Tarbiat Modares University
Modares Civil Engineering journal
2476-6763
18
3
2018
9
1
Effects of side walls and Froude number on flow field over ogee spillway in axial arc condition
1
11
FA
sehagh
Oftad
Department of civil Engineering, Azad University, Bandarabad
cyrus
ershadi
Weirs are one of the common structures for regulating and measuring of flow. Also, this structures with different forms such as normal weir, side weir and ogee spillway are widely used for various purposes. In some cases, due to practical restrictions the ogee spillways with curvature in plan are designed. An ogee spillway is located at the top of the reservoir of dam. Dams may also have bottom outlets with valves or gates which may be operated to release flood, and a few dams lack overflow spillways and rely entirely on bottom outlets. In such situations study of flow distribution over the ogee spillway and other related parameters, might be important. In this study, the flow field over thw ogee spillways in axial arc condition was simulated by FLOW-3D software. In the FLOW-3D model, the Navier-Stokes and continuity equations were discretized using the finite difference method. The computational domain was divided into a mesh of rectangular cells. All variables (except for velocity values) were placed at the center of the computational cells (staggered grid arrangement). To solve the governing equations, control volumes were defined around each dependent variable. The surface fluxes, body forces and surface stresses were computed in terms of surrounding variables. Most terms in the governing equations were explicitly evaluated. To solve the flow field of a non-compressible fluid, the continuity and the Navier-Stocks equations were solved. On the other hand, to validate the numerical results, the experimental measurements that were performed in Soil Conservation and Watershed Management Research Institute at reservoir with dimensions 1.4 m length, 0.30 m width and 0.18 m height. The experimental model was made of plexiglas plates which was a model of prototype at the scale of 1:75. Moreover, to measure the flow discharge, a sharp triangular weir with apex angle of 90˚ in the output of channel was used. The flow field turbulence was modeled using the standard and the RNG turbulence model. According to numerical model results, the RNG turbulence model had more accuracy than the standard turbulence model. Also, variations of flow free surface were reconstructed by volume of fluid (VOF) scheme. In this numerical model, the effects of the side symmetrical walls of ogee spillway were examined for cases 60o, 90o and 120o in discharges 34, 34 and 22.6 lit/s, respectively. The applied boundary conditions were chosen according to the physical model. Therefore, the depth and discharge specific values were for inlet boundary condition. At the outlet boundary of the model, the outlet boundary condition was used. All the solid walls of the model were defined as the “Wall” boundary conditions. Also, a symmetry plane was determined at the top layer of the computation field. According to numerical results, the acceptable agreement was obtained between numerical results and experimental measurements. For example, the relative error percent of longitudinal profiles of flow free surface were calculated 12.83, 13.60 and 3.48 percentage for cases 120o, 90o and 60o, respectively. Also with increasing angle of axial arc, the height of rooster tail increased significantly. In addition, by increasing Froude number, the height of rooster tail reduced.
Tarbiat Modares University
Modares Civil Engineering journal
2476-6763
18
3
2018
9
1
Numerical analysis of effective parameters in direct shear test by hybrid discrete – finite element method
13
24
FA
mohammad
afrazi
civil eng. Dep. of Tarbiat modares university, Tehran, Iran
mahmoud
yazdani
Assistant Prof., Geotechnical Eng., Dept. of Civil and Environmental Eng., Tarbiat Modares University
mahtab
Alitalesh
Ph.D. student in Geotechnical Engineering, Faculty of Civil and Environmental Eng., Tarbiat Modares University
Ahmad ali
Fakhimi
Professor, Mineral Eng., Dept. of Mineral Engineering, New Mexico Tech University
Determination of soil engineering properties such as shear strength is essential to analysis many geotechnical problems. Therefore, determination of the reliable values for this parameter is very important. For this purpose, direct shear test as one of the oldest test to examine the shear strength of soils, is conducted on soil samples. There are too many factors which could affect results of direct shear test. Laboratory tests are expensive, difficult and time consuming, hence using numerical method to simulate experimental test and study effective factors could be useful. In this paper direct shear test was numerically modeled using CA2 hybrid finite element-discrete element method code. CA2 solves explicitly equations of motion together with macro or micro-constitutive equations. In this study, shear box is modeled using finite element grids and a discrete element model is implemented for simulation of soil specimen within the box. Appropriate boundary conditions are assigned to the box, normal stress is applied to the specimen using finite element grid and shear velocity was finally applied to the model. Shear force is applied to the model by a constant velocity 4.5×10-9 meter/cycle. It should be noted that, shear velocity is applied to the upper part of shear box, and applied velocity is considered small enough to confirm that there is a quasi-static condition in numerical solution. In this study using numerical simulation, the effects of box dimension, genesis pressure, normal stress, shear velocity and box wall friction on shear strength of the soil specimen are investigated. Study of box dimension effect, shows that peak effective internal friction angle in small direct shear box and large direct shear box differs about 6°, and cohesion decreases by increasing box dimension but for box dimensions bigger than 20cm, changes in box dimension has no significant effect on resulted soil cohesions. Investigation of influence of genesis pressure shows that, incrementing genesis pressure, cohesion increases too, that can be attributed to the SOCPI model provided in CA2. In SOCPI model by increasing genesis pressure the overlap between cylinders increases. In SOCPI model by increasing overlap the cohesion increases but peak friction angel doesn’t change too much. Normal stress analysis shows that, increasing normal stress, interlocking between soil particle increases and more interlocking causes increasing cohesion of the soil model. Shear velocity is another parameter which is studied in this research. Results show that by increasing shear velocity, soil shear strength increases. It should be mentioned that shear velocity should be considered as small enough to result in a quasi-static solution; for velocity smaller than that model run time increases ineffectively. In this research, friction of shear box wall as one of the important parameters in study of soil shear strength is also investigated. When there is friction between box wall and soil particles, shear strength can be underestimated for contractant soils or overestimated for dilatant soils. In this paper, it is shown that if soil has no significant volume changes, peak shear strength is not affected by friction between soil particle and box wall.
Tarbiat Modares University
Modares Civil Engineering journal
2476-6763
18
3
2018
9
1
Assessment of seismic behavior of RC frames retrofitted using concrete jacket system
25
35
OTHER
The vulnerability and poor seismic performance of the columns and beams of old non-seismically designed reinforced concrete (RC) frame structures has been proven time and again, both at laboratory level as well as by natural disasters in real life situations. On the other hand, a vast majority of existing structures designed with non-seismic considerations is not practically viable so that they designed and constructed with improper reinforcement detailing, with no consideration of the principles of the capacity design, inappropriate of the seismic actions, etc. Such structures need to be correctly assessed to predict their seismic performance and retrofitted, if required.
