A Self Informed Bridge Assessment, Maintenance Management Tool for Bridge Monitorng

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ITRN2011 31st August – 1st September,University College Cork

Hand, Bruton, Foley, Bekic, McKeogh: SELF-INFORMED BRIDGE ASSESSMENT TOOL

A SELF-INFORMED BRIDGE ASSESSMENT, MAINTENANCE ANDMANAGEMENT TOOL FOR BRIDGE MONITORING

Stephen Hand

University College Cork

Aoife Foley


Gillian Bruton


Damir Bekic

University of Zagreb, Croatia

Eamon McKeogh


Abstract

The continual inspection, assessment and maintenance of bridges requires amultidisciplinary approach. Beyond a good understanding of structural engineering, a bridge

inspector must have a good knowledge and appreciation of geotechnics, hydraulics,hydrology, materials and even transport management. A number of international standardsand guidelines exist based on experience, historical events and best practice in industry.However, the risk-informed decision-making process in bridge monitoring is complex. Thus,the application of intelligent assessment measures built using Bayesian Logic controls canassist in ensuring a failsafe bridge inspection programme. This paper provides a review ofsome existing bridge assessment, maintenance and monitoring guidelines and standards. Inaddition, the Self-Informed Bridge Inspection, Assessment and Maintenance ManagementTool (SIBIAM) developed using a Bayesian Logic approach is presented. SIBIAM uses a GIS(Geographic Information Systems) specialised software tool. The purpose of SIBIAM is toprovide for bridge owners, an up-to-date inventory of bridge condition and maintenanceinformation by collecting and monitoring relevant bridge and river parameters for input to amanagement database.

1. Introduction

A number of bridge management systems are currently in use worldwide. In 2010 theIABMAS Bridge Management Committee prepared an overview of the existing bridgemanagement systems [1]. This report assessed a total of 18 bridge management systems, inoperation across 15 countries being used to manage 900,000 objects. The systems all showa strong focus on the structural health monitoring of bridge structures, managing this facet ofbridge stability to varying degrees. Although infrastructure managers are increasingly usingmanagement systems to support their decision making process, most of the owners of thesesystems lacked an up to date view of the capabilities of the most advanced systems and howthese could be adapted and further enhanced to support their requirements [1].




31st August – 1st September,University College Cork

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In general the mandate of a Transportation Network is to provide a safe, economical and

effective network to allow the transportation of people and goods. A risk that has come to

prominence in recent times is the risk of structural damage to a bridge as a consequence of

river flooding and the resulting scouring that can occur due to these increasing flood risks.

Unfortunately many bridge management systems do not place enough emphasis on this riskand some systems disregard the risk entirely.

2. Objectives

The implementation of a bridge management system allows the co-ordination andmanagement of both cyclical and non-cyclical activities and inspections. If the right bridgemanagement system is used, it will facilitate the management of the integrity of the bridgeand the associated budgets. The primary objectives of this current work are to:

Integrate all bridge management activities from inventory and inspection to bridge

maintenance and repair decisions into a single system with appropriate links to

systems.

Improve the current systems in terms of on-line availability, integrity, accurate data

entry, security, integration with other systems and accurate reporting and system

interface so that the system can be accessed and easily used by staff members.

Incorporate the intricacies associated with the bridge scour process such as river

hydrology, morphology, historical flood event data etc into the enhanced bridge

management system.

Supply management and staff with the appropriate tools and information to assist in

decision making and to recognise the risks involved in the decisions being made.

3. The Scour Problem

Bridge scour is defined as the erosion or removal of streambed or bank material from bridgefoundations due to flowing water [2]. The hazards associated with bridge scour aresignificant. Bridge scour is the leading cause of bridge failures in the United States. From1966 to 2005, there have been at least 1,502 documented bridge failures in the US; 58%were the result of hydraulic conditions [3]. The probability of a bridge damage or failure isrelated to the probability of having a flood event, the effect of that flood event on scour andthe effect that the scour can have on the bridge stability. The problem is relevant both toexisting bridges and to the economical design of new bridges to ensure they are resistant tothe effects of hydrological hazards.

Natural scouring can cause dramatic changes in the plan shape, cross-sectional shape andeven location of a river. Different riverbed materials scour at different rates and variations aredifficult to measure after a flood event as the peak scour depth usually occurs at or near thepeak of the flood and may subsequently refill as the flood flows recede. Figure 1 illustratesthe different types of scour. General scour occurs as a result of the natural energy of theflow, constriction scour occurs if a structure causes the narrowing of a watercourse or thereturn of floodplain flow to the main channel, and local scour occurs directly from the impact

of the structure on the flow [4].



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Figure 1. Types of Scour at a bridge [4].

Figure 2. Sketch of typical bridge and immediate environment [5].

