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Transaction on Civil Engineering

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Volume 17, Issue 1, 2010
Transaction on Civil Engineering


Preloaded High-Temperature Constitutive Models and Relationships for Concrete
 
        M. Bastami (PhD.)
  • F. Aslani [MSc.]

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The behavior of concrete structures that are exposed to extreme thermo-mechanical loading is an issue of great importance in nuclear engineering. The structural re-safety capacity of concrete is very complicated because concrete materials have considerable variations. Constitutive models and relationships for preloaded Normal and High Strength Concrete (NSC and HSC) subjected to re are needed, which are intended to provide ecient modeling and to specify the re-performance criteria of the behavior of preloaded concrete structures exposed to re. In this paper, formulations for estimating the parameters a ecting the behavior of uncon ned preloaded concrete at high temperatures are proposed. These formulations include residual compression strength, initial modulus of elasticity, peak strain, thermal strain, transient creep strain and the compressive stress-strain relationship at elevated temperatures. The proposed constitutive models and relationships are veri ed with available experimental data and existing models. The proposed models and relationships are general and rational, and have good agreement with the experimental data. More tests are needed to further verify and improve the proposed constitutive models and relationships.

Keywords:
Preloaded concrete

    Constitutive models and relationships
      Normal and high strength concrete
        Fire
          High temperature.


Characteristics of Second Generation Endurance Time Acceleration Functions
 
        H.E. Estekanchi (PhD.)
  • V. Valamanesh [MSc.]
  • A. Vafai [Professor]

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The Endurance Time (ET) method is a time-history based dynamic pushover procedure in which structures are subjected to specially designed, intensifying accelerograms, and their seismic performance is judged based on the time duration needed to satisfy the required design objective. Second generation refers to ET acceleration functions that are generated by application of optimization techniques in order to produce response spectra compliant linearly intensifying accelerograms. In this paper, the major characteristics of a set of second generation ET acceleration functions (ETA20a01-3) are investigated. The template response spectra of this set of ET acceleration functions corresponds to the design spectra of the Iranian National Building Code (Standard 2800) for sti soil (type II). Results show that a good correspondence can be established between the e ective ground motion parameters of earthquakes and ET acceleration functions at speci c target times. Therefore, it is expected that ET acceleration functions can be used to predict various demand parameters of structures subjected to ground motions whose response spectra is more or less compatible with the adopted template response spectra. Discrepancies between characteristics of ET acceleration functions and ground motions have also been discussed.

Keywords:
Dynamic pushover

    Intensifying acceleration functions
      Endurance time method.


Experimental and Numerical Behavior of Shallow Foundations on Sand Reinforced with Geogrid and Grid Anchor Under Cyclic Loading
 
        N. Hataf (Professor)
  • A. H. Boushehrian [MSc.]
  • A. Ghahramani [Professor]

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There are many cases where the foundations of structures are subjected to cyclic loading in addition to static loading. Oil reservoir foundations with frequent discharges and lling or road embankments under repeatable trac loads are examples of such foundations. Although the amplitudes of the cyclic load is usually less than the permissible static load, the concern still exists for the amount of uniform and non uniform settlement of such structures. The soil under such foundations may be reinforced with geosynthetics to improve their engineering properties. This paper deals with the e ects of using the new generation of reinforcements, grid-anchor, for the purpose of reducing the permanent settlement of these foundations under the in uence of di erent proportions of the ultimate load. Other items, such as the type and number of reinforcements, as well as the number of loading cycles, are studied experimentally. In all cases, the foundation is rst under the in uence of a xed static load equal to the weight of the structure itself and, then, the cyclic load in di erent proportions of the ultimate load is applied to it. The results show that by using grid-anchor and increasing the number of their layers in the same proportion as that of the cyclic load applied, the amounts of permanent settlements are reduced and the numbers of loading cycles to reach it are decreased. For comparison with the experimental ndings, similar to the conditions of the tests conducted, numerical models were made using a 3-D nite element software. The numerical results showed good agreement with the test results.

Keywords:
Reinforced soil

    Shallow footing
      Cyclic loading
        Grid-anchor
          Geogrid.


