of flows. By fixing the grid resolution and the discretization scheme, the difference of computation time is mainly attributed to the turbulence model. The choice of turbulence model depends on factors such as the physics encompassed in the flow, the level of accuracy required, the available computational resources, and the amount of time available for the simulation. It is a fact that no single turbulence model is universally accepted as being superior for all classes of problems. The main purpose of the present study is numerical investigation of two-phase turbulent flow over a triangular flip-bucket to evaluate effects of different turbulence models in this type of flow. Hence, using FLUENT® software, two dimensional Reynolds averaged Navier-Stockes equations have been solved in unsteady state. Different turbulence models consist of k-ε, k-ω and RSM; have been used. To simulate two-phase flow, volume of fluid (VOF) method has been applied. Standard k-ε and stress-omega RSM models with low-Reynolds number modifications have the best performance among the other turbulence models. In standard k-ε model when low-Reynolds number modification was activated, the effects of molecular viscosity were taken into account in near-wall regions. Therefore, in low-Reynolds number k-ε model, maximum dynamic pressure over the bucket was predicted more accurately in comparison with standard k-ε model. Regarding modification in strain-pressure terms in turbulence equations, effects of anisotropic Reynolds stress tensor were taken into account in stress-omega RSM model with low-Reynolds number modifications. Thus, compared to other turbulence models, numerical results of this model are in a better agreement with experimental results. Different k-ε models could not predict the jet trajectory after the bucket very well. Due to blending function in SST k-ω model, this turbulence model effectively blended the robust and accurate formulation of the k-ω model in near-wall regions with the free-stream independence of the k-ε model in the far field. In estimation of maximum dynamic pressure over the bucket, this model had a better performance than standard k-ω model and relatively similar results to k-ε model. In addition, SST k-ω model has shown the best prediction of the jet trajectory among other turbulence models. Eventually, with respect to computation cost and accuracy of results, SST k-ω turbulence model has been introduced as the most suitable turbulence model to predict the flow pattern of a triangular flip-bucket.
Tarbiat Modares University
Modares Civil Engineering journal
2476-6763
16
4
2016
1
1
Experimental investigation of hydraulic parameters in modern horseshoe spillway
83
93
FA
vahid
hasanzadeh vayghan
Ph.D. Student of Hydrualic structures, Department of Civil Eng, Faculty of Eng, The University of Urmia
mirali
mohammadi
Associate Prof , Department of Civil Eng, Faculty of Eng, The University of Urmia
farzin
salmasi
Associate Prof , Water Eng. Dept., Faculty of Agricultural Eng., The University of Tabriz
ali
Hosseinzadeh Dalir
Professor, Water Eng. Dept., Faculty of Agricultural Eng., The University of Tabriz
mohammad
manafpour
Assistant Prof , Department of Civil Eng, Faculty of Eng, The University of Urmia
Abstract:Modern horseshoe spillways are combination of external and internal weirs and determination of hydraulic parameters of this kind of spillways is essential. In this study, by making physical models of Modern horseshoe spillway in laboratory dimension, some hydraulic parameters of modern horseshoe spillways like discharge coefficient, discharge rate through the two weirs and water surface profiles in different parts of horseshoe weir are investigated. The results showed that by increasing the ratio of head (h) to external weir’s Length (Lw) for Lw≤100 cm, the discharge coefficient for internal weir (CO) increases linearly. By increasing ratio of internal weir discharge to external weir discharge Q1/Q2, the discharge coefficient has a decreasing trend between 0.65 to 0.3 . By increasing the external weir length, the discharge rate through external weir increases while it decreases in internal weir. The external weir with Lw=100 cm has better performance than other Lws and If we select it as the better length, on average 31 percent of the flow will pass from the internal weir. Classic and Modern horseshoe spillways are able to reduce the head above the weir respectively to 28 and 50% in comparison to rectangular weir head with the same width. Investigation of the water surface profiles shows that by creating an internal weir, rooster tail hydraulic jump that existed in Classic horseshoe spillway is removed.Keywords: Modern & Classic horseshoe spillway, Discharge coefficient, Internal weir, Physical model.Abstract:Modern horseshoe spillways are combination of external and internal weirs and determination of hydraulic parameters of this kind of spillways is essential. In this study, by making physical models of Modern horseshoe spillway in laboratory dimension, some hydraulic parameters of modern horseshoe spillways like discharge coefficient, discharge rate through the two weirs and water surface profiles in different parts of horseshoe weir are investigated. The results showed that by increasing the ratio of head (h) to external weir’s Length (Lw) for Lw≤100 cm, the discharge coefficient for internal weir (CO) increases linearly. By increasing ratio of internal weir discharge to external weir discharge Q1/Q2, the discharge coefficient has a decreasing trend between 0.65 to 0.3 . By increasing the external weir length, the discharge rate through external weir increases while it decreases in internal weir. The external weir with Lw=100 cm is the optimum length and If we select it as the optimum length, on average 31 percent of the flow will pass from the internal weir. Classic and Modern horseshoe spillways are able to reduce the head above the weir respectively to 28 and 50% in comparison to rectangular weir head with the same width. Investigation of the water surface profiles shows that by creating an internal weir, rooster tail hydraulic jump that existed in Classic horseshoe spillway is removed.Keywords: Modern & Classic horseshoe spillway, Discharge coefficient, Internal weir, Physical model.Keywords: Modern & Classic horseshoe spillway, Discharge coefficient, Internal weir, Physical model.Keywords: Modern & Classic horseshoe spillway, Discharge coefficient, Internal weir, Physical model.Keywords: Modern & Classic horseshoe spillway, Discharge coefficient, Internal weir, Physical model.Keywords: Modern & Classic horseshoe spillway, Discharge coefficient, Internal weir, Physical model.
