Tarbiat Modares University
Modares Civil Engineering journal
2476-6763
14
2
2014
7
1
AC Stray Current and Environmental Effects on Concrete
Power poles
1
12
FA
A.
Aghajani
Isfahan University of Technology
M.A.
Golozar
Isfahan University of Technology
A.
Saatchi
Isfahan University of Technology
S.
Shabani
Isfahan University of Technology
The purpose of this article is study of effects of high voltage AC stray current (380 VAC) and environmental effects on durability of concrete power poles. In Iran, annually spend large costs for replacing them with new concrete poles. In this study, the degradation effects and mechanisms of 380 VAC stray current on water saturated concrete were investigated via field and laboratory tests. They included electrical and electrochemical tests and microscopic inspections. Based on the results of tests both environment and AC stray current reduce durability of the concrete power poles. Formation of galvanic macro cell, carbonation, vibration of concrete pole due to wind force and weight of cable, diffusion of corrosive ions from soil to concrete power pole and its accumulation in near ground level are some of environmental effects on degradation of concrete power poles. In addition pollution of air can form deposit layer on insulations of concrete power pole. In raining condition, the layer absorbs water and surface resistance of insulators reduces considerably. So high voltage AC current can creep in surface of the insulator and can exchange between two other phases and ground. Generally grounding system can't act immediately or have not suitable low electrical resistance. Meanwhile if concrete of power pole has low quality then it absorbs high content of rain and its resistivity decreases considerably. Thus some parts of fault current can pass through water saturated concrete power pole. In an special location around Isfahan with polluted air condition, in raining condition current creep through the surface of some insulators occur and immediately many cracks forms on concrete cover of power pole and in some cases concrete cover start to fall without any corrosion in rebar. Based on field study, AC stray current does not occur in concrete power poles which manufactured by centrifugal method and they have very low w/c ratio in their mix design. In this method excess water of concrete mix is removed before staring curing of concrete. Microscopic study of concrete of the molded pole and centrifugal pole showed that the later had much denser structure and for this reason depth of carbonation in the later was much lower than the first. In addition the centrifugal poles have pre-stress condition and it reduce their vibration against wind force and weight of cables. Vibration has destructive effect on concrete cover of concrete poles, special in their ground level because force of vibration concentrates in this location. For study mechanism of degradation of AC stray current, some concrete samples prepared and in water saturated condition subjected to 380 VAC stray current. Based on results of tests, high voltage AC stray current can reduce the concrete pole durability by thermal and shrinkage stress and also by creating vapor pressure within water saturated concrete and transport calcium hydroxide toward cement paste-aggregate interfaces. Application of new technology for reducing water to cement ratio and proper installation and maintenance of the concrete poles considerably increases durability of them against destructive effects of AC stray current.
Tarbiat Modares University
Modares Civil Engineering journal
2476-6763
14
2
2014
7
1
Dispersive soil improvement with lime, special attention to the
reduction of peak intensity of clay minerals in XRD analysis
13
25
FA
V.R.
Ouhadi
Bu-Ali Sina University
M.
Amiri
Bu-Ali Sina University
S.
Hamidi
Bu-Ali Sina University
Civil engineering projects usually require an improvement on soil properties. In this case, the improvement of soil strength, permeability, and erosion resistance, collapsible or dispersive behaviour might be important. Consequently, engineers are always willing to achieve an optimum way to stabilize/improve soil performance for practical applications. Generally, the presence of dispersive clays in engineering projects is one of the essential problems for civil engineers. Several failures in engineering projects due to the presence of dispersive soils are reported in different parts of the world. In spite of several researches on this subject, there has been little attention to the microstructure of stabilized dispersive soils. In this paper the improvement of properties of a natural dispersive soil is studied with special attention to the solubility of clay mineral peaks in XRD analysis and formation of aluminates and silicates minerals after lime application to dispersive soil. The soil sample of this area is silty clay which has 100% dispersivity potential. Several geotechnical erosions are reported in this area which is attributed to the presence of dispersive soils. Due to the low bearing capacity of soil, and to overcome to its dispersivity behaviour, the application of lime is chosen as soil stabilization means. In this paper, at the first step with application of double hydrometer and cation exchange capacity experiments, the dispersive potential of soil is determined. In addition, the major minerals of soil are addressed by the use of XRD analysis. For the dispersive soil sample, the major geotechnical properties of soil including unconfined compression test, settlement behaviour, and over consolidation stress are measured and reported. Then, the soil sample is mixed with lime and water. After equilibrium and after curing process, several geotechnical and geo-environmental experiments were performed on soil samples to investigate on the dispersivity potential variations of soil. Furthermore, the microstructure of lime treated dispersive soil was monitored by application of XRD analysis on lime treated soil. The results indicate that the dispersivity potential of soil disappears after application of 6% of lime. In this process the dispersive structure of soil changes to the more flocculated structure. Moreover, the results of XRD analysis indicate that following application of lime, parts of clay fraction solubilize, leading to the formation of pozzolanic components. The main conclusion of this paper is the determination of the required percentage of lime for soil stabilization with the application of micro-structural experiments and with the measurement of the quantity of increase in over-consolidation stress. With the performance of several consolidation tests on untreated and lime-treated dispersive soil, it is shown that with application of 6% lime, an increase of 430% in over-consolidation stress is observed. This means for each percent of lime a 50 kN/m 2 increase in over-consolidation stress is achieved.
