Tarbiat Modares University
Modares Civil Engineering journal
2476-6763
17
3
2017
9
1
Effect of Internal Curing On Mechanical Properties and
Durability of High-Strength Concretes
1
8
FA
Jamal
Ahmadi
Alireza
Panahi
HPC has become popular due to its superior mechanical and durability properties.HPC signiﬁcantly reduces maintenance costs and enhances service life. Internal curing of high performance concrete (HPC) by pre-saturated lightweight aggregates is a wellestablished method of counteracting self-desiccation and autogenous shrinkage. However, by introducing the internal water reservoirs strength and durability properties can be injured. Concrete is a strong and durable material that has been utilized since the beginning of civilization. It is understood that many of the early structures are deteriorating or have already deteriorated away; however, with the knowledge of the material properties of concrete that is available, it is hard to imagine that concrete structures is prematurely failing before their intended service life. There are many factors involved in these failures, some of which are due to environmental conditions and others which have arisen from human errors or lack of knowledge. Early age properties of concrete are vital to its long-term performance. Many contractors and owners are interested only on the strength of concrete in 28 days or 90 days, that they overlook the importance of other early age issue especially when the mix design has water to cementitious material ratio (w/cm) lower than ~0.42. Internal curing with LWA has been successfully used recently in large construction projects of normal density concrete structures. For example, in January 2005, about 190 000 m of internally-cured concrete was used in a large paving project in Hutchins, Texas. Although the benefits of internal curing for high-performance concrete structures have been evidenced in the laboratory and some field investigations (such as those previously mentioned), the literature does not provide any significant quantitative information regarding the additional extension of the service life that can be achieved by the use of internal curing in concrete structures. A great concern for design engineers and contractors is whether the concrete will achieve the specified compressive strength and all durability requirements in the structure during service. Cusson & Hoogeveen (2008) demonstrated that internal curing can reduce autogenous shrinkage considerably without affecting the strength and stiffness of high-performance concrete. This was achieved by reducing the amount of mix water in the concrete by an amount equal to that used in the LWA for internal curing, thus reducing the effective watercement ratio (w/c ) of the concrete when using higher quantities of pre-soaked LWA. Tests by the widely accepted methods of durability assessment, such as resistance to chloride penetration, air permeability, water absorption, autogenous and drying shrinkage and mass loss, were conducted on HPC mixes made at water to cement ratios in the range of 0.21–0.33. The effect of internal curing on the durability related properties of high-performance concretes as a function of water to cement ratio is reported.
Tarbiat Modares University
Modares Civil Engineering journal
2476-6763
17
3
2017
9
1
Design of gravity quaywalls via nonlinear analysis of soil-quay ineraction
9
20
FA
There are several quay types parallel to the shore line such as the walls constructed by piles, sheetpiles or gravity walls. Among these types of structures, the gravity quaywalls are widely used because of their simplicity of structure and ease of construction. Usually, it is the best alternative particularly in the locations with acceptable soil strengths. Weight of the blocks provide the stability of the quaywall against overturning and sliding and therefore, their dimensions are determined based on the applied loads on the quay structure. The most important load is the soil pressure that increases the lateral loads acting on the quaywall particularly during an earthquake condition. For design, the soil pressure usually converts into a static load by utilizing the seismic coefficient method. Analytical equations such as the Mononobe-Okabe formula are usually employed to calculate the applied soil pressure. However, some researchers believe that these analytical formula do not appropriately express the real behavior of the soil, and therefore, they can not be used for a proper design. There are, actually, some simplified assumptions in calculating the applied soil pressure those decrease the accuracy of the commonly used methods for quaywall design. The main assumptions are neglecting the nonlinear behavior of the soil and neglecting the flexibility of quay blocks. Due to the importance of the soil pressure in the quaywall design, these assumptions are investigated numerically in this study by making use of two well known FLAC and ANSYS softwares. For this purpose, the quaywall of Shahid Beheshti port is selected as a case study and the soil pressure around this quaywall is calculated by modeling the nonlinear behavior of the soil via using the Mohr-Coulomb constitutive model. In addition, the effect of the block rigidity on redistribution of the soil pressure beneath the quay structure is studied by a 3-D modeling of the lowest block located on linear springs (representing the supporting soil). To study the importance of each above mentioned assumption individually, two seperated models are utilized separately. According to the results, the pressure distribution under the quay wall is more uniform in the case of employing the nonlinearity of the soil. The total pressure is, however, less than the total calculated pressure by analytical formula that shows the Mononobe-Okabe formula are not accurate, but its results are overestimated for the studied problem. In addition, results show that the simplified methods can not be used for design of the lowest block because the value and the location of the maximum moment along this block changes due to its rigidity. As a result, neglecting the block deformations what is done in simplified methods is not acceptable for design purposes. It should be noted that the lowest block is so important is providing the global stability of the quaywall because its failure can lead to a total failure of the quaywall. On the other hand, all blocks are supported on this block and consequently, its repair would be too difficult even in the case of any small failure.
Tarbiat Modares University
Modares Civil Engineering journal
2476-6763
17
3
2017
9
1
Development and Validation of the Modified Barium Chloride Method for CEC Measurement and Determination of Accurate Exchangeable Calcium Cation Concentration in Carbonated Clayey Soils
21
34
FA
Vahid
Ouhadi
Bu-Ali Sina University, Faculty of Eng., Civil Eng. Department, Iran
Cation exchange capacity (CEC) is known as the main geo-environmental characteristic of the soil in which in uncontaminated soils it is a direct function of soil swelling, cohesion, and specific surface area. According to the results of the several researches the current method for determination cation exchange capacity and exchangeable cations in carbonated soils has faced to several problems due to the solubility of calcium carbonate. Carbonated soils can be found in many parts of Iran, therefore, in this type of soil, it is essential to study the impact of carbonate calcium concentration of soil upon the measures quantity of soil CEC. In addition, the selection of optimum soil weight for CEC experiment is a common problem in previous researches. The objective of this research is to develop and to validate the modified barium chloride method for CEC measurement and determination of accurate exchangeable calcium cation concentration in carbonated clayey soils. To achieve this objective, the current method of barium chloride is modified by saturation of exchangeable electrolyte with calcium carbonate. In this step, the dried sample of bentonite with the initial weight of 0.3, 0.5, and 0.7 grams were mixed with 30 ml of 0.1 M BaCl2 for two hours period. Then, samples was centrifuged at 2500 rpm for a 5 minutes period. The achieved electrolytes was used for analysis of exchangeable cations and the measurement of barium concentration. Generally, by the use of the current barium chloride method for CEC measurement of carbonated soils, one observes an increase in the measured quantity of CEC of a single soil sample as its carbonate concentration increases. This can be attributed to the solubility of calcium ions and precipitation of barium ions which cause a wrong measured quantity for CEC. Furthermore, for validation of the proposed modified method, a bentonite sample was de-carbonated with acid. The cation exchange capacity of this sample was measured before and after de-carbonation. Moreover, the CEC was measured for laboratory added calcium carbonate to the de-carbonated bentonite. The achieved results show around 60% reduction in the measured quantity of CEC of bentonite in the proposed modified method in comparison to the common barium chloride method. This is attributed to the prevention of the precipitation of barium in the modified method. In addition, according to the achieved results in the validation section of this research, the reduction of the deviation of CEC and summation of exchangeable cations, in some cases was around 80%. For instance, for a sample with initial weight of 0.3 grams, the measured quantity of calcium ion is decrease from 66.45 to 8.09 cmol/kg-soil. The achieved results show that in the modified method the measured quantity of CEC and the summation of exchangeable cations was independent to the weight of carbonate in the soil sample and soil: electrolyte ratio. It should be emphasized that according to the results of this paper, the standard deviation and coefficient of variation of the proposed modified method show 5 to 12 times reduction in comparison to that of for the current barium chloride method.
