Sharif Digital Repository

The solution for the Duffing equation in a nonlinear vibration problem is studied in this paper. Clearly, in the case of the perturb parameter being a larger value, the traditional perturbation method is no longer valid but the Homotopy Perturbation Method (HPM) is applicable usually. HPM is used to solve the weak and strong nonlinear differential equations for finding the perturbed frequency of the response. The obtained frequencies via HPM and the approximate method have good accordance for weak and strong nonlinear differential equations. Additionally, the calculated responses by use of the approximate method are compared with the responses obtained from the Numerical method in the time... 

Offshore jacket platforms are exposed to environmental loads such as wind, wave, current, and earthquake throughout the lifetime of operation. Due to dynamic and periodic nature and fatigue phenomenon in the structure, wave forces are the most important loads among others. There are diverse methods to explore the fatigue life of jackets, including deterministic, spectral, and time domain analysis. Among these methods, spectral method is a reliable method, which considers the random nature of sea waves in fatigue analysis. In the current study, a spectral method is introduced for assessment and rehabilitation of jacket platform structure. To this end, a computer program has been developed,... 

Offshore jacket platforms are exposed to environmental loads such as wind, wave, current, and earthquake throughout the lifetime of operation. Due to dynamic and periodic nature and fatigue phenomenon in the structure, wave forces are the most important loads among others. There are diverse methods to explore the fatigue life of jackets, including deterministic, spectral, and time domain analysis. Among these methods, spectral method is a reliable method, which considers the random nature of sea waves in fatigue analysis. In the current study, a spectral method is introduced for assessment and rehabilitation of jacket platform structure. To this end, a computer program has been developed,... 

Three dimensional responses of riser subjected to Vortex Induced Vibration (VIV) are investigated. Proportionality relations of stress and fatigue damage are mentioned. A computer code has been developed for time domain modeling of VIV of riser accounting for both Cross-Flow (CF) and In-Line (IL) vibration. The wake oscillator model is used to calculate the VIV of each strip. The wake oscillators are coupled to the dynamics of the long riser, while the Newmark-beta method is used for evaluating the structural dynamics of riser. 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... 

The assessment of existing platforms under past environmental (wave, wind, current and etc.) loads and probable future loads (earthquake) is a relatively new process and has not yet been standardized as the design has. This lack of standardization creates some difficulty in establishing performance requirements which must be developed depending upon the risks (i.e., hazards, exposures and consequences) associated with the future operation of the platform. The present criteria of the offshore structure standards for seismic assessment can be improved using Building prestandards. Recently some documents such as FEMA-356 and ATC-40 are developed for seismic assessment of buildings. However... 

A significant part of torsional effects in buildings is due to disregarding the arrangement of infill walls during the design. This is a challenging issue since on the one hand the structural engineers do not usually consider infill walls in modeling a structure, while on the other hand the architects do not pay attention to the seismic behavior of infill walls during architectural design. In this research, an approximate method for identifying the torsion caused by infill walls was proposed to be used at the final stages of basic architectural design, which is based on the geometric specifications of architectural plans and does not require structural specifications. For this purpose,... 

When the stiffness of a story is much less than the story above it, a significant portion of lateral displacement of the building is concentrated in this story and soft story is formed. One of the main reasons of soft story in buildings is eliminating or reducing the infill walls in lower stories. This reason is more important than the others because structural engineers usually neglect infill walls in modeling the structure, and on the other hand, architects do not consider the seismic behavior of infill walls during architectural design. In this research, based on the provision of soft story in seismic codes, 2277 macro models with different arrangements of infill walls in the adjacent... 

Metal plates are essential parts of structures such as ship hulls and offshore oil platforms. These plates are typically under compressive axial forces. Hence, one of the main mechanisms for failure and collapse of such structures is buckling of plates. Thus, for safe and secure design, buckling strength of plates should be evaluated. Finite element analysis techniques are perfect tools for this purpose because of the accuracy and flexibility for performing simulations with different variables. In this study, the strength of aluminum plates has been studied using finite element analysis software and it was tried to study the influence of variables such as initial imperfections, plate... 

The dynamic response of the leg (tether) of a Tension Leg Platform (TLP), subjected to axial load at the top of the leg, is presented. The structural model is very simple, but several complicated factors, such as foundation effect, buoyancy and simulated ocean wave load, are considered. As an application, the effect of added mass fluctuation on the dynamic response of the leg subjected to such a load is presented. This effect is important in the fatigue life study of tethers. A first order perturbation method is used, in order to formulate and solve the problem. The differential equation is solved by means of non-harmonic Fourier expansion, in terms of eigenfunctions obtained from a... 