Tarbiat Modares University
Modares Civil Engineering journal
2476-6763
14
2
2014
7
1
Use of Walnut and Peanut Shells Activated Carbon in Direct
Blue71 Removal: Kinetic and Isotherm
27
37
FA
S.N.
Larimi
Tarbiat Modares Univ
B.
Ayati
Tarbiat Modares University
Most of dyes used in production processes caused serious environmental pollution when
discharged to the water resources. Azo dyes are the most used synthetic compounds in the
industries such as textile, food, leather and cosmetic. Due to their toxicity and hard
degradation, these kinds of compounds are classi
fied as environmental hazardous materials
that have to be treated before discharging to the environment.
Direct blue 71 (DB 71) is one of azo dyes that is resistant to aerobic degradation and under
anaerobic condition is reduced to potential carcinogenic aromatics.
Different kinds of physical, chemical and biological methods such as adsorption,
ultra
filtration, reverse osmosis, coagulation and electro coagulation are widely used for
efficient dye removal but they just transport contaminants from water to sludge and generate
secondary wastes which need more treatment. Adsorption with many advantages is a proper
method that is applied to treat dye compounds. In recent years, use of low cost materials as
adsorbent for dye removal has been highlighted.
Since natural absorbents are inexpensive and may be achieved without any cost and they
are usually in abundance in nature, absorption of solute ions by these materials are a proper
method to eliminate color from polluted waters and industrial wastewaters.
In this study, removal of azo dye Direct Blue 71 was evaluated with two new natural
adsorbents of walnut and peanuts shells. These adsorbents are produced from agricultural
wastes. The effect of pH, contact time and adsorbent dosage on the removal efficiency has
been studied. According to the results, maximum removal of dyes by the two natural
absorbents (0.75 gr/L walnut shell in 50 mg/L initial dye concentration with pH of 9 in 45
minutes and 1 gr/L peanut shell in 50 mg/L initial dye concentration with pH of 9 in 60
minutes) was 55 and 60 percent, respectively.
Comparison of prepared adsorbent in the laboratory and commercial ones in optimum
condition have similar dye removal efficiency that means suitable and cheap adsorbent could
be prepared in the laboratory. Removal efficiency of DB71 was also obtained 85 and 83 by
the two walnut and peanut shells adsorbent powder, respectively.
The results have also shown that amongst four isotherms of Langmuir, Freundlich, Temkin
and Dubinin-radushkevich, Freundlich isotherm has the highest correlation coefficient which
implying heterogeneous surface of adsorbent for both sorbents. It has also shown that the
maximum surface adsorbent capacity for the monolayer of activated carbon of the walnut and
peanut shells were 26 and 28 mg/gr, respectively. From the n values 3.58 and 3.72 of the
Freundlich isotherm for both adsorbent of walnut and peanut shells, it could be concluded that
physical adsorption process has been happened. The data could also indicate that pseudosecond-
order was the best adsorption kinetics model for the two adsorbents
Tarbiat Modares University
Modares Civil Engineering journal
2476-6763
14
2
2014
7
1
Based Performance Evaluation of Earth Dams under Seismic
Loading
39
53
FA
H.R.
Paseh
Tarbiat Modares University
M.
