International Journal of Structural Stability and Dynamics

The use of finite element methods to determine the collapse behavior of steel portal frames in fire requires temperature, large deformation, complex geometry, boundary conditions and degradation of material stiffness to be taken into account. For such analyses, the cost of computation is important as well as the accuracy, robustness and stability of the analyses. The implicit dynamic method is a rigorous technique that considers the equilibrium of every time step. However, convergence may become an issue, particularly if the frame undergoes structural instability while using a direct time incrementation scheme. In contrast, the explicit dynamic method does not require the equilibrium criteria to be met in every time step, and thus convergence problems are not encountered, although the cost of computation may be tremendous if the natural time scale is used. This paper presents a comparison between the efficiency, stability and accuracy of computations using the implicit and explicit dynamic methods, in determining the collapse behavior of portal frames at elevated temperatures; the models are quasi-static since inertia forces are ignored. It is found that similar results can be obtained using both the implicit and explicit dynamic methods, although the analysis times differ significantly. It is shown that, if the applied artificial inertia forces, in terms of residual forces, are magnified and an automatic time incrementation scheme is activated in the implicit dynamic method, then this method shows significant superiority over the explicit dynamic method both in terms of the cost of computation and the accuracy of results obtained for such structures.

The girth welds of the steel connections in subsea pipelines are subjected to combined fatigue loading and static tensile loading in most of their service life. In this paper, both experimental and numerical studies are presented on the fatigue behavior of Carbon Fiber Reinforced Polymer (CFRP) composites repaired steel connections under combined loads. In the experimental program, each specimen is designed to be formed by two steel plates joined together by single-sided girth welds as a simplification of subsea pipelines, and reinforced by CFRP sheets on one side. The applied loads include a constant amplitude tensile cyclic load combined with a tensile static load, which is perpendicular to the cyclic load. The experimental results reveal that the superimposition of the tensile static load leads to a prolonged fatigue life. The effect gets more noticeable with increased tensile load. To further this study, an analytical model is developed on the basis of the Linear Elastic Fracture Mechanics (LEFM) method. It can be used to predict the fatigue lives efficiently. The comparisons with experimental results reveal that the analytical method is able to reasonably predict the fatigue crack growth life. Parametric studies are therefore performed using the proposed analytical model. The influence of CFRP layers, stress range and tensile static stress on the fatigue life was evaluated.

In this study, experimental tests were performed to investigate the load capacities of tubular steel adjustable shores in setups typically employed in construction sites, both for isolated units and combined systems. As for isolated units, the column curves of isolated tubular steel adjustable shores for a wide range were presented based on the test results. These curves can be used for interpolating the strength of tubular steel adjustable shores in design, instead of relying on extrapolation that may lead to uncertainties. The test results show that the overlap length between the connecting tube and base tube of the shore does not affect substantially the load capacity. Additionally, the load capacity of a 3-section tubular steel adjustable shore is lower than that of a 2-section shore. The load capacity of tubular steel adjustable shores decreases when No. 3 steel bars, instead of tube locks, are used. As for the combined systems, the reinforcing effect for a tubular steel adjustable shoring system with less than 16 shores is negligible when reinforced horizontally. For a tubular steel adjustable shoring system with four shores, the reinforcing effect is obvious when reinforced both horizontally and diagonally. This study also introduces the novel concept of upper and lower limit values of load capacity, which embrace the maximum and minimum strengths of all tested shoring members randomly selected for testing at the construction site. This concept can be potentially applied in shoring design.

Surface effect often plays a significant role in the pull-in performance of nano-electromechanical systems (NEMS) but limited works have been conducted for taking this effect into account. Herein, the influence of surface effect has been investigated on instability behavior of cantilever nano-actuator in the presence of van der Waals force (vdW). Three different methods, i.e. an analytical modified Adomian decomposition (MAD), Lumped parameter model (LPM) and numerical solution have been applied to solve the governing equation of the system. The results demonstrate that surface effect reduces the pull-in voltage of the system. Moreover, surface energy causes the cantilever nano-actuator with the assigned parameter to deflect as a softer structure. It is found that while surface effect becomes important for low values of the cantilever nano-actuator thickness, vdW attraction is significant for low initial gap values. Surprisingly, the increase in the initial gap, enhances the contribution of surface effect in pull-in instability of the system while reduces the contribution of vdW attraction. Furthermore, the minimum initial gap and the detachment length of the cantilever nano-actuator that does not stick to the substrate due to vdW force and surface effect has been approximated. A good agreement has been observed between the values of instability parameters predicted via these three methods. Whilst compared to the instability voltage predicted by numerical solution, the pull-in voltage obtained by MAD series and LPM method is overestimated and underestimated, respectively.

