Integrated Buildings & Structures

Does anyone have experience using Stress Wave Timing for NDT inspection of timber structures. I learned about this at an ASCE 2-day seminar on structural condition assessments. The procedure is outlined in "Wood and Timber Condition Assessment Manual - Second Edition" by USDA.

1. Are there any case studies you found useful? 
2. I would like to know if you found the Fakopp device to be durable and reliable in the field.  Are there any maintenance issues you have experienced?
3. Do you think other equipment is better?
4. I would also be interested in any other thoughts you have on this method.

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Darrell Staaleson P.E., M.ASCE
Principal
Staaleson Engineering, P.C.
Covington WA
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The subject of Condition Assessment  (CA) of existing wood/timber structures is indeed very interesting and the steps to do the CA very challenging. In doing a search in the web one could find tons of information, but there I found a Presentation of CA of Wood by Dr. Robert J. Ross of USDA Forest Products Laboratory where some Case Studies are discussed at the end and other pertinent information presented and briefly discussed. Below the link to download the presentation:

https://www.wisconsinhistory.org/pdfs/hp/2014conference/Condition-Assessment-Wood.pdf
Perhaps some engineers that may had some direct experience on actual projects could shed some light and feedback on the four key items-questions noted above, it would be interesting to know some practical approach to address the NDT of existing wood/timber structures .... Hope this could spark some reaction from professionals that may share some practical guides into the subject ... Looking forward to hear more on this .....

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Pedro R. Munoz Ph.D., P.E., Archineer
Principal Engineer
PRM Engineering, LLC
Methuen MA
(978) 738-8001
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Hi Darrell,

Many of the NDT for timber structural assessment are useful where there is significant damage that is not visible. Unfortunately, for practicing engineers, these techniques are usually not very useful. Many owners want to know if there is any damage that can be proactively treated. Wood members have a high variablility (Coefficient of Variation, COV), on the order of 15% or more. So when we try to do Stress Wave Timing, we may also get a wide variability of readings from timber to timber, with 15% COV on top of any instrument variability. For the practicing engineer, to not know the true condition of the wood within say 20% of the NDS value is a major problem. These test rely on the MOE of the material showing some degradation. Decay is either small holes relative to the size of the member or a reduction in the cellulose content of the cell walls. A 5% degradation is hard to assess because the surrounding sound wood will still transmit the stress wave. Only when degradation is substantial, do these techniques show a reduction in MOE. By this point, the owner and engineer usually see other signs of problems such as large deflections or pre-buckling behavior. So to date, I have not found them useful in practice.

There is a regular Forest Products Society symposium on NDT: ndtesymposium.org

You may be able to obtain the proceedings from a local university library.

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William Kirkham Ph.D. Ph.D., P.E., M.ASCE
Director MCE Program
The University of Kansas
Overland Park KS
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