Assessment of timber structures using the X-Ray technology
, Bettina Franke
, Florian Scharmacher
Bern University of Applied Sciences, Architecture, Wood and Civil Engineering,
Solothurnstrasse 102, CH-2504 Biel/Bienne, Switzerland
Keywords: Timber, Connections, Nondestructive testing, Assessment, X-Ray
Abstract. The assessment of timber structures is a permanent task to check the normal function of
individual structural timber elements. Non-destructive testing methods are preferred but the value of
the information is limited due to the performance of the applied assessment method. However,
X-ray is a technology which allows a view into the structural member or the connections. The
mobile X-ray technology has been used in laboratory tests and practical situations at existing
structures and led to excellent results which allowed detailed analyses. The method and its
possibilities for non-destructive testing of timber structures will be presented. The results reached
show a high potential for an effective assessment of existing structures including connections and
structural timber members.
The structural assessment of timber structures is caused by different reasons, such as regular
inspections, structural modifications, changes in serviceability or historic preservation. The
assessment of timber structures always begins with the visual inspection of the complete building
for the analyses of the supporting structure. The following assessment of the single members,
connections or specific details will take place only after this step. An advantage in assessing timber
structures is that abnormalities are normally relatively easy to detect due to discoloration, cracks or
plastic deformations. Especially in combination with the measurement of the moisture content, first
specification can already be done. Depending on the abnormalities found, specific testing methods
are available and can be used. The test methods can generally be classified into nondestructive, less
destructive and destructive test methods. For the detailed survey of the building and assessment, an
overview of common methods is given in Table 1. Further explanation can be found in e.g. Aicher
, Görlacher , Kasal & Tannert , Köhler et al. , Rinn , Steiger  and Vogel et al. .
Non-destructive testing methods are preferred, but the value of the information is limited due to
the performance of the applied assessment method. Especially the occurrence of internal damages
like cracks, holes, fitting inaccuracy or plastic deformations of mechanical fasteners cannot be
detected reliably with these common methods. However the X-ray technology allows a view into
the structural member or connections. The application of the X-ray technology on wooden
structures was investigated and the results and limitation are presented.
Common assessment methods for timber structu
Crack detection and mapping
Ultrasonic wave or echo
Penetration resistance tests
Withdrawal resistance test
Drill core specimens
Test of glue line quality
Mechanical testing for strength
Advanced Materials Research Vol. 778 (2013) pp 321-327
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Method. The X-ray technology is known from the medical use. Nowadays there are also mobile
X-ray systems available which are used for the in-situ assessment of structures, as shown in , ,
, , , . The adoption of this technology provides the possibility to look inside the
member with a high accurate resolution according to the measuring area of 30 by 40 cm for the film
used. The X-ray technology is a non-destructive testing method and works quasi contactless. The
use of a mobile X-ray technology in combination with the specific digital scanner allows in-situ
assessment of existing structures.
The safety requirements for the use of the mobile X-ray system do not limit the practical use on
existing timber structures. The mobile X-ray system used works with hard X-ray impulse generator
but with a very low dose as against stationary X-ray systems known. Furthermore the exposure
transmitter is only active, meaning X-rays are only generated, while ”taking” the picture. This
process takes only a few seconds and before and after no X-ray exposure happens. In practical use,
the safety zone is specified as follows: 3 meters around the transmitter, 30 meters in measuring
direction and 11 meters perpendicular to it. The users carry a personal dosimeter to register any
Theory and Calculation. X-rays are a form of electromagnetic radiation. The X-rays are absorbed
depending on the material respectively their density. The X-ray absorption parameter is defined by
the Beer-Lambert law as follows:
Where I is the intensity after radiography in [W/m
the intensity before radiography in
], d the thickness in [m] of the material and µ the X-ray absorption coefficient in [m
wood, the X-ray absorption coefficient is defined as follows:
= ′ ∙ (2)
With µ’ as the mass absorption coefficient in [m
/kg] and ρ the density of the material in [kg/m
The absorption capacity depends on the density of the material, the atomic mass, atomic number
and the depth of the material, .