Oliaei
Tarbiat Modares University
Dams shall safely retain the reservoir and any stored solids, and pass environmentally acceptable flows, as required for all loading conditions, ranging from normal to extreme loads, commensurate with the consequences of failure. The new trend for performance-based design is to consider 2 levels of seismic actions and analyze the situation where the limit of force balance is exceeded for high intensity ground motions, associated with a very rare seismic event. For the design, two basic requirements are defined: (i) Non-collapse requirement (ultimate limit states), i.e. after the occurrence of the seismic event, the structure shall retain its structural integrity, with respect to both vertical and horizontal loads, and adequate residual resistance, although in some parts considerable damage may occur, (ii) Minimization of damage (serviceability limit state) , i.e. after seismic actions with high probability of occurrence, during the design life of the structure, some parts can undergo minor damage without the need of immediate repair. This study evaluates the behavior of a typical earth dam by nonlinear seismic analyses, in two performance levels, named “Base Performance Level” and “Desired Performance Level.” The level of seismic action and related acceptance level of damage are defined for each performance level. In “Base Performance Level,” with seismic levels of OBE (0.3g) and MDE (0.5g), the structure shall be serviceable and repairable and in “Desired Performance Level”, with seismic levels of MDE (0.5g) and MCE (0.7g), the structure shall be serviceable and repairable, respectively. Also, the stability of dam has been assessed by the “Strength Reduction Analysis.” The analyses are nonlinear and the constitutive law of the materials was assumed to follow "Finn" and "Mohr-Coulomb" models, incorporated into “FLAC 2D” finite difference analysis program. The factors such as initial shear modulus, variation of shear modulus versus shear strain, generation and dissipation of pore pressure and hysteretic damping are considered in this study. In addition, using the scaling method of applying maximum acceleration, the response of dam is investigated in different maximum accelerations. The results show that the dam needs to be changed in geometrical specifications or seismically improved in “Desired Performance Level”, in contrast with “Base Performance Level.” Results are confirmed by low amount of safety factors of stability in dam, which are calculated for different seismic loads. Also, the behavior of dam is examined by sensitivity analysis for type of accelerograms, constitutive model and the standard penetration number in shell of dam. Two accelerograms, including “Friulli” and “Sakaria” are considered. Maximum acceleration and duration of both of them are equalized and frequencies more than 5Hz are filtered. Sensitivity analyses of “Friulli” and “Sakaria” accelerograms, despite the difference in response spectra and specific energy density, show approximately similar results. “Finn” model predicts the amount of excess pore water pressure to be more than "Mohr- Coulomb" up to %20, and shows the occurring of liquefaction in SPT more than 35 and acceleration more than 0.7g, in shell of upstream of dam
Tarbiat Modares University
Modares Civil Engineering journal
2476-6763
14
2
2014
7
1
Seismic sensitivity evaluation of concrete buttress dams
55
66
FA
S.Zh.
Hosseini
M.T.
Ahmadi
Concrete buttress dams are constructed in large numbers at medium sites in many countries such as Iran because of their considerable technical and economical benefits in previous century. This type of dams is exposed to damages due to earthquakes as other structures. Some buttress dams such as Sefidroud dam in Iran, Hsinfengkiang dam in China and Honenike dam in Japan have undergone some damages due to recent earthquakes. After these incidents, some investigations have been carried out. However, these investigations have just mentioned the manner of incidents and the resulting damages. Therefore, the seismic behavior and sensitivity recognition of these dams with respect to different factors have been ignored; however the study of behavior and seismic sensitivity of this type of dams is important. In this paper, the tallest monolith of the Sefidroud concrete buttress dam is analyzed using a 3D model with massless foundation to study the seismic behavior and sensitivity of this type of dam. The interaction of the dam with the reservoir, the reservoir bottom absorption and upstream radiation of hydrodynamic waves are considered, but the cross-canyon component of earthquake is neglected. The applied accelerograms to the system are scaled according to the Sefidroud dam site DBE response spectrum. To determine the initial conditions before occurring earthquake, a series of detailed static analyses are done under the effect of dam body weight, hydrostatic pressure, uplift pressure and ambient temperature. Seismic loading due to longitudinal and vertical components of earthquake is applied and the nonlinear behavior of dam under various factors such as different seismic loading scenarios and different properties of dam body and also foundation materials is investigated. The results of analyses show that the dam body downstream kink, heel, toe and buttress web are sensitive and vulnerable zones. The results also demonstrate that the compressive stresses in the dam body are usually much less than the compressive strength of concrete. Therefore, the possibility of compressive failure is almost zero. But the conditions of tensile and shear stresses are different and large stresses may occur at the mentioned zones and considerable tensile and shear damages to the dam body are possible. According to the results of analyses, it is apparent that when the ratio of dam body modulus to that of the foundation (called softness modulus) is small, i.e. when the foundation modulus is high and near to that of dam body, the construction of concrete buttress dams at highly seismic zones may cause local failure and unfavorable situations for the tensile stresses at the kink, the heel and the toe of the dam body. Therefore, adaptation of this dam type in such sites should be carefully studied and in these circumstances, the modulus of the concrete of dam body should be kept more than usual practice. Furthermore, the shear damage at the dam-foundation contact surface is highly dependent to the applied earthquake type, but increasing the softness modulus could reduce this type of damage. The compressive strength of concrete has no effect on the shear damage at the dam-foundation contact surface.
Tarbiat Modares University
Modares Civil Engineering journal
2476-6763
14
2
2014
7
1
Seismic performance assessment for hybrid system of tuned
mass & liquid column damper in series and a method to
increase efficiency
71
83
FA
S.M.
Zahrai
University of Tehran
R.