Therefore, the assessment of seismic behavior of RC frames retrofitted using concrete jacket system would be effective. In order to realistically predict the seismic performance of such structures, providing practical and accurate models to simulate the inelastic behavior of structural members are significantly importance. Evaluation of the response of non-seismically designed RC structures by using such tools would estimate the seismic performance of the structures with high degree of confidence. According to such assessments, an efficient and cost-effective solution can be designed and developed.
In order to improve the seismic behaviour of old non-seismically designed RC frame structures, numerous retrofitting techniques of structural members have been adopted and studied in practical applications. Among these retrofitting techniques, the application of concrete jacketing has extensively increased. The current study has focused on evaluating the effects of concrete jacketing on flexural behaviour of RC jacketed members and seismic performance of non-seismically designed RC structures through analytical modeling approaches.
For this purpose, an analytical model including rotational springs in members was developed. The nonlinear characteristics of these springs can be computed using moment – curvature analysis of a RC member before and after retrofitting so that each level of curvature would be converted into rotation based on the first moment-area theorem. Experimental and analytical studies have confirmed that the confinement induced by transverse reinforcements would lead to considerable enhancement in terms of strength and ductility of RC columns and beams. Thus, this effect was taken into account in determining characteristics of stress – strain concrete. On the other hands, it is well known that the response of RC members in terms of ultimate deformation could be influenced when buckling in longitudinal steel bar occurs. Therefore, in this study, a simplified methodology has been developed to take into account this effect as ultimate condition in the calculation of the moment – curvature analysis of a RC jacketed member.
In order to prove the reliability of the proposed model, it has been validated against experiments conducted by other researchers in the literature. The results showed that the proposed model can be successfully predicted the response of columns retrofitted by RC jacketing. Moreover, the application of the developed model to assess and retrofit RC structures has been demonstrated by a case study and using pushover analysis. The response of structures in terms of initial stiffness, strength, ductility and damage patterns corresponding to performance point and ultimate displacement indicated that the seismic behaviour and performance level of the retrofitted structure were significantly improved in comparison with the original structure.
Tarbiat Modares University
Modares Civil Engineering journal
2476-6763
18
3
2018
9
1
Laboratory Study of Factors Affecting the Mechanical Properties and Durability of High Performance Concrete with the Economic Approach
37
47
OTHER
The most important aims in Concrete-1404 are compressive strength more than 50 Mpa, quality and durability. These items seems to ignore before that in Iran. The high strength concrete production always needs to use suitable materials, attention to mix design, optimizing cement percentage and other component in concrete. For reaching to a trustable mix designs and aspects of high strength concrete, it needs to be evaluated by a complete investigation which can provide all three aspects of strength, workability and durability of a 1404-concrete. In this paper, cubic concrete samples are made in a way which is affected by 5 variables. These 5 Effective ranges are included: 1) Three different types of coarse and fine aggregate gradation curves which are made of finnest through roughest grading of aggregates according to ASTM C33. 2) Three types of high water reducers used in exact same mix design to compare the effects of these 3 different kind of superplasticizers on high strength concrete. These three type are grade F with polycarboxilate-ether based, grade F with sodium naphthalene solphonate based and grade G as a super high water reducer admixture. 3) Effects micro silica with 5, 7 and 12 percentage of total cement amount are added to compare the results on high strength concrete. 360 and 410 kg cement per m3 are used in source concrete mixtures as the regular using amount in practical projects in Iran. 4) The effect of cement content, with fixed w/c and 5% micro silica, which are between 375 through 425 kg/m3. And 5) the effects of adding filler which are thoroughly investigated on mechanical properties, durability and economy plan. There are 20 mix design in 6 groups are prepared with total cementitious materials from 360 to 425 kg/m3. To compare the results, in these experiments the water cement ratio has been fixed to 0.25. The tests used in experiments were compressive strength test by 3000 KN hydraulic compressor and 24 hours water absorption test. The mechanical and durability properties are recognized by these tests. For evaluating fresh concrete properties, slump test was done. Economical aspect is studied and compared by comparing price of materials and content that are used in each mixture. The results show that the variables were impressively affected the compressive strength in high strength concrete. Using optimum conditions of the studied mixture design can improve the mechanical properties of concrete. By optimizing components in mixture design, high strength concrete can be made even with cement content of 400 kg per cubic meter. The 360 and 410 kg cement with adding 5% micro silica showed the best improving in compressive strength. Enhancing cement more than 400 kg results to decrease compressive strength. High water reducers with polycarboxilate ether based show better results in mechanical and fresh concrete properties and also in economical aspect. Adding filler to mixture led to decrease compressive strength and increase durability, however it had no effects on economical properties of high strength concrete. It can be concluded that aggregate grading can still lead to changes in compressive strength, durability and workability of HSC.
Tarbiat Modares University
Modares Civil Engineering journal
2476-6763
18
3
2018
9
1
The effect of foundation depth on subgrade reaction coefficient in sandy soils
49
59
OTHER
Subgrade reaction coefficient is one of the important parameters in the design of structures that for the first time was presented by Winkler (1867). In the Winkler method, in order to evaluate and calculate the vertical displacement of the beam, it is assumed that the deformation at each point of the beam fits with the contact pressure of the reactive bed at that point and for mathematical defining of this ratio, the term ”Subgrade reaction coefficient” was used. After Winkler so many researchers such as Baiot, Terazeghi, Vesic and Bowles have studied on this parameter. Researches show that this parameter is not a constant characteristic of soil and depends on many factors. Factors such as size, shape of foundation, soil type, bending stiffness of soil and foundation, level of under ground water and water content are effective on Subgrade reaction coefficient. One of this factor that less attention has been in prior research is the depth of foundation. In this study by using of numerical method, the effect of the depth of foundation on the amount of Subgrade reaction coefficient was studied. For this purpose the results of 4 samples of plate load test at four depths 10, 12 ,14 and 20 meters and an numerical models that consider the depth of foundation placement have been used. Plate load tests have been conducted on sandy soils of Qom city. Numerical model was done by PLAXIS 2D V8.2 software and hardening soil behavior model was used for sandy soil. The results of numerical model in this study show significant effect of the depth of foundation on the value of Subgrade reaction coefficient and that by increasing the depth, this coficient is increased and changes in foundations with smaller dimention is more and with increasing the dimention of foundation, the percentage of increasing of Subgrade reaction coefficient decreases. The increase in the depth of the foundation from a given value to the next has a small effect on the amount of response coefficient, and almost immediately afterwards, the coefficient of reaction of the bed has reached a constant. In the following, results of numerical model is compared with two formula of Terzaghi and Bowles. For the comparing with Terzaghi formula, the results of plate load test is used and for different width of foundation this amount was calculated and compared with results of numerical model. Tezaghi formula for sandy soils first presents smaller amount and then larger amount than the numerical model. Based on the results of four studied soils, with the fewer depth of test implimentation, results of Tarzaghi formula and numerical model have adaptation in fewer width of foundation. In depth of 10 to 20 meters of current research, almost in foundation width of 5 meter, the results of numerical model is adapted to Tarzaghi formula. The results of comparing the numerical model with Bowles formula shows, to a definite depth for foundation, there is good adaptation between results of this formula and numerical model and with increasing the depth, the differences between Bowles formula and numerical model increases. Finally by using of all states data and nonlinear regression method and SPSS V20 software, for studied sand soils a proposed formula was presented.