Tarbiat Modares University
Modares Civil Engineering journal
2476-6763
16
4
2016
1
1
Application of the endurance time method for evaluation of jacket platforms
95
108
FA
Mohammad Ali
Dastan Diznab
Sharif University of Technology
Hamid
Mehdigholi
Mohammad Saeed
Seif
Sharif University of Technology
In this research, seismic performance of Endurance Time (ET) method is considered for assessment of jacket platforms. ET is a new method that can assess the extreme response of the structure at various seismic excitation levels with very low computational costs. For this aim, artificial records have been generated that gradually increase with time. They have been named Endurance Time Acceleration Functions (ETAFs). For determination of the seismic response of the jacket platforms, various nonlinearity such as buckling of the brace members, material nonlinearity, soil structure interaction and fluid structure interaction are important and can be a challenging issue for the ET approach. In this way, a real jacket platform located in the Persian Gulf is studied. Finite element method is utilized to prepare a three dimensional model of this platform with using ANSYS software. Moreover, various nonlinearity sources are considered in this model. Fluid structure interaction is included by using Morison equation that hydrodynamic added damping and added mass are considered by nonlinear drag force and inertia force, respectively. Soil–pile–structure interaction is also considered by near and far field soil effects. Near field soil is modeled by nonlinear spring and elastic solid elements are used to model far field effects. Material nonlinearity is considered by a standard bilinear stress-strain curve with 5% strain hardening and the von Mises yield criterion. Buckling of the brace members is also modeled by the initial imperfections at the mid-span of the braces, as recommended by previous studies. A methodology is also addressed for assessment of this type of offshore structures. For considering the accuracy and the reliability of this approach, the results of the ET method are compared with the typical time history method. In this regard, seven records are selected for soil type C from FEMA 440 and FEMA 695 and scaled to the ELE event such that their spectral accelerations match the ELE spectral acceleration at the main period of the platform. For other excitation levels, scale factors change proportional to the ELE ratio. For example, in this case, the ratio of the ALE spectral acceleration to the ELE one is 1.4; therefore, the scale factors of the ALE event are 1.4 times of the ELE one. ET records are scaled such that the response acceleration spectrum of the ETAF until target time becomes compatible with the ELE spectral acceleration. Initial studies recommend that 10 s is an appropriate target time. Due to linear increase of the excitation of the ET records, each time can be in accordance with the especial level of ELE, for example, 5s and 15s indicate 0.5 and 1.5 times of the ELE event. A comparison between the results of the earthquake records and the results of the ETAFs show that the ET method can accurately estimate engineering demand parameters such as maximum deck displacement, maximum base shear, maximum axial force in the leg and maximum axial force in the brace. The ET method is a vigorous approach that can be successfully estimated the seismic excitation of the buckling initiation. Moreover, the results indicate that despite significant decrease in the computational costs of the ET method, this approach can show appropriate performance.
Tarbiat Modares University
Modares Civil Engineering journal
2476-6763
16
4
2016
1
1
Optimal Parameters of Tuned Mass Damper for Tall Buildings by neural networks
109
122
FA
Meysam
Ramezani
1- Graduate Student of Earthquake Engineering, The University of Tehran, Tehran, Iran.
Seyed Mehdi
Zahrai
Associate Professor at the School Civil Engineering, University of Tehran, Tehran, Iran
There are a variety of tools like vibration absorbers to reduce the vibration of structures by dissipating imposed energy. Tuned mass Damper is a vibration system with a mass and spring, that usually installed on the top of a structure. Tuned mass dampers are kind of absorbers that could reduce vibrations if the parameters of frequency and damping are well tuned. There are many analytical and empirical relations to identify these parameters obtained by structure simplification and loading. The paper demonstrates that neural networks can be used effectively for the identification optimal parameters of tuned mass dampers.In this paper, a new method is proposed to determine these parameters. For this purpose, several buildings with a number of different stories (8 to 80) are first created in MATLAB code. After obtaining the stiffness, damping and mass matrix of the structures, the program enters the Simulink environment and then structure with TMD tuned in the approximate range of optimal parameters is analyzed under different earthquakes. These ground motion records, including two near field and two far field records that suggested by the International Association of Structural control are used. To increase the number of data used to train artificial neural network and reduce uncertainty, in addition to the records mentioned, four other earthquake was considered. The TMD Mass, damping coefficient and frequencies is assumed as the design variables of the controller; and the objective is set as the reduction of the maximum displacement of the building. In the end, the parameters for the maximum reduction in lateral displacement of structures are identified. Then neural network training begins to have a complete database of structures with different stories. The number of hidden layer neural network are ten, and the number of layer's output are two considered. To train the neural network of Six hundred and forty-four date has been used. Of these numbers, the Seventy percents are share of training data, and fifteen percents are share of Validation data, and fifteen percents are share of test data. Accordingly, the neural network after training can evaluate frequency and damping ratio of TMD based on input such as frequency and TMD mass ratio. Using this method, it has been observed that errors in the frequency and damping optimum TMD have been reduced particularly compared to empirical relations of Den Hartog. This reduces the errors caused by factors not considered in relation to the various reasons. For example, these factors maybe not yet identify, or because of the complexity of behavior have been simplified. However, using a neural network without fully understanding the parameters influencing the behavior of TMD exists, these factors indirectly to predict the behavior of TMD considered. So the results of this method can be used to optimize the parameters of TMD, to be used with greater confidence. Finally, these values are compared with those provided by Den Hartog in a high-rise building. The results showed that for the building, parameters provided by neural network approximately 1 percent of the actual value of the optimal values is different.
