Abstract
This thesis is concerned with the structural behaviour of thin walled plastics pyramids which form a shear resisting medium between the upper and lower elements of double layer grids. The several aspects of the work are described under separate headings in seven chapters. In Chapter 1 the various types of construction are discussed and the properties of the most important structural plastics materials reviewed. Chapters 2, 3 and 4 contain a description of the experimental work undertaken on single pyramids and on a roof model consisting of sixteen pyramids. The behaviour of pyramids both before and after initial buckling is investigated and conclusions are drawn whenever appropriate. Chapters 5, 6 and 7 are devoted to the theoretical treatment of stressed-skin pyramids and comparison of the observed quantities with those obtained from calculations. The theoretical part is of a fundamental nature and constitutes the author's own and original contribution. The stress distribution is determined by superimposing on the stresses existing in an infinite pyramid a linear combination of corrective solutions, each satisfying the boundary conditions at the inclined edges of the pyramid walls. By this means any type of simple or mixed boundary conditions at the base and cap can be satisfied. The displacements are derived by introducing displacement functions from which both the stress and displacement components can be determined. A finite difference approach is used for calculating buckling loads, while the ultimate load-carrying capacity is assessed by modifying the known semi-empirical formulae for rectangular plates. The theoretical part includes computer programs, which are essential for the understanding of the numerical analysis. The results for typical cases are reproduced in Appendices A and B. Design aspects and practical considerations have been taken into account throughout the work.