Justifying Structural Health Monitoring

Transcription

1 Peter McCarten, Group Technical Leader, Opus International Consultants Ltd SYNOPSIS Structural Health Monitoring (SHM) is an upcoming technology in bridge and structure engineering and the industry is considering how this technology can be integrated with holistic bridge and structures management. To date the SHM technology is being applied when anomalies or deterioration is detected within a structure. The technology is hence being used as a bridge engineering tool to assist bridge engineers in assessing structure criticality and rates of change in condition or stress. With continuous data flow and interpretation SHM clearly has advantages over traditional monitoring methods. The paper considers this matter and also considers the vision of integrating SHM data into a condition indicator that can be used for bridge asset management reporting and decision making thus increasing the versatility of SHM and allowing the technology to be more readily justified. INTRODUCTION Legislation and society in general are demanding Road Controlling Authorities (RCA s) show good stewardship of all road assets and this requires performance targets to be set, monitored and measured and the outcomes reported. These demands have necessitated RCA s applying asset management principles to day-today activities requiring implementation of processes for managing road network assets over a long period of time, in a manner to satisfy business and road user requirements at the lowest possible cost. Bridges are critical nodes within a road network and need to be operated by the RCA to ensure all structures are functional, serviceable, safe, reliable and stable throughout their lives. Bridges by their nature are complex structural systems and a critical issue to date has been defining appropriate performance targets and measures. Performance targets for bridges need to have apparent simplicity for reporting network achievements to the public yet the sophistication and complexity to reflect the true nature of the assets to allow the Bridge Manager to have the confidence the structure performance is appropriately reflected and for use in making asset management (maintenance, rehabilitation and replacement) decisions. The International Infrastructure Management Manual (IIMM) (2006) provides an outline of the monitoring required for overall performance of assets. The IIMM recognises that the business performance measures are related to customer service delivery and need to be reported at all levels with a focus on assessing and reporting the effectiveness of the operational, maintenance and capital works programme. The IIMM also recognises the difference between events or failures and the need to reflect the full picture in respect to condition or performance. For structural assets there is a clear need to report all five network performance criteria: essentiality, 1 of 9

2 functionality, serviceability, safety and stability. This paper focuses on the structural performance criteria. While performance targets can be readily set for essentiality, functionality and serviceability (e.g. network needs, width between barriers, clearance under beam soffits, clearance above the deck surface, deck surface roughness, deck joint roughness and general load carrying capacity) bridge condition, safety and stability are less well defined and more difficult to monitor, measure and report. It is recognised that these latter three structural characteristics are dependent on the following: As-built strength, stiffness, ductility and durability Current strength, stiffness, ductility and durability which is dependent principally on current element condition Demand strength, stiffness, ductility and durability which is dependent on the environment the structure is located in and the effects that can occur Structure robustness and redundancy or the ability of the structure to accommodate loads or actions by alternative load paths The above considerations show the importance of collecting condition data. With recent asset management sophistication there is now a demand for clear, concise and quantitative performance measures and a desire to move away from qualitative performance measures. Historically the data collected from a bridge inspection programme was defect focussed with the aim to feed into a bridge maintenance management programme. The IIMM shows the need to monitor and report the full picture of structural performance. In recent times many RCA s have supplemented the bridge inspection programme with condition monitoring as this data allows benchmarking between jurisdictions, enhances reporting to key stake holders and offers opportunities for asset management modelling, refer McCarten (2008). These enhanced bridge asset management functions are only possible if there is a robust bridge asset management structure in place. In McCarten (2004) a systems method for best bridge maintenance management was outlined and it utilises a hierarchy of asset preservation performance measures and targets within a condition focussed bridge management regime. While this structure was specifically set up for Public-Private- Partnership concessions its form can be applied to all jurisdictions and for all methods of procuring asset management services. With a structure of this form in place the focus then becomes determining the appropriateness, reliability and accuracy of the data collected for performance monitoring, measuring and reporting. Qualitative versus quantitative data becomes a prime issue and this is where Structural Health Monitoring (SHM) offers some opportunities. BACKGROUND The management practices for bridge and structures assets are evolving. Traditional management methods are changing as asset management practises and processes are being implemented and new technologies and practices in data collection, processing and reporting are being developed. Through this change process the 2 of 9

