Improved Structural Health Monitoring of the DLR Warton Road Bridge using Digital Image Correlation

Jan Winkler1, Chris Hendy2 1 ATKINS, Copenhagen, Denmark 2 ATKINS, Epsom, England

ABSTRACT: As our transportation infrastructure ages and the challenge of keeping it serviceable grows, the need for improved condition information on which to make good cost-effective maintenance decisions becomes ever more vital. Gathering this condition information requires structural health monitoring and inspection on a grand scale and, for it to be useful, it must be accurate, inexpensive, easy to interpret and avoid interfering with traffic flows - whether rail or highways. Digital image correlation (DIC) is a non-contact photogrammetry technique that can be used for monitoring by imaging a bridge periodically and computing strain and displacement from images recorded at different times or operating conditions. This paper discusses the use of DIC for monitoring of the impact damaged bridge deck displacements, with a view to avoiding the need for strengthening. Application of DIC technique in asset management proved to be cost effective.

1 INTRODUCTION

1.1 Bridge monitoring – challenges

The principal difficulty with adding lots of monitoring systems to bridges is that they produce vast quantities of data. Bridge operators typically do not know what to do with this data unless there are very clear trigger levels defined associated with this data and clear interventions defined if they are exceeded. This means that the structural engineer needs to identify what the most likely deterioration mechanisms are for the particular bridge and design bespoke monitoring that can be used to specifically measure performance in such a way that an acceptable level can be set and checked e.g. movement at a bearing or joint, tensile crack lengths/widths at particular discontinuity regions, wire breaks in a cable, fatigue cracking at a steelwork detail, tilt of a column or pylon or deflection of a deck where a bridge is very sensitive to creep variations and other material parameters. This may require the use of many different systems.

Instruments such as strain gauges, accelerometers, fiber optic sensors and displacement transducers are becoming increasingly common in structural health monitoring. These types of sensors can however possess drawbacks such as the need for external power and cabling/antenna for data transmission, high data acquisition channel counts and the limitation of only measuring at discrete points or along a line, so it is necessary to have an idea of where to expect damage when placing the instrumentation. These sensors can be used effectively to

continuously monitor for abnormalities that indicate damage, but the type and severity of the damage can still be difficult to identify from discrete point measurements.

1.2 Digital image correlation (DIC)

DIC represents a photogrammetry technique used for accurate measurements of surface deformation. The digitized images (e.g of a bridge deck) are compared to match facets from one image to another by using an image correlation algorithm (Fig. 1a, b). Image analysis involves capturing a reference image of a bridge component surface in its undeformed state. As the load is applied (e.g truck load), additional images are collected. The algorithm (which can either run in real-time or post-process), involves a stage-wise analysis, in which each stage consists of one image resulting in a description of displacements occurring on the surface of the bridge component. The evaluation of a correlation measurement results in coordinates, deformations and strains of the surface (Fig. 1c, d). DIC method allows high precision surface deformation measurement that can reach the accuracy of a few micrometers.

DIC measurement can provide information about strain (all directions), vertical and horizontal displacement, crack size, rotation and acceleration. DIC is independent of scale (local & global monitoring) and material (concrete, steel) and is especially helpful in monitoring of bridge components with a difficult access. Real time operation, off-line analysis, remote access to equipment and live reporting of results is also possible. As the technology stands, DIC can be operated with as little as one individual and does not have to be in contact with the bridge therefore avoiding any potential conflicts with traffic or difficult geography. Also, DIC takes a fraction of the time to setup compared to traditional instruments.

Figure 1. Digital image correlation method.

1.3 Image analysis in bridge engineering

Research activity on the application of photogrammetry in bridge-related projects has been minimal and widely dispersed within the last 25 years (Jiang et al. (2008)). Early applications of this technique include measurement of the deflection of a continuous three span steel bridge under dead load (Jauregui et al. (2003)) and identification of the bridge deformation (Bales (1985)). DIC was also employed to measure the geometry of a suspension bridge (Li et al. (1988)) and deformation of steel connection of a pedestrian bridge (Johnson (2001)). It was shown that DIC is a complementary tool of the conventional measuring systems such as LVDTs and strain gages. The last few years have seen an increased application of the DIC in the measurement of the vertical deflections of steel and concrete bridges due to traffic (De Roover et al. (2002), Lee and Shinozuka (2006a,b), Santini-Bell et al. (2011), Chiang et al. (2011), Yoneyama et al. (2007), Yoneyama and Ueda (2009)) and train transit (Busca et al. (2012)). In general, it was concluded the DIC system accuracy is comparable to existing displacement measurement techniques and DIC is an easier way to measure displacement of multiple points at once. DIC was also proposed as a method to asses dynamic characteristics of suspension bridge hanger cables (Kim and Kim (2013)). In this study, a non-contact sensing method to estimate the tension of hanger cables by using digital image processing based on a portable digital camcorder was proposed. Moreover, DIC technique has been used to record the strain on a concrete girder during a full scale bridge failure test (Sas et al. (2012)) and for the measurement of the displacement field on a cracked concrete girder during a bridge loading test (Küntz et al. (2006)). In both cases the photogrammetry method was able to detect a change in loading condition and locate cracks. Finally, DIC was used in fatigue testing of monostrands for bridge stay cables. Here, the vision-based system allowed for the measurement of the interwire movement (fretting fatigue) being the governing mechanism responsible for the fatigue life reduction in modern stay cable assemblies (Winkler et al. (2014, 2015)).