Tarbiat Modares University
Modares Civil Engineering journal
2476-6763
18
3
2018
9
1
Fracture modes of a penny-shaped crack in a layer of transversely isotropic solid
61
71
OTHER
Nowadays fracture behavior composites play an important role in geomechanics engineering. Also, it is common knowledge that all existing structural materials contain different inter- and intra-component defect (cracks, delaminations, etc.). On the other hand, analytical techniques can provide a better physical interpretation of problems. In this paper, by using an analytical approach, effects of the fracture modes (opening, shearing and tearing) on a penny-shaped crack in a layer of transversely isotropic solid has been studied. The layer surfaces are fixed from displacement and the system is loaded symmetrically in each mode. In each mode, by substituting the boundary conditions into the governing equations of the medium, the problem reduced to dual integral equations. With the aid some mathematical methods, the dual integral equations are converted to a Fredholm integral equation which is amenable to numerical solution. These Fredholm integral equations are the functions of the thickness of the layer, the radius of crack and the properties of the layer. To evaluate the effect of anisotropic materials on the stress intensity factors(SIFs), several synthetic types of isotropic and transversely isotropic materials are selected. By employing a numerical method the opening, shearing and tearing SIFs for different ratios of layer thickness are obtained. The results for the opening SIF show that by increasing the the SIF decreases substasinaly. On the other hand, an increase in leads to increments in opening SIF. Also, the results demonstrate that the variation in has a negligible effect on the opening SIF. Moreover, an increasing in leads reductions in SIF. For the shearing SIF, has little effect on the results although by decreasing the the shearing SIF increases. Unlike the , the modulus of the young in the plane ( ) of the isotropy has substantial effect on the shearing SIF. An increase in leads increments in the shearing SIF. Also, by increasing the the SIF increases marginally. In the mode III, the tearing SIF is only the functions of (the shear modulus for the plane normal to the plane of isotropy) and . The results show that by reduction in the tearing SIF increases and by increasing the tearing SIF increases. An important point that can be inferred from the results is that by increasing the ratio of layer thickness to the radius of the penny-shaped crack all of the three SIFs increase, this increase for the lower thicknesses is much more in comparison to the greater thicknesses. Additionally, when the layer thickness gets higher, the stress intensity factors for all the materials tend to a constant coefficient. This means that when the layer thickness gets greater and tends to infinity, the SIFs become independent of the material of the layer
Tarbiat Modares University
Modares Civil Engineering journal
2476-6763
18
3
2018
9
1
The effect of concrete slab on the behavior of connections in steel moment resisting frames
73
85
FA
Maryam
Rooin
Tehran-Gisha bridge-Tarbiat Modares University
Ali Akbar
Aghakouchak
Tehran-Gisha Bridge-Tarbiat Modares University
Application of composite sections in structures is increasing in recent years. This type of structure utilizes the strength and ductility of steel as well as strength and low cost of concrete. Composite action is developed when the two load carrying structural members such as concrete slab and supporting steel beam are integrally connected and deflect as a single unit. The extent to which composite action is developed depends on the provisions of shear connectors between steel and concrete. The horizontal shear that develops between the concrete slab and the steel beam during loading must be resisted so that the slip will be restrained. A fully composite section will have no slip at the concrete-steel interface. As we know, the shear connectors provide the interaction necessary for the concrete slab and steel beam parallel to the beam. One of the most important objectives in design and construction of steel moment resisting frames, is the ability of high energy dissipation due to yieldings and plastic deformations in beams in such a way that formation of plastic hinges in beams occurs prior to that of the columns. This procedure leads to fulfilling the strong column-weak beam relation. On the other hand in steel frame structures, it is common to use concrete slab in order to construct floor diaphragms. However in design codes, specially National Building Code of Iran, it is not clearly mentioned how to consider the effects of the concrete slab on connection behavior and the strong column-weak beam relation control. In this study, the behavior of bare and composite beams in steel moment frames under monotonic and cyclic loading has been investigated through numerical modeling in ABAQUS finite element software. The requirements of National Building Code of Iran regarding the ratio of bending moment strength of columns to beams ,α, have been studied. In this regard a direct connection with uniform beam section is simulated. Rotation of beams and steel columns in different sections for formation of plastic hinges are considered as a criterion to measure these relations. In order to determine the effect of the number of shear studs (percentages of composite actions) and their location specially in protected zone within a fixed length of the girder, reduced beam sections (RBS) are modeled. The results indicated that decreasing the initial distance of shear studs arrangement from the column face leads to increasing the composite action. Analytical results also showed that the effective width of concrete slab depends on the load transfer and the force distribution. Based on the results of this study, it is suggested that in calculation of strength ratio of girders and columns, the effect of the floor slabs should be taken into account to ensure that the requirement of strong column-weak beam is fulfilled, otherwise column failure may occure before girder failure. It must also be mentioned that the relative strength of columns to girders can affect the panel zone behavior in such a way that different values of α, require different shear strength of panel zone.
