Iterative method for frequency updating of simple vibrating system

Iterative methods for modification of vibratory characteristics of dynamic systems have attracted a lot of attention as a convenient and more economical way when compared to the traditional and costly structural dynamic optimization processes. Many complicated structures, such as telecommunication towers, chimneys and tall buildings, may be modeled as simple spring-mass systems. This paper presents an iterative method for modification of the frequencies of simple vibrating system consisting of springs and masses. The proposed algorithm may be used to adjust any of the vibration frequencies of a simple vibrating system to the target values within the desired level of accuracy. The method based on the variation of mass and/or stiffness properties of the system is simple yet efficient and needs less computational effort. The efficiency of the method is demonstrated using a numerical example. It is demonstrated that there is a faster convergence for adjustment of the lower frequencies and for the case with stiffness variation of the system rather than mass variation

Wake and structure model for simulation of cross-flow/in-line vortex induced vibration of marine risers

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 existing experimental and numerical results for stepped and sheared current are used to validate the proposed model and the results show reasonable agreement. The proposed model was implemented on Amir-Kabir semi-submersible riser deployed at the water depth of 713 meters of Caspian Sea. CF/IL VIV of this riser is simulated for various current velocities. The results show that although displacement amplitude of IL direction is lower than CF direction but because of higher curvature, stress values of IL direction for some cases can be higher than CF direction. Also because of higher frequency of IL direction, fatigue damage of this direction can be higher than CF one in some cases. It is shown that with increasing of current velocity; however, variation of displacement amplitude of two directions is low but stress increased and fatigue damage also increased with higher rate. For lower velocities which the modes are controlled with tension, stress and fatigue damage of IL direction is higher than CF direction

Comparing the results of dynamic analysis (maximum displacement) with approximate method of Iranian seismic code

Determining structural maximum deformation under earthquake excitation is an important problem in seismic design of structures. There are several approaches in order to estimate an acceptable accurate response for maximum drift of the structure in nonlinear region. Both ATC and FEMA approaches are good ideas to evaluate maximum drift but more simplified approaches should be applied in seismic design codes. Most of seismic design codes suggest a very simple relation for estimating maximum drift in design base earthquake. Iranian seismic code of practice (Standard No. 2800) proposes such a relation. In this paper a comparison between nonlinear dynamic results and code suggested relation is carried out. 3 scaled records have been used for analysis. Based on reduction factor calculated for the structure, a comparison has been carried out with the suggested factor in Iranian Code of Practice (Standard No. 2800)

Seismic retrofit of existing structures using friction dampers

Conventional methods of seismic rehabilitation with concrete shear walls or steel bracing are not considered suitable for some buildings as upgrades with these methods would have required expensive and time consuming foundation work. Supplemental damping in conjunction with appropriate stiffness offered an innovative and attractive solution for the seismic rehabilitation of such structures. This paper deals with the use of friction damper as a passive dissipative device in order to seismic retrofit of existing structures and discusses the design criteria and seismic analysis of a building. The structure is modeled using the finite element program Sap2000 and is analysed using both non-linear static pushover analysis and non-linear time history analysis