Imposed Lack of Fit as a Means of Enhancing Space Truss Design

Abstract

Space trusses with a high degree of static redundancy, such as double-layer grids, are highly sensitive to imperfections. Substantial reductions in load-bearing capacity are caused primarily by random lack of fit (LOF) of members, associated with brittle-type compressive member buckling. A substantial increase in load-bearing capacity and/or reduction in weight can be achieved by imposing lack of fit on selected members in a controlled manner. Such prestressing by imposed lack of fit utilizes the compressive capacity of nominally tensile members and can be used to compensate for the deleterious effect of random lack of fit. A simple algorithm is presented for obtaining LOF patterns that minimize weight or maximize load-bearing capacity for a class of trusses in which members' cross-section properties are proportionally fixed. The algorithm is based on the flexibility method of structural analysis and on the simplex method of linear programming. Some illustrative examples suggest the possibility of doubling load-bearing capacity or halving weight in some design cases.