Tarbiat Modares University
Modares Civil Engineering journal
2476-6763
17
3
2017
9
1
Numerical study of cyclic and failure behavior of built-up concentric braces containing double angle sections
35
46
FA
Amin
Izadi
Ali Akbar
Aghakouchak
The most important characteristic of brace frames is their significant and appropriate stiffness as well as their compression strength against earthquake forces. Built-up special concentrically braced frames (SCBFs), which contain double angle braces, are among the common steel structural systems resisting lateral loads. Along the built-up brace length, the stitch and connector distances make significant role in cyclic and ductility behavior of braced frames due to possibility of out of plane buckling.The results of experimental studies of built-up double angle braces illustrate that setting the stitches closer to each other can improve the post buckling behavior of systems, resulting in increasing the final compression strength, close to box-shaped brace strength. In addition, an individual member buckling is possible by increasing the stitch distances along built-up braces. According to AISC seismic provisions regarding built-up SCBFs, the slenderness ratio of individual elements between the connectors should not exceed 0.4 times the governing slenderness ratio of the built-up member. Also, connecting built-up members by the use of welding is not permitted within the middle one-fourth of the clear brace length. In fact, AISC seismic provision has prohibited the use of stitches and connectors in the protected zones of built-up specially concentrically braced frames such as the center one-fourth of the clear brace length and a zone adjacent to each connection equal to the brace depth in the plane of buckling. In this research, seismic provisions related to built-up diagonal and X-braced SCBFs are numerically investigated under cyclic loading using a single-bay single-story frame. The numerical study is performed on models, which contains parameters such as back-to-back and face-to-face connection types of built-up members. Seismic behavior of these braces are investigated from the view points of cyclic and failure behavior. This investigation is performed on both types of diagonal and X-braced steel frames. The cyclic behavior of systems is studied based on post buckling capacity, structure initial stiffness, and final compression strength. Failure behavior of systems is investigated with regard to failure cycle and ductility capacity. In order to evaluate of seismic behavior and ultimate ductility of the numerical models, regarding to proximity of initiation and propagation of steel cracks, the concept of plastic equivalent strain is used to predict system failure. The results of this study show that increasing the number of stitches or decreasing their distances along the length of the built-up members may not necessarily improve behavior of braced systems.That means inelastic deformation consent will probably occur in individual elements between stitches resulting in earlier failure of braces. Therefore, current seismic provisions such as not exceeding the slenderness ratio of individual elements between stitches from 0.4 times of the governing slenderness ratio of the built-up member for compression sections, are conservative in SCBFs and can be changed according to the type of braces. In addition, Failure of double angle back-to-back diagonal braces occurs sooner in comparison to face-to-face braces. Also, in X-braced frames, cyclic and failure behavior of built-up face-to-face braces are more desirable than the similar back-to-back braces in general.
Tarbiat Modares University
Modares Civil Engineering journal
2476-6763
17
3
2017
9
1
Investigation of the effects of vertical link beam length on steel structures residual displacement
47
60
FA
Akbar
Bathaei
University of Tehran
University of Tehran
There is a debate among earthquake engineers that the structural and non-structural damages initially occur due to lateral loads caused by earthquake excitation. American provisions, including FEMA356 estimates structural performance by means of maximum deformation demand. However, in addition to the maximum deformation, residual displacement plays an important role in structural performance. Amplitude of residual displacement is an important parameter in technically and economically determining rehabilitation of damaged structures for resisting aftershocks. In this study, residual displacements of a five-story steel frame is designed with vertical link beam as well as the effect of vertical link beam length have been investigated. For vertical link beam, the IPE sections with typical steel is considered, instead of using boxes and H-shaped cross-section.The IPE section has some advantages than box section such as lower cost, easier installation and replacement. The Vertical link beams with IPE cross-section has been studied in 5 separate models with length of 20, 25, 30, 35 and 40 centimeters. In this paper, experimental results of a frame model with vertical link beam tested in structural laboratory of Building and Housing Research Center (BHRC) has been used for verification of numerical model. As one of the fastest nonlinear softwares, OpenSees (Open System for earthquake engineering simulation) has been used for structural modeling. The steel material that has been used in this model is uniaxial material steel 02. In the following, seven near field and seven far field earthquake acceleration time histories that scaled by 2800 standard, are used analysis of five-story and five-bay structure with chevron bracing system. According to the seismic design of structures if ductile elements is used in a structure, then beams and columns should remain elastic during earthquake, while ductile elements dissipate input energy by nonlinear behavior of ductile members. By considering of the results, the vertical link beam with length of 20 and 25 centimeter for far field earthquakes and 20 centimeter for near field earthquakes have the best performance compare to the other cases. The Bam earthquake is selected to investigate of hysteresis diagram of the vertical link beam energy dissipation. The results for near field earthquake like the Bam earthquake show that link beam with length of 40 centimeter with moment behavior, has low energy dissipation capability. Furthermore, the vertical link beam with 40 centimeter length causes more residual displacement and yielding. By considering the station with equal 104.28 km distance from center of earthquake can use the Bam record as a far field earthquake. In this case link beams with more than 25 centimeter length have more fluxing. However, the link beams with length of 20 and 25 centimeter have better seismic performance. Considering the RMS (Root Mean Square) parameter as a controller criterion the vertical link beam with length of 20 centimeter is more suitable for near and far field earthquakes. Considering the seismic performance parameters of vertical link beam like appropriate stiffness, high stability, energy dissipation capability, appropriate control of maximum response of structure and less residual displacement, the vertical link beam with length of 20 centimeter has the best seismic performance for near and far field earthquakes compare to the other cases.
Tarbiat Modares University
Modares Civil Engineering journal
2476-6763
17
3
2017
9
1
Study of Backwater and Hydraulic Jump Phenomena in Historical Bridges by Computational Fluid Dynamics
61
69
FA
Babak
Khaiatrostami
Research Manager, Ardabil Regional Water Company
Yousef
Hassanzadeh
Prof., Faculty of Civil Engineering, University of Tabriz
Siamak
Khaiatrostami
University of Mohaghegh Ardabili
Establishing a bridge in a waterway changes flow characteristics. Most of these changes derive from geometric details of bridge. Circular and pointed (nested) arches have been used as opening geometry in historical bridges. Historical bridges are valuable heritage and protection of them is important because of cultural continuity. Hydraulic study is a tool to recognize these structures and their design philosophy. In this research, effect of three opening geometry on backwater and hydraulic jump phenomena was numerically studied by Flow-3D software. The FLOW-3D software was selected because not only previous studies indicated that flow around a bridge as well as in a compound channel involves significant 3D characteristics but also it is a powerful hydraulic engineering design tool to model 3D free surface flows. The performance of FLOW-3D was tested using of experimental data obtained from test series which were conducted at the Hydraulic Laboratory, Birmingham University on two opening semi circular bridge model in compound channel (AMOSEC) in which the width of model was 0.10 m. Laboratory tests were carried out for low flow conditions without flow contact with the lower bridge deck (21 to 35 lit/sec). In order to study submerged (High Flow) condition, a program has been developed in the MATLAB environment to extrapolate discharges and related normal depth for 40 to 60 lit/sec discharges. Three opening geometry with the same area as AMOSEC model designed in the AutoCAD. DWG files converted into the Stereolithography format and imported into the Flow-3D.The computational domain, 18 m long and 1.213 m wide, was divided into structured grids. This domain involved nonuniform rectangular grids of 950, 100 and 26 to 40 cells in the x-, y- and z-directions, respectively. Inflow boundary condition was specified as discharge. The downstream boundary condition was specified with a constant fluid height equal to the uniform depth. The sidewalls as well as the channel bottom were defined to be no slip boundaries. On the top, the symmetry (atmospheric) boundary condition was assigned to describe the free surface flow condition. Measured uniform flow depth with zero velocities for each run was assigned to each computational cell to set the initial flow condition. Free surface modeled by VOF and turbulence by two equation K-ɛ methods. Then, a total of 27 runs carried out until steady state resulted. The results indicate that pointed arch geometry makes maximum afflux for both low flows (sub-soffit) and high flows (super-soffit) conditions in all models. Emerging Location of afflux at longitudinal axis is the same for all of the models. Length of hydraulic jump for pointed arch geometry is maximum under low flow condition and minimum under high flow condition. Hydraulic jump starts near the pier for rectangular opening geometry in comparison with others. Critical shear stress due to hydraulic jump is minimum for rectangular geometry and maximum for pointed arch in all discharge conditions. Circular opening geometry produces less upstream flooding and less possibility of downstream bed destruction, so it has advantages on pointed arch geometry.Out of the structural reasons, whole of these results may be considered as hydraulic reason of evolution of pointed arch to semi circular geometry from Safavid to Qajar era.
