mohammadreza tabeshpour

Engineers usually pay attention to Peak Ground Acceleration (PGA) and the approximate duration of the record when dealing with the acceleration time history. However, according to the basic principles of dynamics, vibrations, and Fourier transform and understanding of the intensity of earthquake and its related parameters from the fundamentals of earthquake engineering, it is possible to obtain a relatively accurate understanding and prediction of the characteristics of the frequency domain and the intensity of the earthquake by taking a careful and deeper look at the time history. The importance of such a skill in performing dynamic analyses is doubled. The process of selecting and scaling records will be also done with higher accuracy if there is such a skill. In this paper, a careful and heuristic view at the time history and its relationship with the frequency domain is presented based on the analysis of Sarpol-e-Zahab earthquake records in the SeismoSignal software. The results indicate that there is a good agreement between the conclusions from the time history and the frequency domain, and by looking carefully at the time history of the earthquake record, an important part of its characteristics can be discovered before performing the frequency domain analyses. For dynamic analysis and understanding the relations between the record and the response of the structures and their failures, visual understanding of earthquake time history is also very important. According to the case study presented in this paper, it is possible to achieve such a skill based on the principles of earthquake engineering and the applied mathematics.

In studying the earthquake record, it is very important to identify the moment when the main energy enters the structure. From a technical point of view, it is very important to find a parameter that shows the intensity of an earthquake. Although the PGA as well as the PGV and PGD are very important parameters in earthquake engineering, but it is very important to know the time of the most intensive ground motion in the time history of earthquake. By taking a careful and deep look at the time history of the earthquake, a good understanding of the time of the most intensive ground motion can be achieved. In this paper, through a case study of the time history of the Sarpol-e-Zahab earthquake, a simple method and a suitable estimation of intensity and its distribution during the record is presented, also, a method based on the derivative of Arias intensity and energy flux has been developed to accurately determine the part of the earthquake record where the most important part of the energy is transferred to the structure. The results of this study show that there is an accurate relationship between the time history and the intensity. Also, by considering the proposed method for determining the effective duration and comparing the resulting response spectra, it can be concluded that in most cases, there is no need to use the entire time history of the earthquake for dynamic analysis, and a significant saving in time and cost can be achieved.

Based on the seismic design codes to prevent soft-story failure, columns of a soft story must be designed for amplified loads due to the discontinuity of braces or shear walls in that story. Because of the masonry infill walls discontinuity, Soft story failure has been reported in the recent earthquakes. Most national seismic design codes don't consider the effect of masonry infill walls for the design of the soft story. This paper aims to investigate the soft story failure and then present a simple formula for the design of soft-story in moment resisting frame structures. In this paper, the different arrangements of masonry infill walls are considered. Structural modeling was carried out based on reliable parameters and some national or international seismic design codes. By using nonlinear static analysis, a simple methodology is proposed and the main result is a simple formula that can be used for the engineering design of concrete moment resistant frames.

Owing to the common use of infill walls in conventional buildings, it is a practical and important topic to investigate the effect of infill walls on the behavior of structures during earthquakes. One of the disadvantages of infilled frames is the presence of large window openings in some of the reinforced concrete frame buildings, which results in the short column phenomenon. The part of the column that is adjacent to the wall is almost integrated with the wall and leads to a reduction in the height of the column. Therefore, the lateral stiffness increases considerably. With increasing stiffness, the lateral force applied to the column also increases. In this study, a 4-story, 3-span reinforced concrete model with a different arrangement of infill walls in the stories and considering short walls on the ground floor, the short column phenomenon has been investigated in ETABS software. Also, the seismic capacity of the structure has been calculated by valid methods based on the capacity spectrum method proposed by ATC40. The results indicate that with increasing stiffness, the maximum shear force applied to the column due to the presence of the infill wall under the designed earthquake load will increase by about 50% compared to the frame without the infill wall. Furthermore, the amount of stiffness difference in the frame with and without infill wall in the analyzed models is about 70%, which can prevent irreparable damage by predicting this event and proper design.