The X-ray radiography depends on the impulse intensity of the X-rays, the distance of the test
object to the transmitter as well as to the film plate and also the thickness of the material. The
principle of the process is shown in Fig. 1, where the test object is located between the X-ray
Fig. 1 Principle process of X-ray technology and investigations
X-ray system RadiogramTest object Film plate
322 Structural Health Assessment of Timber Structures
transmitter and the film plate. The X-rays transmitted travel through the test object and will be
absorbed with different intensities before they hit the film plate. The material specific absorption of
the X-rays leads to the so called radiogram which will finally be transferred in a grayscale picture.
The volume of the three dimensional test object will be reproduced as a two dimensional picture.
Applications and limits of the mobile X-ray technology
Laboratory analyses of the system. The principle of the X-ray process is similar to taking a
picture with a photo camera. The quality of the photo depends on the depth of field, sharpness of
movement and focus. These parameters are not comparable for X-ray systems. Here the impulse
intensity, the distances of the test specimen between the transmitter and the film plate and the
thickness respectively density restrict the results, resolution and accuracy of the method.
According to Eq. (1), the intensity I on the film plate increases linear with the intensity of the
. The gray value of one pixel behaves proportional to the intensity and will increase.
Fig. 2 shows the radiograms taken with different numbers of impulses from a steel screw tip
inserted in wood block as test object. The test object had a constant thickness d of 70 mm.
Increasing the thickness increases the absorption of the X-rays, so that for radiograms with
comparable quality, the number of impulses has to be increased as well. The effect of the number of
impulses was analyzed for two different thicknesses and is shown in Fig. 3.
Furthermore the distance a between the transmitter and the test object and the distance b between
the test object and the film plate, see Fig. 1 was verified related to the accuracy and sharpness of the
radiograms. The same test object with the steel screw inserted in a wood block with a thickness of
70 mm was used. The increase of the distance b results in a smaller projected area where the object
is enlarged compared to the original size, as shown in Fig. 4. On the other hand, the reduction of
distance a leads to a clear “burned” spot and unusable radiograms. A minimum distance a of about
1 meter was necessary for the test configuration with a film plate of 30 by 40 cm. The relation
between the two distances a and b is summarized in Fig. 5.
Finally the thickness of the test object was verified from 70 mm up to 350 mm. Hereby constant
parameters for the number of impulse, distances of the test object to transmitter and film plate were
used. The radiograms of the test object with the metal screw inserted in the wood block with
different thicknesses are shown in Fig. 6. The contrast of the radiogram reduces with the increase of
the thickness of the test specimen. The typical structural elements of wood on macro scale level are
visible for thicknesses up to 200 mm. For greater thicknesses, only major differences are visible in
the radiogram like parts of steel or wood. As summary, the relation between the mean gray value
and the thickness is shown in Fig. 7.
Fig. 2 Radiograms with different number of
Fig. 3 Effect of number of impulse, the shadowed
area marks the not useful configurations
0 50 100 150 200 250 300 350
number of impulse [-]
Advanced Materials Research Vol. 778 323
Assessment of wood and connections. The first investigations are done in the laboratory with
samples of historical wood to wood connections or with mechanical connections. Fig. 8 shows a
wood to wood connection with an internal hardwood dowel. Not only the two wood species,
European spruce and beech, can be clearly distinguished but also differences within one material
like knots and even the annual grow rings are visible. Furthermore the fitting accuracy of such a
connection can be checked. In this case gaps are clearly detectable. As a practical application, a
historical timber construction in a chateau was investigated. Wooden nails could be detected during
the assessment of a multi layered beam construction, as shown in Fig. 9.
The assessment of timber connections with mechanical fasteners is shown in Fig. 10 for a
dowelled connection with inner steel plate. The test specimen shown was loaded/unlodaded in
certain steps at the laboratory and X-rayed after each load step. For every case, the visual inspection
of the outside area (heads of the fasteners) do not indicate any irregularities. But the radiograms
show that inside the connection plastic deformations according to the Johansen theory,  already
occurred, as shown in Fig. 10c)-d). The plastic deformations of the fasteners indicate an
overloading and a failure of the connection. The connections have to be repaired in this case.
