NDTCE’09, Non-Destructive Testing in Civil Engineering Nantes, France, June 30th – July 3rd, 2009

Applications and limitations of NDT: a timber bridge case study

Thomas TANNERT1, Andreas MÜLLER1, Mareike VOGEL1

1 Bern University of Applied Sciences, Biel, Switzerland, thomas.tannert@bfh.ch

Abstract The applications and limitations of different non-destructive and semi-destructive

techniques to evaluate the structural integrity of timber members in a pedestrian bridge are presented as a case study. Sophisticated assessment tools are required to detect hidden damages in timber structures: for example stress-wave techniques are used to evaluate the modulus of elasticity of bending members and resistance to drilling is used to gain knowledge of areas of changed density due to insect or moisture induced damages. Reliably relating the gathered data to the structural integrity of the structure is a complex issue. Bending members and connection details of a decommissioned timber bridge were evaluated using non destructive assessment tools. Eventually these parts were tested destructively to assess their remaining modulus of elasticity and load bearing capacity. The need for improvements in the current practice is highlighted by comparing the results from the non-destructive, semi-destructive and destructive tests.

Résumé Les applications et les limites de différentes techniques d’analyse non destructives de

structures porteuses en bois sont présentées dans le cas concret d’une passerelle piétonne. Des moyens d’investigations sophistiqués ont été mis en œuvre pour déceler des dommages cachés : par exemple, les ultrasons sont couramment utilisés pour mesurer le module de Young d’éléments porteurs et la résistance à la pénétration est souvent utilisée pour déterminer l’étendue des attaques d’insectes ou de pourritures suite à des champignons lignivores qui provoquent de fortes chutes de la densité du bois. Des éléments porteurs et des assemblages d’un pont piétonnier désaffecté ont été évalués par des moyens non destructifs. Par la suite, ces éléments et assemblages ont été testés de manière classique afin d’en déterminer la capacité portante et le module de Young résiduel. Les besoins d’amélioration des pratiques actuelles sont démontrés par la comparaison des résultats des mesures non destructives avec les mesures classiques destructives.

Keywords Timber bridge, wood decay, connections, non-destructive-techniques, structural integrity

1 Introduction Timber has been a structural material for centuries, and numerous examples throughout the

world demonstrate its durability. But timber is biodegradable, and damage attributed to bio-deterioration decreases the capacity of structural members. Replacement of deteriorated members is an acceptable option in some cases, in others economic considerations require decommissioning of the complete structure. Methods for assessing the condition of timber can be nondestructive (NDT); such techniques are useful for rapid screening for potential problem areas and are generally based on correlations between nondestructive and destructive parameters. For strength prediction, a major drawback of NDT is the relatively poor correlation between the measured nondestructive quantity and material strength [1,2].

Semi-destructive techniques (SDT) can bridge the gap between indirect nondestructive and direct fully destructive methods of strength measurement. SDT often require the extraction of

NDTCE’09, Non-Destructive Testing in Civil Engineering Nantes, France, June 30th – July 3rd, 2009

small specimens for subsequent testing to determine elastic and strength parameters while preserving the integrity of the member. Biological deterioration of timber structures is one of the main grounds for maintenance; however the tools that help to detect the deterioration can compound the problem. SDT such as drill resistance measurements can aid in determining the extent of damage yet these mechanisms themselves can create a material weakness allowing damage to set in [3]. The weakness of SDT is the necessarily small size of specimens that leads to increased variability in test observations [1].

For timber bridges it, is recommended that inspections using NDT and SDT are carried out at regular intervals to determine the condition of the structure [4]. Evaluating the results of several hundred projects, visual inspections usually found less than 30% of the total damage. At the same time, many timber constructions evaluated as ‘to be replaced’ by visual inspection were preserved because of internal good condition shown by drill resistance measurements [5].

The objective of the presented research was to evaluate the applications and limitations of NDT and SDT on a decommissioned timber bridge in Dietfurt, Germany.

2 Timber bridge case study 2.1 On site evaluation In 2001 a series of pedestrian and cyclist bridges along the Danube were evaluated by the

district authorities. The bridge at Dietfurt showed signs of neglected maintenance with vegetation at structural elements, which should have been back-cut. Highly stressed construction details were substantially damaged, as shown in Fig. 1, so that the structural safety of the overall construction was no longer ensured. Immediate repair measures were initiated and a consulting engineer was assigned for further investigation.