Confined masonry buildings, in which all or part of gravity loads and all of lateral loads on both main directions of the building are resisted by unreinforced masonry walls, have been widely used in Mediterranean Europe, Latin America, the Middle East, south Asia, and the Far East. Experiences obtained from past earthquakes and experimental results indicate that confined masonry buildings, if properly built, exhibits an adequate seismic response. Consequently, it represents a good choice in those seismic regions where masonry is widely used due to economical or traditional reasons.In this paper, first, the functions of tie beam and tie column and the similarity of seismic behavior of the confined masonry walls and the filled frame have been studied. The structural response of confined masonry and infilled frames under in-plan lateral loading is similar, despite the different construction techniques. In both cases, structural separation occurs at the initial stage. After this separation, a diagonal compressive stress field is formed in the masonry. In the following, the field study of five masonry buildings after the Sarpol-e Zahab earthquake have been presented. The buildings have been selected in such a way that all types according to the scope of this research have been investigated. These types include: 1. Two-story masonry building with tie-beam, 2. Two-story masonry building with tie beam and tie column, 3. Two-story masonry building with tie beam and tie column in first story and tie beam in second story, 4. One-story masonry building with tie-beam, 5. One-story masonry building with tie beam and tie column. The results of this study show that the seismic behavior of confined masonry buildings in comparison with a lot of severely damaged buildings with steel and reinforced concrete structures, is much more desirable. In the following, the behavior of two main types of two-story masonry building in Sarpol-e Zahab including masonry wall with tie-beam and masonry wall with tie-beam and tie-column is analyzed as finite element models in Abaqus. By using finite element linear analysis, the load path and cracks in the confined masonry walls can be studied with good accuracy. In the condition that there are only tie-beams in the building and gravity load and seismic force are applied to the wall, the maximum stress is formed in the diagonal path in the middle of the first story wall. Consequently, the first cracks will form in the same path when seismic force creates a shear stress more than shear strength of the wall under gravity load. In the condition that there are both tie-beams and tie-columns in the building, in the initial earthquake cycles, when the wall and ties have a good connection and seismic force is applied from left to right, the shear stress at the junction of the left side of the tie-columns to the right side of the walls is maximum and as a result, the walls will separate from the ties in these locations. In the next cycle, in which the direction of the earthquake is from right to left, the shear stress at the junction of the right side of the tie-columns to the left side of the walls is maximum and as a result, the walls will separate from the ties in these locations. These changes in direction of the earthquake will lead to the separation of the walls from the tie-columns in the initial earthquake cycles. Therefore, the behaviour of confined masonry wall will be similar to the infilled frame in subsequent cycles. Accordingly, the maximum shear stress path will be formed in the diagonal of the bays confined by the tie-beams and tie-columns. Consequently, the first cracks in the bearing walls of confined masonry buildings will form in this path. The results of the study show that the failure modes obtained from the finite element analyses are well matched to the actual cases damaged in the Sarpol-e Zahab earthquake.

Immersed boundary method is a novel and efficient tool for solving fluid-structure interaction problems. One of important fluid-structure interaction problems is vortex-induced vibrations of cylinders. In this study, we want to introduce immersed boundary method. Philosophy, advantages, disadvantages and applications of the method will be presented. Because of wide range of applications of this method and for not losing focus, we shall limit our study only to vortex-induced vibrations. Next, details of governing equations of immersed boundary method and their discretization will be introduced. Iterative immersed boundary algorithm which has been used for vortex-induced vibrations before, will be used to solve governing equations. Finally, implementation and coding requirements will be discussed. This paper is organized in a way so readers can obtain a comprehensive understanding of the method behavior in fluid-structure interaction problems, especially vortex-induced vibrations. Considering the benefits of immersed boundary method, it has the potential to become the main method for analyzing fluid-structure interaction problems. These benefits are considerable enough that makes the implementation of the method justifiable.

The primary objective of this paper is probabilistic quantification of the fatigue life of tension-leg platforms (TLP) using reliability methods. The need for such methods stems from the significant uncertainty in the loads exerted on offshore structures. The scope of this paper is limited to the study of fatigue in TLP tendons. For this purpose, nonlinear time-history of force response of the TLP tendon under random-wave load is computed via MOSES software and the damage due to fatigue is estimated in accordance with the Palmgren-Miners rule. Assuming a Rayleigh distribution for stress variation and eight different sea states, the ultimate fatigue damage is computed by accumulating the damage over all individual sea states. This cumulative damage enters the limit-state function that is based on the Palmgren-Miners rule. Prevailing sources of uncertainty in this problem are those in the estimation of fatigue stresses, fatigue strength, and the Palmgren-Miners rule. Finally, reliability analysis is carried out for four different service lives using the first- and second-order reliability methods (FORM and SORM) and Monte Carlo sampling. The results indicate that FORM computes the probability of failure sufficiently accurate. It is concluded that the probability of failure increases drastically with the service life. The importance vector from the sensitivity analysis in FORM reveals that the model error is the most influential source of uncertainty on the probability of failure.