Figure 2 shows the flow through a bridge site involving complex interactions between thebridge structure, floodplain and the main channel especially during high flow conditions [5]

4. Existing Assessment Methods

In the United States the Federal Highway Administration offers guidelines for the monitoringof scour at bridges. There are three Hydraulic Engineering Circulars, of which we are mostinterested in HEC-18 “Evaluating Scour at Bridges” [6]. The guidelines offered in thisdocument relate directly to the bridge structure, namely their design, evaluation andinspection. The departments of transport of states across the U.S. carry out scourevaluations of their bridges using these guidelines. While each state can develop their owninspection program, the general approach (Figure 3) used is outlined below:





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Step 1. All bridges over waterways are screened into five categories:

(1) low risk

(2) scour susceptible

(3) scour critical

(4) unknown foundations

(5) tidal

Bridges which are particularly vulnerable to scour failure are identified immediately and theassociated scour problem addressed.

Step 2. Scour susceptible bridges and bridges with unknown foundations are prioritised byconducting a preliminary office and field examination of the list of bridges compiled in Step 1,using the following factors as a guide:

a) The potential for bridge collapse or for damage to the bridge in the event of a major

flood

b) The functional classification of the highway on which the bridge is located, and the

effect of a bridge collapse on the safety of the travelling public and on the operation

of the overall transportation system for the area or region.

Step 3. Field and office scour evaluations are conducted on the bridges prioritised in

Step 2 using an interdisciplinary team of hydraulic, geotechnical, and structural engineers:

Step 4. Bridges identified as scour critical from the office and field review or during a bridge

inspection in Step 2 should have a plan of action developed for correcting the scour problem.

This plan of action should include:a) Specific instructions regarding the type and frequency of inspections to be made at the

bridge, particularly in regard to monitoring the performance and closing of the bridge,

if necessary, during and after flood events.

b) A schedule for the timely design and construction of scour countermeasures

determined to be needed for the protection of the bridge.

Step 5. After completing the scour evaluations for the list of potential problems compiled in

Step 1, the remaining waterway bridges included in the state's bridge inventory should be

evaluated.



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Figure 3. Flow chart of the US FHWA inspection procedure.

In the United Kingdom, the Highways Agency has issued an advice note, BA7406 [7], whichoffers guidelines for assessing the susceptibility of bridges to scour damage. While theHighways Agency developed this note primarily for use on road bridges across waterways inthe UK, they indicate that it may also be applied to other types of bridges.

There are two stages in the approach they suggest, an initial assessment and gathering of

information, followed by analysis. Their outline of these stages is quoted below.

Stage 1 Assessment

• This involves the collection of data regarding the bridge, its foundations and the river and, ifpossible, any information on the history of the bridge and any problems experienced. Thisstage should always include a site inspection.

• The principal element of Stage 1 is the site inspection and an assessment by the Inspectoras to whether the bridge could suffer from scour damage at all. If there are features thatmake the risk of scour endangering the bridge very low, then the analysis need proceed nofurther. Otherwise, the assessment should proceed to Stage 2.





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Stage 2 Analysis

• Stage 2 involves a calculation of the potential scour depths and then an assessment ofPriority Rating. The steps in the calculation are as follows:

(i) An estimation is made of the magnitude of the 200-year flood at the bridge site.

(ii) For that flood the depths and velocities of flow in the upstream river channel and atthe bridge are calculated.

(iii) With those flow parameters, the potential depths of scour adjacent to the bridge

piers and abutments are determined.

(iv) Finally, the Priority Rating is established. This is based on a number of parameters

including the relative depth of scour to the depth of the bridge foundation, the type of

foundation, whether there has been a history or scour problems, the stability of the

river within its channel, and the importance of the road measured in terms of the

traffic volume.

Comparing the two approaches reveals strong similarities. The assessment of a bridge‟svulnerability to scour requires a greater detail and range of data than a site inspection canprovide. Additional information sources, such as orthophotos and hydrological stationreports, are required to more accurately predict the river channel‟s behaviour. Historicmapping, channel soundings, and previous inspection records are essential to understandingthe development of any issues at the bridge site. These are the minimum further sources ofinformation required to adequately assess the scour vulnerability of a bridge structure.Storing, maintaining and accessing a physical catalogue of these documents are expensiveand complicated, yet crucial, tasks for the bridge owner.

The analysis of the information gathered in both the field and office is an onerous task whichrequires a panel of qualified professionals. The appraisals and calculations involved in

assigning a risk rating to a bridge are time-consuming. This limits the anticipative element ofbridge scour protection, as predicting the outcome of real-time events is rarely cost-effective.

5. SIBIAM

There is an opportunity, and furthermore a need, to develop either a bridge managementsystem or a plug-in module for existing software, which includes scour as a factor in themonitoring of the bridge. This would significantly extend the effectiveness of bridgemonitoring, and aid labour-intensive tasks such as the analysis and scheduling processes.Such a tool would require the continued input of field and office research, but would offerdistinct improvements on the current state of the art.

