Truss imperfections in the design of bar and diaphragm bracing systems

Abstract

In the article three variants of roof bracing were considered: bar bracing, diaphragm bracing and the combination of bar and diaphragm bracing. Different analytical and numerical ways of taking into account the imperfections of the truss girder were compared. The entire 3D model of the roof (shell and beam elements with the eccentricities taken into account) was analysed numerically. Selected stressed-skin aspects were considered. Stabilizing forces in the purlins and forces in bracing elements (bar and/or diaphragm bracing, connections) were observed. The importance of the imperfection issues (e.g. shape of the imperfection, method of including imperfection, wind forces) was evaluated numerically to indicate the key points in the design procedure. The biggest forces in purlins occurred for “nonstandard” shape of the imperfection. On the other hand, in case of bracing elements, when wind forces were taken into consideration, “standard” approach of including the imperfection was safe approximation. Moreover, the diaphragm took over significant part of the bracing forces, however the distribution of the forces depended strongly on the flexibility of the bracing and purlin/truss connection.