The Behavior of eccentrically braced frame (EBF) in terms of stiffness and ductility is between moment resisting frame and concentrically braced frame. EBF should be designed in such a way that yielding is only concentrated in the link beam at the non-linear stage. Field survey after the Sarpol-e Zahab earthquake shows that despite several defects in structural design and construction of EBFs, they have remained stable. In this study, one of the damaged buildings in the Sarpol-e Zahab earthquake, in the form of a three-story four-bay frame was modeled in Etabs and its seismic behavior in two cases; with and without infill walls was studied. The results of the analyses show that the presence of diagonal struts of infill walls reduces the axial force of the braces, the shear force, and the bending moment of the link beams. Infill walls also reduce lateral displacement and period of EBF, and they increase the lateral stiffness. Therefore, in the condition that there are several defects in the design and construction of link beams and braces, connecting the infill walls to the structure has a positive achievement. In this condition, if there were not infill walls, there would be a possibility of structural collapse.

The ultimate capacity of offshore structures including jacket type offshore platforms is used to achieve structural performance levels and determine their ductility. Accurate estimation of this parameter is of great importance. Formation of fatigue cracks at the joints, corrosion of members, environmental loads and damages caused by accidental dynamic loads such as impacts of vessels and floating bodies will result in change of the ultimate capacity of these structures over time of their life. These cases should be considered in calculating the ultimate capacity of the offshore platforms at any given time of their life. However, accurate modeling of the global and local buckling of compression members is important at any time of calculating the ultimate capacity. Buckling modes and deformations due to local buckling will be considered, if the compressive braces are modeled by Shell or Solid elements and the imperfections are applied. The purpose of this paper is to achieve the correct compressive behavior of compression members. ABAQUS finite element computer software is used for this purpose. The buckling envelope derives from Marshall Strut theory defines the post-buckling damaged elasticity model and the hysteretic loop response. Finally, by using this modified behavior in Frame elements, the effects of local buckling in compressive braces can be considered.

Recorded earthquakes on near fields have special characteristics different from other usual records. Buildings placed near faults should be designed for the effects of such faults. Because of existing such faults in some cities in Iran such as Tehran and Tabriz, the effect of them should be considered in seismic codes in Iran. In this paper a comparison among various seismic design codes has been carried out almost this effect leads to increase the base shear using a separate coefficient that implies to design spectrum. This coefficient is a combination and a function of distance and period being larger in more near distance and longer periods. This effect has been considered in seismic design codes in some countries such as Taiwan, New Zealand and United States. In this paper a brief review of some seismic design codes and the new version of the Iranian seismic code of practice is presented.

Now a days using catamaran vessel in passenger, military and commercials is increasing.one of the concept design that attracted many researchers is HARTH (high aspect ratio twin hull) vessel. The advantages of this vessel in addition to large deck, this kinds of vessels have good stability and can maintain speeds in harsh seas. However slamming in high waves can create some problems for these kind of vessels. Some different method have used by designers to decrease the impacts of slamming.one of this methods is adding Centre bows in these kind of vessels. Centre bows can decreased the impacts of slamming.one of the more applicable methods to studying the slamming is the drop test.in this research by using this method, we are testing the different Centre bow cross-sections and the acceleration of them are compared. The cross-sections are set in two categories of 1 and 2 from dimensions view.it is recognized that categories 1 have lesser accelerations than categories 2.

Based on numerous studies, using weak and fragile building materials such as traditional brick due to their high weight, low strength, and low ductility during earthquake will cause the most casualties and losses. Observations from past earthquakes indicate that many structures have undergone remarkable damage even in moderate earthquakes. Low ductility and strength, high weight and severe strength degradation under seismic loads are responsible for these buildings failures. In this paper, behavior of a modern reinforced composite gypsum panel is evaluated and compared with corresponding panels made with traditional materials. The cheap and accessible basic materials used for making these panels result in favorable performance including a significant increase in the tensile and compressive strength as well as providing panels with integrity. So that when it breaks down, its particles do not disintegrate. In order to determine the basic mechanical properties of such panels, numerous standard tensile, compressive, and shear tests have been performed on various panel specimens at the materials laboratory of the faculty of mechanical engineering, Sharif University of Technology. Considerable ductility, strength, energy dissipation capacity, minimum cost, and fast construction are among the features of the proposed panel. Applying the finite element simulation, the buckling force of these composite gypsum panels were determined, which shows high buckling capacity. Subsequently, parametric studies were performed to evaluate the effects of openings on the behavior of these panels as load-bearing walls. The results of experimental tests for this type of panel presented that tensile strength is 4.3 Kg/Cm2 and compressive strength of panel is 18 Kg/Cm2, which are more considerable in comparison with the other traditional masonry materials.