Biglarian
Tehran Central branch of Azad University
In recent years, development of effective devices for seismic energy dissipation in structures has become more important to keep response of structure in elastic range. Dampers are used in structures to reduce response and effect of seismic forces. Also, using secondary mass technology can help seismic energy dissipation. Among these systems one can mention tuned mass damper and tuned liquid column damper, working base on secondary inertia in structures. In this paper, hybrid system of tuned mass & liquid column dampers in series was considered with mass ratios 0.035-0.005, 0.03-0.01 and 0.02-0.02. Time history analysis using the Northridge, Tabas and Loma Prieta earthquakes for 20 story structures were modeled in Simulink Matlab software considering shearing structure and damper modeling in every blocks separately. Effect of damper to structure is determined as forces applying on corresponding story. Performance indices using software outputs such as root mean square and Maximum of displacement and acceleration of stories were calculated. Performance of single and hybrid systems has been compared due to different earthquakes. Also effect of hybrid systems in series was studied by increasing head loss coefficient. Results show that performance of hybrid systems is dependent on earthquake characteristics that improves with increasing secondary mass ratio. For example under the Northridge earthquake, hybrid system in series tuned mass & liquid column damper with mass ratios 0.035-0.005, 0.03-0.01 and 0.02-0.02 decrease root mean square of displacement of stories 45, 27 and 2 percent respectively and also by selecting optimum frequency ratio based on responses of structure. For example maximum acceleration of hybrid system of tuned mass & liquid column damper in series with mass ratio 0.035-0.005 is optimum frequency ratio in 2.9 and also by selecting this frequency ratio decrease maximum acceleration of up and down stories in 20 story structure. By comparing effects of hybrid system Tuned Mass & Liquid Column Damper in series with different mass ratios on two structures with periods of 1.5 and 2.44 second are considering where by increasing stiffness of structure, performance of hybrid system was improved leading to decrease of acceleration responses and reduction of displacement responses. For example, J1 in 20 story structure with period 1.5 second is 0.71 whereas in other structure is 0.79 that show hybrid system has better performance in structure with period 1.5 second. Hybrid system in series damper with mass ratio 0.035-0.005 have best performance to reduce displacement stories of 20 story structure with period 1.5 second as J3=0.56 means decrease 44%. Also in other structure, hybrid system with mass ratio 0.035-0.005 has best performance to reduce displacement at top floor with J4=0.56. Also performance of hybrid system to reduce maximum displacement of stories was improved by increasing head loss coefficient in tuned liquid column damper
Tarbiat Modares University
Modares Civil Engineering journal
2476-6763
14
2
2014
7
1
A Numerical Study on the Uniaxial Mechanical Properties of
Metal Foams with Spherical Cellular Structure
85
93
FA
A.
Souri
Tarbiat Modares University
S.
Shahbeyk
Tarbiat Modares University
D.
Rahiminejad
University of Sciences & Technology
Metal foams as a new class of materials with interesting properties such as high stiffness and strength to
density ratios, capacity to absorb impact energy, and reproducibility, are rapidly growing their share in
advanced materials market. However, due to their porous microstructure, experimental investigations of
their properties are not trivial and normally need rigorous procedures and high end equipments.
Accordingly, there is a growing research interest towards the numerical modeling of their cellular
structure in which the following three kinds of models have been commonly employed: (1) structures
based on a unit cell or a building block, (2) random Voronoi diagrams, and (3) CAD data provided by Xray
micro-computed tomography. In the current study, the mesostructure of aluminum foam produced by
the brazing technique is simulated as a connected assembly of spherical shells. The latest inward packing
scheme from the set of constructive algorithms is incorporated to efficiently pack the spheres in space.
The Gamma distribution is used to control the cell diameters. Three mean values of 3, 4, and 5 mm and
two variances of 0.5 and 1.0 mm are assumed for the radii of spheres and cubic specimens of 50 mm are
generated. Two assumptions of constant thickness and constant thickness to radius ratio have been
applied to the spherical shells. Two relative densities of 0.05 and 0.1 have been examined in the current
study. A code is written to automatically transfer these geometrical data to ABAQUS FE program. The
models are then meshed in 1 mm S4R shell elements. Tie contacts are defined between neighbor spheres.
Furthermore, self contact is used to prevent any probable penetrations in the models. The foaming
material is assumed to be AL 3003 H12 with elastic-perfectly plastic behavior. Next, the uniaxial load is
applied by means of two rigid planes and the stress-strain curves are extracted. Main attention has been
paid to the elastic modulus and initial yield stress of foam. It is observed that keeping the mean value of
the radius and increasing its variance lead to the generation of more small spheres within the
microstructure which itself increases the number of interactions inside the foam and thus increases
elastic modulus and yield stress. The results also show that, for both thickness assumptions made here,
increasing the mean radius of spheres decreases the number of spheres and their interaction points and
subsequently weakens their uniaxial mechanical properties. Furthermore, compared to foams generated
based on the constant thickness to radius ratio assumption, the presence of thick small spheres in foams
with cells of constant thickness makes them stiffer and stronger. This effect is more pronounced in foam
with higher densities
Tarbiat Modares University
Modares Civil Engineering journal
2476-6763
14
2
2014
7
1
Correlation of LWD, CBR and Cyclic-Triaxial Tests
95
103
FA
M.