Tarbiat Modares University
Modares Civil Engineering journal
2476-6763
17
3
2017
9
1
Influence of effective chloride corrosion parameters variations on corrosion initiation
69
77
FA
Mohammad
Ghanooni Bagha
Department of Civil Engineering, East Tehran Branch, Islamic Azad University, Tehran, Iran
saeed
Asgarani
M.Sc. of Structural Engineering, Faculty of Civil Engineering., Iran University of Science and Technology
Reinforcement inside the concrete is protected from corrosion and its damages until several years after the construction. After corrosion initiation, the Cross Section of Reinforcement begins to reduce and often load bearing of the reinforced concrete structure will be reduced significantly. Corrosion of reinforcements in concrete in polluted and contaminated areas can be occurred in two ways: Chloride and Carbonation. Chloride ion ingress is one of the major problems that affect the durability of reinforced concrete structures such as bridge decks, concrete pavements, and other structures exposed to harsh saline environments. Corrosion occurrence and development in reinforced concrete structures increase the steel volume and produce products with volume of about 2-7 times the steel initial volume. This volume increase, which is due to cracks, reduces the compressive and tensile strengths in reinforced concrete structures. Therefore, durability based design of concrete structures in marine areas has gained great significance in recent decades and various mathematical models for estimating the service life of reinforced concrete have been proposed. In spite of comprehensive researches on the corrosion of reinforced concrete, there are still various controversial concepts. Effect of environmental conditions on durability of concrete structures is one of the most important issues. Hence, regional investigations are necessary for durability-based design and evaluation of the models proposed for service-life prediction. The Persian Gulf is one of themost aggressive regions of the world because of elevated temperature and humidity as well as high content of chloride ions in seawater. Corrosion of reinforcement due to chloride ions attack causes enormous damages to structures in severe condition of marine environments. Normally, high alkaline property of concrete (PH≈13) forms a protective oxide layer on the steel surface. This is called a passive protection. The dioxide existing in the atmosphere or the chloride in the concrete environment along with the moisture and the oxygen can penetrate via the concrete pores and cracks and can reach the armature surface; then, by reducing concrete alkalinity, they cause armature corrosion inside the concrete by destroying the protective oxide layer on the steel. Chloride ions reach the passive layer according to the explained pattern and they begin to react in the passive layer when the amount of chloride ions go beyond the critical value and cause perforation corrosion. Since each influencing factor in the life time of the structure is subject to random variability and inherent uncertainties, a stochastic approach is utilized to predict the time for initiation of the corrosion. Based on Fick’s law, time for corrosion is a function of surface chloride, critical chloride, concrete cover thickness, and diffusion coefficient. The most common models service-life prediction of reinforced concrete structures under load chloride, only produce a limited definite time for the start of corrosion. In this paper monte carlo simulation use for service-life prediction of reinforced concrete structures of predict the time of corrosion initiation, and shown the influence of mean and standard deviation variations for each of the parameters that affect the occurrence of corrosion, on the time of initiation corrosion and impact of these factors on the probability initiation corrosion.
Tarbiat Modares University
Modares Civil Engineering journal
2476-6763
17
3
2017
9
1
Three Dimensional Hydrodynamics of Sudden Dam Break in Curved Channels
77
88
FA
A.
safarzadeh
m.ardabily
In this paper, the dam break phenomena has been simulated in curved rivers using 3D numerical model, Flow-3D. It utilizes the finite volume scheme for structured meshes was used for solving the unsteady Reynolds-averaged Navier-Stokes equations in conjunction with the RNG k-ε closure model. In the utilized software, the Fractional Area/Volume Obstacle Representation (FAVOR) method is used to inspect the geometry in the finite volume mesh. FAVOR appoints the obstacles in a calculation cell with a factional value between 0 to 1 as obstacle fills in the cell. Fluid surface shape is illustrated by volume-of-fluid (VOF) function F(x,y,z,t). With the VOF method, grid cells are classified as empty, full, or partially filled with fluid. Cells are allocated in the fluid fraction varying from zero to one, depending on fluid quantity. The pressure and velocity are coupled implicitly by using the time-advanced pressures and time-advanced velocities in the momentum and continuity equations, respectively. FLOW3D solves these semi-implicit equations iteratively using relaxation techniques. In this paper the GMRES technique has been used as pressure implicit solver. A flux surface is a diagnostic feature in FLOW-3D for computing fluid flow rates. It can be used to obtain time-dependent information about the flow in different parts of the domain. A typical flux surface is a 100% porous baffle with no flow losses, so it does not affect the flow in any way. This feature gives the opportunity to determine the flood hydrograph at various stations downstream of the dam. Effects of curve angle and radious of curvature on the flood wave propagation and unsteady flow features along the curved reach, downstream of the dam has been investigated. Results showed that at the initial instants of the dam break in the straight channel, due to the effects of the dynamic wave, flood hydrographs at the dam location and at a distance downstream of the dam have local peak values, while in the curved chnnel cases, the flood wave becomes unstable immediately after the dam break and the local peak occures just at the dam section. The curved reach decelerate the flood wave propagation compared to the straight channel. Effect of channel curvature on the movement of the flood wave along the inner bank is higher than the outer bank and also the centerline of the curved channel. By decreasing the central radious of the bend, slope of the rising limb of the hydrograph and also the peak discharge, attenuates. Furthermore, the peak discharge time reduces. Unlike to effects of the curvature of the bend, increasing the bend angle does not affect the peak discharge. Changing the bend curvature and curve angle has no effect on the falling limb of the flood hydrograph at various stations downstream of the dam.
Tarbiat Modares University
Modares Civil Engineering journal
2476-6763
17
3
2017
9
1
Evaluation of the effective parameters on the behavior of shallow RC shear walls considering soil-structure interaction
90
102
FA
mohammad
ghanizade
Abdolreza
Sarvghad Moghadam
Chairof Structural Engineering Faculty, International Institute of Earthquake Engineering and Seismology
Masood
Farzam
Assistant Professor of Structural Engineering Department, Tabriz University
Short reinforced concrete shear walls with aspect ratio less than 2 are commonly utilizes in strengthening of low rise structures. These walls demonstrate adequate lateral load strength while they have low ductility comparing with high rise walls with same lengths. Considering typical span length of such a walls– between adjacent column distances -there are no need to motivate all of lateral bearing strength of them and only taking to account a portion of strength will be sufficient for the purpose of strengthening of the structure. In this paper it will be shown that tacking into account the shape and length of foundation and interaction of the soil-structure the ductility of the wall is increased. Furthermore, effect of the soil stiffness on the behavior of the wall is studied. The short shear wall which has been studied experimentally by the NUPEC of Japan is adopted for numerical simulation by the commercial nonlinear analysis software ATENA 3D. The wall has been subjected to the predefined level of axial load and the increasing cycling lateral deformations. Sensitivity of the behavior of wall to mesh dimensions and the affecting parameters of concrete models such as fracture energy, tension softening and tension stiffening coefficient, shear modulus reduction after cracking, fixed or rotating crack modeling among the other affecting parameters are investigated to verify the model. Because of symmetry only one half of the wall is modeled. Reinforcing bars are modeled discretely taking into account the bond-slip between concrete and bars. The results of verified model are used to study the sensitivity of a proposed short shear wall by IIESE for strengthening of low rise masonry buildings, to the parameters of length and shape of the footing together with and foundation soil property. It is shown that with increasing the length of footing, base reaction coefficient and the embedment depth of footing the bearing capacity of shear walls showing rocking behavior is increased but the ductility is decreased. For structures which need a limited level of strength increase or a demanded ductility, the length or embedment length of the footing may choose intentionally to motivate the rocking behavior of foundation. Short reinforced concrete shear walls with aspect ratio less than 2 are commonly utilizes in strengthening of low rise structures. These walls demonstrate adequate lateral load strength while they have low ductility comparing with high rise walls with same lengths. Considering typical span length of such a walls– between adjacent column distances -there are no need to motivate all of lateral bearing strength of them and only taking to account a portion of strength will be sufficient for the purpose of strengthening of the structure. In this paper it will be shown that tacking into account the shape and length of foundation and interaction of the soil-structure the ductility of the wall is increased. Furthermore, effect of the soil stiffness on the behavior of the wall is studied. The short shear wall which has been studied experimentally by the NUPEC of Japan is adopted for numerical simulation by the commercial nonlinear analysis software ATENA 3D. The wall has been subjected to the predefined level of axial load and the increasing cycling lateral deformations. Sensitivity of the behavior of wall to mesh dimensions and the affecting parameters of concrete models such as fracture energy, tension softening and tension stiffening coefficient, shear modulus reduction after cracking, fixed or rotating crack modeling among the other affecting parameters are investigated to verify the model. Because of symmetry only one half of the wall is modeled. Reinforcing bars are modeled discretely taking into account the bond-slip between concrete and bars. The results of verified model are used to study the sensitivity of a proposed short shear wall by IIESE for strengthening of low rise masonry buildings, to the parameters of length and shape of the footing together with and foundation soil property.