The capacity curve obtained from the pushover analysis of jacket-type offshore platforms gives their structural performance levels, ultimate capacity and ductility. Accurate estimation of structural capacity curve is of great importance. Accurate modeling of the global and local buckling of compression tubular members in a correct form is an effective part of studying the behavior of offshore jackets under all various types of loading conditions at any given time of their life. Modeling of compressive braces by shell or solid elements when the imperfections are applied leads to deformations due to local buckling based on buckling modes. This paper aims to achieve more accurate compressive behavior of compression members. The ABAQUS finite element software has been used for this purpose. Regarding to the results achieved from investigation of buckling in tubular members proper elements have been introduced to investigate the global and local buckling phenomena. Then pushovers results of Ressalat jacket with conventional modeling versus more accurate modeling proposed in this paper for compressive members have been compared as a case study. According to the results applying improper mesh size for compressive members can under-predict the ductility by 33% and under-estimate the lateral loading capacity up to 8%. Finally, ISO equations and Marshall strut theory have been applied to investigate critical buckling load and post-buckling response of tubular braces. The innovation of this paper is investigating the interaction of global and local buckling in the braces of jacket with 1-Dimentional elements using ISO equations and buckling envelope derived from the solid element results, which results in low computational costs.

Fully dynamic analysis of offshore structures under random wave loads in time domain is sometimes necessary for calculating structural responses in design issues. Such analyses are very time consuming and therefor simplified methods for estimation of acceptable response of these structures can be very useful in initial design. In this paper an innovative method to obtain response spectrum of fixed offshore structures caused by extreme waves is represent based on concept of impulse response spectrum. For this purpose the structural system is considered as a simple one degree of freedom structure and the different sea states are equalized to different half sinusoidal pulses. Response spectrum of structure is defined as a plot of structural response to these pulses for different periods. By using this method the cost of computations is decreased significantly while the accuracy of results was preserved.

Infill walls are one of the most important elements in the buildings that should be considered in design and construction stages. In architectural design masonry infills are used to separating spaces and areas, but these elements are effective in seismic behavior because of stiffness and strength of masonry infills adjacent to the structural frames. The interaction of masonry infill with concrete frame create short column. Shear failure is a critical kind of concrete column failure that occurs in short columns during earthquake because of low ductility and brittle behavior of these types of columns. It is important to investigate the behavior of short columns. In this paper interaction of masonry infill with concrete frame with 1 bay and 1 story is surveyed. SAP2000 and OpenSees are used for analysis and comparison. The results show that shear reinforcement of concrete columns in ordinary design is not sufficient so plastic hinges form and shear failure happen and increasing in the stiffness and strength of frame because of infill wall should be considered during both the design and construction stages.

Many procedures suggest for reduction of responses of riser to Vortex Induced Vibrations (VIV). Natural frequencies of marine risers is an important parameter that can affect the responses of riser to VIV. Change of riser properties such as top tension and bending stiffness can alter natural frequencies. In this study effects of riser specifications on the responses and fatigue damage of marine risers were investigated analytically and numerically. For numerically analysis 2D wake-structure coupled model is used for modeling of VIV of riser in two directions of Cross Flow (CF) and In Line (IL). The wake dynamics, including IL and CF vibrations, is represented using a pair of non-linear Van der Pol equations that solved using modified Euler method. The Palmgren–Miner Rule is used for evaluation of fatigue damage. Riser of Amir-Kabir semisubmersible placed in Caspian sea is used for case study. Because VIV is self-limiting, it is showed that lower modes have lower curvature, that in some cases this is lead to lesser stress and also fatigue damage. The results show that for tension dominant modes of vibration, natural frequencies was increased with top tension and for a certain Strouhal frequency, dominant modes of vibration was reduced which leads to reduction of stress and fatigue damage. The results show that stress and fatigue damage increased with module of elasticity of riser and reduction of this leads to reducing of stress and fatigue damage. Therefore suitable procedure for reduction of VIV responses of riser should be selected based on the current velocity.

Stability of Tension Leg Platforms (TLP) is sensitive to tendons situations. Therefore, behavior and exact calculation of stiffness matrix of TLP is important for dynamic analysis and investigation of TLP in intact and damaged tendon conditions is necessary to accurate and reliable design of TLP. In this paper deals with dynamic analysis of TLP when tendons are intact and one or three tendons are damaged. Static stability of TLP in damage condition has been studied and static offsets have been estimated, then stiffness of surge, heave and pitch has been derived. Finally, responses of heave and pitch for three load conditions have been predicted.