Tarbiat Modares University
Modares Civil Engineering journal
2476-6763
16
4
2016
1
1
Kinetics Study of UV-ZnO Photocatalytic Process for Post Treatment of Composting Leachate
123
134
FA
Alireza
Ranjbari
Environmental Engineering Student,Civil and Environmental Engineering Faculty, Tarbiat Modares University
Nader
Mokhtarani
Assistant Professor, Civil & Environmental Engineering faculty, Tarbiat Modares University
Economic and industrial growth of the recent decades in most countries and the changing pattern of life, cause a dramatic increase in the production of industrial and urban waste. Considering population growth and economic, cultural and industrial changes, it`s necessary to have a proper management plan to prevent environmental damage and save the cost of waste management. One of the issues associated with urban waste management is controlling and treatment of leachate. Leachate treatments methods are classified into three groups include: leachate transmission; biodegradation techniques and physiochemical treatment methods. Due to its reliability, simplicity and high cost-effectiveness, biological processes are the most conventional methods in leachate treatments. However, due to high organic load and presence of refractory contaminants in leachate, biological treatment alone cannot remove all of the organic matters from leachate. Therefore, to meet discharge standards, additional treatment is required to remove the remaining materials from biologically treated leachate. Photocatalytic processes have been proven to be an appropriate technology for final treatment of these types of wastes. In this study the capability of UV-ZnO photocatalytic process as a post treatment method for composting leachate was examined at laboratory scale and in batch mode. The effect of some factors such as initial pH, catalyst concentration, light intensity and reaction time on the removal of organic load and color of leachate were investigated. Biological pre-treated leachate samples were collected from the effluent of leachate treatment facility of a composting plant in north of Iran. The leachate samples have been collected in 20 L plastic containers transported to the laboratory and immediately stored in refrigerator at 4˚C to minimize any changes in its physical, chemical, and biological properties until the experiments were carried out. A Plexiglas column with 110 mm inner diameter and 300 mm height were used to conduct experiments. UVC lamps (as the source of irradiation) with different power levels were placed inside a quartz tube (external diameter 0.026m) mounted at the axial centre of the reactor. In each experiment, a specified amount of nano particle was added to leachate into the reactor, at ambient temperature and under a specified amount of UVC radiation. In order to prevent the settling of nano particles, air was continuously injected into the column through a diffuser at the bottom of the reactor. Samples were taken periodically from the reactor for analysis. Prior to analysis, the liquid samples were centrifuged at 3000 rpm for 10 min to remove all suspended particles. In order to prevent reflection and scattering of UV radiation in the environment, the reactor was covered with a thick layer of aluminum foil. Based on the results of experiments, after 120 minutes of radiation with 32 W UVC lamps in pH 11 and in the presence of 1 gr/L of slurry nanoparticles (ZnO), maximum COD and color removal were achieved to be 60% and 68%, respectively. In kinetic studies of batch UV-ZnO photocatalytic process, a pseudo-first order model with reaction rate constant of 0.38 hr-1 was found to fit well (R2= 0.99) with the experimental results.
Tarbiat Modares University
Modares Civil Engineering journal
2476-6763
16
4
2016
1
1
Numerical study of soil compaction and the passive trench barrier depth on vibration reduction due to train movement
135
147
FA
Alireza
Ardakani
Ebrahim
Saadatfar
Vibration due to train movement has been concerned since the advent of rail transportation systems. Nowadays this subject is more important by increasing the speed of trains and wagon weight and development rail way in urban area. Different methods and techniques have been proposed to reduce these vibrations by researchers. In this study, geotechnical modeling of trench barrier isolators to reduce train vibrations are investigated by numerical modeling. Trench barrier according to location are divided into two categories: active and passive. Active trench is located near the source load but passive trench is located near the structure that needs protections. The main focus of this research is the effects of trench depth on efficiency of passive trench barrier. In advance by considering the wagon weight and distance between the wheels, the load in terms of time domain by calculating Fourier spectra and considered effective frequency range, the load-time function is determined. Then, a series of nonlinear finite-element analyses were carried out to study effect of trench depth. The depths of trench are considered 0 to eight meters in analysis. Viscous damping was considered by means of Rayleigh methods. The coefficient of mass and stiffness matrices are defined by modal analysis. Then, modeling of rail, sleeper, ballast, sub-ballast and soil layers is conducted to analyze wave vibration propagation due to train movement and evaluation of amplitude reduction due to trench barrier. Mohr Coulomb failure criterion is supposed for soil, ballast and sub-ballast layer and a linear elastic behavior was assumed for sleeper. Infinite element boundaries were used at the lateral sides of the finite element mesh for prevention of wave reflection. The dynamic response analysis was based on input time historical wagon load which determined in previous section applied at the sleeper.Thereby the studying of deformations and displacement and particle velocity of the elements of the model, the effects of the trench barrier on before and after trench, are considered. Results indicate that increase in trench depth is effective on vibration reduction. This means that by increasing the depth of the trench, amplitude of deformation becomes more reduced. It also increases the depth of the trench, reduces the dominant wave frequency vibrations. On the other hand, by increasing the depth of the trench, amplitude reduction ratio has a significant decreasing that means increase efficiency trenches. The relationship between depth and amplitude reduction ratio (Arr) was introduced as a quadratic function. For validation of modeling, theory equation of amplitude decreasing is compared with numerical modeling. The result showed that there is a good accordance between numerical model and theory equation at different distance.The minimum depth of the trench should be selected based on different parameters. To define depth of trench should be note that the reduction due to trench should be significantly greater than the decreasing wave amplitude due geometric and material damping. Reducing the required to mitigate the potential damage of protected structures and economic and implementation considerations are important factor in determination of final depth of the trench barrier isolator.