A clearly visible dark discoloration indicated constantly high moisture contents (MC). Measurements resulted in MC higher than 30% throughout the lower beams which exceeds the critical fiber saturation point and enables the growth of wood destroying fungi. Resistance drilling confirmed that the density of the timber was severely reduced and that various members required complete replacement. Exposed members exhibited shrinkage cracks with depths up to 30 mm which led to even high moisture contents of the inner sections.

The unprotected timber bridge did not fulfill safety requirements and ultimately had to be decommissioned. Individual parts (3 beams and 16 connection details consisting of wooden members with inserted steel plates and steel dowels) with varying degrees of degradation were transported to the laboratory of the Bern University of Applied Sciences for further tests. NDT (visual inspection, stress wave and computer tomography screening), SDT (resistance drilling and mechanical bending tests) as well as destructive tests were carried out.

Figure 1. Pictures of damaged details: connection (left) and beams (right)

NDTCE’09, Non-Destructive Testing in Civil Engineering Nantes, France, June 30th – July 3rd, 2009 2.2 Non-destructive laboratory tests 2.2.1 Stress wave tests to determine modulus of elasticity A Sylva-test device was employed to estimate the longitudinal modulus of elasticity

(MOE) of the beams. This NDT utilizes an ultrasonic pulse generator to send a stress wave into a member. Two transducers are placed on the member: the transmitting transducer imparts a wave into the member, and the receiving transmitter is triggered upon sensing of the wave. The time of flight is then coupled with additional information, such as wood species, path length, and geometry, to compute the MOE but no direct relationship between the time of flight and material strength exists [6]. On each beam, 15 measurements were performed, nine with the transducers placed on the beam faces, and six with the transducers placed on the sides to get an indication of differences in MOE within and between specimens.

2.2.2 Computer tomography tests to determine interior damages On selected specimen, computer tomography (CT) measurements were carried out at the

Forest Research Institute Baden-Württemberg. A CT uses a rotating x-ray tube that performs hundreds of measurements during one full-circle rotation. From these measurements, a 2D reconstruction is computed. Moving along the specimen, a number of slices result in a 3D voxel block. The inner wood features can be identified using CT scanning [7]. Fig. 2 shows scans of an undamaged glue laminated timber section (left), a solid wood section with clear damage in the form of cracks and deterioration (centre) and the problems that arose when sections close to the inserted steel plates or dowels were scanned (right).

Figure 2. Pictures of CT scans: undamaged section (left), damaged section (centre) and

section close to inserted steel plate (right)

2.2.3 Resistance drilling to determine wood density Resistance drilling is a technique that inflicts minimal damage while giving information on

the internal condition of timbers. It has been successfully applied to tree, bridge and building investigations [8]. A drill bit is advanced at a constant cutting speed and the torque required to maintain the speed corresponds to the resistance, which can be correlated to the wood density. The drill resistance is recorded with respect to the penetration depth; total decay will offer no resistance and the drilling profile will appear as a zero flat line. The method can give reliable indications on the local strength of wood specimens and can easily detect the locations of the internal defects, but no direct quantification of mechanical properties is possible [9]. Four separate drillings were carried out per specimen in order to get an indication on differences in density within one specimen and between specimens. Fig. 3 presents two example graphs; one for an undamaged specimen (top) where the drill resistance is approximately constant over the specimen depths and a damaged specimen (bottom) where the drill resistance shows a clear drop in the centre of the specimen.

NDTCE’09, Non-Destructive Testing in Civil Engineering Nantes, France, June 30th – July 3rd, 2009

Figure 3. Drill resistance output: undamaged (top) and damaged specimen (bottom)

2.2.4 Bending tests to determine modulus of elasticity Quasi-static bending tests on a Schenck Trebel testing machine were performed under

displacement-control at a constant loading rate of 5 mm/min. Each beam was tested four times to get an indication on differences in longitudinal MOE within and between specimens.

