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This thesis presents an investigation into the behaviour of 'Composite Double Layer Space Grid Structures'. In this research a reinforced concrete slab was used as the top layer of a thin steel tube space frame. The stiffness matrix method of analysis was used for the theoretical analysis in which the slab was represented by 'Finite Elements'. Two experimental models were designed and constructed mainly to verify this analysis and to check its convergency and also to study the influence of top joint eccentricity. The research showed that these structures are more rigid and have better stress distribution patterns than the comparable double layer grids, and hence can be used for larger spans. The critical factor in the design of these structures is the buckling of the diagonal shear members, hence an 'economical depth' has been found. Plate analogy was found to be of great value, by which it was possible to explain and predict the stress behaviour, also a criterion is found for designing an 'Efficient Section' where no concrete is subjected to tensile action. Since this type of structure requires large computer facilities, which are neither easily available nor economical even for short spans, approximate methods are suggested, some of which are based on a plate analogy and do not need a computer. Finally a design procedure using these methods has been outlined by which it is possible to have a quick and approximate idea about the appropriate section properties for any given span within the specified loading and boundary conditions.

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