Tarbiat Modares University
Modares Civil Engineering journal
2476-6763
16
4
2016
1
1
Determining Hendurabi Island Boundaries (hazard line) in order to Integrated Coastal Zone Management (ICZM)
149
159
FA
The objective of Integrated Coastal Zone Management (ICZM) is to establish sustainable levels of economic and social activities in coastal areas while protecting the coastal environment. ICZM seeks to reconcile the different policies that have an effect on the coast and to establish a framework that facilitates the integration of the interests and responsibilities of those involved in the development, management and use of the coast. The coast is a link between land and sea, includes diverse geomorphologic frameworks and sedimentary environments. The coastal zone is a dynamic marine area where land and sea influence each other to a significant degree.This area is influenced by different biological, chemical and physical activites especially wind, waves, currents and tides that alter the profile of the shoreline. For the first time, coastal zone management in Iran was introduced officially about a decade ago. Ports and Maritime Organization (PMO), Environmental Protection Agency and National Center for Oceanography are institutions that carried out activities on the coastal zone management in Iran. Also Kish Free Zone Portal decided performing this project for islands under its management.Beaches of Hendurabi Island with high potential of natural resources and abundant mineral and natural talents have great importance among islands of Persian Gulf. In fact,due to the strategic location of the Hendurabi Island, politically, socially and economically, to achieve sustainable development and efficient use of island´s resources, implementation of Integrated Coastal Zone Management (ICZM), is necessary. The Integrated Coastal Zone Management (ICZM) of Hendurabi Island composed of several executive parts such as Shoreline Management Plan, Environmental Management Plan and monitoring and evaluation plan. Directly integrated coastal management is divided into two parts: (a) Land area which is consisted of determining of hazard line. (b) marine area that determined based on the active area of sedimentary and environmental criteria.Usually retreat strategy is proposed on land to hazard line, but in islands due to the small size of the islands and land values, retreat is not the best solution and can be constructed dikes after the tidal action. Calculation of hazard line components such as wind setup, wave setup and runup,depend on relationships and parametersm but these relationships are empirical and have errors. So determining hazard line of islands have great important.In this study relationships of two valid regulations, Cem and Eurotop,investigated and sensitivity analysis was performed on the parameters. Hazard line for 5, 20 and 50 return period years, is determined with a new approach by using simple engineering relations. For the calculation, required raw data is extracted from Iran Seas Wave Modelling project. The influence of various factors around the Island, including area wind speed, wind angle, wave height, beach slope, return period on the hazard line were investigated. Results showed that determining hazard line as a part of direct Coastal Zone Management is very essential.
Tarbiat Modares University
Modares Civil Engineering journal
2476-6763
16
4
2016
1
1
INVESTIGATATION AND COMPARISON OF COMPRESSIVE STRENGTH OF CLAY SOILS STABILIZED BY CEMENT, LIME AND CBR PLUS
161
174
FA
Different types of stabilizers have been used for improving the properties of clay soils. Selection of the appropriate stabilizer depends on the soil type, environmental conditions, costs and the required strength. In this research, the effects of three different stabilizers on the compressive strength of two types of clay soils have been investigated. Cement, lime and CBR PLUS nano polymer, in different proportions, have been added to two types of clay soils, one with low plasticity index, and one with a high plasticity index, and the engineering properties of plasticity index and compressive strength have been evaluated. The compressive strength has been evaluated in wet and moist curing conditions in different ages of 1, 7 and 28 days. It is shown that, for the soil with low plasticity index, cement has the highest effect on the reduction of plasticity index, and, for the soil with high plasticity index, CBR PLUS, has the highest effect. Results also show that, for achieving high compressive strength in moist and wet curing condition, cement is the best stabilizer for the soil with low plasticity index. The compressive strength of the cement stabilized soil after 28 days of wet conditioning is found to be about 6 times higher than that of the lime and CBR PLUS stabilized soil. Also, for the soil with high plasticity index, CBR PLUS has the highest effect on the compressive strength in moist curing conditions, and in wet curing condition, the effect of cement is higher than lime and CBR PLUS. However, the difference between the compressive strength of cement stabilized soil in wet curing condition with lime and CBR PLUS stabilized soil is much lower than that for the low plasticity soil. For all the soils and stabilizers, the compressive strength of the moist curing condition is higher than those of wet conditioned. The effect of curing age on the compressive strength is found to be the same for the stabilizers, for which, the compressive strength of 28 days conditioned specimens is approximately twice those of 1 day conditioned specimens. It is also found that the ratio of compressive strength in moist curing condition to that in wet curing condition depends on the soil type, amount and type of stabilizer. The wet curing condition of high plasticity clay has minor effect on the compressive strength of the soil stabilized by lime, and for the soil with low plasticity index the lowest effect is on the soil stabilized by cement. For the CBR PLUS, the effect of wet curing on he reduction of compressive strength depends on the amount of stabilizer. The most economical stabilizer of the soil with low plasticity index is found to be cement, and that for the soil with high plasticity index is lime. Cost analysis show that CBR PLUS is not economical solution for clay stabilization.
