Determination of the strength properties of wood in the inspection of buildings and structures: methods, problems and solutions

. The paper considers the ways and methods of determining the strength of wood according to the current norms and rules. The required number of wood samples are tested and compared with the strength characteristics in SP 64.13330.2020 "Wood structures". It is concluded that it is impossible to use the obtained results for calculations of building structures, due to the lack of coefficients in the norms, taking into account the vices of wood and the scale factor. The existing methods of nondestructive control of wood were analyzed. The tests were carried out and the dependences between the speed of sound along and across the fibers and the strength of the samples were plotted. The conclusions about the possibility of using the ultrasonic method to determine the strength of wood in the examination of buildings are made.


Destructive method for determining the strength properties of wood
When examining buildings and structures in accordance with GOST 31937-2011 [1,2] wood strength must be determined by the destructive method.
The destructive method of determining the strength properties of wood is the most accurate and quite time-consuming. The essence of the method is as follows: samples of wood without defects are taken from the examined structure. The size and quantity of samples is regulated by GOST16483.10-73 [3] and GOST16483.3-84 [4].
For the test in compression on [3] from the examined structure necessary to cut out specimens-prisms section of 20x20 mm and a height of 30 mm. The minimum number of specimens is 36.
For the bending test according to [4] it is necessary to cut out specimens-beams with cross-section of 20x20 mm and length of 300 mm from the surveyed structure. The minimum number of specimens is 36.
Testing of wood specimens for each type of stress state is a one-factor experiment, where only the load is a variable. The number of tests and their reliability are set by the statistical assurance of 0.95. The results are processed according to the following algorithm: Average strength of samples in the batch (1) ( The accuracy index must be no more than 5%. The short-term wood resistance of clean samples at the static assurance of 0.95 is determined: The short-term resistance of the wood of natural sections: Normative resistance of wood taking into account the long-term action of loads: Calculation resistance of wood: t -multipliers that depend on the confidence level (confidence probability) adopted and the type of the distribution density function; kп -is a transient coefficient that takes into account the effect of vices on the strength of the wood; kмtransient coefficient, taking into account the influence of the cross-sectional dimensions on the strength of the wood; kдл -coefficient, taking into account the long-term action of loads.
As an example, let's test the compression of the prisms and conduct the processing of the results ( Table 1).
As can be seen from the  (Table 2). Then, according to the criteria for which their origin is not explained in the normative documentation, in Table B.1 [1] the transition to the strength depending on the grade of wood is made, and in Table 3 [1] the scale factor is taken into account. However, for the examined wooden structures, the question of determining the true strength has not been solved. Finding the coefficients kп and kм by proportional ratios from Table 3 and B.1 is also not a solution to the problem.  Conclusion: It is proposed to use Note 3 from

Non-destructive methods for determining the strength properties of wood
The use of nondestructive method to determine the strength properties of wood is not regulated by any regulatory document in force in the Russian Federation. However, when examining buildings, it is not always possible and convenient to perform sampling from the body of the structure. The most embarrassing factors are: the tight arrangement of equipment, the inability to stop the production process during the inspection of the building. Also during the inspection of buildings of cultural and historical heritage is not always possible to perform sampling, especially in the amount specified in [3,4]. In this case, non-destructive testing methods can help the expert.
Determining the strength of wood by nondestructive methods is carried out by analyzing the main characteristics of its macrostructure and density. Such a characteristic as density has a sufficiently close relationship with the strength indicators under load for wood without flaws. But, as we know, some blemishes increase the density of wood but reduce its strength. Consequently, preference is given to those types of nondestructive tests that reflect the relationship of strength with the stiffness values of wood (modulus of elasticity) [6].
Together we can distinguish the following main types of control: radiation, acoustic, magnetic, eddy current, radio wave, electrical, optical, thermal, with penetrating substances [7].
It is worth allocating to a separate group of mechanical methods of determination of strength. Depending on the characteristics to be determined, they are divided into the method of measuring plastic deformations (also known as A.H. Pevtsov's method) and methods of local failure assessment (fire method, needle immersion method and E.E. Gibshman and V.G. Donchenko method, associated with drilling the structure).
The main methods available to most organizations for the inspection of buildings and structures can be called the method of microdrilling of wood and ultrasonic (acoustic) method.

