AMIRKABIR JOURNAL OF CIVIL ENGINEERING (AMIRKABIR)
Year:2009 | Volume:41 | Issue:1|Papers Count:13

Throughout the years, tuned mass dampers have been implemented effectively to reduce wind-induced vibrations and earthquake excitations in high-rise buildings. In this paper, the performance of braced frames with rooftop tuned mass damper frame is studied. For this purpose, linear and nonlinear time history dynamic analysis of six steel concentrically braced frames with 3, 5 and 15 stories subjected to near and far field earthquakes are conducted. Based on numerical results obtained in this research, applying a rooftop damper frame is more effective in reducing the seismic response of the braced frames with short periods of vibration than those with moderate and long lateral periods. It is found that rooftop tuned mass damper reduces the floor displacement of the 3-story building by as much as 18 to 66 percent under different earthquake excitations. Results also show that this type of damper frame with appropriate design can result in decreased seismic responses both under near and far field earthquake excitations.

In recent years, the world - wide standard codes such as IBC2000 and NEHRP2000 have paid special attention to the coefficient over-strength factor. The effect of this parameter on saving the safety of structures from earthquake is quite postulated. This parameter associated with the Redundancy/ Reliability factor by which the structural safety is controlled is added into the above mentioned Codes from 1997. Describing the over-strength parameter, the role of effective factors on it, it's role on the seismic behavior of structure, quantitative description of the problem in terms of performing linear and non-linear analyzing of selected structures under incremental push-over analysis and determining this factor in performance pint of the structures constitute some sections of this article. Moreover, an important issue concerning the following question is investigated. That is, "Dose the unknown over-strength factor value incorporated in the structure through the design procedure suffices the structures for being safe or it should be sufficiently induced in structures for responding an appropriate non-linear performance".In this article eight Moment Resisting Frames (MRF) with different number of stories and bays in two different systems, ordinary and special are studied and the results are assessed. These structures have been designed according to The Iranian standard 2800 and American IBC2000 Standard and the required overstrength values produced by the design procedure are determined and the results obtained from both Codes are compared and evaluated.

This paper presents the effect of various parameters on deformation characteristics of a 17-mreinforced wall under seismic conditions with consideration of large strains. The foundation soil strength parameters (angle of internal friction and cohesion) and reinforcing elements (length and stiffness) are varied to investigate their effects on horizontal facing deformation and vertical displacement of the wall toe. The main objective of this research is to investigate the optimized performance of such (high) walls under seismic conditions.Numerical 2-D plane-strain analysis with a finite difference scheme using FLAC-2D is performed to achieve the objectives. A variable amplitude harmonic wave with a 3-Hz frequency is applied to the wall foundation. The results show that an improved foundation soil properties of suitable shear strength and steel reinforcement belts with proper length (with respect to the wall height), the wall deformations are significantly reduced and the wall performance under seismic conditions is improved.

Humans are social creatures who interact with others in doing their activities. The social activities occur in space and time.Therefore, a proper model of human activities must consider the two dimensions of space and time simultaneously.This paper attempts to model human activities in spatio-temporal domain to improve the efficiency of group activities. For this purpose, Hagerstrand theory is evaluated and extended for managing human team activities in space and time. The suggested model can be implemented in a Geospatial Information System and helps resolving multidimensional problem in their proper environment. To assess the proposed model, activities of two teams: life-detection and collapse lifting in two forms of normal and simulation were assessed. The proposed strategy improved the activities by a factor of 18.83.

This research investigates the effect of stiffness degradation, strength deterioration and pinching behavior of hysteresis loops in static nonlinear analysis. One of the inefficiencies of static nonlinear analysis is that nonlinear behavior of structural elements due to cyclic deformations is approximately considered in the analysis, and only one quarter of a full hysteretic loop is considered. For investigating the effect of this inefficiency in analysis results, three intermediate concrete moment frames from regular RC structures are selected. The performed procedures in FEMA-356 and proposed-plastic hinges in this guideline are utilized for performing static nonlinear analysis. A coefficient for consideration of stiffness degradation and strength deterioration is proposed by FEMA-356 in nonlinear static analysis. This coefficient for intermediate RC moment frames is equal to unity. For calculation of this coefficient, in this paper, the nonlinear dynamic analysis is used; Clough and Takeda Hysteretic loop and a Hysteretic loop that. Considers effects of severe stiffness degradation, strength deterioration and pinchings are assumed in nonlinear dynamic analysis.Comparing final results lead to conclusion that revealed the value of this coefficient is obtained 25% more than the VALUE PROPOSED BY fema-356.

