International Journal of Structural Stability and Dynamics

This paper presents analytically obtained free vibration frequencies of castellated beams, that take into account the effects of both web shear and rotary inertia. The results show that the rotary inertia effect on the free vibration frequencies is very important for beams with no or weak effect from web shear. However, for beams where the web shear effect is dominant the rotary inertia effect can be almost ignored.

Rotor internal damping is recognized as one of the main causes for instabilities in rotating systems. In a linear speed dependent model, the stability threshold speed can be calculated numerically. However, this threshold depends highly on the modeling choice and mainly on the relation between rotating and nonrotating damping. In this paper, the importance of a well-founded knowledge of the system parameters is highlighted and a method is proposed to estimate the damping of a rotating system not only influenced by the gyroscopic effect but also by rotating damping. With this method, it is possible to reconstruct the physical damping matrix and distinguish between rotating and nonrotating damping. Moreover, the gyroscopic effect can be identified. After a description of the method, a numerical validation is performed.

This paper presents a simplified modeling strategy for simulating the nonlinear behavior of reinforced concrete (RC) structures under seismic loadings. A new type of Euler–Bernoulli multifiber beam element with axial force and bending moment interaction is introduced. To analyze the behavior of RC structures in the axial direction, the interpolation of the axial strain is enriched using the incompatible modes method. The model uses the constitutive laws based on plasticity for steel and damage mechanics for concrete. The proposed multifiber element is implemented in the finite element Code_Aster to simulate the nonlinear behavior of two different RC structures. One structure is a building tested on a shaking table; the other is a column subjected to cyclic loadings. The comparison between the simulation and experimental results shows that the performance of this approach is quite good. The proposed model can be used to investigate the behavior of a wider variety of configurations which are impossible to study experimentally.

The effect of partially distributed internal damping of the Kelvin–Voigt type on the parametric instability of a Timoshenko beam subjected to periodic axial loads is studied. To model the dynamic behavior of the beam, a coupled set of second-order linear ordinary differential equations with periodic coefficients is established by the finite element method. A quadratic eigenvalue equation is derived for a parametrically excited damped system to determine the instability regions of the beam of concern based on Bolotin's method. The effects of internal damping, size and location of the damped segment, ratio of thickness to length and static load factor on the parametric instability of the beam are studied, along with the stabilizing effect of the Kelvin–Voigt damping on the primary parametric resonance presented. The results reveal that the beam with a larger damped segment positioned near the fixed end is dynamically more stable.

The well-known vibration model of axially moving beam is considered. Both axial moving speed and axial force are assumed to vary harmonically. The Method of Multiple Time Scales (a perturbation method) is used. The natural vibrations of beam are considered for different values of beam parameters. Resonances are obtained for seven different conditions. Solvability conditions for each resonance case are found analytically. Effects of transport velocity, axial force, rigidity and damping are discussed. Stability analysis are obtained for principal parametric resonances. Stable and unstable regions are obtained regarding velocity and force effects separately and together.

This paper investigates the seismic performance of a high-rise telecommunication tower using the endurance time analysis (ETA) method. Conceptually, the ETA is a time history-based incremental dynamic procedure in which the structure is excited using an intensifying acceleration function. The structural performance is then evaluated based on the equivalent seismic intensity level that the structure can endure while satisfying the required performance goal. In this paper, the ETA is applied to the Tehran telecommunication concrete tower. The accuracy of the ETA is assessed by comparing the results with those obtained from the time history analysis (THA) and incremental dynamic analysis (IDA). For this purpose, a two-dimensional fiber element model of the tower is developed and analyzed. It is found that the ETA can estimate the THA results at the equivalent target time, and the general trend of the IDA curves with acceptable accuracy, while ETA requires considerably less computational effort. Also it is realized that the peak ground velocity and spectral acceleration considering the higher modes effect are the most appropriate intensity measure parameters for comparison of the IDA and ETA methods.

The aim of this paper is to highlight and identify the influencing parameters of the nonlinear behavior of highly deformable structures. Therefore, as an example, a large deformable square frame consisting of four slender members of equal length has been investigated experimentally. Based on highly resolving measurements using the digital image correlation method (DIC), the inverse problem of nonlinear system identification has been solved by an automatic parameter identification algorithm. For this purpose, a numerical model is set up with a beam finite element model using the absolute nodal coordinate formulation (ANCF), which enables the modeling of geometrical and possible material nonlinearities. The influencing parameters as well as the system properties have been determined by using a genetic optimization algorithm. The impact of the main influencing parameter is carved out by an included sensitivity study. The final model with automatically identified parameters shows high agreement with the experimental setup. With this approach the influences and nonlinearities, e.g. material parameters, rigid behavior, real boundary conditions, etc., come up to surface leading to a deeper understanding of the structural behavior of the system itself. Therefore, the present approach can be utilized for further investigations of nonstandard structures undergoing large deformations.

Strengthening steel structures using carbon fiber reinforced polymer (CFRP) materials has attracted much attention in recent years owing to their potential for fatigue crack repair and their convenience in construction. However, little is known about the efficiency of this strengthening method when applied to steel plates at different crack propagation stages. An experimental study was carried out on notched steel plates strengthened using CFRP laminates. 20 specimens were tested to evaluate the fatigue performance of the strengthened steel plates with emphasis on various degrees of initial damage, simulated by different lengths of slots, including 2%, 10%, 20%, 30% and 40% of the plate width. The effects of the retrofitted configuration and CFRP stiffness were also investigated. The "beach marking" technique and crack propagation gauges were adopted to monitor the fatigue crack propagation. The experimental results were very encouraging, demonstrating that the CFRP patches could effectively slow crack growth and extend fatigue life, regardless of the initial damage levels. More effective strengthening was found by using ultra-high modulus CFRP laminates, covering the initial cracks with CFRP and repairing at an earlier stage (i.e. smaller damage level).

The physical interaction of fluids and solids is of practical significance in engineering (e.g. flutter of aerodynamic structures, vortex induced vibrations of sub-sea pipelines and risers, inflatable dams, parachute dynamics and blood flow through arteries). In this paper, a finite element formulation is developed for determining the vibration characteristics of beams in contact with inviscid incompressible fluid. The classical, first-order and third-order shear deformation beam theories are used to model the structural response. Numerical results for vibration frequencies are presented showing the parametric effect of thickness and immersion depth on the frequency response. The results indicate that the presence of fluid interaction has significant effect on the dynamic response. The formulation presented herein is also applicable to a vast number of vibration problems related to beams under a variety of excitations.

In this article, we develop a novel stable time integration scheme for the transient analysis of structural dynamics problems. A second-order (in time) differential operator equation (e.g. obtained after finite element discretization in space) is written as a pair of first-order equations in terms of displacements and velocities. Then the solution is sought by minimizing the inner product of the residuals in the two equations (an unconventional approach) over typical time interval to obtain a symmetric set of algebraic equations involving displacements and velocities at two subsequent intervals. The new time integration scheme is termed the cross weighted-residual (CWR) time integration scheme because each of the two residuals takes the other one as a weight function in the minimization. The CWR time integration scheme is developed by using a uniform linear time approximation of the displacement and velocity fields to yield only a single step time integration scheme, which is comparable to the Newmark family of time integration scheme. A reduced integration technique is used to prevent velocity locking, which is caused by linear approximation of both the displacement and velocity fields. For the verification of the consistency and the stability, the CWR time integration scheme is tested with single-degree as well as multi-degree of freedom problems. The scheme performs extremely well compared with those of the well-known Newmark family of time integration schemes.