A new method for identification of structural parameters is proposed using Damped Transfer Matrices (DTM) and state vectors. A new transfer matrix is derived for continuous mass systems including the damping parameters. The state vector at a location is the sum of the internal and external contributions of displacements, forces, and moments at that point, when it is multiplied with the transfer matrix, state vector at the adjacent location is obtained. The structural identification algorithm proposed here involves prediction of displacement responses at selected locations of the structure using DTM and compares them with the measured responses at the respective locations. The mean square deviations between the measured and predicted responses at all locations are minimized using a nonclassical optimization algorithm, and the optimization variables are the unknown stiffness and damping parameters in the DTM. A nonclassical heuristic Particle Swarm Optimization algorithm (PSO) is used, since it is especially suited for global search. This DTM algorithm with successive identification strategy is applied on one element or sub-structure of a structure at a time and identifies all the parameters of adjacent elements successively. The algorithm is applied on numerically simulated experiments of two structures such as ten degrees of freedom lumped mass system and a cantilever with seven finite elements and one sub-structure of a nine member frame structure. Also this algorithm is verified experimentally on a sub-structure of a fixed beam. The main advantage of this algorithm is that it can be used for the local identification in a zone in a structure without modeling the entire global structure.

Geometrically nonlinear free vibrations of closed isotropic cylindrical shells are investigated through an analytical–numerical model. The method developed is a combination of Sanders–Koiter nonlinear shell theory and the finite element method. The cylindrical shell is subdivided into cylindrical finite elements and the displacement functions are derived from exact solutions of Sander's equations for thin cylindrical shells. Expressions for the mass, linear and nonlinear stiffness matrices are determined by exact analytical integration. Various boundary conditions of shell and in-plane effects are considered. Nonlinear responses are analyzed using the Runge–Kutta numerical method. The nonlinear frequency ratio is determined with respect to the amplitude thickness ratio of the motion for different study cases. Detailed numerical results are presented for various parameters for a closed isotropic shell, indicating either hardening or softening types of nonlinear behaviors, depending on the structure data. The present results show good agreement with the published ones for several cases of shells. This research clarifies the current disagreement about various types of cylindrical shells with geometric nonlinearities.

In this paper, higher continuity p-version finite element, in particular two-dimensional C² elements are utilized in studying vibrational characteristics of rectangular Flexural Micro-Plates based on the "gradient elasticity theory". In order to verify the computational procedure, results obtained from the finite element framework are compared to the classical plate theory results and to those of the micro-plate and classical plate investigations that are available in literature. Results indicate that the proposed framework, yields highly accurate results. Moreover, it is concluded that, under certain boundary conditions and length scale effects, the differences in predictions are so gross that the validity of classical theory in predicting micro-plates vibrational characteristics can be ruled out completely.

The loads induced on the spacecraft orbiting the Earth by the deploying elastic arm are investigated. The coupled equations of motion of the arm with the vehicle orbital mechanics are used to describe the 3D dynamic behavior of the flexible-appendage and the related disturbing loads induced on the spacecraft. To this end, an equivalent dynamical system is derived for the arm by applying an attached Non-Newtonian Reference Frame which is subjected to the orbital motion and geocentric pointing maneuver of the spacecraft. With the help of the Assumed Modes Method, the behavior of the arm attached to the spacecraft in Keplerian orbits is studied. The results show that deploying the arm in some specific directions relative to the orbital plane leads to serious coupling between two lateral displacements. In addition, the effects of specific orbital parameters on arm responses and resulting induced loads are studied for the cases of "True Anomaly of spacecraft at deployment time, and Eccentricity of elliptical orbits". The prediction of disturbing loads induced on spacecraft helps design the robust attitude control system. Further, the positioning accuracy of the payloads (installed on the arm-tip) can be estimated by employing the obtained arm responses in the orbital motion, which enables us to determine the undesirable motions and predict any required control system for the arm.

A displacement-based, novel curved beam element is proposed for efficient and reliable analysis of frames composed of curved members. The accuracy of the proposed element is not controlled by the subtended angle of the element with the angle up to π. In contrast to the conventional method, the interpolation function for displacement is based on the infinitesimal straight beam sections extracted from the curved element. Consequently, the strain energy of the curved beam element can be integrated by the infinitesimal sections along the element length. The relationship between the displacements and the corresponding strains in the straight beam is simpler than that in the curvilinear co-ordinate description widely adopted by many researchers in their element derivations. This technique is formulated to avoid couplings between the tangential and radial displacement variables in the strain field and its successful utilization is also demonstrated herein. Furthermore, the relation between displacements and strains of the infinitesimal straight beam section is equivalent to that of the curved beam in the curvilinear co-ordinate description. Finally, the analysis results of several bench marked examples by the proposed curved beam element are presented. The results show the high accuracy and efficiency of the proposed element against the classical curved beam element.