Tarbiat Modares University
Modares Civil Engineering journal
2476-6763
18
3
2018
9
1
Buckling analysis of axially functionally graded beams with variable cross-section
87
99
FA
Masoumeh
Soltani
Owing to efficiency, rising stability of structures, reduction in structural weight and cost, and the improvements in fabrication process, non-uniform beams are extensively adopted in civil and mechanical structures. Furthermore, the use of functionally graded (FG) materials has been increasing in many mechanical components due to their conspicuous characteristics such as high strength, thermal resistance and optimal distribution of weight. In the present paper, a numerical model combining the power series expansions and the Rayleigh-Ritz method is adopted for stability and free vibration analyses of axially functionally graded (FG) columns with variable cross-section. The main purpose of this paper is calculating the critical buckling loads and natural frequencies concurrently for AFG members with exponentially-varying geometrical properties. For this, a mixed power series expansions and the principle of stationary total potential energy as a first endeavor is presented. In this study, the material properties of the non-prismatic beam including Young’s modulus of elasticity and density of material are assumed to be graded smoothly along the beam axis by a power-law distribution of volume fractions of metal and ceramic. Moreover, the cross-sectional area and moment of inertia vary exponentially over the member’s length. In this regard, the power series approximation is applied to solve the fourth order differential equation of motion, since in the presence of variable cross-section and axially non-homogeneous material, stiffness quantities are not constant. All geometrical and material properties and displacement component are developed based on power series of an identified degree. The natural frequencies of the AFG beam with variable cross-section are derived by imposing the boundary conditions and solving the eigenvalue problem. The explicit expression of vibrational shape function is then derived based on this rigorous numerical method. The vibrational mode shapes of an elastic member are similar to the buckling ones. Therefore, the obtained deflected shapes of the considered non-prismatic beams can be used as deformation shape of member for the linear buckling analysis. The critical buckling load of non-prismatic beam can be then estimated by adopting the principle of stationary total potential energy. According to the steps mentioned above, for measuring the accuracy and competency of the proposed numerical procedure, two numerical examples including axially non-homogeneous and homogeneous column with non-uniform section are represented. Numerical results of the critical buckling loads and natural frequencies for various boundary conditions, different gradient index and cross-section variation are represented. Due to lack of similar research for the stability and free vibration analyses of elastic AFG beams with exponential variation of the cross-sectional area and moment of inertia, outcomes of homogeneous members are compared with the results presented in other available numerical and analytical references and those related to non-prismatic beams with material variation are then reported. The accuracy of the method is then remarked. This method has many positive points consisting of efficiency, accuracy and simplicity contrasted with more complex numerical methods. It has to be noticed that the present numerical method can be applied to determine the critical buckling loads and natural frequencies of axially functionally graded (FG) prismatic beams as well as non-prismatic ones.
Tarbiat Modares University
Modares Civil Engineering journal
2476-6763
18
3
2018
9
1
Progressive collapse evaluation of RC structures with Bubble Deck floor system
101
111
OTHER
Usually, progressive collapse is defined as the progress of a primary local damage within the structures that, like a chain chemical reaction, causes to partial or total collapse of the structures. Although, many researches on progressive collapse under blast load have been made, it can be seen that rupture aroused in structures during strong earthquake events will not happen suddenly, but because of failure in structural design or performance, the weak elements will destroy easier. Subsequently, energy redistribution will occur that may disconnect the adjacent members. Further, the progressive collapse phenomenon will take place and subsequently cause to collapse all the structures. In the recent years, the incidence of catastrophic events such as September 11 has attracted a lot of attention to the issue of the progressive collapse and lead to be considered in the design of new structures. In order to prevent damages by reducing the progressive collapse, different strategies for designing against the progressive collapse have been presented in the government documents of USA, such as UFC and GSA. Although, many researches have been made on the progressive collapse in recent years, but the structures deck effect on the progressive collapse has not been considered sufficiently.
Nowadays, due to the increase of the speed of construction and lightweight construction, the usage of new systems has been increased. Among these systems, Bubble Deck system is notable. This structural system functions as a two-way slab and a lightweight structural member. In Bubble Deck system, the plastic spherical hollow core (PSHC) is used instead of the concrete situated in the central zone of the cross sections around the slab’s mid-span, where the shear stress is relatively small, compared to the supports. PSHC creates a hollow space in the slab. The Bubble Deck technology uses spheres, made of recycled industrial plastic, to create air voids, while providing the strength through the arch action.
The objective of this study is to evaluate the progressive collapse of reinforced concrete structures with Bubble Deck floor system. The behavior of structures, the amount, and the mode of the collapse distribution can be studied by various methods. SAP2000 software is used to model, design and analyze the structures. DoD2013 statement has been selected as the reference criteria, and based on that, all the uploading and collapse measurements has been determined. The equivalent nonlinear shell layered element is used to define the slab sections in numerical modeling. 48 structures with the same plans (3 spans on each direction) were modeled. Moment resisting system is chosen as lateral resisting systems. The models were in 4, 6 and 8 stories. Story height of all structures is 3.5 m, and also three types of span length to story height ratios including 1.5, 2.5 and 3.5 are investigated.
The results show by increasing the number of floors, the structure’s performance against progressive collapse will decrease and the middle-rise structures (6 stories) have shown the best performance rather than others. Also, by increasing the ratio of span length to floor height, progressive collapse resistance is reduced. The most observed damage in the Bubble Deck floors is less than 25 percent of floor area which affected by progressive collapse in the middle-column removal scenario. This value of damage satisfy DoD2013 targets.
Tarbiat Modares University
Modares Civil Engineering journal
2476-6763
18
3
2018
9
1
Improving the seismic performance of new composite multi-story moment frames in comparison with conventional concrete moment frames
113
127
FA
mohammad
Sharbatdar
In recent years, composite frames that are consist of reinforced concrete column and steel beam, have been attended by researchers due to their economic efficiency as well as their effect on improving the behavior of tall structures. In such frames, using of concrete in columns, due to the compressive strength of concrete and using of steel in the element of beam, due to high tensile strength of steel, result in reduction of elements section and structure weight and also improving its performance. Previous studies have shown that the most important issue in these frames is their connections. Moment connections of such frames are through-beam-type and through-column-type connections. Details of these connections should be in such a way that prevent the formation of plastic hinge in panel zone, and besides, has adequate ductility. Most studies, so far, have been done on the details of through-beam-type and little researches have been done on the plans of through-column-type connection. Nonetheless, determination of beam-column connection with appropriate seismic performance and also force transmission mechanism is necessary. In this study, amount of the effect of proposed composite connection consist of through-reinforced concrete column and attached steel beam with peripheral diaphragms and inter plates, on the improvement of the behavior of composite moment frames than conventional concrete frames has been investigated that the sample of this connection has been made and tested, laboratorial, by the authors. The survey has been conducted on the amount of displacement and drift of frame’s stories and also on the loading capacity, ductility and energy absorption of frames. For this purpose, firstly, a composite frame of one story-one span with the proposed connection has been modeled by two finite elements softwares called Abaqus and SeismoStruct and the displacement control has been analysed under pushover lateral loading, and its results have been compared with the results of reference concrete frame. To ensure of the accuracy of the modeling that has been done, load-displacement curve that is the resultant of the connection model of Abaqus and SeismoStruct, has been compared with experimental results. Then, three concrete frames and three composite moment frames with four, seven and ten stories, with four 5-meters spans, 3 meters in height of each story, that had been selected from the meddle frame of a construction with dimensions of 20 meters by 20 meters in plan, and had been designed for area with high seismic risk, were modeled by SeismoStruct software, and were analysed under mentioned loading. The results showed appropriate performance of composite moment frames which are consist of mentioned connection, especially in frames with high number of stories. In this study, the drift of the frame and the relative drift of stories, have been reduced in composite moment frames than conventional concrete frames. Moreover, using the proposed composite connections in composite frames has resulted in the increase of the loading capacity of the frame so that the maximum of force that is tolerated by composite frames with 1,4,7 and 10 stories has been increased, respectively, by 59%, 49%, 67% and 46% than concrete frames. In addition, ductility of composite frames with 1,4,7 and 10 stories has been increased, respectively, by 65%, 50%, 37% and 30% than concrete frames with similar number of stories. The results have indicated the increase in initial and final stiffness of composite frames than concrete frames, in average, by 62% and 10%, respectively. Furthermore, a 116% increase in the energy absorption of composite frames than concrete frames has been observed.