Tarbiat Modares University
Modares Civil Engineering journal
2476-6763
17
3
2017
9
1
Improving the Performance of the Square Shallow Foundations using Inferior Skirts
104
116
FA
Masoud
Makarchian
Assistant Professor,‍Civil Engineering Department, Faculty of Engineering, Bu-Ali Sina University, Hamedan
sajjad
gholipour
phd candidate bu ali sina university
Development of the new type of improvement methods of shallow foundations in the geotechnical engineering, it seems necessary to study the performance of vertical plates beneath the periphery of shallow foundations known as skirted foundations. Skirted foundations are steel or concrete foundations, which have a top raft and a relatively thin plate constructed beneath the periphery. The skirts penetrate the soil beneath foundation which encompass and confine the soil very firmly. The skirts and confined soil behave as a unit to transmit loads to the soil at the level of skirt tip. The benefit of skirted foundations compared with deep foundations such as piers and piles lies in their ease and short time of installations. The skirted foundations can be used as an appropriate alternative for shallow foundations, pier and deep foundations in applications such as oil and gas storage tanks, wind turbines, oil drilling platforms, harbor, and offshore and jacket structures. The behavior of square shallow foundations resting on confined sand by vertical plates as skirt, was studied using small scale physical modeling in the laboratory. The effects of parameters including ratio of skirt length to foundation width, foundation size, the shear strength of sand, and roughness of skirt and foundation surfaces on skirted foundations behavior in terms of increasing bearing capacity, the settlement reduction, and improvement of subgrade reaction modulus were assessed under compression loading, and results compared with the performance of shallow and pier foundations. Modeling test analyses revealed the overall improvement of square skirted foundations performance compared to shallow and pier foundations. Results of this study showed that the values of bearing capacity and settlement of skirted foundations are almost close to those of pier foundations of the same width and depth. Also, it was observed that the existence of skirt, increases the bearing capacity and modulus of subgrade reaction, together with decrease of settlement of shallow foundation. The enhancement in the bearing capacity, as well as reduction in the settlement of shallow foundations increases with increasing skirt depth and decreasing the shear strength of sand. So that, bearing capacity ratio (BCR) of skirted foundations to surface foundations was observed in the range of about 2.4 to 5.1 times for the different values of L/B. Furthermore, the value of skirted foundation settlement decreased up to 91% of that a surface foundation in the case of having skirt depth/ foundation width of 2.0. The modulus of subgrade reaction improved in the range of about 1.5 to 4.23 times duo to skirt existence. From the accomplished laboratory tests, it was found that skirted foundations resting on loose sand, are more beneficial than in case of resting on medium and dense sand. In cases where structures are very sensitive to settlement values, the skirted foundations can be used to gain the same allowable bearing capacity a much lower settlement. In this paper, based on the analysis of the obtained results, charts and equations are presented to estimate bearing capacity and settlement and subgrade reaction modulus of skirted shallow foundations in terms of those of surface foundation and sand relative density, skirt depth to foundation width ratio and its roughness.
Tarbiat Modares University
Modares Civil Engineering journal
2476-6763
17
3
2017
9
1
The Effect of Opening and Stud Spacing on Composite Shear Wall Behavior
118
130
FA
Hamid Reza
Kazemi Nia Korrani
1
1
1
Shear walls are resistant to lateral forces such as wind and earthquake. In recent decades, because of proper ductility and high strength of steel shear walls, engineers and researchers are interested to these walls. If the shear yielding occurs before buckling of web plate of wall, the wall will absorb more energy. Reinforcing steel plate shear wall would cause shear yielding occurs before buckling. In order to strengthen of wall can be used steel stiffeners or one layer of concrete. Steel shear walls with a layer of concrete to increase the strength out of plane called composite shear wall. Composite shear walls consist of a thin steel plate, two columns and two horizontal floor beams with one or two concrete layers. Considering the few studies that have been done on this type of walls, in this study, the effect of opening and stud spacing will be discussed in the behavior of the composite shear wall. Firstly, the 10-storey building with composite shear walls designed. Then upper floor selected and finite element models of this floor modeled with Abaqus. The models are one span and one story that length of span (inside into columns) and a height of story (the inside of the beams) is 3 m. Beams and columns are IPB280 and thickness of web plate and concrete layer are 2 and 100 mm respectively. It should be mentioned, to prevent local buckling column flange at the beam-column connection, in line of beam flange, continuity plate is considered. In this study to ensure the accuracy of the finite element model and ability of Abaqus to accurately estimate the actual behavior of shear walls, several shear walls model and analyze, then their results were compared with the results of the tests in the previous studies (Lubell’s test and Valizadeh’s experimental model). The results show that there is little difference between the experimental results and finite element results. So by taking a little difference can be concluded that the finite element results are acceptable. In order to compare the results of finite element models of composite and steel shear wall and steel moment frame, boundary elements and also load all three models are considered equal. Compare the results show that, on a constant displacement, the base shear in the steel shear wall is about 120% more than moment frame. Also, the base shear in the composite shear wall is 23% more than steel shear wall. Considering the results, it is evident that by reducing the distance between the studs, wall strength increases. Also, increasing web plate thickness cause to increases the capacity of the composite shear walls. Opening has always been the creation of composite shear wall resistance decreases. Opening at the sides and corners further reduces the resistance. This is unlike steel shear walls. At the end of this paper was to effect of the stud positions on the composite shear wall behavior. The results indicated that the studs closer to the external environment, resistance of composite shear wall increases. In order to compare the results of finite element models of composite and steel shear wall and steel moment frame, boundary elements and also load all three models are considered equal. Compare the results show that, on a constant displacement, the base shear in the steel shear wall is about 120% more than moment frame. Also, the base shear in the composite shear wall is 23% more than steel shear wall. Considering the results, it is evident that by reducing the distance between the studs, wall strength increases. Also, increasing web plate thickness cause to increases the capacity of the composite shear walls. Opening has always been the creation of composite shear wall resistance decreases. Opening at the sides and corners further reduces the resistance. This is unlike steel shear walls. At the end of this paper was to effect of the stud positions on the composite shear wall behavior. The results indicated that the studs closer to the external environment, composite shear wall resistance increases.
