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Showing 12 results for Rahai

Bakhtiari Nejad F., Rahai A., Esfandiari A.,
Volume 2, Issue 2 (June 2004)

In this paper a structural damage detection algorithm using static test data is presented. Damage is considered as a reduction in the structural stiffness (Axial and/or Flexural) parameters. Change in the static displacement of a structure is characterized as a set of non-linear undetermined simultaneous equations that relates the changes in static response of the structure to the location and severity of damage. An optimality criterion is introduced to solve these equations by minimizing the difference between the load vector of damaged and undamaged structures. The overall formulation leads to a non-linear optimization problem with non-linear equality and linear inequality constraints. A method based on stored strain energy in elements is presented to select the loading location. Measurement locations are selected based on Fisher Information Matrix. Numerical and experimental results of a 2D frame represent good ability of this method in detecting damages in a given structure with presence of noise in measurements.
Kourosh Shahverdiani, Ali Reza Rahai, Faramarz Khoshnoudian,
Volume 6, Issue 2 (June 2008)

Large capacity cylindrical tanks are used to store a variety of liquids. Their Satisfactory

performance during earthquake is crucial for modern facilities. In present paper, the behavior of cylindrical

concrete tanks under harmonic excitation is studied using the finite element method. Liquid sloshing, liquid

viscosity and wall flexibility are considered and additionally excitation frequency, liquid level and tank

geometry is investigated. The results show a value for wall thickness to tank diameter ratio which may be used

as a guide in the consideration of wall flexibility effects.

A.r. Rahai, M.m. Alinia, S.m.f Salehi,
Volume 7, Issue 1 (March 2009)

Concentric bracing is one of the most common lateral load resistant systems in building frames, and are

applied to many structures due to their manufacturing simplicity and economics. An important deficiency in the

bracing members is their irregular hysteretic loops under cyclic loading. In order to overcome this problem, it is

advised to restrain braces against buckling under compression, since buckling restrained frames dissipate a large

amount of energy. One method to restrain braces against buckling is to cover them with concrete. A proper covering

can prevent the core from buckling and provide similar capacities whether in tension or compression which would

produce regular hysteric curves. In this study, the behavior of buckling restrained braces (BRB) has been investigated

by considering different types of surrounding covers. The steel core is encased in concrete with different coverings. The

covering types include steel tubes, PVC pipes, and FRP rolled sheets. Experimental and numerical analyses were

implemented. According to the results, PVC pipes and FRP sheets are suitable alternatives to steel pipes. Furthermore,

the behavior of several types of steel cores was assessed since, applying steel with high ductility promotes the energy

dissipation of the brace. Finally, the effect of the separating layer between the steel core and the concrete on the

performance of bracing was evaluated.

A. Arabzadeh, A.r. Rahaie, A. Aghayari,
Volume 7, Issue 3 (Sept. 2009)

In this paper a new method based on Strut-and-Tie Model (STM) is proposed to determine the shear capacity of simply supported RC deep beams and an efficiency factor for concrete with considering the effect of web reinforcements. It is assumed that, the total carried shear force by RC deep beam provided by two independent resistance, namely diagonal concrete strut due to strut-and-tie mechanism and the equivalent resisting force resulted by web reinforcements, web reinforcing reduces the concrete compression softening effect with preventing from the diagonal cracks opening or concrete splitting. The unknown function and parameters are determined from 324 experimental results obtained by other researchers. To validate the proposed method, the obtained results are compared with some of the existing methods and codes such as ACI 318-05 and CSA. The results indicate that the proposed method is capable to predict the shear strength of variety of deep beams with acceptable accuracy.
Abolfazl Arabzadeh, Reza Aghayari, Ali Reza Rahai,
Volume 9, Issue 3 (September 2011)

An experimental-analytical investigation was conducted to study the behavior of high-strength RC deep beams a total of sixteen

reinforced concrete deep beams with compressive strength in range of 59 MPaOf'c O65 MPa were tested under two-point top

loading. The shear span-to-effective depth ratio a/d was 1.10 all the specimens were simply supported and reinforced by

vertical, horizontal and orthogonal steel bars in various arrangements. The test specimens were composed of five series based

on their arrangement of shear reinforcing. The general behavior of tested beams was investigated. Observations were made on

mid-span and loading point deflections, cracks form, failure modes and shear strengths. The test results indicated that both

vertical and horizontal web reinforcement are efficient in shear capacity of deep beams, also the orthogonal shear reinforcement

was the most efficient when placed perpendicular to major axis of diagonal crack. Concentrating of shear reinforcement within

middle region of shear span can improve the ultimate shear strength of deep beam. The test results were then compared with the

predicted ultimate strengths using the ACI 318-08 provisions ACI code tended to either unsafe or scattered results. The

performed investigations deduced that the ACI code provisions need to be revised.

