The effect of a dynamical aircushion in front of a wind loaded facade structure
: a numerical study

  • M. Casteelen

Student thesis: Master


Wind becomes an increasingly important factor when designing large and tall structures. The construction of a building does not only has to carry its own weight and the live load, but also the horizontal wind forces acting on its facade. Wind is not a static load; it changes in force and duration over time. The construction of a building needs to be dimensioned to withstand the peak values of the wind load in a particular area despite the fact that they might only occur a few times in the building life cycle otherwise, the building would succumb to those extreme wind forces. As a result the building is heavily over-dimensioned for most of its lifetime. If those peak values of the wind-load were to be avoided, the building could cope with a weaker construction, potentially saving weight and construction costs. The idea is to use an aircushion in front of the buildings facade with the intension that its flexible and dynamical behaviour would smoothen out the extreme wind load values, in the same way transported goods are protected by bubble wrap from reckless handling. This is a dynamical system where the aircushion would basically function as a spring and could be represented by one. So the question becomes, can the addition of an aircushion in front of a facade reduce the forces on the supports? And this translates to the following problem statement: Does the flexible and dynamical behaviour of an aircushion facade element reduce the wind load on the underlying structure? From there a parameter study is done to see how the different properties of the aircushion can influence its response to a specific wind load, and what the optimal conditions are of the aircushion for the maximum reduction of reaction forces on the supports. In this thesis only one wind spectrum is used. The shape of the aircushion is a closed strip with air trapped inside, which can be simplified into a twodimensional model. The aircushion membrane is discretized into mass particles connected to each other through springs and dampers. The springs simulate the stretch of the membrane, the dampers simulate the energy loss due to friction and the mass particles simulate the weight of the membrane itself. This is done in stages, where each model is more complex than the previous one. The equations of motion that describe the behaviour of the models are found by setting up the Lagrange's equation of motion. Due to the complexity of these equations, programmes were written in Matlab that could produce and solve these Lagrane's equations numerically. Matlab could also provide the necessary output that showed the aircushions behaviour to the wind load. For the parameter study five parameters of the aircushion were varied one at the time, they were the length of the membrane in its deflated state; the Young's modulus of the membrane material; the internal inflation pressure; the weight of the aircushion and the total height of the aircushion itself. The parameter study showed that none of the parameter values caused a reduction in reaction forces, the best what they could do is approach the reaction forces of the static solution. This was achieved by increasing the overall stiffness of the aircushion reducing its dynamical behaviour; this would indicate that the dynamical behaviour would only increase the domain of the reaction forces instead of smoothing out the peak values. When dynamic behaviour was possible, the reaction forces would oscillate around the average gradient, which would be the same as the static solution. This meant that the reaction forces would only be smaller than the static solution for small periods of time. Using the optimal conditions from the parameter study resulted in a very stiff aircushion that showed little dynamical behaviour resulting in a response similar to that of a static system when no aircushion is placed in front of the facade structure. One of the reasons why the reaction forces did not lower overall was because the aircushion used in this thesis is a closed system where the air cannot escape; there was no real dissipation of energy than only the energy loss due to friction.
Date of Award30 Apr 2013
Original languageEnglish
SupervisorArjan P.H.W. Habraken (Supervisor 1), Akke S.J. Suiker (Supervisor 2) & Patrick M. Teuffel (Supervisor 2)

Cite this