Hélder S. Sousa

Characterization of Cross-Sections from Old Chestnut Beams Weakened by Decay

This work aims to experimentally assess the material properties of old chestnut (Castanea sativa Mill.) cross-sections weakened by decay. Specimens were taken from critical zones of twenty floor beams, mostly corresponding to their ending parts, in contact with granite masonry walls. These specimens were compared with clear wood specimens taken from non-decayed parts. The experimental campaign comprised of visual inspection, non-destructive testing (ultrasound, impact penetration and drilling resistance tests) and uniaxial compression tests, parallel to the grain. Pin penetration test evidenced a depth of penetration 36% higher for decayed specimens compared to non-decayed specimens, proving the loss of capacity in the superficial decayed layer. The results of decayed specimens exhibited a decrease of approximately 30% in the compressive strength parallel to the grain. A reduction of approximately 5% was found for the dynamic and elastic moduli. The coefficients of variation for decayed specimens are significantly higher than for sound specimens. Different decay models are considered and compared aiming at analyzing decay evolution along time and determining the decrease of load bearing cross-section. Finally, the results were used to calibrate a bi-parametrical decay model for above ground timber structures, considering a linear relation between time and decay depth.

Effectiveness and Subjectivity of Visual Inspection as a Method to Assess Bending Stiffness and Strength of Chestnut Elements

Besides the difficulty of assessing an existing timber structure on site, the efficiency and accuracy of visual inspection is often compromised by its subjective nature inherited by the level of expertise of the inspector. This often leads to conservative predictions of the mechanical properties, even with the use of specific visual grading norms. The main objectives of this work are to assess the effectiveness of visual inspection as a method to define different classes of strength and stiffness and to provide a statistical analysis on its subjectivity. For that aim, visual inspection using Italian standard UNI 11119:2004 and bending tests of 20 old chestnut beams (Castanea sativa Mill.) at different scale element, were carried out. Comparisons and effectiveness of visual inspection is analyzed within and between different scales of the timber members, and also regarding different level of expertise of inspectors. The results evidence similar percentages of segments classified with higher and lower visual inspections classes and proved to be a good qualitative indicator of bending strength between sawn beams. An overall 42% accuracy of the most experienced inspectors was found with better differentiation between visual classes, whereas lower level inspectors scored approximately less 5%. Lower level inspectors also evidenced higher concentration of values around a higher mean for each class denoting a more conservative approach. Regardless of the inspector level, knot size was considered the main limiting visual parameter with higher influence in small scales of the timber elements. When studying the characterization of a single knot, coefficients of variation of 15.7% and 21.8% were found for measuring the minimum and maximum diameter. Bayesian probability networks were considered as to individually assess the accuracy in stiffness prediction of different level of inspectors, and by combination of their information, evidencing that parallel combination for prior information may allow the increase in visual inspection accuracy.

Safety Evaluation of Timber Structures through Probabilistic Analysis

This work presents a procedure for the use of probabilistic modelling techniques in the assessment of safety parameters of existing timber structures, when influenced by two types of actions. The parameters of the models have been defined as probabilistic variables, and Monte Carlo simulation technique was taken into account for safety evaluation. The case study here examined concerns four king post trusses on which a safety evaluation method was conducted to determine a target reliability index. Since the structures presented a high level of deterioration, a model considering the variation of the residual cross-section and the influence of environment in resistance was taken into account. Time dependent deterioration models have also been considered. With the results given by the different used models, it was possible to obtain probabilities of failure and respective reliability indices, as well as time evolution deterioration curves for a deteriorated historical structure.

Visual assessment and diagnosis of a timber railway station warehouse in Foz do Tua

Aiming at the rehabilitation of a timber warehouse in the Tua train station for a new use, it was necessary to assess the level of conservation of the building’s elements and determine whether intervention measures were required. This work summarizes the assessment made of the timber elements by means visual inspection and non-destructive testing. The main results, obtained in the diagnostic survey, were the conservation level of the timber warehouse, the visual strength grading of the timber elements and damage maps indicating the main pathologies and its probable causes.