Tarbiat Modares University
Modares Civil Engineering journal
2476-6763
18
3
2018
9
1
Study of effect of freezing and thawing cycles on bonding between fiber-reinforced self-compacting concrete with different water to cementitious ratios and paste volume as a repair layer and concrete substrate
129
139
FA
Ali
Sademomtazi
oveys
ghodousian
Self-compacting concrete as a type of concrete that has no need to vibration can use for complicated frameworks and in conditions that compaction is hard. So this concrete is an excellent choice for repair and retrofitting of many kinds of concrete structures such as marine structures, bridges and so on. These concrete structures may be under aggressive environmental conditions and harmful agents. One of the most important damages of concrete structures is because of freezing and thawing cycles especially in exposed ones, like bridge’s decks. So, study of effect of different parameters on quality of self-compacting repair layer, its bonding to substrate and its durability under chemical and physical attacks is very important. In this study, effect of paste volume, water to cementitious materials and polypropylene fiber dosages in mix design on rheological, hardened properties and bonding of repair fiber-reinforced self-compacting concrete (FRSCC) to concrete substrate and its durability for freezing and thawing cycles has been assessed. Bonding between FRSCC as a repair layer and concrete substrate had been evaluate using pull-off test. The tests that failed exactly from the bond surface considered as a successful ones, and all tests that failure had been occurred in repair layer or concrete substrate is eliminated from the results. Freezing and thawing test was conducted according to ASTM C666. Both freezing and thawing processes were made in water. To assessment of bonding of FRSCC as a repair layer to concrete substrate, we made 15 cm cubes of substrate layer with compressive strength more than 50 MPa to ensure that the failure doesn’t occur in this layer while pull-off test. After six months (to have concrete substrate without shrinkage),we saw the cubes divided them to 3 pieces. With analyzing of effects of fiber dosages on rheological properties of the mix designs, we found that an increase in fiber percentage that leads to smaller diameter in slump flow test, higher flow time in T50 test, higher time in V-Funnel test, and lower ratio in L-Box test. Also, while compressive strength had no significant changes, tensile strength and modulus of elasticity experienced a big increase through adding polypropylene fibers. Shrinkage of repair layer had a great decrease after adding polypropylene fibers. The optimum dosage of polypropylene fibers for hardened properties of the mix designs was found 0.1% by volume. We can see that because of the positive effect of fibers on decreasing of shrinkage and increasing of tensile strength of repair layers, bonding between the repair layer and concrete substrate increased greatly especially by adding 0.1% polypropylene fibers (by volume). Also, increase of paste volume and water to cementitious materials (summation of cement and micro silica) had negative effect on bonding of repair layer to substrate. That is because of increase of shrinkage of repair layer. Although, adding polypropylene fibers improved bonding of repair layer and substrate in freezing and thawing cycles, we see smaller results for bonding. We defined debonding index (DI) that presents the rate of debonding during freezing and thawing cycles. Higher DI, higher rate of debonding. As we can see, for mixes that containing polypropylene fibers, DI is bigger in comparison with the ones without them.
Tarbiat Modares University
Modares Civil Engineering journal
2476-6763
18
3
2018
9
1
Experimental Evaluations of Stabilization Methods for High Sulfate Soils in Iran Desert Roads
141
151
FA
Hossain
Noferesti
null
In this paper, the approach proposed methods for soil stabilization in road construction projects has been explored in the literature. Geological studies, chemical status and soil salinity, along with relevant tests, analyze data and present the results of theoretical studies, field and laboratory tests have been done. As a case study, Khur- Arababad Desert Road (KADR) is used. KADR is located in South Khorasan province, which has a desert climate and the plain- hills are evaluated in terms of topography. The annual average daily traffic volume of the road between 8500 to 9000 has fluctuated. A total of 11 soil samples were taken for studies of soil chemistry. In this context soil samples to determine the texture, pH, Electrical Conductivity (EC), the percentage of lime and plaster were tested. Results showed that most samples are placed in the category of saline soils. Defining what type of soil salinity that the region is facing, to determine the exact concentration of sodium, calcium and magnesium ions in the samples, advanced atomic absorption method was used. The soil is very high sulfate content at all stations. To study the physical and mechanical properties of soil 7 samples were chosen. The experiments included sieve test, the sand equivalent, the determination of specific gravity, determine the plastic limit and liquid limit, proctor compaction, California Bearing Ratio (CBR) and swelling test. The results showed that the use of lime for soil stabilization with respect to the necessary CBR number is quite responsive. It also seems that even in relation to the amount of lime, Lime 2% would suffice, although higher percentages such as 5 percent of lime will obtain more reliable results. The volume changes average with increasing the percentage of lime almost showed an increasing trend. Soil stabilization options including limestone, blast furnace lump slag and Ground-Granulated Blast-Furnace Slag (GGBFS), copper and magnesium oxide tailings. To determine the optimal amount of lime, examples of the 2, 5 and 10 percent of lime was prepared. The addition of four other factors, according to previous studies was 5% and 2% lime was added to half of the samples as a catalyst. Three modes including lime 5%, fine-grained magnetite 5% with lime 2% and copper tailings 5% showed the best performance in terms of increasing the CBR, uniform effect on all the samples, and a little swelling in treated samples in 7 days. Iron slags, especially when they are combined with lime, have demonstrated effective performance to CBR increasing, But this performance fluctuated sharply and not uniform in different sampling stations. Another reason was that the exclusion of iron slags was the economic disadvantage due to the long transport distance. There is also concern regarding the use of fine-grained magnetite that Inflationary in the long run to the formation of the mineral magnesium sulfate (MgSO4) in soil. Copper tailings and lime performance in preventing soil volume changes in processed samples approximately the same and a small amount of swelling has been observed in some samples. Therefore, the use of two additives including lime 5% or copper tailings 5% more is recommended.
