Abstract
The research reported herein compares five types of grid structure of similar span and subject to the same external loading, each under up to four conditions of boundary restraint, designed by elastic theory to near their maximum span/weight efficiency. The structural medium used is tubular steel, the design standard BS 449. To facilitate design two devices (hitherto unreported by anyone else, to the author’s knowledge) have been developed, namely (a) accurate consideration of lattice beams as single members in structures; (b) automatic design by digital computer allowing for simple changes in geometry. The skeletal structures are analysed by stiffness method and other possible methods discussed. The possibility of applying linear programming to the search for the most efficient structure is also dealt with. Prom the analyses the most efficient span/depth ratio for each boundary condition is suggested and the mode of load distribution, the effect of pattern, pattern intensification and the inclusion of edge beams are discussed. A model diagrid composed of lattice beams in one direction and prismatic beams in the other, the chords of the former having the same second moment of area when considered as a single section as that of the prismatic beam, has been tested and used to illustrate the fallacy of the common assumption made for such beams. The model was designed as a control to the derivation of lattice beam stiffness matrices mathematically, graphs and tables of which have been included in the report. Further work on load factor analysis, pattern effect and topology are suggested.