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Wind Load Analysis of Buildings
Published in Syed Mehdi Ashraf, Practical Design of Reinforced Concrete Buildings, 2017
Galloping and flutter are two important wind-induced motions. Galloping is a transverse oscillation of a structure due to the development of aerodynamic forces, which are in phase with the motion. It is demonstrated by the progressively increasing amplitude of transverse vibration with increase of wind speed. The structural elements that are not circular are more prone to galloping. Flutter is an unstable oscillatory motion of a structure due to the coupling between aerodynamic force and elastic deformation of the structure. Combined bending and torsion are among the most common forms of oscillatory motion.
Dynamic Loading on Structures and Structural Response
Published in Suhasini Madhekar, Vasant Matsagar, Passive Vibration Control of Structures, 2022
Suhasini Madhekar, Vasant Matsagar
Structural innovations and lightweight construction technology have considerably reduced the stiffness, mass, and damping characteristics of modern buildings. A modern skyscraper, with lightweight partition walls, and high-strength materials, is more prone to wind motion problems than the early-day high-rise buildings, which had the weight advantage of masonry partitions, heavy stone facades, and massive structural members. Wind produces three types of effects on structures: static, dynamic, and aerodynamic. Following are three common forms of wind-induced motion: GallopingGalloping is the term used for self-excited transverse oscillations of structures due to the development of aerodynamic forces which are in phase with the motion. It is characterized by the progressively increasing amplitude of transverse vibration with increase in wind speed. Noncircular cross sections are more susceptible to galloping.FlutterFlutter is the self-excited unstable oscillatory motion of a structure due to coupling between aerodynamic force and elastic deformation of the structure. It is the most common form of oscillatory motion due to the combined effect of bending and torsion, generally leading to structural failure. Long-span suspension bridge decks or members of a structure with large value of dt are prone to low-speed flutter; where d is the depth of structure or a structural member parallel to wind stream, and t is the least lateral dimension of the structure or a structural member.OvallingThin-walled structures with one or both ends open, such as oil storage tanks, and natural draught cooling towers, in which the ratio of the diameter of minimum lateral dimension to the wall thickness is of the order of 100 or more are prone to ovalling oscillations. These oscillations are characterized by periodic radial deformation of hollow structures.
Dynamic stability of non-conservative systems
Published in Kurt Ingerle, Non-Conservative Systems, 2018
The clamped free column, shown in Fig.2.2, loaded by a tangential load P, is stable for p <= pcs = π2. For p > pcs, non trivial equilibrium states are absent. The undamped column starts automatically to vibrate, influenced by inertia. At the beginning of p > pcs, the increase in amplitude is very small, and is small for a wide range of pst < p < pflutter = 20.05. This kind of motion will be denoted as ‘vibration’. Damping decreases the amplitudes toward its trivial state. For p > pflutter any small disturbance increases abruptly to large displacements. Damping decreases the flutter amplitudes. Summarizing the types of response for this column, three distinct cases exist: static, vibration and flutter. Hence, this column without damping behaves in the following way: Until p < pcs the column is stable, for p > pcs the column starts to move with small amplitudes (vibration), and with p > pflutter the column will oscillate with large amplitudes. The presently used stability criteria for conservative systems are not suited for non-conservative systems, since they do not consider the peculiarities of non-conservative systems. This difficulty will be discussed later on. The above short description of the different behavior of conservative and non-conservative systems demonstrates that stability criteria applicable for conservative systems may not be suitable for non-conservative systems. Hence, new stability criteria are needed. To show the large energy input of the tangential load, Ziegler’s column with p = pcs = 2.0 (still stable), disturbance a0/l = 0.001, damping ζ = 0.01, mass m1/m2 = 1/1 (see Fig. 2.12,b2) is used. The motion starts with tip amplitude a0 and the amplitudes get larger with every cycle. Die energy introduced by the tangential load increases the potential elastic energy of the system and the kinetic energy of both masses. Damping always dissipates energy. (Fig. 2.3). After approximately 100 sec the largest tip amplitude amax/a0 = 35 is reached. According to Lyapunov, the system is not stable, since the factor 35 can hardly be regarded as ‘near’ or ‘close’. The continued motion shows, however, that now the tangential load extracts system energy (potential) and mass energy (kinetic) together with the present damping energy, and the column will finally reach its trivial equilibrium state (see also Fig. 2.12,a2). This shows that the criteria by Lyapunov do not lead to a correct statement on stability. Hence, the criteria by Lyapunov are not suited for non-conservative systems and might lead to incorrect stability limits.