Tarbiat Modares University
Modares Civil Engineering journal
2476-6763
18
3
2018
9
1
Investigation on the Correlations Between Different Physical and Mechanical Properties of Concrete Made with Recycled Concrete Aggregate
153
167
OTHER
In the last few years, increase in construction rate has caused several new challenges, including waste storage problems, environmental pollution problems, excessive mining of natural resources, and shortage in sand and gravels sources. One way to address these problems and reduce the negative environmental impacts is reusing concrete waste as a new material in construction. Concrete waste could be crushed, graded, and used as aggregate, referred to as recycled concrete aggregate (RCA), in producing new concrete. There is a common agreement that recycled aggregate concrete (RAC) has inferior properties compared to natural aggregate. Nevertheless, structural usage of RAC has been growing in the last decade. However, due to different engineering properties of RCAs compared to natural aggregates (NA), it seems that the existing correlations between different mechanical properties for normal concrete cannot be used for RACs. This research focused on this topic with the aim of finding suitable correlations between different parameters such as water absorption coefficient, compressive strength, tensile strength, and flexural strength for RACs. In this research, two different types of coarse recycled concrete aggregates [CRCA-1 and CRCA-2] and one type of fine recycled concrete aggregate [FRCA] were considered. Also, for improving the mechanical properties of RACs, two types of supplementary cementitious materials (SCMs), namely 8% silica fume and 15% zeolite were used. A total of 18 concrete mixes were considered and various physical and mechanical tests were conducted on concrete samples made according to the designated mix designs. Concrete mixes were categorized in two different groups. In the first group, both FRCA and CRCA-1 were used. As for the second group, 11 mixes were considered in order to investigate the effect of SCMs on the mechanical properties of RACs. Correlations between results of water absorption coefficient, compressive strength, tensile strength, and flexural strength were investigated and compared to those for normal concretes. The results showed that substitution of NA with CRCA resulted in inferior mechanical properties of concrete. It is worth to note that FRCA had a more significant impact on the mechanical properties of concrete compared with CRCA-1. Also, utilization of 8% silica fume or 15% zeolite as a SCM had a positive effect on the mechanical properties of RACs, such that mixes containing 30% CRCA-2 and 8% silica fume or 15% zeolite had similar 28-day compressive strength to the reference mix. On the other hand, the results showed that existing correlations for normal concrete cannot be used for RACs due to different behavior of RACs compared to normal concrete. The correlation between 28-day tensile strength and 28-day compressive strength for RACs had a bigger coefficient (0.52) compared to normal concrete (0.49). As well correlation between 28-day flexural strength and 28-day compressive strength for RACs in the chosen range of compressive strength (22-43 MPa) overestimated the flexural strength compared to ACI 318's. On the basis of this research, using the equations proposed by ACI 318 for estimating the mechanical properties of RACs is not recommended, and leads to misleading results.
Tarbiat Modares University
Modares Civil Engineering journal
2476-6763
18
3
2018
9
1
Assessment of Local Scour at Bridge Piers in Cohesive Soils
169
179
FA
The main reasons of bridge failure are local scour around the piers. In fact, they will be appeared as a scour hole in the river bed. Moreover, lack of control during the time, local scouring will threaten stability of structure. Therefore, determining of depth and dimension of the scour hole, also finding effective factors on scouring are important. Lots of researchers have studied the local scour around the bridge piers. Actually, they have proposed lots of appropriate techniques to control and to reduce scour around piers and bridge abutments. These proposed approaches are divided into two methods which are altering the flow and bed-armoring. In this research, the local scour around the bridge piers in a cohesive soils have studied. Particularly, the cohesive soils in form of mixture of Bentonite (montmorillonitic clay from 0 to 15%) and bed sediment (fine sand) used. In fact, this approach is a solution to challenge bed-armoring against existence shear stress. As a result, the proposed method will be used for controlling the scour. The experiments were carried out at the hydraulic laboratory of the Water Engineering Department, at Shiraz University, Iran. The laboratory flume was a rectangular cross section with 18 m length, 1.2 m wide and 0.4 m deep. According to channel geometry, the discharge and the depth of flow were determined to be 0.034 m^3⁄( s ) and 13 cm, respectively. In all experiments, the discharge and flow depth were constant. This study focused on the best compaction conditions, optimum clay content and the bed shear strength. The undrained shear strength of the soil was measured using an in situ miniature van shear apparatus. The best compaction conditions including optimum moisture and compaction energy. From standard Proctor test, the optimum initial water content Wopt and the optimum dry density ρdopt was determined. The compaction was expressed as the ratio of dry density ρd to maximum dry density ρdopt of the mixture. Therefore three relative compaction equal to70%, 80% and 90% used. Also, three water content optimum initial water content, optimum initial water content – 3% and optimum initial water content + 3% are used. The results indicate that for smaller clay content (5%) the shape of the scour hole was similar to that in sand sediment and is regular and symmetrical. For clay content equal to10%, scour hole is geometrically much irregular compared to that observed in sand bed and it is observed that a steeper slope of the scour hole in sediment mixture. The slope became steeper whit an increase in relative compaction. In fact, the higher relative compaction and the higher clay content increases shear strength and decreases the maximum scour depth. The samples compacted at optimum water content creates a structure with the most resistant to scour. The erodibility of samples compacted dry of optimum water content is less than samples compacted on the wet side of optimum. The results showed that under conditions which the amount of Bentonite equals at least 10% of dry weight in the mixture (Bentonite and sand sediment), relative compaction equal s to 90% and and water content equals to optimum moisture, simultaneously the local scour reduces by 100% in single pier. In addition, the influence of the compaction and type of clay mineral was investigated. The results show that Bentonite was more effective than Kaolin. If the 15% of dry weight Kaolin clay mixed with bed sand sediment to be used and relative compaction equal to 90% and water ontent equals to optimum moisture, simultaneously, the maximum scour hole was reduced only by 34%.
Tarbiat Modares University
Modares Civil Engineering journal
2476-6763
18
3
2018
9
1
A Finite Volume Formulation of Hydrodynamic Pressure in Dam-Reservoir Systems with non-uniform reservoir geometry
181
194
FA
University of Tabriz
In the present paper a new numerical simulation method based on finite volume is developed for calculating hydrodynamic pressure distribution in the reservoir of dams during earthquake excitation. An explicit finite volume scheme is applied for discretization of dynamic governs equation. In the proposed method the asymmetry effect of reservoir shape on hydrodynamic pressure distribution can be considered. In the simulation quadrilateral elements with center cell algorithm is used. Because of the negligible changing of hydrodynamic pressure in the cross direction with averaging, the average differential partial equation in central vertical plan of reservoir is solved. The absorption effects of bottom sediment and lateral wall are included in the analysis and an exact far end boundary condition is applied in the truncation boundary. Different approaches to the solution of the coupled field problems exist solution of the entire set of equations as one discretized system, referred to as the monolithic approach. This approach is often inefficient due to its attempt to capture with one discretization methodology the completely different spatial and temporal characteristics of fluid and the structure. The second approach often mentioned is the notion of strong coupling, referring to solvers which might use different discretizations for the fluid and the structure but which employ sub-iteration in each time step to enforce coupling between the fluid and the structure. In these methods, the governing equations for fluid and structure are discretized separately in each of the sub-domains and coupled using a synchronization procedure both in time and in space without sub-iteration. Weakly –coupled schemes have been extensively applied to a variety of different fluid-structure interaction problems of engineering interest in past ten years. wo vital issues when coupling two domains are: the method of data transformation between domains and what information must be transferred. The property of fluid adjacent of a structure such as density and viscosity are also key parameters in the efficiency of a numerical scheme.A dense fluid coupled with a structure cause a strong coupling and required some special technique to overcome corresponding difficulties. Key questions with this approach include properly enforcing boundary conditions at the solid-fluid interface, and accurately transmitting tractions between the solid and fluid. The biggest complaint about the explicit staggered partitioned solution procedure is the typical instability associated with the method,that is generally caused by the time lag between the integration of the fluid and structure equations. In the typical partitioned method, the fluid and the structure equations are integrated in time, and the interface conditions are enforced asynchronously. In the solution of coupled problems using partitioned methods, it is necessary to find a cost-minimization (optimization) compromise between a few passes solution with small time steps and a more iterated solution with larger time steps. This compromise may depend, among other things, in the degree of nonlinearity of the structural problem, which may require equilibrium iterations independently of the interaction effects. From the computational point of view, a one–pass solution with no iteration would be optimal, but stability consideration may prove this impractical.
