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WOOD STRUCTURE ASSESSMENT
Testing program for assessing the effectiveness of micro-drilling used to determine the soundness of wood structural members: this article describes the Incodo testing program used to study the reliability of micro-drilling in the measurement of the condition of wood timbers, beams, studs, etc. This article series describes the various methods used to test & evaluate the structural integrity of wood-framed buildings. The original authors, Probett et als., propose a technique to provide objective pass-fail data on the condition of in-situ but suspect structural wood using micro drilling to examine wood density.
Also see our insect damage & wood-destroying insect type identification articles beginning at INSECT INFESTATION / DAMAGE and STRUCTURAL DAMAGE PROBING and see TIMBER FRAMING, ROT where we describe the relationship between construction details and the occurrence of structural rot in timber frame buildings.
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Paul Probett, Clinton Craig, Blake Probett, Incodo Forensic Building Specialists 
This article series on methods for assessing structural wood rot & damage is adapted & expanded from the author's "An Introduction to Micro-Drilling Technology for N. Z. Structural Timber Assessment" and is used with permission. We [DF] have added comments, some illustrations, and additional article citations.
At the references section we include a link to the original article as well as contact information for the authors and Incodo Ltd., a Tauranga, New Zealand forensic engineering firm.
[Click to enlarge any image or illustration]
Our discussion of micro-drilling is divided into three segments. Below is segment 3.
Description of the Incodo Testing Program for Micro-Drilling of Wood Structural Beams & Timbers
Phase 1: Initial set up (completed)
Determining consistency of results using sound timber and establishing preliminary variable settings for software to determine Pass/Warning/Fail, Quality Infraction levels and identification of decay type voids.
Phase 2: Site testing and integration into report template (completed)
Initial site testing on buildings with wall framing exposed as part of remediation or demolition works. Three buildings – all in Auckland
Build larger database of sound timber graphic levels to use as controls / base levels.
Phase 3: Outside consultant and further field testing (completed but ongoing consultation)
Consult ex FRI staff and engineers as to draft reports and use of terms.
Determine further testing requirements.
Phase 4: Effect of moisture on consistency and reliability of test results (Completed – report at draft stage as at 05/12)
Assessment of impact of varied moisture levels on microdrilling results.
Undertake in-house testing using resistance moisture meter, H3.1 framing (to ensure no internal decay to skew results), Contherm drying oven, etc. Limited to 4 samples dried from 88-100%+MC to 12-14% Results (currently being tabulated)
Preliminary indications are that moisture level effects are limited and do not impact on micro-drilling assessments sufficiently to be of concern. (likely reason is considered to be that drill readings are a resistance reading and this is a combination reading of lateral penetrative resistance as well as shear loads required by the bit tip to slice through timber fibres at various angles to fibre orientation eg shearing occurs parallel and at random angles to timber fibre direction )
Sample 2 graph at left, at 88.0% Moisture Content (MC) indicated
and next graph (below) at indicated 14.4%MC.
Readings taken from new SG8 pinus radiata using Protimeter resistance meter.
Readings taken from points at 20mm centres, along plane of timber grain to ensure minimal impact to test in regard to variance of earlywood and latewood.
Net result indicates minimal graphical difference values between high and EMC (equilibrium moisture content) approximated moisture levels.
Phase 5: Fractometer comparison with micro-drilling including relationship to compliance documents. (Equipment sourced)
Comparison between test results and timber testing properties in accord with NZS: 3622:2004.
Further testing involving taking samples from critical points in sample(s) identified by test graphs, where there is a transition from sound to decayed wood and determining the degree of correlation to timber strength when tested for compression, bending and tension using an IML Fractometer Mark
Programmed testing also involves the same methodology being applied to other timber species as samples become available and demand and demand for micro-drilling testing is required.
Specific testing involves has involved sourcing dendochronological level dry wood core drills capable of extracting parallel to grain cores to test bending stiffness.
Phase 6: Microbiological comparison with micro-drill test results in timber transitioning from sound to decayed. (Possible test methodology)
Microbiological comparison of samples and phase 5 results by independent laboratory such as Biodet.
NOTE: The value of this testing is being considered and is not currently scheduled.
Arguably the decision to retain or replace building elements in leaky homes should be driven by timber meeting or failing the requirements of NZS:3602, :3603, :3604 and :3622. These are “strength” related criteria – not decay presence criteria.
As micro-drilling is considered to evaluate strength – it becomes a moot point whether decay impact is a Building Consent / Safe & Sanitary criteria that warrants consideration.
[Click to enlarge any image]
Argument in simple terms is - if the timber meets Building Code requirements, that is all that is required to meet for compliance.
This is the same for supplied timber for remediation which is tested for strength only.
Presence of incipient, surface or minor decay is irrelevant to decisions to retain or replace as effective remediation practice should ensure future moisture levels remain at or below 18% at which level any remaining timber decay fungi reportedly cannot thrive.
Current best practice remediation also includes in situ preservative treatment and removal of decayed timber which severely limits opportunities for framing to become structurally non-compliant at a later date. (If this was not the case complete exterior frame replacement should be the norm)
Phase 7: Blind test of hidden framing (Possible Test)
Assess building ready for extensive remediation and reclad using NDT moisture meter to help determine focus points from inside only and using micro-drill to test largely through external wall skirtings and or window liners etc
Note drill used in concert with metal detector/ stud finder for safety etc
Compare report(s) with actuals after cladding removed
Watch out: for moisture meter limitations: we [DF] have tested and demonstrated that while moisture meters are an invaluable tool in assessing building conditions, none of the moisture meter designs presently on the market can reliably detect all wet or leak in conditions in all sorts of building structures and cavities.
Note: Above information is copyright, commercially sensitive and is yet to be peer reviewed. It is not to be copied or circulated without the express and written permission of Incodo Limited. Adapted here by InspectaPedia.com, with permission from Incodo 8/5/2012, technical review is in process.
V4a 06/12 - ####
Author follow-up 10/8/2013
We’ve been playing with our Resistograph a little more and it’s; firmware has been updated. Have also started using a digital Fractometer manufactured by IML as well and developed some software in house that integrates everything (an example is on our www.flameback.net site)
The main advantage of this integration is that when microdrilling and Fractometer testing are done closely together on the same plane of a piece of timber the resultant graphic profiles are very similar so we have two very different test methods crosschecking each other – the key inference is that microdrills are profiling, but not measuring compressive strength parallel to grain, but a digital Fractometer can supply actual compressive strength measurements.
... this is a new methodology that advances timber testing from qualitive and opinion based to quantitive and measurement based enabling checking against code compliance documents. [1a][1b][1c] [Italics ours - Ed.]
We are using it regularly here for government agencies and other consultants and developing modules to determine density and assess other timber strength/ performance indicators.
Continue reading this article at WOOD BEAM ROT ASSESS: MOISTURE METERS or select a topic from the More Reading links shown below.
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