2.3 Destructive laboratory tests to determine connection capacity The load capacity of the connection details was determined on a Schenck Trebel testing

machine. The connections consisted of timbers of 180 mm x 180 mm with one inserted central steel plate of 10 mm thickness and steel dowels of 16 mm diameter. In order to obtain comparable results, only four dowels per specimen were kept in the connection. These dowels complied with the spacing requirements according to Eurocode 5 (EN 1995-1-1:2004). The specimens were fixed in the test machine and the steel plate was pulled with displacement-controlled loading at a constant rate of 5 mm/min until failure.

The characteristic load capacity of the connection was expected to be 125 kN according to Eurocode 5. 12 of the 16 tested specimens exceeded this value regardless of their degree of deterioration. In most specimens, ductile dowel bending was observed before ultimate failure due to block shear failure. The four specimens that failed below 125 kN were those with the highest degree of deterioration. Fig. 4 shows pictures of a specimen clamped in the testing machine (left), a specimen failed due to splitting (centre), and a specimen with premature failure due to deterioration (right).

Figure 4. Pictures of connection detail during test (left), splitting failure (centre)

premature failure due to deterioration (right)

NDTCE’09, Non-Destructive Testing in Civil Engineering Nantes, France, June 30th – July 3rd, 2009 2.4 Comparison between different tests techniques Table 1 summarizes the results from the techniques applied to the beams. The CT scan

gave excellent images of the interior of the beam and confirmed the findings from the visual inspection. The resistance drillings gave a local result on density and deterioration but did not allow to quantify the extent of damage or predict MOE. The Sylva tests allowed estimating the longitudinal MOE but did not correctly quantify the differences between the tested beams.

Table 2 summarizes the results from the techniques applied to the connections. The CT scan again confirmed the findings from the visual inspection but the large differences in contrast between wood and steel did not allow scanning the sections close to the steel plates and dowels that are of greatest interest. The resistance drillings gave local results on density and deterioration, but a regression analysis between average specimen drill resistance and specimen capacity delivered an R-square value of only 0.21, indicating a poor correlation.

Table 1. Test results for beams Element Visual inspection CT Scans Res. drilling MOESyilva (MPa) MOBending (MPa)

B1 only discoloration not scanned 13.7 (COV=24%) 16.0 (COV = 6%) 14.8 (COV = 3%)B2 deep cracks confirmed findings 12.5 (COV=30%) 11.8 (COV = 9%) 6.9 (COV=14%)B3 rod on one side not scanned 12.4 (COV=20%) 14.5 (COV=14%) 8.2 (COV = 7%)

Table 2. Test results for connection details Element Visual inspection CT Scans Resistance drilling Capacity (kN)

1d high degree of deterioration confirmed findings 14.2 (COV=28%) 141 1z rod on one side close to steel confirmed findings 15.7 (COV=20%) 160 2d discoloration and cracks not scanned 15.7 (COV=22%) 183 2z no visible deterioration not scanned 14.5 (COV=25%) 206 3d high degree of deterioration confirmed findings 13.8 (COV=37%) 156 4d discoloration and cracks not scanned 14.7 (COV=22%) 182 4z no visible deterioration confirmed findings 13.9 (COV=26%) 166 5d discoloration and traces of fungi not scanned 14.3 (COV=34%) 169 5z discoloration and traces of fungi not scanned 14.7 (COV=30%) 123 6d only discoloration not scanned 14.3 (COV=23%) 171 6z only discoloration not scanned 12.9 (COV=22%) 112 7d high degree of deterioration confirmed findings 14.8 (COV=32%) 87 7z high degree of deterioration confirmed findings 12.8 (COV=22%) 96 8d high degree of deterioration confirmed findings 15.5 (COV=21%) 185 8zI advanced rod on one side confirmed findings 14.0 (COV=26%) 149 8zII discoloration and cracks confirmed findings 13.9 (COV=17%) 175

3 Conclusions The applied NDT and SDT can be used to identify potential problem areas, to provide

images of the internal condition of timber, and to give reasonable comparative measurements; but these techniques cannot be used to reliably predict material properties such as MOE of beams or capacities of connections due to weak correlation between destructive and nondestructive parameters. Destructive tests can be used to obtain direct strength data, but they lead to complete destruction of the individual members.