Tarbiat Modares University
Modares Civil Engineering journal
2476-6763
18
3
2018
9
1
Numerical Modeling of Groundwater Flow in Steady State Using Isogeometric Method
195
206
FA
Solving the governing equations of a system is the most important issues that is always discussed in science and engineering fields. Since there are few equations that have analytical solution, many numerical methods have been proposed for solving the equations that have no analytical solutions. Numerical methods are developed by the advent of computers. Today, with using computers and these methods together, complicated equations in diverse areas can be resolved. Several numerical methods such as finite element method (FEM), finite difference method (FDM) and meshless (MFree) method have been suggested for solving partial differential equations. In this study, Isogeometric analysis method is engaged as a numerical method. Isogeometric analysis was developed by Hughes in 2005 in order to eliminate the gap between the world of finite element analysis and computer modeling. This method uses the same basis functions, in the process of modelling. Isogeometric method provides the possibility of simulation in irregular and complex geometry domains and also removes errors due to the multiple elements. Two variable NURBS basis functions are defined by B-spline basis functions. B-spline basis functions are calculated by the Cox–de Boor recursion. In this study, Birjand aquifer is modeled in two dimensions by the Isogeometric analysis using four-point Gauss integration method. The mentioned aquifer is defined by 1274 points and 836 control elements. After creating the geometry of the aquifer by control points and knot vector, NURBS basis functions and their derivatives were calculated. Then, with using input information, such as hydraulic conductivity coefficients, boundary conditions, precipitation rates and the sources and sinks, water table is computed. In order to allocate hydraulic conductivity coefficients of the aquifer, the domain is divided by the GIS software to multiple homogeneous Thiessen. According to the location of NURBS elements in the aquifer, a value has been assigned to NURBS elements. In Birjand aquifer there are boundary conditions with constant head. For enforcing the boundary conditions, 83 points of control points were defined as fixed head. There are 190 wells in the Birjand aquifer, the Extracted water from the wells were used as the discharge rate in the model. Also, 15 percent of the amount of rainfall was considered as the recharge rate in 2011-2012 period, the value of recharge rate is 0.0000727 m/day based on rain gauges. In order to ensure the accuracy of modeling the results of Isogeometric method is compared with finite difference method solutions and observation data, the relative mean error of Isogeometric method is 0.000256. With using Isogeometric method the consumed time for running the MatLab code is around 400 seconds. In order to evaluate the model, three criteria is calculated. Mean error (ME), mean absolute error (MAE) and root mean square error (RMSE) whose values are 0.09, 0.34, and 0.459 respectively. The values of the error and computation time has shown the power of this method in modeling of groundwater flow. Finally, Birjand aquifer groundwater balance was calculated using the input values, extracted water and water storage in plain. By studying the model balance and actual balance of aquifer and comparing them with each other, it is determined that the change in the volume of the aquifer in the time period considered is close to that of the aquifer, which indicates the accuracy of the model.
Tarbiat Modares University
Modares Civil Engineering journal
2476-6763
18
3
2018
9
1
Statistical analysis of simultaneous removal of salinity and organic loading from wastewater using phytoremediation process
207
220
FA
Hossei
Ganjidoust
P.O.Box 14155-4838
High salinity wastewater is one of the main environmental issues. Using biological treatment systems to treatment this kind of wastewater is hard due to loss of microorganisms. Phytoremediation is one of the main methods to treat this type of wastewater, which is an environmental friendly and a low cost process. In this research we aim to study operation of two plants, including chrysopogon zizanioides known as Vetiver grass and Cyperus alternifolius know as umbrella, to simultaneous removal of salinity and organic loading. We assume five different combination of wastewater which are called statistical treatments and characterized by different amount of salinity and COD. Statistical treatments contain wastewater with Electrical conductivity 0,2.5,5,7.5,10 (ds/m) and 0,75, 150, 250, 300 (mg/L) COD respectively. Pure water is used as control treatment. Results show that in reactors with low level of salinity, both plants have better operation in removing of salinity and organic loading. For electrical conductivity above 5 and 7.5 ds/m, signs of salinity stress including Chlorosis and withering seen in leaves of both plants. For chrysopogon zizanioides, different statistical treatments results in different removal of organic loading range from of 60 to 12 percent. Salinity removal is ranged from 53 to 4 percent. For Cyperus alternifolius reduction in organic loading and salinity are respectively varied from 49 to 5 and 20 to 1 percent. Results of this study in laboratory scale shows that using phytoremediation method in constructed wetland for different combination of salinity and organic loading results in 17 and 15 percent reduction in salinity and wastewater for each unit of chrysopogon zizanioides and Cyperus alternifolius respectively. By increasing the level of salinity the trend of electrical conductivity is decreased. For the wastewater with 2.5 ds/m electrical conductivity, the most absorbing salinity absorbing is seen for both plants, which is 53 percent for chrysopogon zizanioides and 30 percent for Cyperus alternifolius. Reduction in capability of absorbing salt occurred for both plants due to completeness of salt aggregation capacity of the plant. By increasing the electrical conductivity of wastewater solution, wither and reduction in operation of the plant is seen as well. Increasing the salinity level results in decreasing the trend of organic loading reduction. COD removal for chrysopogon zizanioides is greater than Cyperus alternifolius in all reactors, which is related to natural characteristics of this plant, higher tolerance of salinity fluctuations and different phytoremediation mechanism. In statistical analysis of morphological characteristics of plants, the statistical population is all plants for different combination of wastewater and statistical controls. For each plant, results of different statistical treatments are compared to each other. The null hypothesis for independent tests assumes that the waste water treatment doesn’t have effect on morphological characteristics of the plant. So, rejecting this hypothesis means that all combination of wastewater doesn’t have the same effect on morphological characteristics. This hypothesis is considered separately for each of morphological characteristics by use of two sided independent sample T-test. These characteristics included dry weight, mean leaf area, mean stem diagonal and root volume. Results indicate that increasing salinity will affect the morphological characteristics with 95% confidence. Fitting the generalized linear regression shows that existing of organic loading doesn’t have meaningful effect on morphological characteristics, and level of salinity of wastewater affect the growing characteristics of the plant.
