Tensile structural systems as for example cables nets, suspended bridges or membranes usually have nonneglecting p-delta effects under wind action. Consequently, great displacements give a geometrically nonlinear structural behavior. In addition, the wind action is nonlinear spatially and time depending. This closely affect the dynamic interaction between loads and structure. For this reason, the calculation process needs to take into account the timedepending nonlinear response of the structure. Finally, the wind action can be critical for structural and it can induce global or local instability. Analyses has to be able to capture and to describe the instability response in term of displacements, in the case of cables net, or rotations, in the case of suspended bridges. The paper is focused on the wind-structure interaction analyses carried out by wind tunnel experiments. The wind action is generally given by wind tunnel experiments, and it is time depending. For roofs, it generally is given as pressure time histories measured by some pressure taps on the roof. The process from experiments to structural response is described using a case of study of cables net. The dynamic geometrically nonlinear analyses were performed using the TENSO non-commercial program, which can execute dynamic step-by-step integration of the nonlinear three-dimensional structure with geometric nonlinearities by the Newmark-Beta method with Rayleigh damping.
The paper investigates the modelling and calculation of the flutter instability condition due to the windstructure interaction in the case of suspended bridges. In particular, the paper is focused on FEM (Finite Element Model) analyses on three dimensional models that compute the flutter instability. Geometric non-linear flutter instability analyses were carried out using a research and design software program (TENSO), which enables nonlinear dynamic analysis of wind-structure interaction. All the bridge structure is modelled. In particular, the bridge deck model was simplified by a beam model located in the deck section's center of gravity and two massless rigid links to simulate the connection of the deck to the hangers and cables. The cables are represented by rectilinear cables element. The critical flutter speed was estimated using quasi-static approximation of the unsteady wind loads calculated by aerodynamic coefficients (i.e. lift, drag and moment coefficients) estimated by aerodynamic forces (i.e. lift, drag and moment) directly measured by wind tunnel tests. The executed analyses are by dynamic step-by-step integration of the nonlinear three-dimensional structure with geometric nonlinearities. The global stiffness matrix is updated at each load step by assembly of the stiffness sub-matrices of the elements, updated to account for the strain found at the previous time step, taking into account the geometric nonlinearity of the structure.Firstly, the calculation provides for the static equilibrium of the structure under dead, gravity and construction loads by nonlinear static analysis. It was computed simultaneously using two methods: step-by-step incremental method and a "subsequent interaction" method with variable stiffness matrix (secant method). The secant method is a finite-difference approximation of the Newton-Raphson's modified method for systems of nonlinear algebraic equations. The solution under gravity loads is subsequently used as the initial step of the dynamic wind load analysis. The Newmark-Beta method with Rayleigh damping is used for numerical integration of the dynamic equations. Wind loads on the bridge deck are time dependent and they are simulated by applying the aerodynamic coefficients as a function of the time-dependent angle of attack at a given mean wind speed U. Displacements and rotations of the bridge deck at progressively increasing values of U, are estimated. The velocity at incipient flutter is fixed when the reference deck vibration amplitude exceeds ±5°. Details of the analyses results and calculation are given for an example of suspended foot bridges. Vertical displacements (δ) and rotations of the deck about the longitudinal bridge axis (αx) in normalized format (i.e. upward deck displacements are positive and counterclockwise rotations are positive) are discussed.
<p>Emergencies like post hurricane or earthquake need of buildings quick and easy to build. These buildings have to be great open spaces in order to host as many people as possible. These buildings can be non-temporary because they are useful for one, two years or more. For this reason, they have to be designed like permanent buildings. This exclude a great number of typologies as for example inflatable structures. The typology suggested is cables net and membrane roofs with hyperbolic paraboloid shape. These typology permits to cover very large span and it is built easily and quickly. In order to encourage a sustainability approach to build, the paper promotes recent developments in the field of material engineering that have allowed for the use of natural materials for common structural elements instead of traditional materials such as steel or concrete. In this context, hemp is a very interesting material for structural building design. This paper proposes the use of hemp cables for roofs with hyperbolic paraboloid cables nets, which sees the use of a sustainability material for structure thus having a very low environmental impact in terms of structural weight. The paper discusses five different plan sizes and two different hyperbolic paraboloid surface radius of curvatures. The cable traction, which gives the cable net stiffness, was varied in order to give a parametric database of structural response. Three-dimensional geometrically nonlinear element analyses were carried out on different geometries and a parametrization of the borders structures of anchorage is given.</p>
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