Tensairity: The effect of internal stiffeness on the buckling behaviour of an inflatable column

Tensairity is a new lightweight structural concept which can be classified as a pneumatic structure. By adding a compression element and cables to an inflated tube a structural system is obtained in which the strut and cables carry the applied load and the air tube transfers forces between both and stabilises the compression element against buckling. The structure combines the favourable features of pneumatics (light, deployable, compact storage and transport volume) with good structural properties and moderate air pressures. Thus, Tensairity is interesting for a wide range of applications, from roof structures to (temporary) bridges and tent structures. Next to beams the Tensairity concept can also be applied to columns and arches. Tensairity research currently focuses on structural improvements which can contribute to the applicability of the concept. One of these improvements is the use of a stiffening membrane strip inside the pneumatic tube. This ‘web’ is ascribed a positive effect on the stabilisation of the compression element. This thesis investigates the stabilising effect of the inflated tube on the compression element in Tensairity, and particularly the effect of the web on this behaviour. The approach is essentially experimental. First the concept of pressure induced stability and the web effect are investigated analytically and experimentally by means of small models. The models appear to display higher buckling loads for increasing pressures, even approaching the theoretical yield load of the compression element. Furthermore, the web improves the buckling load with a factor 2 in practice. Next, full-scale experiments are performed on two 5 m long Tensairity columns, both consisting of three aluminium struts supported and stabilised by an inflated membrane hull: one without and one with internal webs. For different hull pressures the columns are subjected to an axial compressive load. The responses are analysed and compared, also with FE predictions from ANSYS. For both columns the axial stiffness and the buckling load appear to improve for higher pressures in practice, emphasising the stabilising effect of overpressure. The maximum registered buckling load is over 25 kN, which is a promising result. The column with webs performs better than the one without webs: the increase in stiffness and capacity is around 35 % for an average pressure of 150 mbar. In general this improvement is lower than expected from the small models and the FE results, and not sufficient to counteract the drawbacks of the web model regarding applications, like an increased weight and more complicated fabrication. Thus, the application of a web is at this point not an effective contribution to the functioning of a Tensairity column. Yet, in comparison with a conventional truss column of similar dimensions both Tensairity columns display a sound structural behaviour, and they offer clear advantages in terms of deployability and architecture. A Tensairity column would therefore be especially interesting for temporary applications, e.g. acting as a pole in a tent structure.Design and ConstructionCivil Engineering and Geoscience