Non-linear behavior of tubular members with axial pre-load is studied against dynamic lateral impact.Nonlinear behavior of these members would be dut to material non-linearity (NLM) or geometric nonlinearity (NLG). Study of the curves of load displacement of arbitrary tubular member by analytical solutions indicates that changes in parameters such as pipe thickness, magnitude of axial pre-loading, and end conditions of the member under study has significant affect on their residual strength subject to application of dynamic lateral impact. Based on this analysis, a reliable method for calculation of residual strength of tubular member is proposed. The results of the comparison between the two methods using quasi-static and dynamic analyses are shown in a diagram. In addition, to measure the reliability of the model it has been validated using available test results from the literatures.

In this paper, a modeling procedure is carried out to numerically investigate the end bearing capacity of drilled shafts in layered soils. For the case of sand overlying clay, the reliable depth for full mobilization of the end bearing capacity of drilled shafts is also discussed. The modeling procedure in homogenous soil and the constitutive law are first described. The modeling procedure in layered soils is explained next. Numerical analyses show that when the pile tip is located in the sand and close to the clay layer below, the distance that pile tip senses the presence of the clay layer depends on the ratio of the fully mobilized end bearing capacity of clay to the fully mobilized end bearing capacity of sand. Finally, numerical results are compared with experimental data suggested by other researchers.

Projects in oil and power plant industries as well as other infrastructure projects are highly affected by technological developments.Therefore implementation of modern methods and new management technique is inevitable. Recent improvements have enabled the project managers dispense with often unefficient traditional methods and to fulfill the projects with higher quality within pre-assumed duration and budget.In Iran, investment in oil, gas and petrochemical industries is growing exponentially. Therefore, to achieve the economical and social development, it is vital to improve the methods of managing theses projects.This paper compares different methods of constructional contracts in Iran with each other. Comparison of the methods has been assessed on different owner's viewpoints using AHP model.

The prediction of local stability of thin plates under compression has become increasingly important in structural design.Numerical methods are employed to overcome the difficulties of exact solution for partial differential equations of plate's stability.Spline Finite Strip Method is one of these numerical methods and different elastic supports attached to the plate may be handled using this method. In the present paper, local buckling of orthotropic plates resting on elastic supports are studied and results are compared with known solutions. It is shown that rapid convergence of the solution is obtained.Local buckling coefficients of plates are calculated for various effective parameters. The optimum position of the elastic supports with specific stiffness is proposed in order to maximize the local buckling coefficients for plates subjected to in-plane loading.

In this paper, the improvement potential of calcareous sand produced in Kish Island by using a chemical grout is investigated.The main characteristics of this sand are its high voids ratio and tendency to be crushed under moderate stresses. The improvement process of sand is conducted using a sodium silicate grout injected with additives such as formamide and sodium aluminate. Samples were prepared in different initial relative densities and then grouted. It is observed that the maximum uniaxial strength, initial tangent modulus and failure strain are obtained in water/silicate ratio of 0.5. Uniaxial strength and initial tangent modulus are increased with time but failure strain variation with time is a function of water/sodium silicate ratio and additives content. Formamide increases and sodium aluminate decreases the uniaxial strength and initial tangent modulus. Furthermore, increasing of grain size decreases grouted sand uniaxial strength. Uniform grading results in brittle and non-uniform grading results in ductile stress-strain behavior. Presence of sulphates and chlorides solved in water reduces the uniaxial strength and increases the initial tangent modulus of grouted sand.

Hydroplaning phenomenon in rainy areas has caused the reduction of safety in the roads. Using a layer of porous asphalt with void about 18% can drain the water from the road and send it out, It also can decrease sound pollution. Low stability and durability and mechanical characteristics of this asphalt and high permanent deformation are limitations of porous asphalt. In these study effects of SBR and APP polymers on porous asphalt regarding permanent deformation was evaluated.Choosing the best aggregate for creating void above 18% and mix design of asphalt through use of cantabero method were the important parts of the study. The results of experiments showed that adding polymer increases specific weight, improving percentage of the void in asphalt, and amplifying strength and stability of porous asphalt.During the creep test, it was determined that porous polymer modified bituminous mixtures will have the least primary and permanent deformation than the specimens were made with pure bitumen. Adding polymer causes more amount of viscoelastic recovery, increase of stiffness modulus in all temperatures and all stresses and also decreases of creep deformation in porous asphalt.