Fakhri
K .N. Toosi University of Technology
S.
Brazvan
Science and Research branch, Islamic Azad University
The resilient modulus of subgrade is an important factor in pavement design which is
determined by cyclic triaxial test on cylindrical samples. Due to test complication and
difficulties, other tests could be useful if there is an appropriate correlation between results.
Nowadays non-destructive testing of pavement is growing in the world as they are rapid,
without destruction and have precision measuring equipment. In recent years the application
of LWD has been widely developed. This device evaluates pavement structure of several
locations in short time with high accuracy. The main objective of this paper is to evaluate the
LWD feasibility in order to estimate subgrade resilient modulus and the quality control of
construction. So the in-construction road path have been chosen, then LWD field test, CBR
laboratory test and finally cyclic triaxial have been done. Also the compaction percentage of
each point determined to compare with LWD results.
Comparing modulus obtained from LWD results to other test lead to establish models and
equation which the best relation between LWD modulus and resilient modulus acquired. Also
LWD result scattering is less than CBR. Moreover LWD device has enough efficiency for
structural evaluation of unbound material and quality control of pavement sublayers
Tarbiat Modares University
Modares Civil Engineering journal
2476-6763
14
2
2014
7
1
Drawing Capcity Spectrum and Determining of Target
Displacement without Control Point For Horizontally Curved
Bridges
117
128
FA
A.
Golafshar
Tarbiat Modares University
F.
Daneshjoo
Tarbiat Modares University
Nonlinear Static pushover (NSP) analysis has become a popular tool for the seismic
assessment of buildings and bridges. One of the well-known methods to determine the target
displacement during NSP is Capacity Spectrum Method (CSM).In the Conventional CSM,
capacity spectrum is drawn based on the location of control point to determine the target
displacement. Because of dependency of displacement capacity spectrum on the location of
control point, determination of displacement capacity spectrum needs further evaluation and
can be time consuming specially for complicated structures. Previous studies have been
proposed different choices for location of control point in the bridges such as the point with
the maximum displacement or the top point of critical pier. In this paper the CSM has been
developed for drawing capacity spectrum. The proposed method is based on determination of
displacement capacity spectrum from pushover analysis of MDOF system without selection
of any control point. The proposed method is based on derivation of frequency of SDOF
system based on current situation of MDOF system for each mode and step. Moreover,
nonlinear behavior of MDOF system is considered in frequency of SDOF system step by step.
After derivation of frequency of SDOF system, displacement capacity spectrum of SDOF
system at each mode and step is determined based on the current frequency and current
acceleration capacity spectrum. In this paper for both of the proposed and conventional
methods, demand curve is obtained by time history analysis of a SDOF system and
specifications of bilinear capacity spectrum. In the proposed method, determination of
displacement for each point is based on NSP step that SDOF bilinear capacity curve intersect
to demand curve. For evaluation of proposed and conventional methods a horizontally curved
bridge have been selected and critical pier and earthquake corresponding to displacement
capacity of bridge is determined through Incremental Dynamic Analysis(IDA). Then the
proposed and conventional methods in the horizontally curved bridge are applied during NSP
with force distribution based on first transverse mode shape. Assessing the result reveals that
displacement from proposed and conventional method are different because of different
formulations and dependency of proposed method to displacement vector,force vector and
mass matrix of structures during NSP while the conventional method depends only to
displacement of one point. Another feature that distinguishes proposed from conventional
method and causes different results is the way that target displacement is determined from
target spectrum displacement for top of critical pier. Finally comparison of top displacement
of critical pier from proposed and conventional methods with those derived from (IDA)
shows the efficiency of proposed method to reduce difference between the NSP and IDA
results
Tarbiat Modares University
Modares Civil Engineering journal
2476-6763
14
2
2014
7
1
Experimental and Numerical Modeling of Soft Soil
Improvement using Compacted Lime Mortar Columns
129
141
FA
Gh.R.