Structural control under seismic excitation is becoming an important problem in earthquake engineering. Among several control systems, semi-active control is usually possible and efficient. In the conventional bracing systems it is assumed that the braces buckle under little compressive forces. In this research a semi-active on-off brace strategy is implemented to improve the conventional braces performance. A new completely tensile ribbed bracing system (RBS) has been developed to use the whole capacity of the member. In the proposed system the buckling of compressive member is prevented using a ribbed shape cylinder. RBS causes a permanent-stiffness-presence system that leads to less... 

Torsion is one of the most important causes of building collapse during earthquakes. Sometimes, despite the symmetric form of the building, infill walls disturb the symmetry of the lateral resisting system. The purpose of this research is to investigate the effect of urban layout on developing torsion caused by infill walls. For this purpose, a typological study was conducted based on the conditions of perimeter walls on 364 buildings and then 9 cases were selected. The dimensions of the selected buildings are constant and the conditions of the perimeter walls including facades with openings and cantilevered facades are variable. The selected buildings with 60 different layouts of infill... 

Experiences of past earthquakes show that the infill wall is one of the non-structural elements which has the highest collapse potential of the structure. Although architects are responsible for designing the form and arrangement of walls, little research has been done in this regard with the aim of architectural design. In this research, a method was developed to control the infill walls in the basic architectural design. In this method, architectural design features are preserved, and only by applying minor modifications to the specifications of walls, undesirable effects such as short column, soft story, and torsion are prevented and without using more materials, additional potential for... 

Recently, various approaches have been introduced to estimate the response of offshore structures in different sea states by stepwisely intensifying records. In this article, a more practical approach entitled Modified Endurance Wave Analysis (MEWA) considering the random and probabilistic nature of wave loading and utilizing optimal time duration is introduced. Generation procedure of this approach is described based on two practical wave theories: random and constrained new-wave. In addition, assessment of a simplified model representing a typical fixed offshore platform under extreme wave conditions in the Persian Gulf is performed making use of MEWA. A comparative analysis has been also... 

A friction damper device (FDD) is used for vibration control of an existing steel jacket platform under seismic excitation. First, the damping is presented for vibration mitigation of structures located in seismically active zones. A new method for quick design of friction or yielding damping devices is presented. The effectiveness of the damping system employing such FDDs in a jacket platform is evaluated numerically. The influence of key parameters of the damping system on the vibration suppression of the offshore structure is studied in detail. To examine the vibration control effectiveness of the FDD for the jacket platform, performance of the controlled structure under the seismic... 

Floating wind turbines are subjected to highly dynamic and complicated environmental conditions leading to significant platform motions and structural vibrations during operation and survival conditions. These motions and vibrations alter the induced loading characteristics; and consequently, affect the dynamic behavior of the system. In order to better understand the influence of such motions and structural vibrations, herein elastic structural disturbance of tower, on the system behavior, the spectral and statistical characteristics of a floating wind turbine dynamic responses under operational and survival conditions are fully explored using a fully coupled... 

In this article, wave endurance time (WET) is addressed as an applicable method for performance-based assessment of fixed offshore structures under extreme waves. In this method, inspired by the endurance time method in the field of earthquake engineering, artificial wave records called wave functions are designed so that their excitations gradually increase with time. Therefore, the main advantage of the proposed method is that it can assess the structural performance under various wave load conditions through a single time-history analysis. Moreover, the reliability of structures can be evaluated on the basis of the time that the structural response is still acceptable. In this study,... 

The main objective of this investigation is to modify endurance wave analysis (EWA) based on New-wave theory for convenient assessment of offshore structures under extreme waves. EWA is a novel time history based approach to evaluate structural performance in various excitation levels of wave loading by intensifying wave records. In this article, modifications are proposed to determine appropriate time duration and intensifying trend for reliable and practical assessment of offshore structures. In this way, a simplified model of a real jacket platform is utilised to investigate the application of this approach under extreme wave conditions in the Persian Gulf and results are compared with... 

In this article, wave endurance time (WET) is addressed as an applicable method for performance-based assessment of fixed offshore structures under extreme waves. In this method, inspired by the endurance time method in the field of earthquake engineering, artificial wave records called wave functions are designed so that their excitations gradually increase with time. Therefore, the main advantage of the proposed method is that it can assess the structural performance under various wave load conditions through a single time-history analysis. Moreover, the reliability of structures can be evaluated on the basis of the time that the structural response is still acceptable. In this study,...