Abstract
Glass reinforced plastics, for structural application, came into use approximately thirty years ago. Their adoption in the elemental forms of laminate plate and beam construction gave way to folded and curved plates and covered skeletal grid formations, the geometric configurations providing the required stiffness to a material of relatively low elastic modulus. To further improve the material stiffness, high modulus carbon fibres have been introduced, in nominal ratio to the glass reinforcement, and have shown suprising increase in efficiency. At the present time no standard procedures are laid down, in this country, for testing the mechanical properties of glass reinforced plastics. This investigation sets out proposals for standard testing to determine tensile, flexural, torsional and interlaminar shear properties of the material. Such basic properties are a necessary adjunct to the prediction of stress parameters in fibrous composites. Much research has been carried out in attempting to predict, analytically, for design, which, though admirable for normal laminate construction becomes increasingly complex for multi-layer composites. The standard testing procedures also include for glass and carbon fibre combinations to appraise the suitable combined ratio of the reinforcing materials. This is shown for laminates and the simple application to grid structures. Where geometry has been employed to gain flexural stiffness for the structural application of reinforced plastics, loading conditions are themselves complex, particularly for wind loading. Model analysis is usually undertaken to determine the pressure distribution, which is then applied to a laboratory model in order estimate deflections and stresses on the structure. The wind tunnel technique is well established in the field of aerodynamics, but in civil engineering it has not been utilised to any great extent until just recently with the increasing use of lightweight structures of complicated geometrical shapes. This thesis describes the wind tunnel techniques and details an investigation to determine the pressure distribution over a shallow dome; this distribution is then related to the applied load on a laboratory experimental model to enable the stresses and deflections to be estimated on the actual structure. lt shows that the wind tunnel technique applied to civil engineering structures is permissible provided certain basic assumptions are accepted. Civil engineering structures are often exposed to wind gusts and eddies which cause considerable stresses to be developed in the structure; this method,at present, is not able to deal with such extreme loading conditions. The results of this analysis have shown that high negative pressures are developed on the windward side of the model; these pressures are reduced to 80% of their original values fairly rapidly in areas away from this region. The high suctions exerted on the dome will set up large tensile stresses which will not be counterbalanced by the deadweight of the light structures. lt is therefore important in shallow plastics domes to design for high tensile stresses,whereas in conventional materials, due to their greater deadweight, the stresses will invariably be compressive. In order to provide a valid prediction without constant recourse to experimental investigation, analytical solutions may be programmed to electronic computation, and, in some cases, simple hand analysis. This latter is shown for the simple grid, whereas in the case of the shallow dome a semi-finite element programme has been developed. This assumes a basic skeletal member configuration conforming to the unit area triangulated zone s. These zones are then idealised into a thin continuum in order to determine the stress parameters of the load variations. It is clear that all load simulation is idealised and simplified, but hopefully shows a common sense approach. Variables, such as temperature and non-homogeneity of the shell however naturally produce differences in results, but with suitable safety factor introduced, such differences can be accounted for Further investigations immediately developing from this work that for a stiffened scalloped shape adopting the same rise; span dome profile; also the introduction of a skeletal web of carbon fibres in combination, in nominal amount, with the glass reinforced plastics material. programmes for wind pressure distribution coefficients; experimental stress parameters and analytical skeletal solution for the shallow dome, are appended to the thesis.