The visual inspection proved to be an indispensible method to direct the attention to damaged sections, but little correlation between visual damage and capacity of connection details was found. The CT scans provided an excellent insight into the internal structure but the used stationary CT is not suitable for on-site evaluation of structures. Portable Ultrasonic-

NDTCE’09, Non-Destructive Testing in Civil Engineering Nantes, France, June 30th – July 3rd, 2009

echo, CT, or X-ray devices provide more appropriate alternatives but were not used in the present study.

The Sylva-test allowed estimating the differences in longitudinal MOE between different beams but could not be used to quantify the influence of deterioration on MOE. Furthermore, the variation of results for measurements within single specimen was considerably high.

With resistance drilling, it was possible to accurately detect areas of deterioration; however neither MOE of the beams nor capacity of connections could be predicted. A limitation of the technique is that it provides a very localized measure of density; therefore this technique is best employed if used together with visual inspection, which provides a qualitative assessment and stress wave inspection that provides a sectional assessment.

The presented case study demonstrated that only a simultaneous combination of several evaluation techniques can lead to reliable results when inspecting timber bridges. Existing NDT and SDT need further development in order to quantify the remaining capacity of damaged structural elements. To improve the assessment of in situ timber members, more research is required to obtain both accurate quantification of deterioration as well as estimate individual member strengths.

Acknowledgements The funding received from the Bern University of Applied Sciences is gratefully

acknowledged.

References 1. Kasal, B., Anthony, R.W. (2004) “Advances in in-situ evaluation of timber structures”,

Progress in Structural Engineering and Materials. Vol. 6, pp.94-103. 2. Graham T. (1995) “Overwiew of non-destructive evaluation technologies“, Proc. of the

Nondestructive Evaluation of Aging Bridges and Highways”, Ed by S. Chose, pp.5–9. 3. Duwadi, S.R., Ritter, M.A. (1998) “An Overview of the Wood in Transportation Program

in the United States”, Proc. of the 5th World Conf. on Timber Engineering, Montreux, Switzerland, Ed by J. Natterer and J.L. Sandoz.

4. Wilkinson, K., Thambiratnam, D., Ferreira, L. (2005) “Non Destructive Testing of Timber Bridge Girders”. Proc. of the Int. Conf. on Structural Condition Assessment, Monitoring and Improvement, Perth, Australia.

5. Rinn F, Schweingruber, F.H., Schär, E. (1996) “Resistograph and X-ray density charts of wood. Comparative evaluation of drill resistance profiles and X-ray density charts of different wood species“, Holzforschung, Vol.50, pp.303-311.

6. Peterson, M.L, Downs, J., Gutkowski, R.M. (1996) “Nondestructive Inspection of Timber Bridge Structures”, Proc. of the conference on Nondestructive Evaluation of Civil Structures and Materials, Boulder, Colorado, pp.55-67.

7. Brüchert, F., Baumgartner, R., Sauter, U.H. (2008) “Ring width detection for industrial purposes - use of CT and discrete scanning technology on fresh roundwood”, Proc. of the Conference in COST E53 ‘Quality control for wood and wood products’ Ed by W.F. Gard, Delft, The Netherlands, pp.157–164.

8. Brashaw, B., Vatalaro, R.J., Wacker, J.P., Ross, R.J. (2005) “Condition Assessment of Timber Bridges: 1. Evaluation of a Micro-Drilling Tool”. General Technical Report, FPL-GTR-159. Forest Products Laboratory. Madison, Wisconsin.

9. Ceraldi, C., Mormone, V., Russo Ermolli, E. (2001) ”Resistographic inspection of ancient timber structures for the evaluation of mechanical characteristics”, Materials and Structures, Vol.34, N.1, pp. 59-64.

IntroductionTimber bridge case studyOn site evaluationFigure 1. Pictures of damaged details: connection (left) and

Non-destructive laboratory testsStress wave tests to determine modulus of elasticityComputer tomography tests to determine interior damagesFigure 2. Pictures of CT scans: undamaged section (left), da

Resistance drilling to determine wood densityFigure 3. Drill resistance output: undamaged (top) and damag

Bending tests to determine modulus of elasticity

Destructive laboratory tests to determine connection capacitFigure 4. Pictures of connection detail during test (left),

Comparison between different tests techniquesTest results for beamsTest results for connection details

Conclusions

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