Microdrilling method of wood
The method of structural drilling is based on the drilling of wood with thin drill bits (small diameter of the cutting part, long shank), operational determination of the energy-power or speed parameters of the drilling process, potentially correlating with the density, deformability, mechanical and technological properties of wood or wood-based materials.
Due to the use of thin drill bits that allow little impact on the workpiece, the drilling resistance measurement method belongs to the group of minimally invasive.
The drilling resistance measurement method can be recommended as a quick nondestructive method for determining density and modulus of elasticity, as well as for predicting the static bending strength [8].
A significant error in determining the physical and mechanical characteristics of wood is introduced by the feed rate of the drill bit, humidity and wood species. It is worth noting the high cost of the equipment and the need for retraining personnel to work with the equipment.

Ultrasonic method
The method is based on the correlation between the speed of ultrasonic pulse wave passage through the structure and the compressive strength of wood [7].
Due to the pronounced elastic anisotropy of wood, the velocities of longitudinal waves strongly depend on the direction of their propagation relative to the fibers of the material. Due to the high attenuation of elastic waves in wood, the attenuation coefficient at a frequency of 0.5 MHz is about 40 dB/m, which is much higher than for reinforced concrete.
According to the results of field tests 150 kHz is the optimal frequency of the sound wave [7].

Construction of the relationship between sound velocity and compressive strength for wood
For testing and construction of dependence of strength to speed of sound the device of ultrasonic control of concrete strength (Pulsar 2.1. (manufacturer "InterPribor"), and test presses with a maximum compression load of 500 kN and 1250 kN were used.
Initially, it is necessary to determine the speed of sound transmission through the studied samples at different moisture content: natural humidity, humidity at exposure to water for 7 and 28 days, humidity after drying cabinet (0 %).
According to the requirements [3,4] to determine the compressive strength of wood along and across the wood it is necessary to use specimens with a cross section of 20x20 mm and a height of 30 mm.
When determining the speed of sound along the fibers, a constant speed of sound was observed regardless of the moisture content of the samples. According to the results of gradual increase of the length of the tested sample, the minimum possible value for the frequency of the sound wave used by the device Pulsar 2.1 was obtained. The minimum length when the sound passes along the wood fibers is 60 mm.
To comply with the conventional ratio between the length and cross sectional area of specimens -1.5, the cross sectional area for testing along the fibers is increased to 40x40 mm.
Compression tests of the specimens were performed at 0% moisture content, in order to accurately observe the physical and mechanical properties of wood during the entire experiment.
Test results of specimens along the fibers are shown in Table 3, Table 4 presents sorting of specimens by decreasing speed of sound and by decreasing compressive strength along the fibers.
The results of tests across the fibers are shown in Table 5, Table 6 presents the sorting of specimens by decreasing speed of sound and by decreasing compressive strength across the fibers. As can be seen from the test results, there is a correlation between sound velocity and strength both along and across fibers.
The test error is about 1% for wood along and across the fibers. Figures 1, 2 show the linear dependence of strength on the speed of sound for wood with a moisture content of 0% along and across the fibers, respectively.  The following conclusions can be drawn from the test results: 1) Application of ultrasonic method allows to determine with high accuracy the compressive strength of wood both along and across fibers; 2) Application of ultrasonic method with correlation of values between speed of sound and strength of specimen at field tests is incorrect, as it is necessary to enter conversion factors between speed of sound along and across fibers; 3) It is necessary to build a graduation relation between the speed of sound and the strain modulus of wood, as a consequence of which it is possible to estimate the strength characteristics of wood more correctly.
4) It is possible to apply in practice the constructed graduation dependences only at control of humidity of the tested wooden element; 5) The surface of the wooden element to be tested must be prepared for the tests; 6) The minimum measurement base along the fibers is 30 mm, along the fibers -60 mm; 7) Using only ultrasonic method to determine the strength of wood does not take into account possible wood defects hidden inside the wooden element.

Conclusion
The destructive and nondestructive methods of determining the strength properties of wood are considered. The existence of only non-destructive methods at the moment is impossible, since it is necessary to build graduation dependences between the strength and the results of destructive control.
As a result of the research the following conclusions are obtained: 1) It is proposed to use Note 3 from Table B.1 [1] (Table 2) and take the strength of wood not by the results of wood testing, but by Table 3 [1] in accordance with the visual evaluation of wood by grade attributes according to GOST 8486-86 [5].
2) Ultrasonic method of determining the strength properties of wood allows in the field to obtain data on the condition of the examined structure; 3) Application of the ultrasonic method is limited not only by the quality of surface preparation of the examined structure, dirt, roughness, finish coating introduce a significant error in the test results, but also by the size of the cross-section of the samples; In the future, additional research is needed in the field of application of the ultrasonic method to determine the strength of wood.