Improving the Assessment of In Situ Timber Members with the Use of Nondestructive and Semi-Destructive Testing Techniques

Abstract

Common practice in evaluating existing buildings and their in situ timber members is in situ grading, where structural grades and the associated design values are assigned based on a visual inspection of the member and current grading rules and standards. This can be useful in that it provides a design value to architects and engineers that can be used in design and calculation, however, there are some drawbacks. Published design values are representative of entire species or species groups, not individual members, they are based on the testing of second generation growth, and they may be conservative to account for the natural variability of wood. Additionally, deterioration can be difficult to locate and even harder to quantify through visual inspection.

As a result of this practice, member strengths are typically underestimated which can cause unnecessary remedial work or member removal. To improve the assessment of in situ timber members, nondestructive and semi-destructive testing should be used to arrive at more appropriate strength values for individual members, as well as to locate and quantify the presence of deterioration.

Five nondestructive and semi-destructive testing techniques were researched including stress wave, radiography, resistance drilling, core-drilling and tension micro-sampling, of which all but stress waves were tested. A two part experiment was perform to assess the ability of these techniques to either gain a better estimate of in situ timber strength or detect and quantify deterioration.

Findings showed that core-drilling could substantially improve the estimate of in situ member strength over the practice of assigning design values based on current standards and grading rules. Research on stress wave and experimentation of tensile micro-specimen sampling showed that further research and improvement to the techniques needs to be done before these methods can be used to reliably estimate member strength.

X-ray investigation and resistance drilling both proved their ability to locate deterioration, and resistance drilling gave reasonably accurate quantification of void presence. Although not tested during experimentation, research showed that stress wave investigation may be useful for the detection of deterioration.