Seismic Risk Analysis of Iranian Construction Projects
 
        M. Mofid (Professor)
  • M. Shokri-Ghasabeh [MSc.]
  • A. Bakhshiani [PhD.]
  • K. Hansen [MSc.]

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In this paper, the project earthquake occurrence risk coecient is determined for each construction project that is located in one of Iran's twenty seismic regions. This coecient is allocated, regardless of the current situation of the project, being in the plan or execution phase or even completed. This coecient indicates the possibility of an earthquake occurrence during a project's life time. To nd this coecient, the Gutenberg-Richter linear relationship has been applied, in conjunction with the Poisson distribution. The Gutenberg-Richter linear equation expresses the relationship between the magnitude of an earthquake and the number of occurrences, during a xed time, of that magnitude. To nd the linear relationship for a series of earthquakes with di erent magnitudes occurring in the same seismic region, the Ordinary Least Square (OLS) has been used. Two linear regression assumption violations, which are variance heteroscedasticity and autocorrelation, have been tested on the available data. In the case of nding one or both of these two violations, The Generalized Least Square (GLS) has been applied to produce a better regression line. Moreover, the second order type of the Gutenberg-Richter relationship has also been determined to validate the linear one. In conclusion, by application of the Poisson distribution and by having the design earthquake's magnitude and project life time, the third parameter, which is the design earthquake occurrence risk, can be determined for a given construction project in a speci c location in Iran.

Keywords:
Earthquake risk analysis

    Construction management
      Project management
        Risk management; OLS, GLS;


Optimization of Semi-Active Control of Seismically Excited Buildings Using Genetic Algorithms
 
        S. Pourzeynali (PhD.)
  • T. Mousanejad [MSc.]

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In this paper, the performance of semi-active viscous dampers in reducing the response of tall buildings to earthquake acceleration is optimized using genetic algorithms. Torsional effects due to irregularities exist in the building and due to unsymmetrical placement of the dampers are taken into account through 3-D modeling of the building. For the numerical example, a twelve-story building is chosen. The building is modeled as a 3-D frame. The equations of motion of the building with semi-active viscous dampers, subjected to earthquake acceleration, is written, resolved in state space and the results are compared with those of the uncontrolled building. Moreover, in order to minimize building responses such as top story displacement and base shear, the required number and location of dampers are optimized using genetic algorithms.

Keywords:
Optimization

    Genetic algorithms
      Fluid viscous damper
        Semi-active control


Comparison of the Cosserat Continuum Approach with Finite Element Interface Models in a Simulation of Layered Materials
 
        A. Riahi (PhD.)
  • J.H. Curran [Professor]

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This paper compares the formulation of the nite element Cosserat smeared approach with the combined nite element-explicit interface element approach, when both applied to the analysis of layered continua. The fundamental equations of both formulations are presented. Also, using three examples, the nature and accuracy of the displacement eld predicted by both techniques are investigated and discussed.

Keywords:
Cosserat theory

    Layered continuum
      Internal characteristic length
        Transversely isotropic; Interface element
          Finite element method.


Improving Mechanical Properties of High Strength Concrete by Magnetic Water Technology
 
        M. Gholizadeh (PhD.)
  • H. Afshin [PhD.]
  • N. Khorshidi [PhD.]

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The most important challenge for concrete technologists is to improve the properties of concrete. In the last two decades, in Russia and China, a new technology, called magnetic water technology, has been used in the concrete industry. In this technology, by passing water through a magnetic eld, some of its physical properties change and, as a result of such changes, the number of molecules in the water cluster decrease from 13 to 5 or 6, which causes a decrease in the water surface tension. Using magnetized water in concrete mixtures causes an improvement in the workability and compressive strength of concrete. Also, this processed water causes a reduction in the cement content required for the speci ed compressive strength value. In this research, in the concrete laboratory of Sahand University of Technology, the e ects of magnetic water on some mechanical properties of high strength concrete, such as workability and compressive strength, have been studied. For the production of magnetic water, a magnetic treatment device (made in Germany) has been used. This device mostly is used for the softening of water and, for the rst time in this research, it has been used by the authors for the production of concrete. The results of tests showed that, in most cases, concrete made with magnetic water (magnetic concrete), has higher slump values than those of control concrete (up to 45%). Also in some cases, the compressive strength of the magnetic concrete samples was higher than that of the control concrete samples (up to 18%). Also, in some cases, with the same slump and compressive strength, cement content can be reduced by 28% in the case of magnetic concrete.

Keywords:
High strength concrete

    Magnetic water
      Compressive strength
        Slump
          Clean concre