Afshin Firouzi, Ali Reza Rahai,
Volume 9, Issue 3 (September 2011)

Corrosion of reinforcement due to frequently applied deicing salts is the major source of deterioration of concrete bridge decks, e.g. severe cracking and spalling of the concrete cover. Since crack width is easily recordable in routine visual inspections there is a motivation to use it as an appropriate indicator of condition of RC bridge elements in decision making process of bridge management. While few existing research in literature dealing with spatial variation of corrosion-induced cracking of RC structures is based on empirical models, in this paper the extent and likelihood of severe cracking of a hypothetical bridge deck during its lifetime is calculated based on a recently proposed analytical model for corrosion-induced crack width. Random field theory has been utilized to account for spatial variations of surface chloride concentration, as environmental parameter, and concrete compressive strength and cover depth as design parameters. This analysis enables to track evolution of cracking process, spatially and temporally, and predict the time for the first repair of bridge deck based on acceptable extent of cracked area. Furthermore based on a sensitivity analysis it is concluded that increasing cover depth has a very promising effect in delaying corrosion phenomenon and extension of the service life of bridge decks.

A. Arabzadeh, R. Aghayari, Ali A. R. Rahai,
Volume 10, Issue 4 (December 2012)

Strut-and-Tie Model (STM) can be used to model the flow of compression within a concrete strut. Concrete struts are formed

in various shapes such as prismatic or bottle-shaped. In order to study the behavior of concrete struts, a series of simple tests

were performed. Eighteen reinforced concrete isolated struts with compressive strength of 65 MPa were tested up failure under

point loading in the plane of specimens. The tested specimens were reinforced by various reinforcement layouts. The behavior

of tested beams was investigated. Observations were made on transverse displacement, primary cracking and ultimate failure

load and distribution of strain on the face of tested panels. Based on these observations, the geometry of the concrete struts was

examined. a new model to analysis of concrete struts was proposed based on modified compression field theory (MCFT). A

database of 44 tested specimens was compiled to evaluate the proposed model. The results indicate that using the ACI and CSA

codes expressions regarding the amount of minimum required reinforcement in a strut produces conservative but erratic results

when compared with the test data. Conversely, the new proposed model presents a more accurate prediction for the strength of

44 tested struts.

A. R. Rahai, S. Fallah Nafari,
Volume 11, Issue 4 (Transaction A: December 2013)

The seismic behavior of frame bridges is generally evaluated using nonlinear static analysis with different plasticity models hence this paper tends to focus on the effectiveness of the two most common nonlinear modeling approaches comprising of concentrated and distributed plasticity models. A three-span prestressed concrete frame bridge in Tehran, Iran, including a pair of independent parallel bridge structures was selected as the model of the study. The parallel bridges were composed of identical decks with the total length of 215 meters supported on different regular and irregular substructures with non-prismatic piers. To calibrate the analytical modeling, a large-scale experimental and analytical seismic study on a two-span reinforced concrete bridge system carried out at the University of Nevada Reno was used. The comparison of the results shows the accuracy of analytical studies. In addition, close correlation between results obtained from two nonlinear modeling methods depicts that the lumped plasticity approach can be decisively considered as the useful tool for the nonlinear modeling of non-prismatic bridge piers with hollow sections due to its simple modeling assumption and less computational time.
P. Vahabkashi, A. R. Rahai, A. Amirshahkarami,
Volume 12, Issue 1 (Transaction A: Civil Engineering March 2014)