A Holistic Methodology for Probabilistic Safety Assessment of Timber Elements Combining Onsite and Laboratory Data

ABSTRACT The assessment of existing timber structures comprises several fields of knowledge, each one providing its own piece of information aiming at an effective answer. Even if general guidelines are available, there is still a need for a holistic approach that is able to combine information from different sources and infer upon that information allowing also, in that process, for an updating scheme aiming at a more substantiated decision process. In that scope, this article proposes a methodology for a holistic assessment of timber elements. The methodology combines information gathered in different scales and follows a probabilistic framework that allows for the structural assessment of existing timber elements with possibility of inference and updating of the mechanical properties of the elements, through Bayesian methods. The described methodology is based in four main steps: (i) scale of information; (ii) local and global data; (iii) probability assignment; and (iv) long-term structural analysis. After the description of the methodology, its limitations and possible future modifications are discussed.

Repairing of a timber truss through two different techniques using timber elements and screwed metal plates

Structural reinforcement of timber buildings may be needed due to different reasons such as change of use, deterioration due to lack of maintenance, exceptional damaging incidents or loading, after changes in regulatory specifications, or interventions to increase structural resistance. In this work, two different techniques were considered for repairing a timber truss that was previously assessed on laboratory (test facilities of University of Minho) and taken up to failure during a load-carrying test. A collar beam truss, with more than one hundred years, was tested considering a vertical point load on each main rafter. Failure of the timber truss was located in the sections of the rafters near the loading positions by bending. Repairing techniques, based on the use of timber elements for one of the rafters and on screwed metal plates for the other rafter, were evaluated and compared to the original unstrengthened condition. The efficiency of the combined repairing techniques was evaluated taking into consideration the structural performance of the collar truss, namely its displacement and ultimate load capacity. In this paper, the results of the experimental tests are discussed attending to the analytical calculation of the contribution of the repairing techniques. Also, the different failure scenarios, for original and strengthened truss, were analyzed and compared.

Reliability based design of interventions in deteriorated timber structures

ABSTRACT When accommodating new uses or mitigating the consequences of deterioration, the strength increase of existing structures is significantly more onerous than a similar increase at the design stage of new structures. The safety methods prescribed in current standards were defined for the design of new structures and are frequently conservative for the assessment and repair of existing structures. This work will introduce the fundamental aspects of structural reliability and their application in the context of existing timber structures regarding the use of target reliability indices. First, the fundamental methods of structural reliability are introduced. The use of reliability methods requires the use of more detailed information in respect with material properties, loads, and model uncertainty. The main sources of such information are described. After overviewing the fundamental methods of structural reliability, methods to introduce additional information, namely results of non-destructive tests in the structural assessment are discussed. Finally, the intervention on a timber structure will be analyzed, within a case study, by considering different repair scenarios that lead to discussions on different suitable safety thresholds for existing and repaired structures.

FP1101 and RILEM TC 245 RTE Training School on Assessment and Reinforcement of Timber Elements and Structures: The Role of the Academic Community in Disseminating Knowledge

In the past decades, timber has been considered as a secondary construction material owing to the emergence of other materials such as concrete and steel, and as a consequence a large amount of knowledge on how to use timber has been forgotten or even lost. This has led to badly designed interventions on timber structures, which have misinterpreted the past structural solution and led to significant structural performance problems. However, the European research community has made a significant effort towards the dissemination of knowledge regarding the design and assessment of timber structures. Within that scope, COST FP1101 had the objective of increasing the acceptance of timber in both the design of new structures, as well as in the repair of old buildings by developing and disseminating methods to assess, reinforce and monitor them. To that aim, FP1101 and RILEM TC 245 RTE in collaboration with the University of Minho (Portugal) organized a Training School (TS) on the assessment and reinforcement of timber elements and structures during 11–14 May, 2015. The training school had the objective of disseminating knowledge to young researchers, as well as to others interested in wood and timber construction and the rehabilitation market. During the TS, several experts provided information on the assessment of existing timber structures, visual grading, interventions on heritage buildings and on analysis, repair and reinforcement of traditional and dowelled timber connections and structures. Also, full-scale tests on an old timber trusses and on connections were made examining both the original conditions and post failure reinforcement. This work presents the different areas of knowledge that were scope of that training school and its main achievements on the dissemination of knowledge to young researchers and other interested groups.

Assessment and Diagnosis of Two Collar Timber Trusses by Means of Visual Grading and Non-destructive Tests

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.