3 opportunity needs to be taken to review current practice and determine which aspects are suitable for use in the new environment. In New Zealand the New Zealand Transport Agency Bridge Inspection Policy is set out in specification S/6 and other principles and practises are outlined in the Bridge Inspection and Maintenance Manual (2001). This documentation is subject to ongoing review and updating. The inspection policy currently aligns well with international practise in respect to the type of inspections and their frequency and there is no guidance on monitoring requirements, it is the responsibility of the bridge management team (Bridge Engineer, Bridge Evaluator and Bridge Inspector) to develop specific monitoring programmes. The data collected from the periodic inspections and reported is currently defect focussed. The New Zealand Transport Agency is currently funding a research project addressing the data collection requirements for bridge management in New Zealand. This research is in the early stages and is currently reviewing international and local bridge inspection and management practises with a focus on collecting condition data and using SHM. The results of this research will not be available for some time. The justification for using SHM has been considered in this paper under two different bridge management regimes, being: Defect focussed Condition focussed The differences between these two bridge management regimes are indicated in the table below: DEFECT Monitor wellness Negative view of condition Bottom up (Inspector) perspective Measures actual defects Benchmarking?? Asset management modeling?? Reporting to key stakeholders?? CONDITION Monitor wellness Positive view of condition Top down (Asset owner/manager) perspective Measures condition by an index or rating Benchmarking Asset management modeling?? Reporting to key stakeholders?? HEALTH INDEX Number of condition states related to components Component weightings dependent upon economic value CONDITION INDEX Fixed number of condition states Component weightings based on rational risk Table 1. Features of Defect and Condition bridge management regimes. 3 of 9

4 The term SHM is starting to be commonly used but what does it actually mean and how does it differ from traditional Non-Destructive Testing and Evaluation (NDE) methods for monitoring structures. Two definitions are given below: Wikipedia: SHM is an emerging new field within various branches of engineering. SHM is a new approach to collect data about critical structural elements using sensors so as to provide indicators of condition when anomalies are detected in a structure. The technologies will collect data on a range of properties (chemical, electrical, ph, stresses, strains, accelerations, cracks, etc) and service environment or exposure characteristics (chloride, ion concentration, humidity, loadings, etc). SHM refers to the broad concept of assessing the on-going, in-service performance of structures using a variety of measurement techniques. Aktan et al (2003): This guide defines Performance, Health, Health Monitoring and Structural Identification separately and in a comprehensive manner. In the guide the most succinct SHM definition is considered to be: Health Monitoring is tracking of any aspect of a bridges health by reliably measured data and analytical simulations in conjunction with heuristic experience so that the current and expected future performance of the bridge for at least the most critical limit-events can be described in a proactive manner. The guide reports the most important distinction between current typical in-depth NDE and SHM are: Minimum standards are required for analytical modelling for reliable computer simulations and interpretations The methods used for designing and implementing the loads and tests to be used with the analytical modelling Comparing these two definitions highlights four important differences, they are: Wikipedia suggests SHM focuses on collecting data and assessing in-service performance where as Aktan et al (2003) states that SHM should go beyond the present and be used to predict future performance in a proactive manner Wikipedia suggests the new approach is hardware dominated where as Aktan et al (2003) suggests that SHM uses new approaches for all aspects of the monitoring (design of the monitoring, hardware used and its set up, data management and analysis and interpretation of the information gathered) Wikipedia suggests SHM is a technologist dominated field of engineering where as Aktan et al (2003) suggests SHM uses a holistic bridge engineering approach requiring inputs from the entire bridge management team Wikipedia suggests SHM is for constructed systems where as Aktan et al (2003) suggests the monitoring be installed during construction so the structure performance during the construction phase can be monitored and compared to designer expectations, while assessing built-in stresses and strains 4 of 9

5 It is interesting to note that both definitions refer to SHM as monitoring bridge condition and not defects and refer to health as overall bridge reliability. The four differences above are not insignificant. It is considered there needs to be industry acceptance of the SHM definition, vision and practises. In this paper the more comprehensive definition presented by Aktan et al (2003) has been adopted. It is clear that for any specific bridge evaluation the full SHM characteristics will not be available and should all RCA s adopt SHM then its full implementation and performance reporting will not be achieved until all bridges have been replaced. In the guide, Aktan et al (2003), four SHM application categories are presented, they are: Implementation to major bridges Implementation to large numbers of existing common short to medium span bridges Integrated structural and operational health and security monitoring Implementation to new bridges of new materials or using systems not yet industry accepted (research) This paper considers these applications and the justification for their use within the different bridge management regimes. It is considered a combination of regular visual inspections, reporting exterior condition, in conjunction with SHM reporting interior condition will be cost effective. The visual inspection while reporting condition can be used to verify some SHM data and allows structure aesthetics to be managed. SHM includes a wide range of tools both experimental and analytical. The guide, Aktan et al (2003), provides an excellent treatise on the technologies, materials and techniques with these ranging from simple/low cost to sophisticated/high cost options. Since the full costing of SHM has not been developed for use in New Zealand this paper has evaluated the various SHM options in terms of the degree of sophistication in experimentation and analysis. Taking the more holistic or systems view of the structure when developing SHM programmes aligns well to a Condition focussed bridge management regime. In this regime it can be expected that general reporting on the structure condition will be more reliable, presents the full picture of structure condition and aligns well with asset management performance measurement reporting of bridge stock. McCarten (2008) research used 10 years of inspection condition data from British Columbia Ministry of Transportation, Canada, for a group of 40 bridges to review the appropriateness of Bridge Condition Index (BCI) weighting factors and determinations. The research was calibrated by comparing changes in condition index trends to variations in the respective annual maintenance funding programmes, The findings of that research shows that BCI determinations based on weightings assessed using structure risk considerations give the best correlation. This finding is significant in that it shows there is reasonable reliability with the inspection condition data used and the condition index method of reporting structure condition. Even with this finding it is considered that using quantitative data is more reliable than just 5 of 9