Poorebrahimi
University of Kerman
Abstract:
Construction of roads, railways and other engineering structures over soft soils usually involves
consideration of excessive settlements, deformations, and stability problems. To avoid or reduce such problems, several ground improvement techniques are available including the use of column type
elements. Stone columns represent the most known column-type techniques for improving soft soils. It
is well established that the stone columns derive their load carrying capacity from the lateral confining
pressure provided by surrounding soil. Since intermediate layers consisting of organic or very soft to
soft soils can not provide proper lateral support to the stone columns, the external lateral support can
be replaced in these columns by an internal bond between the stones. This can be done by applying
binders such as lime during the installation of the stone/gravel columns, which bind the gravels
together and form a solid column. An alternative method is suggested in the present study to improve
the performance of stone columns in soft soils by using compacted lime mortar columns. Laboratory
tests are carried out on large scale composite specimens composing of compacted lime mortar columns
and soft clay surrounding it. Experiments were carried out on the both end bearing and floating
columns with dimensions of 100 mm diameter, and 600 mm height. The columns were surrounded by
soft soil in cylindrical tanks of 600 and 800 mm height, and a diameter varying from 230 to 320 mm to
represent the required unit cell area of soft soil around each column assuming triangular pattern of
installation of columns. In the field, the entire of the column treated ground will be subjected to
loading from the superstructure. The same was simulated in the laboratory by loading the entire area
of the specimen to study the stiffness of improved ground with regard to the influence of different
parameters such as: area ratio (as the ratio of the area of the column (A
S
) to the total area within the
unit cell (A)), and shear strength of the clay. Finite element analyses have also been performed with
the software package PLAXIS V 8.2 using 15-noded triangular elements. Tests results show that
compacted lime mortar columns exhibit a stiffer and stronger response compared to conventional
stone columns installed in soft soils. These columns improve the load carrying capacity and decrease
the settlement of soft soils. The performance of these columns is significantly enhanced by increasing
the area ratio. However, when the area ratio exceeds 15%, the rate of increment of the load carrying
capacity is negligible. When the entire area of the composite specimens is loaded, because of the
confining effect of the boundary of the unit cell, failure does not take place even for settlement as high
as 15 mm. Comparison of numerical and experimental data shows a good agreement between the
results.
Tarbiat Modares University
Modares Civil Engineering journal
2476-6763
14
2
2014
7
1
A Stochastic Approach to Freeway Capacity Estimation:
Tehran-Karaj Freeway Case Study
143
153
FA
A.R.
Mamdoohi
Tarbiat Modares University
M.
Saffarzadeh
Tarbiat Modares University
S.
Shojaat
Louisiana Sate University
Capacity of a road facility as an important characteristic in transportation and traffic studies is defined as the maximum rate of flow that could be held by that facility, which has been supposed to have a constant and certain value. This assumption, although necessary for most traffic studies, has also caused some problems, like that of demand exceeding capacity in many road facilities. Researchers have recently shown that capacity is not necessarily the maximum flow rate held by a facility. They have also demonstrated that capacity has a stochastic nature rather than a constant and deterministic value. Stochastic approach to capacity is more complicated and comprehensive. In this approach, capacity is treated as a random variable generated from a population, and having corresponding distribution function. Knowing more about breakdown phenomenon, as transition from acceptable to unacceptable flow, plays a key role in this approach. To obtain breakdown flow rates, threshold speed as the quantitative measure is used to distinguish congested and non-congested flow rates. Flow rates occurring immediately before decrease of average speed below the threshold speed, are regarded as breakdown flow rates and their value in addition to non-congested flow rates are used to estimate the distribution function. Product Limit Method with analogy to life time data is used to estimate non-parametric function. The main advantage of this method is that it considers censoring data. In capacity estimation, if a time interval is followed by a breakdown, it will be regarded as uncensored interval; if it is non-congested it will be regarded as censored interval, meaning that capacity of the road is bigger than incoming demand. If it is located in a congested area, it would not be used in the estimation process. Two common parametric estimation methods are (OLS) ordinary least squares and (MLE) maximum likelihood estimation. Since binary data is used to estimate capacity distribution function, the ordinary least squares method is not useful with such data. Maximum likelihood estimation with a presumption about the type of distribution is used to estimate the parameters. Distribution function with the maximum log-likelihood value would be the function that has most likely produced the sample, and is known as the capacity of the freeway. In this paper, both non-parametric and parametric capacity distribution functions of Tehran-Karaj freeway as the oldest and the busiest freeway in Iran, serving and average of 100,000 passenger cars a day, are estimated. Threshold speed is found to be respectively 70 km/h and 75 km/h in two sections under investigation located in the direction to Karaj. Based on the data gathered for four months by traffic cameras; and refining to meet standard criteria, a sample of 229 and 169 breakdowns were detected at each section. Different distribution functions are fitted to the data, and with trial about different kinds of functions, Gumbel distribution is found to be the best distribution fitting the observed data.
Tarbiat Modares University
Modares Civil Engineering journal
2476-6763
14
2
2014
7
1
Nonlinear Seismic Analyses of Median Ductile RCMRF
Structures Based on Variation of Columns Acceptance
Criteria
155
164
FA
A.A.
Tasnimi
Tarbiat Modares Uni
H.R.
