Thomas Lechner

Assessment of Density in Timber Using X-Ray Equipment

Wood density has a strong relationship with several mechanical properties, such as strength and stiffness. An x-ray image calibration procedure, which enables the determination of density properties for the in situ assessment of timber structures, has been developed. This nondestructive method is useful for evaluating the internal condition for global assessments of the structure. For this study, 14 wood specimens with significant density differences were exposed to x-rays. The specimens were set up in a number of image configurations. The images were analyzed and evaluated using an image toolbox. A similar procedure was applied to six built-up blocks with different thicknesses in order to calibrate the thickness of the specimens. Finally, a calibration wedge was set up to verify the procedure. The density calibration procedure from x-ray images was verified on a timber beam specimen, resulting in good agreement and an average accuracy of 97%. The values obtained from the image calibration presented a very good linear correlation between the measured density and the grayscale from x-ray images with coefficients of determination (R2) ranging from 0.90–0.98. The main advantages compared with conventional techniques are the detection and quantification of internal damage, defects, disturbances, and deterioration that may reduce the mechanical properties of the structure. This study shows good potential when it comes to the development of a viable tool for in situ assessments of timber structures. This technique could be used indirectly in analyses of structural behavior.

Strategy for developing a future support system for the Vasa warship and evaluating its mechanical properties

To maintain the integrity of the Vasa warship after salvage in 1961, conservation treatment with polyethylene glycol was carried out to prevent the collapse of cell walls. This treatment had negative effects on the strength and stiffness of the oak and the hull structure has been found to slowly deform over time. It is of interest to construct a three-dimensional numerical computer model to model and predict the deformation of the warship. This creates difficulties related to the complexity of measuring the detailed material properties that are required as input. In this context, a non-destructive methodology to predict the stiffness of the Vasa oak in terms of moduli of elasticity in the three principal directions of timber at critical positions in the ship would be useful. The twofold aim of the paper is to propose a strategy for a support system and to conduct an on-site assessment of the warship to predict the mechanical properties of the Vasa oak material. This paper also contains an up-to-date review of all essential mechanical data measured on the Vasa oak. The preliminary investigation using an X-ray technique to investigate the density properties produced promising results for future use in the evaluation of the mechanical performance. Based on these results, a procedure to establish the stiffness properties of the Vasa oak in terms of MOE was suggested, using a combination of data from previous measurements, in combination with extended tests on Vasa oak specimens and an X-ray-based density calibration procedure. The general complexity of the Vasa warship can be mainly attributed to large variations in the properties of Vasa oak due to surface degradation, chemical treatment and the disintegration of the cell-wall structure originating from centuries of waterlogged conditions. That causes difficulties when assessing mechanical and physical properties on a structural level. A combination of visual inspection together with X-ray investigation is of great importance to evaluate those properties and to obtain more accurate estimates. The results from evaluating mechanical properties can serve as input in a numerical model and serve as a foundation for decision-making relating to the modification of the support system.

NON-DESTRUCTIVE TESTING OF THE HISTORIC TIMBER ROOF STRUCTURES OF THE NATIONAL MUSEUM IN STOCKHOLM, SWEDEN

The National Museum in Stockholm is Sweden’s leading museum of art and design. Behind the now closed doors of the National Museum and its construction coverings a renovation project is taking place, which began in 2014 and will be completed in 2018. The renovation project will create a modern museum, a brighter atmosphere for the arts and for its visitors. Part of the renovation project was to de-sign and upgrade the timber roof structure for new loads according to Eurocode 5. As the roof structure was originally built in 1860, it was important to evaluate the condition and mechanical properties of the original load-bearing timber members. The roof structure, which is of main interest, comprises roof trusses and lantern structures.The main aim of the project regarding the timber roof structure in the National Museum was a safety verification in both the ultimate state (ULS) and the serviceability limit state (SLS). The preliminary capacity calculations for the roof structure showed that the roof members were under the new load conditions (with increased loads due to security and environmental requirements) utilized above 100%. Therefore, an investigation into the properties of the old timber members and connections was of vital importance. The need for non-destructive testing (NDT) of structural timber is well known under the name of strength grading and the concept has been used for many years to classify timber with respect to mechanical performance. The stress wave technique used in the project was based on commercial instruments such as FAKOPP®. The timber quality investigation for the attic roof structures showed that the material was generally in very good condition. As a result, according to SS-EN 338-2009, the timber strength could be assigned strength class C27–C30. Based on the results of the investigation, further design of strengthening works and refurbishment of the roof structure were being undertaken

Stress Level Prediction in Axially-Loaded Timber Beams Using Resonance Frequency Analysis: A Pilot Study

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.