A two-level strategy for aeroelastic optimization of a 3D wing with constant and variable stiffness skins
Published in Engineering Optimization, 2023
On the other hand, when designing lifting surfaces, special attention must be given to aeroelastic phenomena. From the viewpoint of the structural reliability of aircraft components, divergence and flutter must be highlighted. Divergence is a static instability, where the terms of aerodynamic loading that are proportional to displacement act in a way that the effective stiffness reduces, and the displacements increase beyond predicted structural limits. Flutter is an aeroelastic instability that occurs when one or more structural vibration modes couple with unsteady aerodynamic forces, resulting in harmonic oscillations of rising amplitude. Both phenomena depend on the stiffness distribution on lifting surfaces and are potentially destructive. Therefore, the engineer must guarantee that the instability onset airspeed is outside the aircraft's flight envelope.
Aeroelastic flutter behaviour of beam: effect of graded GPL and porosity
Published in Mechanics Based Design of Structures and Machines, 2022
Gourav Kotriwar, Jeyaraj Pitchaimani
Aeroelasticity is a branch that studies the influence of aerodynamic forces that act on aircraft structures during its flight. Flutter is a condition where structure executes self-excited vibrations under the influence of aerodynamic, inertia and elastic forces. Flutter induces fatigue damage to the structure and culminates into catastrophic failure. Flutter instability risks the safety of aircraft structures and their flight performance; hence it becomes very important to do investigation of aeroelastic flutter in airplane structures. Zhou, Wang, and Zhang (2021) studied the flutter and vibration characteristics of GPL-reinforced functionally graded porous cylindrical-shaped panels subjected to supersonic flow. Saidi, Bahaadini, and Majidi-Mozafari (2019) performed aeroelastic analysis of the smart porous plates under the influence of supersonic flow. Aditya et al. (2020) studied flutter behavior of 2-D curved porous panels reinforced with GPLs using FEM. Huang et al. (2020) investigated aeroelastic flutter behaviors of GPL embedded quadrilateral shaped laminated composite plates. Ganapathi et al. (2022) studied flutter characteristics of variable stiffness laminated panels using the finite element method considering the nonlinear effect.
Multimodal Flutter of a Long-span Suspension Bridge in Service and During Construction
Published in Structural Engineering International, 2022
Claudio Mannini, Niccolò Barni, Salvatore Giacomo Morano
Flutter assessment is usually carried out by measuring aerodynamic derivatives in the wind tunnel and then calculating the critical wind speed at which the trivial zero-amplitude solution loses stability. Rigorously, several vibration modes should be included in this stability calculation,2 but often a simple bimodal calculation is deemed adequate to estimate the flutter stability threshold, provided that the vertical bending and torsional natural mode susceptible to couple are carefully selected, and the corresponding effective inertial properties are considered (in particular, including the contribution of main cables). This simplified approach complies with the point of view of the bridge engineer during the preliminary design of the bridge, and in Ref. [3] all of the assumptions that theoretically need to be verified for a bimodal calculation to be accurate are examined. Among these, a key point is that the two natural modes that contribute to flutter instability present similar shapes. Otherwise, in order not to underestimate the critical wind speed too much, it is important to include in the bimodal calculation the effect of non-homothetic mode shapes through mode coupling coefficients.3,4 This may be particularly important for the construction phase, when mode shapes progressively change while advancing in deck erection.