Tarbiat Modares University
Modares Civil Engineering journal
2476-6763
17
3
2017
9
1
Effect of curing temperature and SiO2-nanoparticles on the engineering properties of lime treated expansive soil
132
144
FA
Amir reza
Goodarzi
Expansive clayey soils can undergo periodic volumetric changes in the form of ground heave and settlement when subjected to moisture fluctuations. Such changes may lead to exert stress and serious problems to geotechnical structures if not adequately taken care of. Lime continues to be commonly used for treatment of these type soils; however, some restrictions are associated with its application. Therefore, in the present study a series of macro and micro level tests including swelling potential, unconfined compression strength (UCS), consolidation, pH and electrical conductivity (EC), adsorption, X-ray diffraction (XRD) and scanning electron microscope (SEM) analyses were carried out at various curing periods (1, 7, 28 and 90 days) and different temperatures (10, 20, and 40 °C) to assess the effects of environmental condition and SiO2-nanoparticles (NS) on the performance of lime treatment. To achieve the stated objective, lime and lime/NS (LNS) mixture were separately added to highly expandable clay at wide ranges from 0% to 30% by mass, respectively. The results obtained show that the environmental temperature, especially in the initial time of curing, has a prominent role on the geo-mechanical properties of lime treated soil samples. It was also found that the reduction in the temperature particularly at inadequate curing (lower than 28 days) provides a deleterious impact on the pozzolanic activity and decrease the formation of cementing compounds such as Calcium-Silicate-Hydrate (CSH) and Calcium-Aluminate-Hydrate (CAH) gels. In this case, the process of soil modification is mainly due to the short-term reactions (i.e. cation exchange and increase in osmotic pressure), increasing the amount of additives (up to two times) to control the swelling power. On the other hand, the incorporation of SiO2-nanoparticles into the binder system causes a reduction in the detrimental effects of low temperature on the engineering parameters of lime-treated products and decreases their sensitivity to the time of curing. The samples amended with the LNS blend exhibit a continuous development of soil mechanical capacity as the additive content increased. They are less deformable and show a decrease in their compression index by nearly 40% as compared with sole lime. Based on the XRD and SEM experiments, the superior influences of LNS are mainly ascribed to the higher and faster formation of cementitious compounds. In fact, at the presence of LNS, due to direct interaction of lime and silica from NS, the silicate gel can be immediately formed to coat and bind the clay particles together; whereas, with the addition of lime alone, the gel produces only by the removal of silica from the clay minerals that needs further time to complete its formation and hence the lower modification was occurred, especially at curing time shorter than 28 days. Moreover, NS reduces the pores sizes and serves to distribute the new crystalline phase (e.g. CSH gel) in a more homogenous fashion in the available space. This micro-structural reorganization upon the LNS treatment could rapidly block off the soil voids and greatly interlock the clay particles together that provide higher environmentally-stable materials with lower cost and energy as compared to standalone lime. It is finally concluded that the utilization of LNS mixture gives a promising way for increasing the efficiency of lime stabilization and decreasing the additive consumption.
Tarbiat Modares University
Modares Civil Engineering journal
2476-6763
17
3
2017
9
1
Dynamic relaxation with concentrated damping
146
156
FA
S. Reza
Sarafrazi
University of Birjand
Solving a system of linear or non-linear equations is required to analyze any kind of structures. There are many ways to solve a system of equations. They can be classified as implicit and explicit techniques. The explicit methods eliminate round-off errors and use less memory. The dynamic relaxation method (DRM) is one of the powerful and simple explicit processes. The important point is that the DRM does not require to storage the global stiffness matrix. It just uses the residual loads vector. Utilizing the virtual masses, damping and time steps, the DRM convert a system of static equations to dynamic ones. The process is started by assuming an initial solution. The next steps are done in such a way that the residual forces are decreased. The proper value of fictitious mass and time step guarantees the convergence of the proposed DR procedure. On the other hand, the convergence rate is dependent on value of damping factor, which is calculated using the lowest eigenvalue of artificial dynamic system in the common dynamic relaxation method. It is evidence; the dynamic system oscillates when damping is zero. The convergence of DRM with zero damping factors is achieved utilizing kinetic damping or -damping. In the kinetic dynamic relaxation process, the velocities of the joints are set to zero when a fall in the level of total kinetic energy of the structure occurs. However, it is difficult to calculate the extreme point of kinetic energy. Topping suggested assuming the peak point at the mid-point of the previous time-step, when a fall down in kinetic energy is occurred. The factor in the -damping method is time step ratio of two sequence steps. The time-step ratio can be calculated in such a way that the responses converge to exact solutions. In this paper, a comprehensive review of dynamic relaxation algorithms is presented. Of these, the popular and kinetic damping DR methods are described in detail. Then, the new dynamic relaxation algorithm is proposed. In this procedure, the artificial mass and time steps are similar to the DR methods that have been recently introduced. However, the damping factor is different with these methods. Damping factor is calculated in some specified steps. In other words, damping is zero in the most step of DR algorithm. Therefore, the total number of calculations is reduced. The concentrated damping is imposed, when the value of total kinetic energy of system is at its peak point. Utilizing the proper values of concentrated damping factors, the kinetic energy converges to zero. The presented formulation shows the relation between common and kinetic dynamic relaxation processes, too. It should be noted; the procedures of minimizing the kinetic energy of proposed method and Topping algorithm are different. The kinetic technique is required more calculations. Finally, some benchmark problems of truss and frame structures are selected. The linear and geometric nonlinear analyses are performed. The numerical results also show that the convergence rate of the new DRM increases in the majority of cases with respect to kinetic damping and also popular damping.
Tarbiat Modares University
Modares Civil Engineering journal
2476-6763
17
3
2017
9
1
Modeling integrated clothing shopping destination choice using structure equation models
158
170
FA
Alireza
Mahpour
The destination choice problem is an essential element in transportation planning processes. The problem is to find the probability that a person traveling from a given origin will choose a destination among many available alternatives. The focus of this paper is the destination choice of non-work (shopping purpose) trips, as part of the transportation planning process, in particular in trip- based and activity- based models. In general, destination choice models are estimated and applied at the traffic zone level, although the actual destination is an elemental alternative inside a traffic zone. Therefore, the number of explicitly modeled choice alternatives is usually the number of traffic zones. Most destination choice models assume a Multinomial Logit (MNL) form for the problem. The Multinomial Logit is not capable of accounting for unobserved similarities among alternatives, since the covariance matrix of the MNL model has only elements in the diagonal. The purpose of this paper is to investigate alternative destination choice model structures, focusing on structure equation models. The non-work destination choice problem is studied in spatial choice modeling. The literature concerned with spatial choice models covers several disciplines and important insights can be found in spatial behavior and spatial interaction models. Trip distribution models are expressed indirectly in terms of behavior models and this issue in trip generation and trip distribution is bolder. Considering new approach to transport planning (activity- based) and modeling behavior of passengers, the activity location choice is more attended and usually discrete choice models are used. Many studies describe zonal utility (simple decision structure) by using land- use and socioeconomic variables and thus cannot describe individual behavior in disaggregate level. For describe more accurate of individual utility, recent studies, have used simultaneous choice process concept and in other hand few studies used structural equation models and latent variables in describe choice of activity location. Investigating of individual features in activity location choice by using of structure equation models considered in recent studies. Considering the importance of determining activity location in activity- based approach, use of exogenous and explainer variables are bolded. Variable in classic destination choice models firstly are supposed independently and secondly have less attention to psychological and personal feature of passengers. Considering these two points, the power and efficiency of representation of behavior are reduced. Studies on the consumer behavior in shopping centers, showed that in addition to observable demographic and socio-economic variables, latent individual variables like to psychological variable, Attitude lifestyle and shopping orientation are important and must be attendant (complex decision structure).the idea of applying these variables in modeling the individual clothes shopping destination choice by using structure equation models was sourced from ethology studies on customers of shopping centers (novelty of paper). In this paper 213 sample are collected by internet- based questionnaire and individuals socio- economic, attitude, lifestyle and shopping orientation were asked. This integrated model is able to correctly predict the 42 percent of observation in which destination number 1 (Bazar of Tehran and Plasko shopping center) has the highest percent correct.