6 qualitative data. It is therefore expected that supplementing qualitative inspection condition data with quantitative SHM data into a new risk based condition index will significantly enhance the reliability of bridge condition reporting. This in turn is expected to enhance the reliability of reporting bridge performance under an asset management environment. DISCUSSION In the guide, Aktan et al (2003), it is recognised that the application of SHM to existing structures is limited since information about the initial forces/stresses and previous loading history cannot be objectively measured. The guide does recognise that it is still possible to collect a wealth of data and information that offers benefits in more effective and enhanced management, monitoring and reporting of structural performance. In this regard SHM can be considered to be essentially NDE but with a longer term data collection and analysis regime and with a broader focus on the elements to be monitored. The manpower resources needed to manage a typical (non-shm) bridge management system in Australasia has been investigated and reported by McKechie (2006). The resources needed for inventory management, condition management, physical works management, deterioration modelling and network analysis were identified separately. The condition management activities are most relevant to this paper. On the assumption that SHM can fully replace Condition Assessment, Structural Assessment and Load Rating then McKechie (2006) analysis suggests that approximately 3 4 hours of a bridge/structural engineer with at least five years experience is the maximum cost saving available, this approximates $NZ450/bridge/year. With the current discount rate for projects in New Zealand being 8% and assuming an initial instrumentation set-up cost for simple/low cost/defect focussed SHM of $NZ3000/bridge it is determined the annual SHM management cost should approximate $NZ210/bridge for economic equivalency to the existing bridge condition management regime. For existing bridges with uncertain built-in stress levels and unknown loading history it is considered the cost savings required for economic equivalency are unlikely to result. However it is considered the benefit of continuous data flow which will show subtle changes in structure response and provide the bridge management team with an early warning of change would outweigh the cost difference. A significant advantage of SHM is the monitoring design views the structure holistically, in particular considering resilience and redundancy, by taking into account alternative load paths and effects that they may cause. Any monitoring programme which fails to consider the consequences of the deterioration being monitored is unlikely to be cost effective. Options for optimising SHM costs for existing bridges are available. One option presented in the guide, Aktan et al (2003), for monitoring large numbers of standard bridges is that of using a statistical sample. Probabilistic methods are used to identify the sample size and network distribution of the bridges to be tested and this process can take into account the type of data collected, methods of analysis used and the overall costs of the SHM programme. 6 of 9

