Identification of closely spaced modes in space truss structures as base for the determination of member normal forces

Abstract

Abstract Space truss structures are a type of versatile lightweight systems that are applied in different fields such as civil and aerospace engineering. For some of these systems that were built decades ago, for example as roof structures for relatively large spans, not all proofs regarding the load-bearing capacity and serviceability can be satisfied according to current codes of practice as the requirements may have changed since the initial design phase of these systems. In such cases, it may become necessary to obtain the actual stress status of the structural skeleton based on experimental investigations. From the identification of forces in many members, a general stress status can be derived, which leads to an estimation of the current status and possible reserves in the load-bearing capacity of space truss structures. One option for the identification of element forces is their identification from modal parameters of the structure. In literature, the main focus has been put so far on the identification of longitudinal forces and boundary conditions inside a single tension member/cable utilising vibration-based methods. Very good results were also reported for the identification of longitudinal forces in elements of two-dimensional truss structures of moderate complexity. Fewer studies documented in the literature concentrate on space truss structures. For these modular systems consisting of standardized elements, not only the identification of element forces based on identified modal parameters is very challenging, but also the modal identification itself. The latter is often caused by the situation that space truss structures often have numerous modes characterised by both local and global vibrations at close natural frequencies. The study presented in this contribution focuses on the identification of mode shapes and natural frequencies required as a prerequisite for a model based approach to estimate the stress state of the complete considered space truss structure. Apart from aspects with respect to the instrumentation for modal testing, specific issues within a parametric modal analysis such as the choice of model order are discussed. It is explained by means of a case study, why the minimal model order to be taken into account in the modal identification of space truss structures can become considerably higher than those commonly applied in the analysis of, for example, bridge structures, towers or floor systems. These descriptions are illustrated by the results of both numerical and experimental analyses.