Tarbiat Modares University
Modares Civil Engineering journal
2476-6763
17
3
2017
9
1
Analytical Investigation of Proposed Semi-Rigid Beam to Column Frame Connections
172
182
FA
Precast concrete structures have been widely used since the last century. Fast production, quick erection, higher quality, economical aspects, lower labor costs etc. are of noticeable advantages of such structures compared to that of in-situ concrete structures. Considering frame structures, connections play a vital rule in local and global behavior of precast concrete structures. Catastrophic failures and losses are incurred globally due to failure in connection regions, so connections are considered to be the weak spots in precast concrete structures. Consequently, a great amount of attention and care is required in designing and forming connections, especially in precast concrete structures. In addition, compared to monolithic structures, it is relatively more difficult and more time consuming to achieve rigidity in connections due to the nature of precasting. Plus, difficulties arising from construction and structural details will neutralize inherent characteristics of precasting. Thus, obtaining a connection with details that are simple enough to be constructed easily on site, which, of course, satisfies demanding mechanical characteristic, can be of great importance. In this paper, two new types of beam to column connections are proposed. These connections are designed, modeled and analyzed numerically using nonlinear finite element software, ABAQUS. Main goal of the research was to achieve constructible and easily erectable connection detail which can provide satisfactory lateral strength, stiffness, ductility and energy absorption. Embedded steel corbels are used as members which transmit tension due to imposed positive moment and shear in negative moment in addition to their role as seating in initial stages of construction. Continuity is provided with bolting or welding of bottom bars to the corbel and then connection area is filled completely with expansive grout. Eccentricity of transmitted forces is a decisive factor especially in dynamic loadings, thus, in design, it is minimized by adjusting bar and corbel size and position and welding locations, size and shapes. Top bars are passed through holes, previously cast into the precast concrete column and are embedded in in-situ concrete of slabs. T shaped assemblies of the connections are modeled and laterally loaded until ultimate concrete strain is reached. In terms of strength, both connections were capable of achieving 95 percent of equivalent monolithic assembly. Considering lateral stiffness, proposed connections were able to provide initial stiffness of more than 80 percent of equivalent monolithic connection. Precast connections were 20 to 30 percent less ductile than their monolithic counterpart. Noticing relative geometric complexity and difference in force transmission mechanisms of connections, lower ductile behavior of connections is justifiable. Effects of axial column load are studied on response of the assemblies. Compressive axial load relatively improves lateral stiffness and energy absorption of the connections. By imposing axial tension on column, lateral stiffness and strength is significantly reduced. Comparing before mentioned mechanical characteristics of proposed connections with their equivalent monolithic assembly, satisfactory response under lateral monotonic loading is observed. Based upon results derived from this study, proposed connections may be used as semi rigid beam to column connections in precast concrete frames, instead of fully rigid connections.
Tarbiat Modares University
Modares Civil Engineering journal
2476-6763
17
3
2017
9
1
Sedimentation Position Prediction in the Swash Zone of Sandy Beaches using Response Surface Method
183
184
FA
mohammadreza
motalebizadeh
PhD student of Hydraulic, Faculty of Civil and Environmental Eng., Tarbiat Modares University
Mehdi
Shafieefar
Prof., Coastal Engineering. Faculty of Civil and Environmental Eng., Tarbiat Modares University
masoud
ghodsian
Prof., Hydraulic Structures. Faculty of Civil and Environmental Eng., Tarbiat Modares University
Reflective beaches requires a combination of lower waves, longer periods and particularly coarser sands. They are typically steep in beach profile with a narrow shoaling and surf zone, composed of coarse sediment. Coarser sediment allows percolation during the swash part of the wave cycle, thus reducing the strength of backwash and allowing material be deposited in the swash zoneThe Swash zone, as extreme area of inner surf zone, influences coastal area and coastal structures. It defined as the part of the beach between the minimum wave run-down and maximum wave run-up. It constitutes a beach area where waves dissipate or reflect their remaining energy after traveling towards the shore. The role of Swash zone is influenced by incoming waves from surf zone, the geometry of beach face and the interaction between beach groundwater and surf zone.The review of Laboratory researches indicated that wave height and period, beach slope, grain size distribution of beach material, still water level (SWL), beach groundwater level, the hydraulic conductivity of beach influence on the evolution of sand beaches. In a few laboratory researches, experiments is designed with One Factor At a Time method (OFAT) and the qualitative effect of parameters of regular wave height and period, SWL and beach groundwater level, and beach slope are investigated on nearshore evolution. In this research, experiments are designed using Central Composite Design (CCD) of Response Surface Method (RSM). CCD is a type of response surface design that present very good predictions in the middle of the design space. Important properties and features of CCD are orthogonality, rotatability and uniformity. The quantitative effects and interactions of irregular wave height and period, beach groundwater level and SWL, and beach slope on beach profile evolution is examined in a sandy beach by 50 experiments designed with CCD. The experiments are carried out in laboratory flume in Faculty of Civil and Environmental Engineering, Tarbiat Modares University with high accuracy. The experimental setup is designed to simulate varying beach groundwater level and SWL and course sand (d50=0.8mm) is selected for beach material. Analysis of hydrodynamic data of the experiments indicated that the type of breaking waves is plunging wave and the hydrodynamic status of the swash zone is intermediate condition. The starting position of swash sedimentation (SWS) is extracted from mean of the beach profiles evolution.By analyzing of experiments' SWS using CCD, a cubic model is suggested with %95 confidence level and predicted R-squared of 0.86. The results of model revealed that groundwater level has no significant effect on SWS. Wave height is the most influential factor affecting SWS and increasing wave height result to this position moves to upper beach rapidly. In addition, increasing beach slope causes the movement of SWS toward the beach. Increasing sea level lead to the displacement of SWS toward the sea.This model indicated that the effect of wave height on SWS depends on wave period strongly and there is significant interaction between them. In addition, there is slightly interaction between the SWL and wave height and these variables influence on the role of each other in SWS.
Tarbiat Modares University
Modares Civil Engineering journal
2476-6763
17
3
2017
9
1
Evaluation of Support Vector Machine Performance for Carbon Monoxide Prediction
195
202
FA
Roohollah
Noori
Saber
Moazami
Mohammad
Salimian
Mahmudreza
Momeni
In Carbon monoxide (CO) is one of the main air pollutant parameters in the atmosphere of Tehran, Iran. Generally, it is difficult to predict and control CO concentration because it is essentially nonlinear time-varying system. Recently, in particular, environmental control such as CO concentration level control is regarded as one of the most important factors in environmental protections. This paper describes forecasting and more specifically uncertainty determination of CO concentration during the modeling process using a support vector machine (SVM) technique. Uncertainty of the air pollution modeling studies highly affected the simulation results. In this regards, it is very important to determine the uncertainty of air pollution models due to consequences on health of people exposed to the pollution. Therefore, this research aims to calibrate, verify, and also determine the uncertainty of support vector machine (SVM) in the process of air pollution modeling in the atmosphere of Tehran. To achive this goal, the SVM model was selected to predict arithmetic average of daily measured CO concentration in the atmosphere of Tehran. In this regards, the SVM model was calibrated and verified using six daily air pollutants include particulate matter with diameter equal or less than 10 micrometer (PM10), total hydrocarbons (THC), nitrogen oxides (NOx), methane (CH4), sulfur dioxide (SO2) and ozone (O3) and also six daily meteorological variables include pressure (Press), temperature (Temp), wind direction (WD), wind speed (WS) and relative humidity (Hum). The data was collected from Gholhak station located in the north of Tehran, Iran, during 2004-2005. Thereafter, the best developed SVM model for predicting the CO concentration was chosen based on determination of coefficient (R2). Finally, to determine the SVM uncertainty, the model was run many times with different calibration data. It led to many different results because of the model sensitivity to the selected calibration data. Then, the model uncertainty in the CO prediction process was evaluated using the width of uncertainty band (d-factor) and the percentage of measured data bracketed by the 95 percent prediction uncertainties (95PPU). Generally, the results confirmed the strong performance of the SVM model in predicting CO concentration in the atmosphere of Tehran. The predicted average daily CO concentrations by SVM model had a good agreement with the measured ones in the Gholahak air quality monitoring station. It was found that the determination of coefficient for calibration and validation of SVM model were equal to 0.89 and 0.88, respectively. Furthermore, the results indicated that the SVM model has an acceptable level of uncertainty in prediction of CO concentration in which the level of d-factor and the percentage of measured data bracketed by the 95PPU in the validation step were 0.74 and 76, respectively. Therefore, The obtained results indicated that the SVM model had an acceptable level of uncertainty in prediction of CO concentration. Therefore, it can be concluded that the SVM model is able to predict the CO concentration in the atmosphere of Tehran while it resulted an acceptable level of uncertainty. Finally, due to the proposed methodology is general, the authors suggest to apply it for analyzing the uncertainty of SVM model in other fields of science and engineering.
