Research On The Mechanical Performance And Wind-induced Response Of ETFE Cushions Considering Interaction Between The Enclosed Air And The External Membrane

ETFE cushion structure is a new envelope structure which has been widely used since the beginning of this century. Analytical methods on the behavior of the ETFE cushions have been developed and the mechanical properties have been studied. However, existing studies only focused on the static problems including the form-state and the loading analysis, and the wind-induced dynamic response analysis of the ETFE cushions have not been reported. The dynamic problems of the ETFE cushion under wind load involve two types of coupling effects. One is on the interaction between the enclosed air and the ETFE films, and the other one is on the fluid-structure interaction (FSI) between the cushion and the external flow. The problem associated with the first interaction is how to fully consider the influence of the enclosed air on the dynamic characteristics of the ETFE cushion. The problem associated with the second interaction is how to calculate the added mass, aerodynamic damping and additional stiffness originated from the external flow more accurately.With the aim to implement the wind-induced dynamic response analysis of the ETFE cushions considering these interaction effects, several key issues in the dynamic analysis of ETFE cushions are studied systematically with theoretical analysis, experimental research and numerical simulation in this paper. The main contents and innovative developments are as follows:(1) The coupled equation of the enclosed air and external membrane of the inflatable membrane structure is derived, and a numerical analysis method for the inflatable membrane structures is proposed on the basis of the coupling model with numerical verification. The dynamic equation of motion of the external membrane is obtained by introducing an internal pressure in the equilibrium equation of the tensioned membrane structure. The wave equation to describe the motions of the enclosed air is derived based on potential-flow theory. The coupling equations for the ETFE cushion system are finally obtained considering the interface compatibility conditions and the combined equations of motion of the internal air and membrane. Then two typical inflatable membrane structures are simulated based on the interaction model. The accuracy and applicability of the interaction model are verified in the dynamic analysis of the inflatable membrane structures.(2) A method for the determination of the form-state of the ETFE cushions is proposed based on accurate measurement of the internal pressure and the membrane shape, and it is used in the form-state tests and loading tests. The validity and feasibility of this method are demonstrated with results from the tests and analysis. A numerical finite element modeling on the interaction model of the ETFE cushions is proposed and verified by experiments. The trend of variation of the internal pressure and membrane shape with external loads are studied through load analysis and bearing capacity analysis, and the failure modes and criterion of the load-carrying capability are defined. The influences of size, span ratio and membrane thickness of ETFE cushion on the load-carrying capability-internal pressure curves are discussed. Results show that:(a) the internal pressure of ETFE cushion is very sensitive to external loads. The distribution and location of external loads have significantly influence on the deformation and internal pressure of the ETFE cushions;(b) there is an optimal value of internal pressure which associates with the maximum load-carrying capacity of the cushions within the range of design pressure; the change of size, span ratio, and membrane thickness will obviously affect the load-carrying capability of the ETFE cushions.(3) The modal parameters and their variations with internal pressure and span of the ETFE cushions are obtained by using peak picking method, cross-spectrum identification method and exponential fitting method with the help of non-contact video records, and the vibration modes, frequency distribution and damping characteristic are studied. Then the parametric analyses on the self-vibration characteristics are performed and the influence of internal pressure, span ratio, span, membrane thickness and external air on the modal parameters of ETFE cushions is analyzed. Results show that:(a) the first mode shape of test cushions is of squeezing motion, but the subsequent modes are not the same;(b) the ETFE cushion is a low damping structure in still air environment and its modal damping decreases nonlinearly with increasing internal pressure;(c) the natural frequencies of ETFE cushions increases nonlinearly with increasing internal pressure; the frequencies decrease rapidly with increasing span and a higher density;(d) the internal pressure and span have significant influences on the frequencies but have limit influence on the mode shapes;(e) the membrane thickness and span ratio have different influences on the natural frequencies of lower modes and higher modes;(f) the frequencies of the wet modes considering external still air are smaller compared to the frequencies of dry modes, and (g) the influence of external air on the dynamic characteristic of the ETFE cushions increases significantly with increasing span.(4) The accuracy and applicability of linear potential flow elements in random vibration analysis with nonlinear and large deformation effects are verified. It lays down the foundation for the FSI dynamic analysis of the ETFE cushions. Then the wind pressure history from a rigid CFD model is applied on the upper surface of the ETFE cushion for a linear random vibration analysis. The vibration modes and the distribution of membrane response of the ETFE cushions under random wind loads are studied. Results show that:(a) the vibration of ETFE cushions in fluctuating wind shows strong integrity. The two membranes show vertical synchronous vibration with the link of the enclosed air;(b) the peak response on the upper membrane is shifted in the windward direction and the shape of response is nonsymmetrical along the wind direction. The response distribution of the lower membrane is nearly symmetrical due to the uniform pressure it takes.(5) The FSI analytical model of the ETFE cushions including the whole building is established based on the structure-fluid synchronously modeling techniques. The wind-induced dynamic response considering the FSI effects is calculated, and the influences of the FSI effects on the time history, spatial distribution, PSD and the wind excitation factor of responses are discussed. Finally, the FSI effect coefficient for the ETFE cushions is defined, and its behavior and range of variation with common wind velocity, internal pressure, span ratio and span are suggested. Results show that:(a) the FSI effects of ETFE cushions in fluctuating wind are mainly embodied in the one-way effect of the external flow on cushion, and changes of the external flow caused by cushion movement are very small;(b) the FSI effects cause large aerodynamic damping which inhibits the wind-induced vibration of the ETFE cushions;(c) the response spectrums of cushions with FSI exhibit broadband distribution with mainly low-frequency vibration, which are very different from the results without FSI; this indicates that the wind-induced vibration mechanism from with and without the coupling FSI conditions are different. The former is mainly forced vibrations from fluctuating wind, and the latter is mainly resonance near the structural natural frequencies;(d) the FSI effect coefficient of the ETFE cushion is significantly affected by wind velocity, internal pressure, span ratio and span. In the common ranges of wind velocity, internal pressure, span ratio and span, the ranges of variation of the FSI effect coefficients are respectively0.62-0.725,0.6-0.83,0.58-0.87and0.56-0.89.Found on theoretical derivation and experimental verification of the interaction model, numerical simulation of the whole process of wind-induced dynamic response of the ETFE cushions are realized considering the FSI effects between the cushion and external flow. The static and dynamic performance and FSI characteristics of ETFE cushions are studied. These results may provide useful references for the ETFE cushion design.