Advanced Materials Research Vol. 778

Abstract: Old timber structures represent an important portion of the World cultural heritage: wooden buildings materials and building techniques are part of our history and their conservation is an essential contribution to cultural diversity and global cultural wealth. In recent times, the methods and approaches used to assess and to maintain timber in historic buildings have evolved considerably and various techniques for timber repair and strengthening were developed and deeply investigated. A very common and widely used technique for the timber strengthening-repair is realized by inserting reinforcement materials in grooves cut in the original sound wood. The proposed reinforcement materials are various (fiber reinforced plastic materials, steel bar/s or plate/s, wood and wood products) and the grooves number and depth are variable. Several researches were conducted to determine the Modulus of Elasticity (MOE) of old timber members strengthened with the described technics. A common approach is to consider the strengthened timber beam as a composite one and the MOE of the wooden part correspondent to the original. In practice, the timber members work together with strengthened materials, and the strengthening intervention is planned taking into account both original timber and strengthening material MOEs. The basic assumption of this approach is that the slots executed along the timber length have no effect on the timber MOE. The aim of this research is to investigate the effects of the routed grooves cut along the wooden beams, on the bending MOE of old timber members, to provide important information for the strengthening interventions plan.

Abstract: Service life modelling of cracked timber beams can be performed using modified damage accumulation models that describe the combined effect or long term loads (mechanical) and biological of physical wood degradation. The combined model allows for the estimation of residual service life and an analysis of crack development. The model can also be used to analyse safety factors that may need to be applied. Also, a sensitivity analysis can be performed for future risks. It is shown that the failure risk is very sensitive to the level of the applied loads, similar to time to failure analysis of non-degraded timber. Failure in timber structures occurs within a very short time frame. A practical case of cracked glulam beams is included in the paper.

Abstract: This paper deals with the procedure followed in order to define a visual grading protocol to determine the mechanical properties of existing old pitch pine timber beams with large sections. The commercial name pitch pine comprises several species in the group of southern yellow pines, being the Longleaf pine or Georgia pitch pine (Pinus palustris) the most important. Old growth Pinus palustris, was very much appreciated for its rigidity, density, strength properties and natural durability. In Spain, many of the buildings constructed with this timber (named as pino tea) are nowadays under rehabilitation processes which, in some cases, will change the previous use of the building. All these interventions require understanding the mechanical properties of the timber. The structural properties of the aged pitch pine timber are unknown and there is not any European stress grading standard to enable the classification of the existing elements. For this reason, a total of 45 large section pitch pine pieces were visually analyzed and graded according to Spanish standard UNE EN 56.544 [. After this, the beams were tested according to UNE EN 408 [ in order to determine their modulus of rupture and flobal modulus of elasticity. All beams were tested keeping their original positions in the building and a new classification system for old beams is proposed based on a reduced number of chief visual grading parameters and taking into account their position into the beam.

Abstract: A special software simulating changes to wood due to various processes (either treatment or degradation) has been developed within the SWORFISH (Superb Wood Surface Finishing) project. The definition of the material modifications due to processes is based on the expert knowledge and/or experimental data. The dedicated algorithm simulates material modifications (with a special focus on surface) taking into account original material characteristics (evaluated by means of NDT techniques) and setting of process parameters. In this way, it is possible to analyze the sequence of processes (i.e. material modifications) and to estimate properties of the resulting product. Two case studies are presented for illustration of the potential uses of the SWORFISH approach in the field of timber structures.

Abstract: Timber constructions with glulam members have regularly to be proofed for their performance to avoid structural collapse. For the assessment of glued laminated timber, it is important to know reliable methods and criteria. The requirements given in standard EN 386:2001 are valid for the quality control of the glulam production. The use and application of these two different methods at existing timber structures were investigated and discussed. The experimental test series comprise different adhesives as well as specimen from new material and existing structures. Problems and issues noted during the test series and analyses of the results are discussed. The correlations found provide advice for the assessment of existing structures.

Abstract: The use of self-tapping screws with continuous threads to reinforce and join timbers in addition to glulam elements represents an effective, simple and economic method. The high withdrawal resistance of the screws as well as its continuous bond with the wood enables an effect similar to steel reinforcement in concrete structures. Within the framework of a research project, funded by the German Research Foundation (DFG) at the Chair of Structures and Structural Design in cooperation with the Institute for Building Material Research of the RWTH Aachen University, several tests have realized to investigate the bond behaviour, the force transfer and the anchorage length of the screws. Herewith the stress distribution will be determined with an optical 3D field measuring system, based on the digital image correlation (DIC) method, which measures the strain by observing the surface of the test specimens. To ensure the accuracy of the measurements for a wide measuring field of wood surface, a comparison study was conducted involving traditional electrical sensors (i.e. strain gauges and LVDTs). The results of this study, which confirm the accuracy regarding the determination of stress distribution for small deformations on the wood surface with optical 3D field measuring system, will be presented in this article.