Tarbiat Modares University
Modares Civil Engineering journal
2476-6763
16
4
2016
1
1
Determination of optimal dimensions of flip buckets by the simulation-optimization approach
Case Study: The Jareh Dam
175
186
FA
Dean of Dept. of Civil Eng. Engineering School, Shiraz University.
Energy dissipation downstream of large dams is one of the most important concerns in the design procedure of dams. Flip buckets are employed whenever the velocity of flow at the downstream of the spillway is in excess of typically 20 m/s because of problems with stilling basins in terms of cavitation, abrasion and uplift. One of the most important issues in flip buckets is determining their optimal dimensions in order to increase the energy dissipation and reduce the maximum pressure on the surface of the flip bucket, simultaneously. Nowadays, the increasing computational power of computers to analyze complex problems, development of numerical modeling techniques and artificial intelligence models have caused them popular, in contrast with physical models which are often very time consuming and expensive. Hence, most of the researchers use these methods to analyze the complex engineering problems. In this research, by developing a new simulation-optimization methodology, the optimum dimensions of the flip bucket were determined based on the FLOW-3D model, artificial neural network (ANN) model and Genetic Algorithm (GA) models. The aim of determining the optimum dimensions is to calculate the radius of curvature and also the deflection angle of the flip bucket such that the maximum pressure on the surface of the bucket be minimum and the relative energy dissipation be maximum. Based on this methodology, in the first, the flip bucket of the Jareh dam was simulated for different radii and deflection angles using Flow-3D software. The calibration process was done in the basis of the experimental results which were obtained from the laboratory model. This laboratory model was built in the Water Research Institute of Tehran. Then, an artificial neural network (ANN) as a meta-model was trained using the maximum pressure on the surface of the bucket and the amount of energy dissipation after the impact of trajectory jet with the downstream channel bed. Then it was evaluated by the data that were not used during the training phase. The ability of this meta-model is to predict the values of the maximum pressure on the surface of the flip bucket and the amount of energy dissipation after the impact of trajectory jet with the downstream channel bed for different dimensions of flip bucket. Then by combining this neural network meta-model, with the genetic algorithm (GA) optimization model, the optimum dimensions of the mentioned flip bucket were determined. The optimum dimensions of flip bucket based on the mentioned objectives were found to be equal to the radius of 14 meters (0.28 m at physical model) and angle of 44.5 degrees. The results showed that despite the reduction of dimensions compared to the original size of flip bucket, the rate of energy dissipation has been increased (about 14% for the PMF flow).
Tarbiat Modares University
Modares Civil Engineering journal
2476-6763
16
4
2016
1
1
Effects of the number of Concrete panel and Concrete strength on the behavior of Composite Shear Wall
187
198
FA
Hamed
Ahmadi Taleshian
Composite steel plate shear wall -or in short Composite shear wall- is a lateral load resisting system which was investigated widely during past decade. Steel plate is the most important component of the system in carrying shear, but it would buckle under relatively low levels of loading due to its instinct high slenderness. To avoid such a buckling and obtaining a suitable seismic performance, Concrete panel is attached to one or both sides of the plate. The composite performance of the system would be achieved by using shear connectors, usually in the form of bolts when pre-cast concrete is utilized. Surrounding beams and columns act as a framework for the infill composite panel. However, the behavior of theses boundary members together with their connection to shear wall is of main concern, especially in high-seismic design when a dual action is needed and the beam-to-column connection should be rigid. Furthermore, these members would also participate in resisting most of the overturning moments and depends on their stiffness, a portion of the shear. In this paper, the number of concrete panels (whether on one or both sides of steel plate) and Concrete strength are the parameters that their effects on the behavior of composite wall would be investigated numerically. For this purpose, a valid experimental specimen was modeled in the Finite Element Software ABAQUS, with the capability of considering different elements for different parts of the composite system, and after verifying the accuracy of the numerical model, new specimens were constructed and analyzed using push-over method in order to demonstrate the effect of the required parameters. Results show that specimen with concrete on both sides would have considerably less ductile behavior than specimen with one-sided concrete, although its shear strength is higher at lower levels of inelastic displacements. However, both specimens reach the same ultimate shear strength and as was expected, they have also same initial stiffness. Although steel plate is likely to buckle in specimen with concrete on one side of the plate at high levels of stress near yielding, but results of the present paper shows that not only this kind of buckling has no stability sequences and does not result in performance similar to thin steel plate shear walls due to prevention of early buckling, but also leads to a more ductile behavior than the specimen which reaches the full shear yielding without the possibility of such buckling. Furthermore, Concrete strength has little effects on overall behavior of the system, although can reduce the damage to concrete panel. Based on the obtained results, it can be concluded that using normal-strength concrete (such as 4000psi or 28Mpa) and just on one side of steel plate is sufficient for obtaining a desirable seismic performance of the system.