Tarbiat Modares University
Modares Civil Engineering journal
2476-6763
17
3
2017
9
1
Strengthening of concrete
columns by near surface mounted method with different material bars and confined with CFRP sheet
203
212
FA
Seyed Roohollah
Mousavi
Assistant Professor, Sistan and Bloochestan University
In recent years, Fiber-reinforced polymer (FRP) materials are increasingly used for strengthening and retrofitting of reinforced concrete structures. Fiber-reinforced polymers in the form of jackets with the fibers typically in the columns’ circumferential direction, are quite effective in carrying shear and in providing confinement. In addition, the previous studies show that the FRP jacket can greatly enhance the dissipation capacity and ductility of reinforced concrete columns. However, FRP jackets are not effective for the strengthening of columns in flexure. To overcome difficulties associated with FRP jacketing, recent research efforts have focused on the use of near surface mounted (NSM) FRP or stainless steel reinforcement through a combination of externally bonded (EBR) FRP sheets and anchors for the flexural strengthening of columns. NSM strengthening technique consists of FRP rods embedded in grooves made on the surface of the concrete and bonded in place with epoxy. The NSM technique is not effective in terms of enhancing the energy dissipation capacity of RC columns. Therefore, by combining NSM technique with FRP confinement sheets, a high effective technique (hybrid FRP-based strengthening technique) can be obtained. FRP and steel bars have been used together for reinforcing the concrete beams in previous studies. However, the combination of both materials for strengthening of reinforced concrete columns is not common. The present paper experimentally investigates the effects of combined NSM-GFRP bars with the CFRP confinement sheets and also NSM-GFRP bars with NSM-Steel rods on strengthening of reinforced concrete columns. It should be noted that in combinatory method of NSM-GFRP with NSM-Steel, NSM-Steel to NSM-GFRP ratios is different. The experimental program includes five square columns with a cross section of 250*250 mm and the length of 1200 mm. The columns were tested to failure by applying constant axial compressive and cyclic lateral loading. According to the results, it is indicated that using these two combinatory reinforcing methods cause an increase in load capacity, ductility, dissipated energy and initial stiffness of columns compared to the control column and also columns reinforced by using NSM-GFRP bars. Furthermore, NSM-GFRP technique increases the load capacity 43 percent in comparison with the control specimen. In the case of combined retrofitting methods, load capacity increase was up to 60 percent. Also, using NSM-GFRP reinforcement causes 7 percent increase in ductility and 6 percent decrease in dissipation capacity. In contrast, in the specimens reinforced by the combinatory methods, the ductility parameter and dissipation capacity increase 3 to 150 percent and 24 to 133 percent, respectively. In addition, hysteretic diagrams corresponding to specimens strengthened by combinatory methods has less pinching effect than those of the control specimen. Experimental results indicate that the reinforced concrete column strengthened by combinatory method of NSM-GFRP with NSM-Steel is a viable solution toward enhancing the behavior of RC columns subjected to simultaneous axial and seismic loads. This is especially the case when the retrofitting scheme contains higher NSM-Steel to NSM-GFRP ratio. Finally, due to corrosion of steel reinforcement, NSM-GFRP bars combined with CFRP confining sheets is selected as the best choice of strengthening.
Tarbiat Modares University
Modares Civil Engineering journal
2476-6763
17
3
2017
9
1
Investigation of the Seismic Behavior of Reinforced Concrete Beam-column Connections Made of High Performance Fiber Reinforced Cementitious Composites (HPFRCC)
213
226
FA
farzin
moludi
Ali
Kheyroddin
Ali
Hemmati
Investigation of the Seismic Behavior of Reinforced Concrete Beam-column Connections Made of High Performance Fiber Reinforced Cementitious Composites (HPFRCC) In recent years, the use of the HPFRCC materials has been taken into consideration in order to construct safe structures against earthquake. High performance fiber reinforced cementitious composites (HPFRCC) refer to the materials, including cement mortar with fine aggregates and fibers. The distinctive feature of the materials is that they exhibit strain hardening behavior under tensile loading unlike normal concrete and fiber reinforced concrete. The HPFRCC materials can be used for seismic retrofitting of structural components, construction of structural fuses and in areas susceptible to degradation in structures, such as beam-column connections and shear wall interface beam. As the beam-column connections are considered as one of the points of damage in concrete flexural frames, the use of the HPFRCC materials in the beam-column connections, which have high strength and ductility, can lead to the formation of the structures with higher strength and ductility compared to the conventional concrete structures. This study first introduces the materials and then determines the effect of the use of the HPFRCC materials in the beam-column connection performance. Therefore, the results of laboratory studies conducted by Chao at University of Michigan were used to verify the finite element model. The effect of the different parameters of beam-column connection, including HPFRCC materials length area in the beam, HPFRCC materials length area in the column, compressive strength of concrete and HPFRCC materials, the distance between stirrups in the beam and the distance between stirrups in the column, individually or combined, and performance of connection were investigated in the base model. The results showed that maximum strength, yield strength and ductility ratio of beam-column connection, if the HPFRCC materials are used in some parts of the beam or column (with panel zone), are respectively 36.9%, 10.4% and 53.1% greater than the beam-column connection made of reinforced concrete. Furthermore, the concrete compressive strength parameter has a significant effect on the connection ductility ratio so that the ratio of ductility of the connection with 35 MPa concrete is 40.7% greater than the base connection (University of Michigan). It is notable that in the laboratory connection of the University of Michigan where the HPFRCC area length in the beam is twice the beam depth (711.2 mm), the reduction of the HPFRCC area length just in the beam led to the 40.7% increased ductility ratio of the connection from twice the beam depth (711.2 mm) to the length equal to the beam depth (355.6 mm) compared to the base connection (University of Michigan) while it had minimal effect on connection strength. Moreover, the use of the HPFRCC area just in the column led to the 50.1% increased ductility ratio of the connection compared to the base connection (University of Michigan). The results indicated that when HPFRCC materials were used in the beam, the use of the HPFRCC materials in the column did not have a significant effect on the strength and ductility ratio of the connection. Keywords: HPFRCC, Reinforced Concrete, Beam-Column connection, Seismic Behavior, Ductility.