Kazemi
Tarbiat Modares Uni
Based on experimental evidence and empirical models the proposed supplement to ASCE 41-
06 is developed for the purpose of updating provisions related to existing reinforced concrete
structural buildings. Several experimental research programs have demonstrated that many
older-type columns are capable of sustaining limited plastic deformation due to flexural
yielding prior to shear failure. This proposed supplement concentrates on this failure mode
and includes the categorization of columns based on failure mode, the selection of target
probabilities of failure for each failure mode and revisions to modeling parameters and
acceptance criteria for reinforced concrete columns. In this research, the effect of new
provisions on seismic evaluation of reinforced concrete moment resisting frame is
investigated. In this regard three medium ductile MRCRF structures with 4, 8 and 12 stories
and two direction median moment resisting frame systems are considered. These structures
initially have been analyzed and designed according to ‘Iranian Standard 2800, for seismic
design of buildings’ and ‘Iranian concrete code of practice’. Then nonlinear static analysis
and nonlinear time history analysis methods have been utilized to evaluate the seismic
performance of these structures. In nonlinear static analysis there are several methods for
determining target displacement among them some reliable methods are displacement
coefficient method (FEMA-356), Capacity spectrum method (ATC-40), and equivalent
linearization method and modifies coefficient method (FEMA-440). The target displacements
with these methods are compared with maximum displacement in nonlinear time history
analysis. It is observed that capacity spectrum method given by ATC-40 reports target
displacement values higher than time history analysis. Furthermore, results obtained from the
equivalent method and the modified coefficient methods suggested by FEMA-440 are closer
to time history analysis values. The performance levels of these structures have been
evaluated based upon target displacement of nonlinear static analysis that obtained from
FEMA-440 methods and maximum displacement in nonlinear time history analysis. The
effect of the variation of reinforced concrete columns modeling parameters and acceptance
criteria on performance levels of reinforced concrete structure is investigated. For this purpose
in nonlinear seismic design, the modeling parameters and acceptance criteria have been
considered with those from FEMA-356 for columns “controlled by flexure” and then with
those from proposed supplement to ASCE 41-06 for flexure failure (flexural yielding without
shear failure) and flexure-shear failure (shear failure following flexural yielding). The
obtained results indicate that these structures are to some extant conservative in their seismic
performance due to the modifications of ASCE41-06.
Tarbiat Modares University
Modares Civil Engineering journal
2476-6763
14
2
2014
7
1
Application of genetic algorithms in the optimization of
empirical relations to estimate of geometrical characteristics
of the scour hole downstream of grade control structures
165
173
FA
A.A.
Dehghani
Gorgan University
M.
Abdi Dehkordi
Gorgan University
Grade-control structures are aimed at preventing excessive channel-bed degradation in
alluvial channels and at the outlet of a grassed waterway to stabilize the waterway outlet. The
erosive action of the weir overflow may, however, cause significant local scour downstream
and may endanger the stability of these structures. The scour process can be split up into
different time phases. In the beginning the development of scour is very fast, and eventually
an equilibrium situation is reached depending on clear water or live bed flow condition.
Appropriate design of grade-control structures requires estimate geometric characteristics
of the downstream scour. The local scour downstream of a grade-control structure located on
an alluvial bed is a very complex and dynamic phenomenon which is influenced by many
parameters. Most of these parameters are related to each other and determination of them is
difficult. The estimation of maximum scour depth is very important in planning, design and
management of hydraulic structures. Therefore, there is a great need to develop good prediction methods for geometric
characteristics of scour hole downstream of grade control structures. Some of researchers
studied scour downstream of grade-control structures and presented different empirical
equations for estimation of maximum scour depth based on experimental data. The results of
literatures show that the equation of D'Agostino and Ferro (2004) can be used as one of the
most important empirical equations to estimate the maximum scour depth.
In this study, coefficients of D'Agostino and Ferro (2004) equation to estimate of the
maximum scour depth, horizontal distance between the weir crest and the maximum scour
depth position, horizontal distance between the weir crest and sand deposition and maximum
height of sediment deposition, were optimized by applying genetic algorithm.
For this purpose, the field and laboratory data of Veronese (1937), Bormann and Julien
(1991), D’Agostino(1994), Mossa (1998), Lenzi et al. (2000), Missiaga stream (2004),
Falciai and Giacomin (1978), Some large-scale data ((Veronese 1937), Scimemi (1939),
Whittaker and Schleiss (1984)) and shahabi (2010) were used for training and validation of
the equations.
The efficiency and accuracy of each equations were examined based on statistical
Analyses such as percent error, root mean square error, mean bias error and correlation
coefficient using independent data sets of training data and were compared with the results
obtained from the equations proposed by D'Agostino and Ferro (2004).
The results show that the value of percent error, root mean square error, mean bias error
and correlation coefficient for maximum scour depth, horizontal distance between the weir
crest and the maximum scour depth position, horizontal distance between the weir crest and
sand deposition and maximum height of sediment deposition were 46.09, 11.78, 10.28 and
118.05, respectively. The corresponding values are 128.1, 27.73, 15.23 and 194.46 by using
D'Agostino and Ferro relation. The results showed that the root mean square error and mean bias error are decreased by
using proposed equations and the accuracy of obtained results by presented equations is
higher than D'Agostino and Ferro (2004) equations. So it is recommended to use the proposed
relations for assessing the characteristics of scour hole downstream of grade control
structures.