Tarbiat Modares University
Modares Civil Engineering journal
2476-6763
16
4
2016
1
1
Beneficial and detrimental effects of soil-structure interaction according to damage spectrum
199
212
FA
Hamzeh
Shakib
In spite of extensive studies since 1970 on Soil Structure Interaction (SSI), there is still controversy regarding the seismic performance of the structures rested on soft soil. SSI is known as a phenomenon influencing beneficial and sometimes detrimental effects on the response of structures. The response of soil–structure system depends basically on the size of the structure, its dynamic properties, and the soil profile as well as the applied excitation. Under strong shaking the response of soil–structure system is nonlinear and a combination of different types of nonlinearity as well as sliding, uplift and soil yielding will occur. Although foundation uplift in conjunction with soil yielding may dissipate energy during earthquakes, they may also lead to excessive permanent deformations in the structures. In such case, soil- structure interaction (SSI) is not only beneficial, but also detrimental to the seismic response of the structures. In order to investigate the beneficial and detrimental effects of SSI on the nonlinear response of building, damage spectra on the basis of Park and Ang damage Index for the SDOF models are provided by considering and neglecting the SSI effects. The bilinear SDOF models are supported by Beam on Nonlinear Winkler Foundation. Two non-dimensional parameters are used to control the modeling. (I) non-dimensional frequency a0 which is a statement of the structure-to-soil stiffness ratio (II) the aspect ratio of the structure h/r. The systems are subjected to three earthquake ground motions recorded on soft soil. For each period, first the yield strength demand of the structure is calculated by iteration in order to reach the specified target ductility in the fixed-base model within 1% of accuracy when subjected to the ground motions. The dissipated hysteretic energy in the structure is also calculated accordingly. Then, the ductility and the hysteretic energy demands are calculated for the soil–structure systems under different values of a0 and h/r subjected to the same ground motion providing the same yield strength for the structure. Consequently, the damage index is calculated for the structure in the fixed-base state as well as for the structure when located on soil. The result showed that for most of structures with long period SSI decrease the damage index. How ever It is observed that in some cases of structure to soil stiffness, SSI increases the damage index before a threshold period which is closely related to the predominant period of the ground motion. It means that the conventional fixed-base model underestimates the damages sustained by buildings having periods less than this threshold period. In particular, the SSI substantially increases the damage index of short-period buildings located on soft soils. It is also observed that increasing the aspect ratio of the structure increases this effect. However, the trend is reversed after the threshold period. it is observed that the increase in the slenderness ratio and ductility ratiob of structures leads to increase in the maximum damage.
Tarbiat Modares University
Modares Civil Engineering journal
2476-6763
16
4
2016
1
1
Effect of Loading Protocol on the Steel Moment Connections in Iran
213
224
FA
m
Ghassemieh
Professor, School of civil engineering, University of Tehran
mohamad
hassani sokhtesaraei
Ph.D. Student, School of Civil engineering-University of Tehran
Premature rupture of steel structures in seismic Northridge connections led to investigate the connection performance against the forces of earthquake, many researchers are studied. The performance of these joints and connections against forces caused by various kinds of earthquakes, accurate determination of the used parameters and bearing capacity parameters of connections. The fact that no two earthquakes are similar adds emphasis to adopting a coordinated approach for testing different structural elements. These requirements results in developing standard loading histories called loading protocols. Today, the ever-increasing development of using steel in the construction industry in Iran has become clearly evident. Considering the fact that Iran is an earthquake-prone country, identifying the behavior of these structures against earthquake forces assumed importance. Seven, twelve and twenty-story steel moment frames were selected as research models, and after preparing a proper list of accelerograms recorded in the course of earthquakes occurred in Iran during 1978-2007 , the seismic demands applied to structural components were estimated. Regarding the catastrophic aftermath of earthquakes, having a reliable approach to assess the seismic demands proper to the region of interest seems inevitable. The scope of this study is to provide a list of far-field seismic records applicable to 2-D nonlinear time history analyses in Iran. To do that a set of 2000 seismic records from Iran's seismic records database has been investigated based-on their characteristics, amongst are focal distance, magnitude, rupture mechanism, frequency content, and soil profile, resulting in compilation of a list of 20 accelerograms for the purposes of the two dimensional and non-linear analyses in Iran. The proposed accelerograms portray as a true area’s seismic movement as possible. In continuation of study, the final coefficient of scale for the accelerograms for the steel moment frames is calculated with a point to point differentiation method in periodic intervals corresponding to each frame. For the coefficient of scale, an operation conducted in a way that for each accelerogram in the average spectrum, an appropriate response is determined. Time-history dynamic analysis has been carried out by using the list of proposed accelerogram records aiming at preparing the loading protocol of steel moment connections, and the rainflow cycle counting technique has been employed to determine damage parameters for research models. Considering the fact that strength and deformation capacities depends on cumulative damage in earthquake engineering, every component has a permanent memory of past damaging events and at any instance in time it will remember all the past excursions (or cycles) that have contributed to the deterioration in its state of health. A loading protocol is developed specifically for this purpose.For developing the loading protocol of moment-resisting connections, the time history is analyzed using the proposed set of accelerograms recorded, and by applying the method of rainflow cycle counting, damage parameters are determined for the research models. The number of damaging cycles, total deformation range, deformation range and peak of deformation range are the employed parameters, which will be evaluated and compared for all the layers of the research models. Afterwards, statistical estimations are performed on damage parameters and target values are established. Finally, a loading protocol is recommended for moment-resisting joints commonly used in Iran which has the capacity of being applied to the considered structural elements under one general state.