Tension leg platform is a compliant structure with vertical moorings and added buoyancy force. In this paper، effective parameters on experimental modeling of TLPs have been investigat. experimental modeling with correct scale factor has been done in Sharif University of Technology marine engineering laboratory test basin. deep water simulation capability were discussed and finally the model، launched in the basin. Pretension modeling is another conceptual component that could be easily analyzed. Ultimately Sea keeping results of ISSC TLP model duo to the regular laboratory waves with different heading angles and two amplitudes were extracted. The main purpose is ISSC TLP frequency response and surge beating analysis by model testing.

Wave endurance time (WET) is a time domain method for evaluating the response of offshore structures under different storm conditions. In this method، intensifying artificial records named wave functions (WFs) are generated in place of various random records of extreme sea states. In this regard، the computational costs are reduced، and reliability of structures is evaluated by the endurance time. In this study، a description of WET concept is presented، and also generation of initial type of WFs is thoroughly addressed for the Persian Gulf region. To consider the accuracy of this method، the results of WET method are compared with typical 3h extreme wave simulations for maximum base shear and deck displacement. The results indicate that there is acceptable agreement between two approaches.

In this paper، the modification of Endurance Wave Analysis (EWA) in assessment of offshore platforms based on the new-wave theory is introduced. EWA is a new method to estimate the response of offshore structures in different sea states by use of incremental records. This study is offered practical procedures to select time duration and intensifying trend of the records. In addition، a simplified multi-degree of freedom model of a jacket platform under extreme waves is assessed through current study. A comparative study between this analysis and 3-hour random wave simulation to investigate the accuracy has also been carried out. The results show that the proposed procedure is a convenient method in design and assessment of offshore platforms.

This paper evaluates the hydrodynamic performance of a damaged ISSC TLP which is caused by a tendon disconnection. Performance evaluation of a TLP with a disconnected tendon in a rough sea state is the major aim of this paper. First off، modeling of a platform in a proper sea state is carried out and then after disconnecting one of the tendons، the transient effect on the other tendons is assessed. Also، the steady state forces are validated using analytical results. Additionally، time history of heave and surge accelerations، TLP six degree of freedom motions and also tendon forces are determined in regular waves. In order to assess the hydrodynamic performance of the platform، a numerical simulation is conducted، using multipurpose boundary element software. At last the analysis shows that the modeling of a damaged platform is carried out properly and also the conclusion can be drawn that it is possible to evaluate the performance of a damaged platform in Regular waves.

Efficiency of numerical methods is an important problem in dynamic nonlinear analyses. It is possible to use of numerical methods such as beta-Newmark in order to investigate the structural response behavior of the dynamic systems under random sea wave loads but because of necessity to analysis the offshore systems for extensive time to fatigue study it is important to use of simple stable methods for numerical integration. The modified Euler method (MEM) is a simple numerical procedure which can be effectively used for the analysis of the dynamic response of structures in time domain. It is also very effective for response dependent systems in the field of offshore engineering. An important point is investigating the convergence and stability of the method for strongly nonlinear dynamic systems when high initial values for differential equation or large time steps are considered for numerical integrating especially when some frequencies of the system is very high. In this paper the stability of the method for solving differential equation of motion of a nonlinear offshore system (tension leg platform, TLP) under random wave excitation is presented. In this paper the stability criterion and the convergence of the numerical solution for critical time steps are presented.

However it is possible to use of numerical methods such as beta-Newmark in order to investigate the structural response behavior of the dynamic systems under random sea wave loads but because of necessity to analysis the offshore systems for extensive time to fatigue study it is important to use of simple stable methods for numerical integration. The modified Euler method (MEM) is a simple numerical procedure which can be effectively used for the analysis of the dynamic response of structures in time domain. It is also very effective for response dependent systems in the field of offshore engineering. An important point is investigating the convergence and stability of the method for strongly nonlinear dynamic systems when high initial values for differential equation or large time steps are considered for numerical integrating especially when some frequencies of the system is very high. In this paper the stability of the method for solving differential equation of motion of a nonlinear offshore system (tension leg platform, TLP) under random wave excitation is presented. The key point of suitability of MEM for solving the TLP system is that the maximum frequency of the system is about 0.5 Hz. The stability criterion and the convergence of the numerical solution for critical time steps are numerically discussed.