Tarbiat Modares University
Modares Civil Engineering journal
2476-6763
18
3
2018
9
1
Evaluation of Drift Distribution in Steel Moment Frames Designed Based on Performance-Based Plastic design Approach Considering Soil-Structure Interaction Effects
221
236
FA
Gholamrezar
Ghodrati Amiri
School of Civil Engineering, Iran University of Science & Technology, Tehran, Iran
Behnoud
Ganjavi
school of civil engineering, babolsar, Mazandaran, Iran
Abolfazl
Gholamrezatabar
School of Civil Engineering, Center of Excellence for Fundamental Studies in Structural Engineering, Iran University of Science & Technology, Narmak, Tehran, Iran
It is well known that structures designed by current codes experience large inelastic deformations during major earthquakes. However, current seismic design practice in almost all seismic design provisions such Iranian Seismic Code is based on elastic structural behavior and accounts for inelastic behavior only in an indirect manner through certain modification factors such as strength reduction factor. Under moderate to severe earthquakes, inelastic activity, including severe yielding and buckling of structural members can be unevenly distributed in the structure, which may result in global collapse or costly repair work. Recently, a new design method has been developed and referred to as Performance-Based Plastic Design (PBPD). This method directly accounts for inelastic behavior by using pre-selected target drift and yield mechanism as key performance limit states. In this paper, for the first time, the effect of soil-structure interaction (SSI) on drift demands distribution along the height of the steel moment frame (SMF) structures designed with performance-based-plastic design (PBPB) approach under strong ground motions are parametrically investigated. The soil beneath the structure is considered as a homogeneous elastic half space and is modelled using the concept of Cone Models. For the the steel moment frame structures, the design base shears are calculated by using the modified energy balance equation and new lateral force distribution based on inelastic analyses. The new distribution of the lateral forces for performance-based plastic design was used from shear proportioning factors that were derived from the relative distribution of maximum story shears obtained from nonlinear dynamic analyses. Then, plastic design method is employed to design the beams and columns with the calculated design base shear and to satisfy the requirements for strong column-weak beam mechanism, which results in the pre-selected yield mechanism. The plastic design procedure proposed herein can be one of the simplified approaches without requiring sophisticated computer nonlinear analyses, by using the preselected yield mechanisms and target drifts. The energy dissipation capacity of the structure designed by these procedures can be less than that required to prevent collapse under severe ground motions. The purpose of this design procedure is to avoid collapse mechanisms characterized by poor energy dissipation capacity, such as soft-story mechanisms. To this end, the plastic hinges should be developed only in beams and at the column bases of the structure during severe earthquakes. The system is then subjected to 20 different earthquake ground motions and the analyses are performed directly in time domain using direct step-by-step integration method. Effect of various parameters including fundamental period, inelastic behavior, SSI key parameters on strength reduction factor and structural damage distribution are examined. The adequacy of different lateral loading patterns is also parametrically investigated. Results indicate that only mid-rise SMFs designed based on current PBPB approach could have the best performance in SSI systems. It is also demonstrated that under slight and moderate SSI effects, SMFs designed according to various load patterns tend to more uniform distribution as compared to the fixed-base counterparts. However, all of them lose their efficiency when the SSI effects and inelastic response are pronounced. Moreover, the influence of SSI key parameters, fundamental period and ductility ratio on dispersion of the drift results are evaluated and discussed.
Tarbiat Modares University
Modares Civil Engineering journal
2476-6763
18
3
2018
9
1
Experimental Investigating on the Effects of Two Obstacles on Behaviour of Turbidity Current
237
249
FA
mandana
naji abhari
Turbidity currents account for transporting sediments into reservoirs, seas, and oceans. Therefore, understanding dynamics of these currents for sedimentation and erosion is very important. In this paper, the effects of two obstacles on the behaviour of turbidity currents investigated experimentally. An 11 m long rectangular channel (11 m×0.6 m×1.0 m) with the bottom slope of 0.25% was used to run the experiments and a 3 m3 tank along with a constant head tank were served as the turbid water supplier. Two triangular obstacles were installed at predefined locations from the sluicegate. Then the experiments were carried out and the results compared with those from without obstacle condition. Velocity and concentration profiles at the upstream of first obstacle and between the first and second obstacles are measured by Vectrino at quasi-steady conditions and compared to those of without obstacle conditions showing a significant decrease of velosity in the presence of the two obstacles specially between the two obstacles and also at the downstream of the second obstacle. Fluid volume discharge per unit width and suspended sediment transport rate are calculated based on measured velocity and concentration. Also, the effects of inlet Froud number on the fluid volume discharge per unit width and suspended sediment transport rate was investigated. The results show that presence of obstacles introduces new regions to velocity profiles and two ponds of turbidity currents are formed at the upstream of the first obstacle and between the two obstacles. The hydraulic conditions at these ponds make a suitable condition for the suspended particles to be trapped and hence the sedimentation. Variation of the suspended sediment transport rate and the fluid volume discharge per unit width depend on obstacle location. These parameters at the upstream of the first obstacle are directly in proportion to the inlet Froud number while at the downstream the second obstacle and between the obstacles are inversely proportional. By decreasing the inlet Froud number, the volume discharge per unit width increases at the upstream of the first obstacle wheras, the amount decreases between the obstacles. Also, as the inlet Froud number decreases, the suspended sediment transport rate increases at the the upstream of the first obstacle but the value decreases between the obstacles and downstream of the second obstacle resulting the increase of the trap efficiency. The obstacles become more effective in controlling the turbidity currents when the inlet Froud number decreases. The first obstacle is 1.8 times more effective on reduction of local sedimentation rate than the second obstacle. These parameters at the upstream of the first obstacle are directly in proportion to the inlet Froud number while at the downstream the second obstacle and between the obstacles are inversely proportional. By decreasing the inlet Froud number, the volume discharge per unit width increases at the upstream of the first obstacle wheras, the amount decreases between the obstacles. Also, as the inlet Froud number decreases, the suspended sediment transport rate increases at the the upstream of the first obstacle but the value decreases between the obstacles and downstream of the second obstacle resulting the increase of the trap efficiency. The obstacles become more effective in controlling the turbidity currents when the inlet Froud number decreases. The first obstacle is 1.8 times more effective on reduction of local sedimentation rate than the second obstacle.