Tarbiat Modares University
Modares Civil Engineering journal
2476-6763
16
4
2016
1
1
Numerical study of effect of Crest shape on Discharge Coefficient of Trapezoidal Arced Labyrinth Weir
225
238
FA
ALi
Gholizadeh
M.Sc. of Water Engineering, Faculty of Civil and Environmental Eng., Tarbiat Modares University
Masoud
ghodsian
Professor of Hydraulic Engineering, Faculty of Civil and Environmental Eng., Tarbiat Modares University
Rouzbeh
Panahi
Assistant Professor of Marine Structures, Faculty of Civil and Environmental Eng., Tarbiat Modares University
Weirs have important roles in dam safety in which they should spill floods with high return period. Designers generally enhance width of the weirs to increase their discharge capacity. This procedure involves topography as well as economic limitations. Here, arced weirs can be considered as an alternative. In plan view, arced weir is part of a circle that increases the crest length for a given channel width. This increases the flow capacity at a similar heads. Such structures are also recommended for modification and increasing the capacity of the existing spillways. Discharge capacity of labyrinth weirs is a function of flow height, effective crest length, height of weir and shape of the crest. Discharge coefficient is also a function of height of flow, height of weir, weir thickness and crest shape. In this study, hydraulic characteristics of arced labyrinth spillways are numerically investigated. Here, the effect of crest shape on the discharge coefficient of the labyrinth spillway is included. In the first step, dimensionless parameters affecting the performance of arced weirs are introduced using Buckingham π theorem. To analyze this problem, a commercially available CFD code; Flow 3D by Flow Science; was selected. Flow 3D is known for its ability to accurately tracking free surface using Volume of Fluid (VOF) method. A similar method called Fractional Area to Volume Ratio (FAVOR) is used to define labyrinth within the model. Also, Reynolds averaged Navier Stokes (RANS) equations are solved using a finite volume method. Besides, The Renormalized Group Theory (RNG) model was implemented for turbulent simulations. The laboratory data of Crookston and Tullis (2012a) is used to validate the numerical model. These researchers conducted experiments on physical modeling of the labyrinth spillways at the Utah Water Research Laboratory at Utah State University. Comparison of numerical simulations with those of experimental results validates the ability of this software to simulate the complex flow over labyrinth spillways with an acceptable accuracy. In this study, result of 16 geometry models was used to develop a hydraulic design and analysis formulation for arced labyrinth weirs. Discharge coefficient data for Half-Round, Quarter-Round, Sharp-crest and Flat-crest arced labyrinth weirs are presented for 6̊ ≤ sidewall angles ≤ 24̊ and various head water ratio (0.1≤ H0/P ≤ 0.9). The study has shown that half round crest shape could increase the discharge coefficient about 22% compared to other crest shapes. Also, the results show that factors such as local submergence and nappe interference near an upstream apex has a negative impact on performance of arced labyrinth weirs. The local submergence area is directly related to the flow head, crest shape and sidewall angle. For high head conditions, the local submergence may decrease the efficiency of a labyrinth spillway. Efficiency parameter is defined as the ratio of discharge of arced weir to that of liner weir with the same width. From efficiency curves indicates that reduce of sidewall angle can improve efficiency. As a result, the highest efficiency related to arced labyrinth spillway with sidewall angle (α = 6̊ ).
Tarbiat Modares University
Modares Civil Engineering journal
2476-6763
16
4
2016
1
1
Investigating the Amount of Structural Gap For Base-Isolated Structures, Subjected To Near-Fault Earthquake Records
239
250
FA
Benyamin
Mohebi
Imam Khomeini International University
Mojtaba
Jani
Imam Khomeini International University
One of the important effects of earthquake is impact of adjacent buildings. To eliminate or reduce the damage and destruction caused by the impact of adjacent buildings to each other, they should be separated by a gap or should be constructed with a minimum distance from the border. In this issue, the properties of near field records of earthquakes, with close distance to the source of the wave propagation makes them different from other records. Seismic Code 2800 Iran has made no mention of near-field earthquake response spectrum and seismic design spectrum in 2800 code is for far field earthquakes. Furthermore, a new method to reduce the effects of earthquakes on structures is using base isolators. In this study, the effects of base isolators in prediction of the behavior of concrete moment frames (medium ductility) located near the source of earthquake have been investigated and two-dimensional frames with same bays and different number of floors (2 4, 6, 8, 10 and 15 story frames) which represent the short, medium and high rises buildings have been considered. Results indicate that the required gap for isolated buildings located near faults is more than the predicted amount in Iranian Seismic Code (2800 standard). All the models are intermediate reinforced concrete frames which are separated from regular structures. One of the modern techniques for designing seismic resisting structures is using base isolations. There are several types of isolators which can be used practically in the structures. The damping of the isolators depends on the type of them and there are wide ranges of damping amounts based on isolators’ types. The main idea in this regard is to reduce the strength demand in the structures and also reduce the nonstructural damages even during severe earthquakes. Also it is possible to reduce non-structural damages by using base isolators. Base isolators with high damping, absorb earthquake energy using displacement. Although it is very good idea to use base isolators in seismic areas, but the gap should be produced between adjacent buildings to prevent buildings contact. This is very dangerous especially when the story levels are not same in the two buildings. In this case a concentrated load will induce to middle of column of another building. In this research it is tried to find the suitable amount of gap for base isolated structures. Also for considering near-fault effects, some of records with near-fault behavior have been selected and used. The structures considered to be retrofitted by base isolators. For calculating displacement demand of the structures under earthquakes the nonlinear time history analyses have been used. All the analyses performed using opensees software and scaled records. The results shows that the amount of Gap in Iranian Seismic Code (2800 standard) is suitable for structures which are not in near-fault regions. Of course there are some differences between the code and the results achieved in this research. But for structures in near-fault regions the proposed amount in the code is less than required amount of gap from the results of this research.
