The Wood NDT laboratory of the University of Sopron has developed a technique for the evaluation of old, historical wooden structures, where the residual bending strength of individual wooden beams are estimated with +/- 9 MPa accuracy. The strength predictor parameters are stress wave velocity (V) and screw withdrawal resistance (Fscrew). The next figure shows the good correlation between the strength predictor and the actual strength of full size specimens.
Correlation between MOR and strength predictor. The and 
represents soft and hard wood specimens.
The figure above shows the setup for stress wave determination using FAKOPP stress wave timer. The stress wave is induced by a hammer impact. The read out of the machine is the transit time in microsecond. FAKOPP is designed for foresters, but working well in
the evaluation of old wooden structures.
In-situ determination of the stress wave velocity.
The fundamental strength predictor parameter is the MOE. The dynamic MOE determined by rV^2 term, where r is density, V is stress wave velocity. Our strength predictor is given by a similar term: FscrewV^2.
The stress wave velocity determination by FAKOPP microsecond timer
Determination of the screw withdrawal force (Fscrew).
As an example of our results, the strength distribution over a 214 years old ceiling structure (a system of doweled, closely aligned beams) is shown on the last figure. Material is larch. Numbers represents the residual bending strength in MPa unit.
The strength distribution over a wooden ceiling.
If you have any question regarding the in-situ evaluation of wooden structures or the applied equipment, please do not hesitate, contact us:
Ferenc Divos and Laszlo Nemeth, e-mail: divos@fmk.nyme.hu
University of Sopron, H-9401 Sopron, P.O.Box:132
FAX: +36-99-311 103 or +36 99 33 00 99