Tarbiat Modares University
Modares Civil Engineering journal
2476-6763
14
2
2014
7
1
Estimation of the Pattern of Debris Load Caused By an
Earthquake on Steel Beams
175
186
FA
P.
Zarparvar
Power and Water Uni
M.
Safi
Power and Water Uni
During an earthquake and its effect, what happens on buildings is local damage and destruction which is different due to different characteristics such as diverse earthquake and geotechnical characteristic, different methods of analysis for earthquake-resistant structures, etc. Therefore, in addition to removing structural elements and their falling as debris, a shock and impulsive load could be added to the dynamic load imposed by the earthquake. Impact loads with extreme severity are loads with very low frequency of occurrence, but with extraordinary consequences. Undoubtedly appearing greater success in designing buildings resistant against extraordinary loads is required to forecast the real pattern and the impact of the impulsive loads. In other words, identify all possible failure modes of structures under extraordinary loads is necessary, which requires nonlinear analysis of structures under various loading conditions. Detailed modeling of the impact of an upper floor onto the floor below is feasible using current sophisticated nonlinear dynamic analysis software. Yet the computational effort in the case of large and complex structural systems can be excessive, especially if a detailed model of the whole structure is considered. Moreover, such analysis requires structural engineers with considerable expertise in nonlinear structural dynamics. Due to these limitations, detailed impact modeling is not practical for design applications. Hence, there is an evident need for simple, yet sufficiently accurate methodologies that can be used to establish whether the strength, ductility supply and energy absorption capacity of the lower impacted floor are adequate to withstand the imposed dynamic loads from the falling floor(s). This paper proposes a new design methodology for progressive collapse assessment of floor within multi-storey buildings subjected to impact from an above failed floor. The amount of debris and its scattering and distribution on different floors of a building play an important role in the operation of the impact load. The important point is that how these parameters should be considered in the dynamic analysis and how much the structure’s respond is sensitive to the impact characteristics such as impact velocity, the mass of debris and its distribution. This study has attempted to calculate the dynamic load factor (DLF) for samples of steel beams with an elastic-plastic behavior, and to extract the pattern of debris impact load in order to provide an equivalent loading pattern to estimate the performance of structures subjected to above failed floor(s). The study of dynamic load factors for the samples illustrates that the period of beam, the height of falling and the mass of debris have a noticeable effect on the result so that the DLF decreases by increasing in amount of the period of beams or the mass of debris, as well as it increases by increasing the height of falling. About the achieved patters for the impact load what is important is providing multi-line graphs in order to estimate the dynamic effect of the debris impact
Tarbiat Modares University
Modares Civil Engineering journal
2476-6763
14
2
2014
7
1
Evaluation of Seismic Performance of Reinforced Concrete
Frames Using Pall Friction Damper
187
203
FA
ل
vaseghi
f
tamjid
Abstract: Considering the seismicity of most of the areas in Iran, it is inevitable to confront the earthquake because of its major property and life damages. Therefore researchers put a great effort on designing and strengthening against earthquake. The researches conducted so far for improving the reinforced concrete structures using different types of steel braces and analytical studies and widespread experiments has been done for confronting destructive effects of earthquake on structures, results show the proper effectiveness of different mechanisms of inactive seismic control of structures as an efficient option for confronting the earthquake forces. Of different control methods, using friction damper could be counted as one of the best methods for improving the seismic behavior of current structures, as it uses a simple mechanism and doesn’t need specific materials or technology. By using friction dampers both the rigidity and the structure’s hysteresis energy loss ability is increased. These dampers by their inelastic behavior in different points of the structure cause the loss of incoming energy of earthquake. Also in high importance structures, by selecting the proper design parameters, it is possible to prevent the main structural members to enter the inelastic behavior limit which causes local damages to some parts of them or minimize that. This system was first introduced by Pall and March in 1982. The mechanism of this system is creating slippage friction surfaces at the intersection of braces. For building frames, these dampers could be used in crossed tension bracing, single diagonal bracing and Chevron bracing. The first model of pall friction damper was tested in chevron bracing against earthquake in Eaton Building in Canada. The purpose of this study is to investigate the role of Pall friction dampers in reducing structural response during the earthquake. Therefore, modeling of the damper is based on the model used by the Pall Corporation in Eaton Building. The functioning of this damper is by generating friction under lateral shear force which causes the movement of the damper and generation of slippage in it. Therefore, three concrete moment frames with 5, 8 and 10 stories, have been designed according to the Iranian National Codes. Using SAP2000, v14, several static nonlinear analyses were done to get the performance point of frames on the basis of the capacity spectrum method. Adding chevron braces to the mid span, the target displacement of frames were determined. Considering the fact that none of the braced moment frames satisfied the Life Safety criteria under Design Based Earthquake, Pall friction dampers have been added to the frames and static nonlinear analysis were done by several slip loads such as 1%, 10%, 25%, 50%, 75% and 100% of frame weights. Evaluation showed that in optimum slip Load, the performance level of the frames improves.