Initial Bar Tensions in Pin-Jointed Assemblies

Abstract

This paper is concerned with initial bar tensions in pin-jointed statically indeterminate assemblies, for example the grid shown in Fig. 1. Initial tensions are present in such assemblies on account of the lack-of-fit of the constituent bars and joints due to unavoidable inaccuracies in manufacture: and they may contribute to premature collapse of an assembly under increasing load, by predisposing particular bars to early failure by buckling. An algorithm for computing initial stresses due to given lack-of-fit is used to make statistical estimates of the magnitudes of initial bar tensions in a grid assembled from bars whose lengths are specified in the form of a mean and standard deviation. A scheme for computing the attenuation of these initial bar tensions by allowing for a limited amount of “backlash” at the bar-joint connections is also investigated. The theoretical and computational work is supported by a series of detailed experiments on a double-layer space grid of the type shown in Fig. 1, and built from standard MERO spherical bolted junctions: these experiments involve the measurement of initial bar tensions both with and without some deliberate loosening of bolts to produce limited “backlash” at the joints.