Tarbiat Modares University
Modares Civil Engineering journal
2476-6763
17
3
2017
9
1
Effects of return mixed liquor recycling ratio on A2/O process
227
234
FA
mahdieh
raji
Seyed Ahmad
Mirbagheri
Professor /civil and environmental engineering K.N.Toosi University of technology
Discharging wastewater effluent to surface water or groundwater is so dangerous for environment, while it includes nutrient. As the phosphorus and nitrogen combinations in the aquatic environments have harmful impacts (mainly the poisonousness of ammonia, overgrowth of aquatic plants, groundwater pollute to nitrate and diseases caused by drinking the polluted water, and also eutrophication, resulting in frequent outbreaks of algal blooms and threatening the reliable supply of drinking water resources), some limitations were imposed on the consistency of these combinations in the entry waste. So nowadays removal of these combinations must be considered in designing of the treatment plants and also systems designed for treating the municipal wastewater must be able to remove nitrogen and phosphorus combinations to reach the standard limit. Therefore, in order to good performance of the aerobic-anaerobic A2/O method, it is proposed in this research and a study in advanced treatment of municipal wastewater using the A2/O method to remove nitrogen and phosphorus in the pilot scale in Ekbatan WWTP has been done. In this research, firstly the principals of biological removal of nitrogen and phosphorus, and secondly the basis of designing biological treatment plants have been investigated. Then for laboratorial studies, an A2/O pilot has been made. This pilot consists of anaerobic, anoxic and aeration tanks and also sedimentation tank. The volume of these 4 tanks are 40, 60, 170 and 120 L, respectively. In order to simulate the real condition, this pilot has been set up in Ekbatan plant and the experiments were done to observe the effect of hydraulic residence time on nitrate, ammonia and phosphorus removal and also the effect of oxic mixed liquor recycling ratio on nitrate removal has been conducted. In order to observe the nitrate, ammonia and phosphorus removal process efficiency, the experiments were done in a period of three months and in 5 aeration hydraulic residence times, 4, 6, 8, 10 and 12 hours. In these experiments, the returned sludge was 25% and the oxic mixed liquor recycling ratio was 75%. After determining the best hydraulic residence time, experiments continued in 5 different oxic mixed liquor recycling ratios, 75%,150%, 225%, 300% and 375%. It was concluded that at aeration hydraulic residence time of 8 hours, 96% COD , 95% ammonia and 79% phosphorus (effluent: 9 mg/L COD, 0.87 mg/L ammonia, 2.1 mg/L phosphorus, 18.7 mg/L nitrate) removal were achieved and that was the best HRT. Furthermore, according to the mixed liquor recycling ratio experiments, when the oxic mixed liquor recycling ratio was about 180 - 200%, optimum removing nitrate has been occurred. Although the mixed liquor recycling ratio of 225 - 275% resulted better efficiency for nitrate removal, it is not proposed, because effluent limitations in Iran for the nitrate is up to 10 mg/l which it is resulted in the mixed liquor recycling ratio of 180 -200%, and the other reason is that, with increasing the oxic mixed liquor recycling ratio, energy costs will increase, too.
Tarbiat Modares University
Modares Civil Engineering journal
2476-6763
17
3
2017
9
1
Evaluation of Effective Parameters on the Axial Load Carrying Capacity of Preloaded Strengthened Steel Box Columns
235
246
FA
Mohsen
Naseri
Amirkabir university of technology
Saeed
Erfani
Amirkabir university of technology
Strengthening of existing structures is one of the most important issues in the field of structural engineering. Due to avoiding any interruption of service on a structure and economic issues, strengthening process usually occurs when a member is under service loads. On the other hand in the loaded steel columns, it is really difficult to weld plates after unloading the column from existing loads, Therefore one of the important issues being neglected in the redesign process of strengthened columns is the significant axial load existing in the column, caused by service loads before strengthening them. This paper aims at numerically investigating the behavior and ultimate load bearing capacity of in-service strengthened steel box columns with continuous welded plates. Effects of different parameters on the capacity of preloaded strengthened columns are presented and discussed. Included in the result are the effects of initial imperfection; magnitude of preload before strengthening; slenderness ratio of the strengthened column and ratio between cross sectional area of reinforcing plate and unstrengthened column. To investigate the effect of these parameters, each un-retrofitted specimen is exposed to the preloading levels of 0.0, 0.2, 0.4 and 0.6 of the load carrying capacity of unstrengthened column. Then results of this preliminary analysis are defined as a predefined field for the column of same retrofitted model and ultimate bearing capacity of the strengthened model is calculated using a modified Riks analysis method. The critical load carrying capacity of models without pre-existing axial load was set to the theoretical value presented in ANSI/AISC 360-10 and suitable imperfection for each model was calculated. This is because the main objective of this study is the variation of results with respect to the existing design curves. Based on the results of numerical analysis, application of preload to unstrengthened column magnifies the initial geometric imperfection of the column and consequently decreases the ultimate bearing capacity of strengthened column. Also as the magnitude of axial load existing prior to addition of reinforcing plates increases, the ultimate bearing capacity of the strengthened column decreases with respect to the calculated theoretical value. The maximum amount of this reduction for the preload ratios of 0.2, 0.4 and 0.6, is respectively up to 2%, 5% and 9.5% of the load-bearing capacity of strengthened column. As another result, slenderness ratio is one of the main parameters that affect the bearing capacity of specimens with a specified preload level. This means that at a constant preload level the maximum reduction in bearing capacity occurs for models with median slenderness ratio. Also models with cross sectional ratio of reinforcing plates ranging from 0.4 to 1.0 were studied and it was shown that inside this range the cross sectional ratio of reinforcing plates parameter does not have remarkable effect on the ultimate bearing capacity of column. At the end, an empirical relation is proposed to calculate reduction of ultimate bearing capacity for columns with different slenderness ratios and preload level. Results of this study may be utilized to increase the accuracy of redesigning process during in-service strengthening of steel box columns.
Tarbiat Modares University
Modares Civil Engineering journal
2476-6763
17
3
2017
9
1
Sensitive Analysis of Nodal Force Function in the Elastostatic Problems in Decoupled Equations Method
247
257
FA
mahdi
yazdani
The elastostatic problems are a significant subject in the analysis and design of solids and structures. As most of the complicated elastostatic problems do not have closed-form solutions, numerical methods such as finite element method (FEM), boundary element method (BEM), discrete element method (DEM), meshless methods, scaled boundary finite element method (SBFEM), and hybrid methods are the current approaches dealing with these types of engineering problems. This study presents a novel application of the decoupled equations method (DEM) to assessment elastostatic issues. In the present method, the so-called local coordinate's origin (LCO) is selected at a point, from which the entire domain boundary may be observed. For the bounded domains, the LCO may be chosen on the boundary or inside the domain. Furthermore, only the boundaries which are visible from the LCO need to be discretized, while other remaining boundaries passing through the LCO are not required to be discretized. In this method, only the boundaries of problems are discretized using specific higher-order sub-parametric elements and higher-order Chebyshev mapping functions. Implementing the weighted residual method and using Clenshaw-Curtis quadrature result in diagonal Euler’s differential equations. So, the coefficient matrices are diagonal, which provide a system of single Euler’s differential equations for the ith degree of freedom (DOF). If n indicates the number of DOFs of the problem assumed to be analyzed by the proposed method, only n Euler’s differential equations (with only one unknown differential equation for each DOF) should be solved. In the proposed method, the LCO is the same for all nodes, for which the LCO has the same displacement components. Therefore, the physical concept of this fact may be considered as some semi-parallel springs adjoining to each other at the LCO. Therefore, the proposed procedure is called “redistribution” of the stresses in the present method. At the final step, using the calculated displacement field along ξ, the displacement at any point of the problem’s domain is interpolated by using the proposed special shape functions. Although the governing equation of each DOF is decoupled from those of other DOFs, however the “redistribution” of the stresses at the LCO and resolving the problem for each DOF, represents the connection between all DOFs of the domain. In the solution procedure, the order of displacement function u(ξ) depends on nodal force function F^b (ξ). To analysis of elastostatic problems in the classical Decoupled Equations Method, F^b (ξ) varies in the undertaken domain like a body force. Therefore, F^b (ξ) is defined as a linear function. In this study by proposing new forms of force function, the response of elastostatic problems is assessed. In the following Sensitivity of this method via proposed nodal force functions is fully demonstrated through two benchmark problems. The results show that stress and displacement fields totally depend on the form of force function. Also, the results show to get optimum results, proposing an appropriate nodal force function corresponding to physical concept is necessary. For example in the cantilever beam which is subjected to a shear force at its free end, by considering the linear form for nodal force function results in minimum error. In the other hands, in the Kirsch’s problem with a central small circular hole, considering the nonlinear form for nodal force function leads to minimum error.