7 A variation on the sampling technique outlined above is one which focuses on specific defect monitoring. In this option selective sampling can be used for the bridge subset so that structure condition in respect to as-built standards and the respective environmental hazards can be quantified to allow critical indicator bridges for monitoring to be identified. This technique was used by McCarten (2006) for identifying critical durability performance for the bridge subset of pre-tensioned prestressed I beam bridges in New Zealand. Due to the advanced state of chloride ion ingress, the small size of the critical bridge group, the isolated locations of the bridges, the high cost of installing SHM and the degree of redundancy within the bridge beams low cost inspection monitoring with NDE has been adopted for this subset of bridges. It has been accepted that this monitoring regime will result in a reactive maintenance regime. For existing bridges it is considered that until the cost of installing and managing SHM programmes reduces general use of the techniques will be limited. It is considered that if the data collected can be integrated into condition indexes and used for regular performance monitoring the use of SHM will be more readily justified. In a defect focussed bridge management regime the justification for full SHM implementation will be limited. For major or landmark existing bridges which are nearing the end of their life or exhibit premature aging, distress or performance problems then it is considered there will be strong justification for implementing SHM. Applying SHM to new bridges presents several challenges for the common or standard bridges but for major or landmark bridges significant benefits are available. In the last decade SHM technologies have matured with support from the industry. In the guide, Aktan et al (2003), it is reported SHM can be applied in the following fields: Control of geometry, material properties and construction processes, especially when segmental construction with complex erection and/or posttensioning processes are involved, may be greatly enhanced by a well designed monitoring programme SHM may help manage safety risks during construction as incomplete structural systems are more vulnerable and exposed to accidents and hazards SHM offers the opportunity to validate design assumptions regarding forces, reactions, displacements and drifts during construction SHM documentation can fulfil the requirements for recording the as-built structure and with current technology that can be in terms of a calibrated 3D analytical finite element model of the bridge, which of course will be available for future analysis of the bridge The above SHM uses readily apply to new major or landmark bridges. Recent major new bridges constructed implementing SHM have been termed smart bridges and these have proven the feasibility of the technologies. The justification for using SHM on new bridges needs to take into account contractor requirements, construction management requirements, designer requirements as well as the bridge management team requirements for post-construction activities. It is considered there 7 of 9

8 needs to be full industry awareness of SHM capabilities and benefits in order to assess the scope of monitoring and the justification for each project. Justification for construction management SHM requirements will be independent of the bridge management regime but post-construction SHM requirements will depend on whether the bridge management regime is defect or condition focussed. While the above new bridge benefits are available for all new bridges the cost of implementing full SHM to the group of small/medium standard bridges are considered high and difficult to justify, especially with the new monitoring technologies. The justification would obviously change should RCA s or bridge owners make a policy decision for SHM implementation. Options for optimising SHM costs are available and need to consider the network stock of bridges but until all bridges have been replaced the stock will include a mix of new and existing bridges. The statistical analysis of the group of small/medium bridges will need to make some assumptions on the future bridge replacements to maximise the efficiency of the optimisation. With knowledge of historic bridge design and construction practice and the respective in-service performance there may be an opportunity to implement focussed SHM but it is considered this would need to be on a trial and error basis. The application of SHM to new bridge technologies or bridges built of new materials or involving new practices that are not yet codified for industry acceptance is considered good risk management and readily justified. It should also be recognised that this application has the benefit of gathering information which can advance the general practice of bridge engineering or the wider civil engineering. For new bridges it is considered there are many significant benefits for implementing SHM but the actual use will still depend on the costs of the SHM programme. For major or landmark bridges SHM will be more readily justified. As for existing bridges if the SHM data collected can be integrated into condition indexes and used for regular performance monitoring and reporting the use of SHM will be more readily justified. In a defect focussed bridge management regime the justification for postconstruction SHM will be limited. CONCLUSION The main aim of this study is to review structural health monitoring application within both defect and condition focussed bridge management regimes. The study has considered implementing SHM to both existing and new bridges. Options for optimising SHM costs have also been considered. This study has found the implementation of full SHM will be dependent on the costs of the technology as well as the requirements of the Road Controlling Authorities. For the group of common small to medium bridges full SHM implementation is unlikely, it is considered more likely a sample of indicator bridges will be selected. For major or landmark bridges full implementation of SHM is more readily justified. Recent research has shown that risk based condition index analysis correlates well with bridge funding investment and if the SHM data can be integrated into condition indexes and used in regular performance monitoring and reporting the use of SHM will be more readily justified. 8 of 9

9 REFERENCES McCarten P.S., 2004 A systems method for defining best bridge maintenance management practice. Proceedings 2nd International Association of Bridge Maintenance and Safety Management Conference, Kyoto, Japan October 2004 McCarten P.S., Bruce S. and Freitag S., 2006 Deterioration of Prestressed Concrete Bridge Beams. Proceedings 6th Austroads Bridge Conference, Perth, Australia September 2006 McCarten P.S., 2008 Stock Condition Index Analyses response to Bridge Condition Index Determinations. Proceedings 4th International Association of Bridge Maintenance and Safety Management Conference, Seoul, Korea July 2008 The NAMS Group, International Infrastructure Management Manual Edition New Zealand Transport Agency, Bridge Inspection and Maintenance Manual, July 2001 Aktan A.E., Catbas F.N. Grimmelsman K.A. and Pervizpour M., 2003 Development of a Model Health Monitoring Guide for Major Bridges. Report to FHWA Research and Development DTFH61-01-P-00347, Drexel University July 2003 Wikipedia McKechie K., Staffing the Bridge Management System. Proceedings 6 th Austroads Bridge Conference, Perth, Australia September of 9