Fig. 4 Radiograms with different distances, top
row distance a and bottom row distance b
Fig. 5 Effect of distances of impulse, the shadowed
area marks the not useful configurations
Fig. 6 Radiograms with different thicknesses of
the test object
Fig. 7 Effect of thickness, the shadowed area
marks the not useful configurations
Fig. 8 Wood to wood connection with hardwood dowel
0 10 20 30 40 50 60 70 80 90
mean gray value [%]
0 50 100 150
324 Structural Health Assessment of Timber Structures
Assessment of restored glue lines and fungal/insect decay. Glulam is a common used engineered
wood product for large span timber constructions. The assessment of these structures is a permanent
task in order to ensure the integrity and performance. In some cases, the glue lines or cracks have to
be restructured or supports and high stressed areas have to be reinforced. The assessment of
restructured glue lines was therefore investigated with the mobile X-ray system within a research
project. A glulam member with two restructured cracks was X-rayed in different directions to check
the restoration. In the first radiogram, taken in a direction perpendicular to the glue-line planes and
shown in Fig. 11b), a clear failure at the outside of the beam can be seen. But the allocation to one
of the glue-lines or even the evaluation if there are more failures in the same direction is not
possible. Fig. 11c) shows the final radiogram inclined to the glue-line plane. Here, the two
restructured glue-lines can be separated from each other and the failure spots and injection holes are
clearly visible for each glue-line. In this case, both glue-lines show the failure at the same position.
Furthermore, the assessment of this member also shows voids and bubbles along the glue-line plane
as well as in the injection holes. Depending of the size of these defects, the structural capacity of the
beam and the strength of the restructured glue-line can be influenced. In a practical application, also
voids and bubbles within a glued-in rod connection could be detected.
a) b) c)
Fig. 9 Historical wood nail in multi layered wooden member, a) position of X-ray shot, b) original
radiogram, c) wooden nail marked in radiogram
a) b) c) d)
Fig. 10 Connection with mechanical fasteners, a) Test specimen, b) unloaded connection, c) and d)
connection with plastic deformations
Advanced Materials Research Vol. 778 325
In general fungal or insect decay can be observed within the visual inspection. But in some cases
structural elements are covered or only viewable from one side, so that the mobile X-ray system can
be used for detailed analyses or specification of assumptions. Fig. 12 shows as example of a glulam
member with fungal decay in the top layers. The typical cubic failure structure is visible in the
radiogram observed and allows estimating the dimension of the decay.
Discussion and conclusion
The X-ray system has been used in laboratory tests and practical situations at existing structures and
led to excellent results which allowed detailed analyses going further as common non-destructive
assessment methods. It was shown that the mobile X-ray technology offers a high potential for an
effective assessment of existing structures including connections and structural timber members.
Deformations of mechanical fasteners like the formation of plastic hinges due to overloading are
visible as well as the macroscopic structure of wood, knots or different wood species. Also glued
connections like finger joints or restructured glue-lines were checked for quality and/or damages.
Voids or bubbles but also cracks due to overloading could clearly be detected.
The practical examples presented, give an overview of the ability and the limits of this method
and show that the mobile X-ray system is a novel successful non-destructive testing method of
timber structures. With increase of the differences of the density of the investigated materials, the
contrast is getting more and more intensive. However, reliable analyses of the resulting radiograms
should be done by people who have experiences with the system and are professionals in timber
structures in order to be able to identify irregularities from inaccuracies even in less contrast
a) b) c)
Fig. 11 Gulam member with two restructured cracks, a) test specimen, b) X-ray direction
perpendicular to the glue-line (top view),
c) X-ray direction inclined to glue-line
Fig. 12 Fungal decay in X-ray, a) test specimen, b) radiogram
326 Structural Health Assessment of Timber Structures
We would like to thank the master student Mr. Scherler for his effort and contribution to these
results within his project paper.
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