Visual strength-grading and NDT of timber in traditional structures

Abstract

Maurizio Piazzais Full Professor at the Department of Mechanical and Structural Engineering at the University of Trento and a member of a number of international and national technical committees. His research activity is mainly devoted to the themes of steel, composite steel -concrete, timber -concrete and timber structures. As regards the latter theme and in particular the existing timber structures, the research areas deal with traditional connections, mechanical characterisation, static and seismic behaviour, rehabilitation procedures as well as fi re resistance. Mariapaola Riggiois an architect and has a PhD in Structural Engineering. She is a member of the RILEM technical committee AST for the ' In-situ assessment of structural timber ' . She has carried out research activities in fi elds typical of structural rehabilitation, initially at the University of Calabria (UNICAL) and currently at the University of Trento. Her research and professional interests are mainly focused on the appraisal and restoration of existing structures. AbstractIn order to assess the safety of old timber structures and eventually to select appropriate strengthening strategies, the evaluation of mechanical properties of the material is of key importance. Visual strength-grading coupled with the use of nondestructive in situ testing allows these properties to be determined. Nowadays, the defi nition of both nondestructive testing (NDT) procedures and criteria for the results interpretation are almost exclusively in charge of the producer. In Italy, effort has been made to develop codes for grading timber on-site, and also by means of NDT. The aim of this paper is to highlight the diffi culties and shortcomings that still occur in the assessment of traditional timber members following the criteria established by the legislation in force in Italy. The presented results can be helpful for the building surveyor who is in charge of the inspection of existing timber structures. Indeed, the paper gives some indications on the effectiveness of the grading criteria applied on historical timbers, as well as on the level of accuracy of some nondestructive methods.

Inference on stiffness and strength of existing chestnut timber elements using Hierarchical Bayesian Probability Networks

“…Therefore, when no evidence is provided the results of bending strength are more influenced by the higher grade segments results as they represent a larger number within the sample considering all results. These results are also consistent with the consideration that clear wood has a higher influence on stiffness as it is mostly determined by average properties rather than by local weak sections, whereas bending strength depends mainly on the variation of the material properties and local defects [32]. The statistical parameters of the posterior distributions, with evidence in VI board , are presented in Table 2.…”

Application of imaging techniques for detection of defects, damage and decay in timber structures on-site

“…Knots often cause material discontinuity, because of the presence of checks in the knots themselves as well as in the surrounding material. A comparison of some European standards regarding timber visual grading highlights the differences concerning the way in which knots are accounted [2].…”

Inspection and condition assessment of existing timber structures

“…Natural defects in combination with deterioration of wood have a significant effect on mechanical properties of the wood. Deterioration caused by biotic attack affects the original quality of wood, not only because of the general decrease of density but also because of chemical alteration of the wood texture, as in the case of decay caused by rot [9]. Visual inspection has also definite limitations: inadequate or limited experience of personnel, problems with access to critical parts of the structure, random cross-checks, and inspection sometimes limited to the surface of the timber only, etc.…”

“…The time it takes for the wave to travel from one piezoelectric sensor to another situated on the opposite side of the timber element is used to calculate speed of ultrasonic waves. Longer times generally reveal the presence of defects, rotten wood or wood of lower density [9]. Stress wave techniques are also affected by other factors, including moisture content, wood species, and growth-ring orientation [9].…”

“…Longer times generally reveal the presence of defects, rotten wood or wood of lower density [9]. Stress wave techniques are also affected by other factors, including moisture content, wood species, and growth-ring orientation [9]. The speed of ultrasonic waves is directly correlated to the modulus of elasticity (MoE).…”