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The assessment of the mechanical and physical properties of in situ timber

Bather, Mike

Authors

Mike Bather



Abstract

The estimation of the properties of in situ timber elements is an essential part of the structural appraisal of many existing buildings and structures around the world. Current methods of doing this are (i) inappropriate, as they utilise visual grading codes of practice intended for use on large batches of new timber and not for use on individual pieces of timber, (ii) inaccurate, as the visual grading parameters used are only weakly correlated with timber’s mechanical and physical properties, and finally, (iii) imprecise, as they utilise strength classification, which groups all timber into a small number of classes with associated characteristic properties. Additionally, no current methods of NDT nor SDT adequately account for the immense variability of in situ timber in the UK to estimate properties in accordance with the Eurocodes. Therefore, better methods are needed.

This study (i) researches the contexts and background of in situ structural timber in the UK and (ii) takes a practical structural engineering approach to develop models that combine visual observations, NDT and SDT to estimate characteristic values of MoE, MoR and density, in a manner consistent with the Eurocodes and in a way that accounts for the variability of in situ timber. This exploratory study also demonstrates the ineffectiveness of the practice of treating knots as voids, as the resulting strength reduction factors are shown to be only very weakly correlated with MoR (regardless of whether the factors are derived from first principles or calculated using the US code ASTM D245).

Statistical techniques such as quantile regression (and bootstrapping to find the confidence intervals around quantiles) are applied to timber data in a novel way to develop new predictive models. New knot measures and ratios are developed with predictive powers superior to current measures and ratios. Significant factors such as selection bias and potential prior grading and the deterioration of wood during its life in service are considered and accounted for in novel ways in relation to the models. The study is based on a sample of new UK grown structural sized timber joists of four lesser used species (n=527) and so, further work is required to improve the models. The most useful outcomes of the study are (i) a methodology for the appraisal of the properties of in situ timber in accordance with the Eurocodes and (ii) an outline of what is required in the future to improve them.

Thesis Type Thesis
Deposit Date Nov 3, 2022
Publicly Available Date Nov 3, 2022
DOI https://doi.org/10.17869/ENU.2022.2947919
Public URL http://researchrepository.napier.ac.uk/Output/2947919
Award Date Jul 7, 2022

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