For the assessment of building components, it is preferable to use non-destructive or less destructive or less destructive to the building fabric. For example, moisture measurements on the building site are carried out as standard using capacitive, resistance or microwave measurement methods. In contrast to the carbide and Darr method, no destructive intervention takes place here. The disadvantages of the non-destructive measuring methods are only the indirect statement about the moisture content and the interference factors influencing the measurement. In the case of timber components, the question often arises as to whether the strength or load-bearing capacity is impaired due to infestation by wood-destroying fungi and insects. Particularly in areas that are not or hardly visible or difficult to access, the examination by means of drilling resistance measurement offers a measurement method with comparatively high information content that interferes little with the building fabric. As with all tests, the measurement result only represents a random sample and is only meaningful for the area tested.
The procedure
In the type of device used by the author, a drilling needle is driven in, which has a diameter of approx. has a diameter of about 1.5 mm at the shaft and a width of about 3.0 mm at the tip. The needle is driven by two motors which feed the needle and rotate it.
the rotation of the needle. The electrical power consumption required for this (for feed and rotation) is measured and stored.
The values obtained can be read out using special software and displayed graphically. The power consumption of the motors correlates with the strength of the drilled wood. This allows defects in the wood, whether cracks, cavities or infestation by wood-destroying fungi and insects, to be detected and visualized. With increasing drilling depth, the drilling resistance is also influenced by the so-called shaft friction due to jammed wood chips (more so with hardwood than with softwood). The advantage of the additional registration of the feed force is that it is hardly influenced by the shaft friction and thus direct information about areas with reduced strength is obtained. The drilling resistance measurement is used both for tree inspection and for checking installed timbers. For static evaluations, an estimate is obtained of what proportion of the cross-section of a timber (or tree) has the usual strength or is damaged.
Description of measurement examples
Various measurement curves are shown and described below as examples (source for photos and measurement curves: Dr. K. Geith). The brown-filled curves represent the drilling resistance, the blue-filled curves the feed force. The drilling depth in cm is shown on the x-axis and the amplitude of drilling resistance and feed force on the y-axis. The diagrams are to be read from right to left, as the needle is inserted at a drilling depth of 0 cm. Graph 1 shows the diagram for a piece of wood with normal, unaffected strength throughout. The numerous, narrow peaks result from the natural differences in strength between less dense earlywood and denser latewood within an annual ring. The combination of a peak with a “valley” thus represents a growth ring. In the area between about 7.5 cm and 10 cm drilling depth, the growth rings are wider. Here, growth rings of juvenile, near-marrow wood were probably drilled into.
Figure 1
Drilling resistance curve of a timber intact over the entire cross-section. Late and early wood of the growth rings can be easily recognized by the narrow peaks and “valleys”. Similar ratios are shown in Figure 2. This is also an intact wood, in which the pith in the middle, which naturally has a significantly lower strength, was probably hit by chance.

Figure 2
Drilling resistance curve of an intact piece of wood. It is very likely that the pith, which naturally has a significantly reduced strength, was hit.

Diagram 3 shows the result of drilling a hole in wood infested with house longhorn beetle. This wood-destroying insect only attacks the sapwood of softwoods. The maximum possible damage and impairment of the load-bearing capacity of the wood infested by the house longhorn borer essentially depends on the proportion of sapwood. This can be very different and individual for each timber, depending on whether a timber was cut from the top of a tree with a high proportion of sapwood, for example, or rather from the area of the trunk close to the ground with a high proportion of heartwood or mature wood. The measurement result shown in Figure 3 shows that the infestation is only present in the areas of the examined wood close to the surface. In contrast, Figure 1 and Figure 4 show a beam severely damaged by house longhorn beetle infestation.
Figure 3
Drilling resistance curve of a timber damaged on both sides near the surface by house longhorn beetle infestation.

Picture 1
Example of a beam severely damaged by house longhorn beetle infestation.

Figure 4
Drilling resistance curve of a beam severely damaged by house longhorn beetle infestation.

Time and again, timbers appear healthy on the outside when visually inspected (Fig. 2), while on the inside there is massive damage caused by so-called internal rot (Fig. 5). This is often caused by fungi from the Gloeophyllum genus. As these fungi are very tolerant of changing temperatures and humidity levels, playground equipment, carports, conservatories, windows and maypoles, for example, are affected (Figure 3 and Figure 6).
Picture 2
Example of a beam that appears to be intact on the outside, but shows massive damage due to internal rot (see diagram 5).

Figure 5
Typical drilling resistance curve of wood with internal rot (Fig. 2).

Picture 3
Example of a maypole infested by a leaf blight. The majority of the cross-section of the wood is damaged (Figure 6).

Figure 6
Evaluation of a drilling resistance measurement in the area of a maypole close to the ground (Fig. 3). There is a reduced strength over the majority of the wood cross-section.

Even more obvious damage to a beam is shown in Picture 4 and Figure 7. Here, only the area near the surface on one side of the wood is still intact, the rest of the cross-section is completely destroyed. In this case, there was an infestation by the spread house fungus (Donkioporia expansa).
Figure 4
Drilling resistance measurement (arrow) on a chair column. See diagram 7 for the result of the measurement.

Figure 7
Wood that appears to be intact on one visible side, but is completely destroyed from a depth of approx. 3 cm.

Another task is shown in Figures 8 and 9. Here, the mortise and tenon joints of the rafter feet of the roof structure of a church tower were to be checked by means of drilling resistance measurements. Figure 8 shows the evaluation of a drill hole on an intact mortise and tenon joint. The tenon (drilling depth 6.8 cm to 11.4 cm) and the cavities between the tenon and the adjacent timber are clearly visible. In contrast, the drill hole shown in Figure 9 shows a significantly reduced strength of the tenon, caused by an infestation of a wood-destroying fungus.
Figure 8 Drilling resistance measurement on the mortise and tenon of a rafter with no evidence of damage.

Figure 9
Drilling resistance measurement on the mortise and tenon of a rafter with significantly reduced tenon strength.

Possible problems with measurements
The measurement results do not always have to be completely reproducible. Drilling needle sharpness and drilling speed can lead to different measurement curves. Small-scale wood anomalies, such as knots, can deflect the drilling needle and thus influence the measurement curves.
Summary
Drilling resistance measurement is a method for testing the strength of timber that has little destructive effect on the building fabric. Taking into account the wood anatomy, information about cracks, cavities and other defects as well as the degree of damage caused by wood-destroying organisms and thus the load-bearing capacity of timber can be obtained for areas that are not or not sufficiently visible or accessible. The method can also be used successfully for tree inspection.
Details of the author of the article
Dr. rer. nat. Dipl.-Chem. Klaus Geith
www.svb-geith.de
Telefon: 08458/6719
E-Mail: dr.geith@web.de