Abstract: In this paper a new formalism based on the complementarity between the optical full field techniques and integral invariant Mtheta is proposed in order to evaluate the fracture parameters in cracked specimen made of wood, under mixed mode loadings. The coupling between the experimental and numerical approaches allows identifying the fracture parameters in terms of energy release rate without any the material elastic properties such as the elastic modulus and the Poissons ratio. The proposed formalism allows also determining, in addition with the fracture parameters, the local elastic properties in terms of reduced elastic compliance. The fracture mixed mode tests are realized using a Single Edge Notch sample made in Douglas with the Arcan fixtures and dried to 11% moisture content and the crack is cutting in Radial-Longitudinal system.

Abstract: Significant scientific work has been dedicated for exploration of the infrared applications within wood science and technology, not much has been done linking it to the mechanical testing. It is expected that due to mechanical stresses (and related deformations) the interaction between constitutive elements of wood changes, proportionally to the stresses applied. The response of the material to mechanical stresses (such as tensile loading) on the molecular level should be therefore detectable by means of infrared spectroscopy. Dedicated tests have been devoted for proving this hypothesis. Self-developed testing machine has been integrated with infrared spectrometer in order to conduct a series fully controlled mechanical tests. It was possible to predict stress level of wood during tension by applying proper chemometric analysis (partial least square (PLS) models). Implementation of infrared spectroscopy in to timber engineering and mechanical testing of wood provides very essential supplement to the typical information collected during standard tests. More additional tests and reference data is necessary in order to create more reliable and universal model suitable for routine assessments. Nevertheless, it was demonstrated that prediction of the stress on the basis of the infrared spectra is possible.

Abstract: The structural integrity due to problems in the uncertainties of load, the load-carrying capacity of timber structures is of great importance, since peaks of increased loads might occur. One solution of these problems lies in the evaluation of timber structures using non-destructive testing (NDT) methods, and in this special case frequency based identification methods. The aim of this paper was to investigate if it is possible to estimate the axial loads in timber beams using resonance frequency analysis to evaluate on whether the beams have sufficient load-bearing capacity. This was achieved by performing transversal frequency measurements on 32 timber specimens and an aluminium bar under tension. The latter hereby served as a homogeneous reference for better interpretation of results. The two first frequencies, together with different values for the E-modulus were then used to estimate the axial load S and the rotational stiffness k at the boundaries. The stress levels for the timber ranged from 2 MPa to 11 MPa, whereas for the aluminium reference bar the frequencies were measured for stress levels from 4 MPa to 32 MPa. The numerical model behind the calculations was based on Timoshenko beam theory, including effects of shear deformations and rotary inertia. Finally, a sensitivity analysis was carried out to investigate the influence of errors in input parameters on the final results. The best results were obtained using the E-modulus derived from transversal vibration tests and showed a mean error ranging from 7.6% to 46.6%, where the results generally improved for higher loads. The results of the sensitivity analysis showed that the sensitivity of the estimated axial load decreases for higher stress levels, which could also be observed in the test results. The most influential parameters on the quality of the results were the measured frequencies and the clear beam length, followed by the density and the E-modulus.

Abstract: This paper presents a novel approach for the determination of the loss of composite action for timber composite systems using only measurements from non-destructive vibration testing. Traditionally, the composite action of a system is evaluated from static load testing using deflection measurements. However, static load testing is expensive, time consuming and inappropriate for existing flooring systems. The method proposed in this paper is based on the Damage Index (DI) method, which uses changes in modal strain energies, to detect locations and severities of damage. In the proposed method, a new Loss of Composite Action Index (LCAI), which is derived from direct mode shape measurements obtained from dynamic testing, is introduced to evaluate the loss of composite action. The proposed method is tested and validated on numerical and experimental models of a timber composite beam structure, which consists of two timber components that are connected with different numbers of screws to simulate various degrees of partial composite states. The results obtained from the new method are very encouraging and show a clear trend of the proposed dynamic-based LCAI in indicating the loss of composite action in the investigated timber composite structure.