Jorge M. Branco

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.

Structural grades of timber by bending and compression tests

This paper presents the results obtained in a series of tests on Pinus Pinaster Ait. timber specimens, using the prEN408:2000, to estimate the local and global Young’s modulus and strength both in bending and compression parallel to the grain. The results obtained are compared with the values presented in the Portuguese Nationally Determined Parameters of Eurocode 5, for the quality classes assign by Portuguese Standard NP4305:1994 by visual grading.

In-plane stiffness of timber floors strengthened with CLT

Five full-scale timber floors were tested in order to analyse the in-plane behaviour of these structural systems. The main objective was an assessment of the effectiveness of in-plane strengthening using cross-laminated timber (CLT). To this end, one unstrengthened specimen (original), one specimen strengthened with a second layer of floorboards, two specimens strengthened with three CLT panels, and one specimen strengthened with two CLT panels, were tested. A numerical analysis was then performed in order to analyse the composite behaviour of the timber floors in more detail. Due to its importance as regards composite behaviour, the first phase of the experimental programme was composed of push-out tests on specimens representing the shear connection between the timber beams and the CLT panels. This paper describes the tests performed and the numerical modelling applied to evaluate the composite behaviour of the strengthened timber floors. The use of CLT panels is revealed to be an effective way to increase the in-plane stiffness of timber floors, through which the behaviour of the composite structure can be significantly changed, depending on the connection applied, or modified as required.

Analysis and Strengthening of Timber Floors and Roofs

In many countries, traditional construction comprises floor and roof systems in wood. Current knowledge assumes the need to preserve and to protect existing wood systems as a cultural value with important advantages to the overall behaviour of the building. In this chapter, after a description of the most common systems used in traditional wooden floors and roofs, strengthening techniques are presented. For that, the analysis and design of the existing structures is discussed and the effectiveness of the strengthening techniques is evaluated through experimental results obtained in laboratory.

In-Plane Stiffness of Traditional Timber Floors Strengthened with CLT

Five full-scale timber floors were tested in order to analyze the in-plane behaviour of these structural systems. The main objective was to assess the effectiveness of the in-plane strengthening using cross laminated timber (CLT). For that, one unstrengthened specimen (original), one specimen strengthened with a second wood board, two specimens strengthened with 3 CLT panels and one specimen strengthened with 2 CLT panels were tested. Moreover, because of its importance in the composite behaviour, the first phase of the experimental program was composed by push-out tests on specimens representing the shear connection between the timber beams and the CLT panels. This paper describes the tests performed and the numerical modelling aimed to evaluate the composite behaviour of the strengthened timber floors.

Single step joint: overview of European standardized approaches and experimentations

In the field of Built Heritage Restoration, engineers have to work with old structures made of poorly preserved timber elements. The assessment of timber elements and connections is a major issue for engineers involved in a restoration project. Before thinking about any intervention techniques, engineers have to properly understand how the carpentry connections fail, which parameters influence the failure modes (e.g. geometry of the joint, mechanical properties of the wood) and how the internal forces are distributed into the joint to finally figure out how to design the traditional carpentry connections. The present paper aims at raising those questions focusing on the Single Step Joint (SSJ) design. Even if this common joint between the rafter and the tie beam is geometrically simple, one may pick up three SSJ families from the past till today: the Geometrical Configuration Ideal Design, the Geometrical Configuration Perpendicular to the Tie Beam and the Geometrical Configuration Perpendicular to the Rafter. The first one is more recent because its geometry requires accurate timber cutting, using new technologies (e.g. Computer Numerical Control). For each one, some general design rules about the SSJ geometrical parameters are defined by some European standards (e.g. Eurocode 5 in Design of timber structures—part 1‐1: general—common rules and rules for buildings. CEN, European Standardisation Institute, Brussels, 8) or authors (e.g. Siem and Jorissen in Shatis’15: 3rd international conference on structural health assessment of timber structure, vol 1. Wroclaw, Poland, 9–11 Sept 2015, 11), but no detail is available on how to design this connection in order to prevent the shear crack at the heel depth in the tie beam, or the compressive crushing at the front-notch surface. Hence the design rules and the emergence of failure modes must be defined according to the SSJ geometrical parameters. In order to check the design equations and the failure modes, lab tests on the three SSJ families have been carried out, modifying the heel depth, the shear length and the inclination of the rafter.

Load-Bearing Capacity of Traditional Dovetail Carpentry Joints with and Without Dowels: Comparison of Experimental and Analytical Results

Post-disaster field studies widely suggest that historical timber structures are seismically resistant, and a growing number of experimental studies support this observation. The joints between structural members, which are the major energy dissipation mechanism within the structure, play a crucial role in the overall robustness and the way that a structure handles the seismic demand. Joints mostly fail when the timber members are still in the elastic range, therefore a thorough understanding of their behaviour under various loading schemes is of utmost importance to gain deeper insight about the overall structural performance of timber structures. This paper summarizes the findings from a series of testing carried out on dovetail joints, which is one of the most common traditional carpentry joints, during the 5th COST FP 1101 Training School, held in University of Minho, Portugal. Within this framework, a dovetail joint (with and without dowel) was tested under compression and tension. The experimentally obtained load-bearing capacity of the joints was then compared to the capacity values calculated using analytical models, and the failure modes were further discussed. The results showed that the experimentally obtained capacity values can be successfully reproduced by analytical models for dovetail joints without dowel. On the other hand, the capacity of a dovetail joint with dowel under compression or tension is always underestimated by analytical models.

Seismic Analysis of a 2-Storey Log House

The current paper deals with the analysis of the results yielded by a series of tests performed to evaluate the seismic behaviour of a model log construction. The study was based on an experimental investigation performed to improve the existing knowledge on log houses subject to seismic events. The main part of the experimental work is based on a full scale shaking table test, conducted on a two-storey log house designed by the Portuguese company Rusticasa® in compliance with design rules for timber buildings. The test was performed by the University of Minho within the framework of the SERIES Project Multi-storey timber buildings and was coordinated by the University of Trento, at LNEC, Lisbon, Portugal. The geometry of the specimen, the design of the test, the setup and the instrumentation layout are first presented in this paper. The test procedure was conducted in stages with maximum accelerations (bi-directional) of 0.07g, 0.28g and 0.5g. During this incremental test procedure, whenever damage occurred, identification tests were performed to assess any variation in the fundamental period of the house. The experimental results of each test have been analyzed and the resultant values of inter-storey drift, wall slippage and uplift measurements, shear deformations and hold-down forces measured are presented. Most importantly, the dynamic properties (fundamental period and mode shapes) of the system have been determined.

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.