This paper discusses the use of DIC for monitoring of the impact damaged deck displacements, with a view to avoiding the need for remedial measures.

2 DOCKLANDS LIGHT RAILWAY (DLR) BRIDGES - FATIGUE ASSESSMENT

Docklands Light Railway (DLR) has implemented the running of three-car trains on the DLR network between Poplar station and Stratford station, known as the north route. The ‘DLR Capacity Enhancement & 2012 Games Preparation Project’ report identified that the fatigue life of some of the underbridge structures should be reviewed. Atkins has been commissioned to prepare fatigue management strategy for a number of DLR structures including the Warton Road Bridge, located close to the Olympic Stadium (Fig. 2). The bridge structure had its own site constraints and monitoring challenge. DIC monitoring was used as a part of overall assessment strategy for global monitoring to compare deck deflections to those expected.

Figure 2. Docklands Light Railway (DLR) - Warton Road Bridge.

2.1 Measurement methodology

The DIC system comprised of a camera mounted on a geared head on top of a surveyor’s tripod and the pan. Lens was adjusted to provide required field of view.The sampling frequency was +30 Hz and the system was set to have a higher resolution than 1/50th of a pixel. The normalized grey-scale correlation tracking algorithms were used for the image analysis. Calibration was achieved by measuring a distance within the image. The existing features on the bridge surface were used as the tracking objects. No editing of the raw data has been undertaken. All ‘processing’ was related to optimizing contrast levels and exact positioning of reference points for the virtual strain gauges

2.2 Warton Road Bridge

The structure is a single span half through simply supported bridge constructed in 1937. The span of the longitudinal girders is 13.56 m. A plan view of the structural layout taken from as-built drawing records is shown in Fig. 3a. The structure consists of two longitudinal girders and a concrete slab with encased steel beams. Each longitudinal girder is formed from flange and web plates that are riveted together using angle sections. In addition to the DLR track the southern girder also supports a walkway cantilevered from the girder top flange. The typical cross-section is shown in Fig. 3b.

Figure 3. Plan view (a) and cross section (b) of the Warton Road Bridge.

The structure carries a single DLR track and the alignment is effectively straight in plan. Three-car trains are in operation on this line. The abutments are brick abutments founded on spread footings.

2.2.1 FE model of the bridge

The bridge structure was analysed with a grillage model using a proprietary analysis computer program (LUSAS). The model is shown in Fig. 4.

Figure 4. A simplified FE model of the Warton Road Bridge.

A grillage model is necessary to analyse axle load distribution between the transverse beams encased in the concrete slab as it was initially found to be too conservative to model these encased beams with simplified line beams. Cracked section properties are used for the members representing the longitudinal concrete and the members representing the encased transverse beams.

2.2.2 Global DIC monitoring of the bridge (displacement measurement)

A fatigue management strategy for the Warton Road Bridge (Fig. 5a) involved detailed site based works to check using non-destructive testing (NDT) for damage assessment. For this purpose DIC technique was employed to capture the deflection of the main beam (Fig. 5b), without instating a road closure to the road underneath which was a main route used for a large nearby shopping centre, and compare this to the deflection calculated under the fatigue assessment loading. The camera was located 30 m away from the bridge. The measured peak deflections at mid span (Fig. 5c) were of similar magnitude to the deflections from the fully factored assessment loading assumed in FE analysis. In case of the Warton Bridge, it was possible to set up, record data on deflections, pack up and leave site within few hours. This was possible due to a useful recognisance site visit to plan the monitoring in advance. The dynamic monitoring using DIC technology was performed in summer 2015.

Figure 5. Warton Bridge (a), close up on main beam and DIC measurement (b), analysis of DIC data (c).

3 CONCLUSIONS

The Warton Road Bridge (Docklands Light Railway structure) required site based monitoring to contribute to the assessments of the structures. Atkins proposed undertaking site investigation using DIC to measure actual beam deflections. The outcome of the DIC measurements informed the fatigue management strategy. The results from the monitoring of the impact damaged deck displacements obtained with DIC, quantified the actual behaviour of the investigated structure, leading to the conclusion that the defects could be tolerated.

