Assessment and diagnosis of two collar timber trusses by means of visual grading and non-destructive tests

Abstract

The main objective of the present work is to evidence the importance of the inspection and diagnosis phase on the assessment of existing timber structures through visual grading and non-destructive testing (NDT) of its elements, especially for old structures with historical importance. To that aim, two collar timber trusses were inspected and assessed. These trusses were part of an old roof with over than one hundred years old that was decommissioned and then reassembled in laboratory conditions in the University of Minho (Portugal). The timber trusses were then tested on full scale at laboratory conditions until failure. The first part of this work presents the visual grading procedure complemented with NDT. During that part, all the elements of the trusses were visually inspected on each 40 cm segments with consideration of its geometric characteristics (exterior cross-section, wane and deformation), its defects (cracks, knots and decay) and critical sections. Non-destructive techniques, such as impact penetration, ultrasound and drilling resistance tests, were carried out in order to characterize the level of conservation of the elements at its present condition. The combination of the results of all NDTs allowed for a more substantiated grading of the segments and consequently of the elements. The results are presented through damage maps indicating the main pathologies of the structure and its residual cross-section, as well as correlations between different NDTs. In the second part of this work, a comparative analysis between the two trusses is made considering the prior results of the NDTs and the performance of the trusses on the full scale tests. To that aim the results of the visual grading and NDTs are correlated to the sections were failure of the segments was visible.