نشریه مهندسی عمران فردوسی
سال بیست و ششم شماره 2 (بهار و تابستان 1394)

In this research، the effectiveness of multiple tuned mass dampers (MTMDs) in mitigating the response of nonlinear hysteretic structures has been studied where the optimal parameters of MTMDs have been determined based on minimization of maximum relative displacement (drift) of structure. For solving the optimization problem the genetic algorithm (GA) has been used successfully. For numerical analysis، an eight-storey bilinear hysteretic shear frame subjected to a white noise excitation and optimal MTMDs have been designed. The results of numerical simulations have shown the effectiveness of the proposed method in designing optimal MTMDs for nonlinear structures. Also، it can be said that the damage of structure can be reduced using optimal MTMDs where the performance has been affected by earthquake characteristics and mass ratio.

In this paper، a new analytical method is proposed to study the effect of crack and axial load on vibration behavior and stability of the cracked columns. Using the local flexibility model، the crack has been simulated by a torsional spring with connecting two segments of column in crack location. By solving governing eigenvalue equation، the effect of crack parameters and axial load on the natural frequencies and buckling load as well as buckling load are investigated. The results show that the presents of crack cause to reduction in natural frequencies and buckling load whereas this reduction is affected by the location and depth of the crack. Furthermore، the tensiel and compressive axial load increase and decrease the natural frequencies، respectively. In addition، as the compression load approaches to certain value، the fundamental natural frequency reaches zero and instability occurs. The accuracy of the model is validated through the experimental data reported in the literature.

Among the systems used for Retrofitting and Strengthening of structures، FRP systems seem to be more popular. When using FRP، the surface strength of concrete plays major role in producing sufficient adhesion between FRP laminates and concrete. Therefore، this paper is devoted to the findings of research on five different concrete strength that were used in manufacturing concrete beams for Retrofitting. In total twenty beams were prepared for studying the changes in adhesion between concrete and CFRP layers. While half of the beams were wrapped with CFRP''s، the other half were left as they were for comparative studies. The adhesion of CFRP layers on concrete beams were measured by new Twist-off method and the results were compared with those obtained from standard flexural testing of the beams. Finally، the effect of concrete surface strength and the bond strength of the CFRP on the ultimate strength of the strengthened beams have been assessed.

The world largest adobe citadel in Bam (Arg-e Bam) was damaged seriously in the 26 December 2003 earthquake. Since then many studies and researches have been done to restore this historical complex. One of the first structures in the citadel for which a restoration plan was prepared over a project was adobe tower No. 32. The objective of this project was studying the seismic behaviour of the tower to find out the cause of damage، the weakest parts of the structure in future earthquakes، the best way to improve the seismic performance، and the most approperiate restoration plan. This paper presents the study on seismic behaviour of tower No. 32 and its restoration plan.

While significant advances have been made on the blast analysis، applications of nonlinear analysis of structures under explosion have been limited to the frames and structural behavior of bridges subjected to blast is an issue that has attracted attention of researchers in recent years. Nonlinear dynamic analysis can be lead to the most accurate responses of the structure under blast but the use of it is not justified in many cases because of its complexity and excessive computing times. Pushover method is often seen to be a viable and attractive alternative to nonlinear analysis because of its simplicity and ability to estimate deformation demands with acceptable accuracy without the intensive computational and modeling effort of a nonlinear dynamic analysis. In this study، a rectangular load pattern is developed to estimate nonlinear responses of composite bridges subjected to blast using pushover method. For this purpose، first، an efficient method is proposed to determine equivalent static blast loading and then the accuracy of pushover analysis based on the developed load pattern is evaluated. It is shown that the proposed method can be easily used to conservatively estimate maximum displacement of composite steel girder bridges under explosion.

This paper deals with new method for designing the tuned mass dampers (TMD) with considering both transient and steady state responses، simultaneously. This formulation is presented for undamped vibrations. For this purpose، the absolute value of the structural displacement ratio is minimized numerically. This procedure leads to optimum value of TMD''s specifications. For numerical verification، the vibrations of some structures a controlled by proposed method. Results show the efficiency of the proposed technique in comparison with other existing approaches.