Piles or drilled shafts used in bridge foundation, waterfronts, and high rise buildings are generally subjected to lateral loads. In order to study the effect of concrete pile geometry on the structural behavior in layered soils, several models with different shapes and dimensions for piles and different properties for two soil layers with variable thickness were selected and analyzed using the finite difference method. The performance of piles situated in layered granular soil with different compaction and thicknesses were studied in two cycles of lateral loading and unloading. The applied finite difference procedure is also validated based on experimental and published results. The pile head displacement of different models due to their overall deformation and rotation were calculated under maximum loading. For a comparison of pile head displacement due to their overall deformation and rotation in different models, the "performance index” is defined as the ratio of “displacement due to deformation” to the “total displacement”.
A. R. Rahai, M. Mortazavi,
Volume 12, Issue 4 (Transaction A: Civil Engineering December 2014)

During the past years the use of buckling restrained braces (BRBs) have had a dramatic growth due to their better performance comparing to conventional braces. BRBs have more ductility and energy absorption capacity by excluding the overall brace buckling. However, even these kinds of braces have some problems restricting their use in some projects, i.e. high tolerance of applying unbonding material, concrete placing difficulties and their weight. Accordingly, many researchers have conducted experiments to find the possibility of shortening or even eliminating the infill material of the braces. The following study has addressed the effect of debonding material friction ratio, shortening the concrete fill, and finally eliminating it if possible, by reshaping the core element with constant section area. The operated analysis has been carried out both numerically and experimentally. ABAQUS finite element software was applied for numerical analysis and the results were verified by an experimental study in two groups of models each including four full-scale brace models. With a constant core section area, results revealed that without the risk of buckling, the concrete cover length could be reduced. With a special core profile, the infill may be fully omitted and the restrainer would be made up of only a steel tube, which may happen without any changes made to the cross sectional area of the core profile.
Mohammad Reza Saberi, Alireza Rahai, Masoud Sanayei ,
Volume 15, Issue 1 (Transaction A: Civil Engineering 2017)

Steel bridges play a very important role in every country’s transportation system. To ensure that bridges perform reliably, engineers monitor their performance which is referred to as Structural Health Monitoring (SHM). An important element of SHM includes the prediction of service life. There is ample historical evidence that bridge damage is pervasive and their life time is decreasing. To manage costs and safety, service life prediction of bridges is necessary. We present a statistical method to predict service life for steel bridges. A nonparametric statistical model based on the bootstrap method for stress analysis for fatigue life prediction of steel girder bridges is proposed. The bootstrap provides a simple approach for reproduction of the probability distribution of measured strain data. The bootstrap is sensitive to the number of events in the verification sample (data), thus we introduce a stable survival distribution function (SDF). An index is presented in this paper for inferring the service life of steel bridges, which can be known as the Life Index (µ). The life index function shows variation of the age of steel bridges under daily traffic loads. A regression model is developed which relates the service life of steel bridges using a bridge life index based on measured operational strain time histories. The predicted remaining service life derived from the model can contribute to effective management of steel bridges. The proposed method assists bridge engineers, bridge owners, and state officials in objective assessment of deteriorated bridges for retrofit or replacement of deteriorated bridges. Timely repair and retrofit increase the safety levels in bridges and decrease costs.

Hamid Reza Ebrahimi Motlagh, Alireza Rahai,
Volume 15, Issue 5 (Transaction A: Civil Engineering 2017)

This paper tried to analyze the behavior of a typical bridge and the effect of its skew degree on its behavior to near-field earthquakes. To this end, the seismic behavior of a number of typical bridges with same spans and different skew degrees was studied under near-field and far-field earthquakes. Non-linear static analyses (pushover analyses) were performed to determine the performance parameters of the bridge in each model. Non-linear time history dynamic analyses were also performed on the models to analyze the dynamic behavior and deformations of bridge components under near-field and far-field earthquakes. The responses of models, such as their displacement, base shear, and axial forces of columns to earthquakes under study are presented in the following sections. Results indicated that the base shear and displacement of the superstructure in near-field earthquakes without velocity pulse and far-field earthquakes are about or less than the corresponding values of the bridge performance point. Moreover, in the case of near-field earthquakes with velocity pulses the values of these parameters showed an increase. It was also revealed that an increase in the skew degree of the bridge led to an increase in the axial forces in columns and transverse displacement of the bridge.

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