Monitoring of the bridge structure using traditional methods would require construction of an access platform, working at night and at height on a road and rail closure. There were also additional requirements related with completing the monitoring in a timely fashion, avoiding disruption to local stakeholders (adjacent construction site and large shopping complex) and avoiding changes to appearance of structures (survey targets). DIC measurement (setting up, capturing and packing away) was completed within few hours. All of the client’s requirements and concerns regarding closures, stakeholder disruptions, safety risks and structural appearance were avoided. Throughout the monitoring, the structure was left untouched and was in full service operation. Overall costs related with the DIC measurement were significantly lower than those associated with the traditional method.

The ability to capture a bridge’s behavior with DIC and calibrate a structural model with the collected data provides bridge designers and managers with an easy-to-collect objective measure of bridge performance. Application of the DIC monitoring technique for the overall fatigue management strategy proved to be cost effective. To the authors knowledge this is one of the first examples of the DIC strain measurement performed in outdoor conditions.

DIC is becoming very versatile and cost effective due to the dramatic improvement over the digital cameras and is therefore a promising solution to low-cost structural monitoring of existing infrastructures. A combination of DIC and other advanced monitoring technologies could finally give us the information we need to make effective whole life management decisions that keep our assets in service in a cost-effective way.

References

Bales, F.B. 1985. Close-range photogrammetry for bridge measurement. Transportation Research Record, Washington, D.C., 950, 39-44.

Busca, G., Cigada, A., Mazzoleni, P., Zappa and Franzi, M. 2012. Cameras as displacement sensors to get the dynamic motion of a bridge: performance evaluation against traditional approaches. Proc., 6th International Conference on Bridge maintenance, Safety and Management (IABMAS 2012), Stresa, Italy.

Chiang, C., Shih, M., Chen, W. and Yu, C. 2011. Displacement measurements of highway bridges using digital image correlation methods. Proc., SPIE – The International Society for Optical Engineering, 8321(1), 83211G-6.

De Roover, C., Vantomme, J. , Wastiels, J. and Taerwe, L. 2002. Deformation analysis of a modular connection system by digital image correlation. Experimental techniques, 26 (6), 37-40.

Jauregui, D.V., White, K.R., Woodward, P.E. and Leitch, K.R. 2003. Non contact photogrammetric measurement of vertical bridge deflection. J. Bridge Eng., 8(4), 212-222.

Jiang, R., Jauregui, D.V. and White, K.R. 2008. Close-range photogrammetry applications in bridge measurement: Literature review. Measurement, 41(8), 823-834.

Johnson, G. W. 2001. Digital close-range photogrammetry - a portable measurement tool for public works. Proc., 2001 Coordinate Measurement Systems Committee Conf., Coordinate Measurement Systems Committee.

Kim, S. & Kim, N. 2013. Dynamic characteristics of suspension bridge hanger cables using digital image processing. NDT &E International, 59, 25-33.

Küntz, M., Jolin, M., Bastien, J., Perez, F. and Hild, F. 2006, Digital Image Correlation Analysis of Crack Behavior in a Reinforced Concrete Beam During a Load Test. Canadian Journal of Civil Engineering, 33(11), 1418-1425.

Lee, J.J. & Shinozuka, M. 2006a. A vision-based system for remote sensing of bridge displacement. NDT&E Int., 39(5), 425–431.

Lee, J.J. & Shinozuka, M. 2006b. Real-time displacement measurement of a flexible bridge using digital image processing techniques. Exp.Mech., 46(1), 105-114.

Li, J.C. & Yuan, 1988. B.Z. Using vision technique for bridge deformation detection, Proc., International Conference on Acoustic, Speech and Signal Processing, New York, 912-915.

Santini-Bell, E., Brogan, P., Lefebvre, P. Peddle, J., Brenner, B. and Sanayei, M. 2011.Digital Imaging for Bridge Deflection Measurement of a Steel Girder Composite Bridge. Transportation Research Board (TRB 90th Annual Meeting), Washington, D.C , USA.

Sas, G., Blanksvärd, T., Enochsson, O., Täljsten, B. and Elfgren, L. 2012. Photographic strain monitoring during full-scale failure testing of Örnsköldsvik bridge. Structural Health Monitoring, 11(4), 489-498.

Winkler, J., Fischer, G., and Georgakis, C.T. 2014, Measurement of local deformations in steel monostrands using digital image correlation. Journal of Bridge Engineering (ASCE), 19(10).

Winkler, J., Georgakis, C.T., Fischer, G., Wood, S., and Ghannoum, W. 2015, Structural response of a multi-strand stay cable to cyclic bending load . Journal of Structural Engineering International 2/2015. (DOI: 10.2749/101686614X14043795570138).

Yoneyama, S. & Ueda, H. 2009. Bridge Deflection Measurement Using Digital Image Correlation with Camera Movement Correction. Materials transactions, 53 (2), 285-290.

Yoneyama, S., Kitagawa, A., Iwata, S., Tani, K. and Kikuta, H. 2007. Bridge deflection measurement using digital image correlation. Experimental techniques, 31(1), 34-40.