Design of bridge deck with long-term strength، durability and permanence is a significant interest for engineers. One applicable solution to this challenge could be via using hybrid system consisting of conventional materials such as concrete and steel with FRP plates which is also known as composite deck. Since these deck are relatively new so their performance is not completely known. The present study is dedicated to composite deck consists of a steel core and GFRP plates. This composite sandwich bridge deck system is composed of wrapped hybrid core of GFRP grid and multiple steel box cells with upper and lower GFRP facings. The structural performance of deck was evaluated by nonlinear finite element method and numerical results have been compared with available experimental results where possible. After ensuring the validity of numerical modeling of composite deck، parametric studies such as change in geometry، steel core shape and mechanical properties of materials have been done. It was found that failure mode of the proposed hybrid deck was more favorable because of the yielding of the steel tube when compared with that of absolute GFRP decks. Increasing the thickness and changing the steel core geometry can improve the ultimate load capacity of the deck. The grid layer has the maximum thickness among the GFRP layers and therefore ultimate load capacity of the deck enhanced by increase the elastic modulus of grid layer.

Recently، mixtures of clay-tire chips are used as construction material in civil engineering projects. With respect to the tending for using these materials، it is necessary to know their mechanical behavior. Thereby، a number of undrained monotonic triaxial tests are carried out on the specimens of pure clay and clay-tire chip mixtures. Mixed specimens prepared by mixing pure clay with 10%، 20%، and 30% tire chips in weight. The results of the tests indicate that shear strength and pore water pressure due to shearing depend on the amount of tire-chips and clay type. As dependent on the clay type used in the mixtures، there is an optimum content for tire-chips to mix with the clay.

Nowadays FRP composite sheets for structural repair، as an efficient and cost-effective technique for restoring and upgrading the load bearing of concrete structures are have become very common. In addition to high strength to weight ratio and the use of FRP sheets while structure is in use، these sheets have good fatigue resistance. Furthermore، their easy application is also an undeniable advantage. When exposed to cyclic temperature changes، wetting and drying and freezing and thawing، the FRP retrofitted structures do not show acceptable durability. Although، considerable researches have been reported on failure modes and increment of strength and changes in ductility of the FRP strengthened elements، but experimental data on their durability are scarce. Hence، this investigation focuses on the durability of these sheets under temperature changes، wetting and drying and freeze - thaw cycles. During this investigation، CRFP sheets were glued to all 8 surfaces of 15×15cmconcrete cubes for every environmental condition. At the end of every thirty cycles، adhesional strength of CFRP sheets were determined using the Twist-off method. The results show that، while the effects of wetting and drying and temperature changes on the CFRP/concrete interfacial adhesion are not considerable، the decreasing effect of freezing and thawing is enormous.

The rotational friction dampers can be improved the seismic behavior of the steel structures، and they can be reduced the internal forces in the bracing members، columns and the other structural members. In this research، a simple method for modeling and analysis of braced frames equipped by these dampers is proposed. The proposed model describes seismic behavior of dampers very well and it is applicable to analysis of braced frames with common sections in Iran. Braced steel frames with dampers and without dampers are considered for time-history dynamic analysis، and some parameters such as: base shear، axial forces of the columns and the bracing، drift of stories etc. for two different cases are compared. In the entire analyzes، the structure with damper has been shown the better performance and the more suitable seismic behavior than the structure without damper. Then، use of frictional dampers for rehabilitation of existing buildings or designing of new structures with minimum weight is very useful.

The Caspian Sea shoreline especially Mazandaran coastal area is very important area because of its economic، environmental، and fishery parameters as its high length is always exposed to the negative effects of oil spill. So، determination of its sensitive coastal areas due to oil spills، presentation of some approaches to develop the models، localization the methods and determination of sensitive coastal criteria to oil spills is much necessary. For creating this model، after recognizing the hazard line and creating the objective layers، the processed layers are weighted using analytical hierarchy process (AHP) and Fuzzy AHP methods، then according the fuzzy model the layers are combined by using geographic information system (GIS) software and the final sensitivity maps are produced. The fuzzy results show that the most area of Mazandaran province has a high sensitivity during the oil spills.