Tarbiat Modares University
Modares Civil Engineering journal
2476-6763
16
4
2016
1
1
new mechanical connection for retrofitting concrete piers of bridges with composite materials
251
264
FA
Navideh
Mahdavi
Studies on reinforced concrete bridges damaged in the last earthquakes, confirm that weak piers are the main reason for the collapse of these structures. Retrofitting concrete piers with FRP plays an important role in enhancing axial and shear strength. However, it will have an insignificant effect on raising the flexural capacity of the members. To increase the bending capacity of reinforced concrete piers, various methods have been suggested by researchers. However, each method has different shortcomings such as difficulty in implementation, high cost, low operation speed, and uncertainty of performance. Considering the problems of the existing techniques and after comprehensive studies, a new mechanical connection to enhance the bending strength of reinforced concrete piers using FRP was proposed in this study. The proposed detail is easy to perform and is cost effective. Meanwhile, it can be applied in a short time. In addition, suggested mechanical connection eliminates detachment of FRP layer from the surface of concrete and develops proper connection between FRP layer and the surface of the concrete column. So, the detail can be used to improve the flexural capacity of the column. To assess the proposed mechanical connection, a reinforced concrete column was modeled in Abaqus software. The concrete column was built in 2007 and was tested at IIEES. To validate the model, calculated results were compared with experimental results and the accuracy of the model is ensured. Using the validated model, the other models were simulated. The models consisted of a reinforced concrete column retrofitted with longitudinal FRP layers, a concrete column retrofitted with longitudinal FRP layers and transverse FRP layers in flexural plastic hinge location and the other model is a reinforced concrete column retrofitted with longitudinal FRP layers and proposed mechanical connection. So, in order to evaluate and study the detail, four finite element models were created in Abaqus software. The performance of the concrete columns under vertical and lateral loading was studied using force-deformation curves. According to the results, flexural capacities of concrete columns retrofitted with longitudinal FRP layers were increased. By comparison of the results obtained from force-deformation curves of the models, wrapping columns in the plastic hinge locations partially raises flexural capacity. However, Due to the linear elastic behavior of FRP material, damage mode is sudden in both second and third models. Based on the results, the general behavior of the concrete columns retrofitted with longitudinal FRP layers with or without transverse FRPlayers in flexural plastic hinge location did not improve and failure damages occurred immediately. Based on the observation, the best behavior has been shown by the concrete column retrofitted with longitudinal FRP layers and proposed mechanical connection. Using the proposed detail, the flexural strength of the reinforced concrete column was increased. As force-deformation curves show, the proposed mechanical connection can inhibit efficiency of the longitudinal FRP layers and enhance the flexural capacity of the reinforced concrete column. Moreover, the detail improves the overall performance of the concrete column retrofitted by FRP layers and prevents instantaneous failure damages.Keywords: Rinforced Concrete, Bridge Piers, Flexural Rehabitation, FRP Layers, Mechanical Fasteners.
Tarbiat Modares University
Modares Civil Engineering journal
2476-6763
16
4
2016
1
1
Investigation of Steel Column Behavior with Different Cross Section under Blast Loading
265
278
FA
Mohammad Ali
Hadianfard
Shiraz university of technology
Adel
Nemati
shiraz university of technology
Ali
Johari
Shiraz university of technology
Terrorist attacks and explosions in the vicinity of buildings and vital areas are happening in different countries repetitively. Most of these incidents lead to global and local failure in the main elements of the buildings and in some cases due to intensity of explosions can occur entire structure collapses. Columns are the key bearing elements in the building, and between all columns, the exterior of them are more vulnerable to terrorist attacks. Usually blast resistant design of structure is carried out by simplifying the models and considering a single column with nonlinear behavior under blast loading. Explosion is a complex phenomenon with high strain rate, which affects strongly on behavior and material property of structural elements. Operation of experimental test on structures under blast load is very expensive, difficult and dangerous. Hence, simulation of experimental models using nonlinear finite element software is very useful. In this paper, to achieve better performance of columns under blast loading, the response of steel columns with different cross-sections has been investigated. In addition, effects of blast wave incidence angle, blast distance and different boundary conditions are considered. For this purpose, wide flange steel column of experimental test has been simulated under axial force and blast load using LS-DYNA software. Numerical model is simulated using shell elements and its result has been validated with the full scale blast experimental data. In the finite element analysis the effects of high strain rate and material nonlinearity are considered. The columns with different cross sections of wide flange, cross-IPE and box sections are simulated under two angles of blast waves extensive, zero and 45 degree. Also, two support conditions of fixed-fixed end and pinned-pinned end have been considered. The results show in the both boundary conditions for blast with zero angle, the dynamic response of column with wide flange section subject to blast load has been less than the other cross sections. Also, the box section has better performance than cross-IPE. In 45 degree blast angle and fixed end boundary conditions, the displacement time history of box column is less than two other sections and it shows better performance respect to other sections. But, under pined end boundary conditions, cross-IPE section has better and stronger behavior respect to wide flange and box sections. In addition, the displacement of wide flange section (section with non-identical strong axes) in 45 degree blast angle has more than zero degree. However, in the columns of box and cross-IPE section under the same explosion situation in 45 degree blast angle, the dynamic response is less than zero degree, because they have two identical strong axes. Then for corner columns of buildings that direction of blast wave propagation may be 45 degree the best section (based on minimum deflection criteria) is column section with two strong axes such as box and cross-IPE, however for peripheral middle column of building that bending moment of explosion may be accrued about strong axis, the wide flange section with only one strong axis is better. Various distances of explosion from column cause different nonlinear behavior, therefore investigation of optimum column cross section under blast loading depends to distance of explosion from the column. Then displacement criteria may be not enough and use of additional criteria such as residual load bearing capacity can be appropriate.