Improving the Analysis of Historic Wooden Structures through the Non-Destructive Testing of Connections

The restoration or rehabilitation of timber structures requires that the structural system be analyzed for determining load-bearing capacity. This analysis requires that properties of both the members and the connections be determined for accurate modelling. A number of non-destructive approaches have been developed to determine the structural properties of the wood. To date, little progress has been made in determining the structural properties of timber connections. A method based on analyzing the frequency of vibration of the member is presented. The theory, lab validation, and application to six historic structures are shown. The results from field testing provided a number of significant findings: First, the wide-spread assumption that timber joints have insignificant rotational stiffness (alpha < 1) is not valid. 70% of the 20 joints tested had stiffness values that classified them as semi-rigid (1 < alpha < 100). Second, the finding from lab work was confirmed: the ability to accurately predict the stiffness improves as the joint stiffness increases from pinned to semi-rigid (Percent error of 84% for alpha < 1, and 36% for 1 < alpha < 100). The results of the sample analysis using this information found that calculated stresses in members were reduced by up to 21%.