Risk Analysis methods will form an integral part of the system. Estimating the risk of failureinvolves correlating historic rates of failure with the potential for a given hazard at a site, inaddition to indicators of a bridge‟s vulnerability to failure [8]. A bridge‟s vulnerability to failureis generally influenced by two basic factors, the degree of stress or degradation that a bridgecan safely withstand and the corresponding severity of the hazardous event required toinduce this degree of stress or degradation. Components of the risk determination willinvolve the product of the estimated probability of failure (which includes hydrological,hydraulic and geomorphological factors) and the total cost of failure (bridge replacement,workarounds, loss of life) A screening analysis technique will be used to select the mostappropriate prioritisation plan for the sample size of bridges which will be used i.e. those withknown vs. unknown foundations, previous history of failure etc. Once the risk of failure hasbeen quantified, the estimated risk will be used to select an appropriate course of action.



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As data is gathered on a bridge, it would be input to a database. This database would collatereports and inspection records compiled on the bridge. Outside data sources, such as theOPW for hydrological station data, could be monitored to maintain a continuous log ofrelevant data for the bridge (Figure 4). This would both make the information easily available,and provide a log of changes at the bridge site. The costs and effort associated with storingand retrieving this data would be significantly reduced due to the centralised database.

A large proportion of the analysis that is carried out when considering a bridge‟ssusceptibility to scour would be automated and accelerated by SIBIAM. While furtherconsideration of the results will still be required by the appropriate professionals beforeassigning a final scour risk rating, the key data would now be prepared and highlighted.Once the bridge has been fully assessed, a PoA and inspection schedule is normallyoutlined. There is now an opportunity to add key criteria to monitor at the bridge structureand site. Calculations can be carried out real-time, and the system can flag parameterswhich exceed constraints determined during the bridge‟s assessment. Depending on theurgency set for the flag, the system may issue a notice for action to the relevant personnel,or simply log the event as information to be considered during the next bridge assessment.

One example of SIBIAM‟s potential application involves inspection scheduling. Thefrequency of inspections for each bridge is set during its assessment. Keeping track of this isa complicated task, particularly when the number of individual bridges becomes significant.Planning the inspections once they are due holds further challenges and requireconsiderable manpower. SIBIAM would address this by cataloguing and automating a rangeof processes. As a suggested inspection date approaches, the system will begin planningthe range of tasks involved. Firstly, it will consider the inspection dates of any other bridgeson the system which are located in close proximity to the flagged bridge. If an inspection isdue on any of these bridges within two years, or if it is close and the schedule has leeway,they will be added to the inspection list for that day. This will optimise the use of resources.The system will then notify the relevant individuals, such as the inspectors, the engineering

department of the transport authority, and the local supervisors for the bridge structure. It willprepare a schedule based on optimal travel times between the bridges flagged forinspection, and list the necessary resources and personnel. This automated planning is oneexample of the extensive labour savings SIBIAM could make on crucial yet low-impactdecisions. This leaves the management and engineering teams free to concentrate on fullyassessing the comprehensive bridge data collated by SIBIAM.

Figure 4. Outline of potential database inputs and outputs.





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6. Conclusion

An effective bridge management system must recognise that all bridges over watercourses

are potentially at risk of scour. The assessment of the level of risk and appropriate action putin place to reduce that risk are crucial to the success of the system. It is not economicallyfeasible for a bridge owner to protect a bridge from all conceivable flood events and potentialscour related risks. It is crucial however that every bridge over a watercourse is appropriatelyassessed and risk mitigation measures are put in place. The need to ensure public safetyrequires us to continually improve and provide the most state of the art practises needed fordesigning, monitoring and maintaining our bridge structures to ensure that they resist theeffects of scour. The system may not prevent failures due to scour during the more extremeevents but it will ensure that the bridge owner has fulfilled its „duty of care‟ to users

References

1. The IABMAS Bridge Management Committee „Overview of Existing Bridge ManagementSystems 2010‟, July 2010.

2. Kattell, J and Eriksson, M., Bridge Scour Evaluation: Screening, Analysis and

Countermeasures. U.S. Department of Agriculture Forest Service San Dimas California.

3. Transportation Research Board of the National Academies. Monitoring Scour Critical

Bridges. Washington D.C. : s.n., 2009.

4. Whitbred et al, Cost Effective Management of Scour Prone Bridges., Proceedings of the

Institute of Civil Engineers, Transportation, May 2000.

5. National Co-operative Highway Research Program, Evaluation of Bridge Scour

Research, Pier Scour Processes and Predictions, NCHRP Project 24-27 (01), March2011.

6. Richardson, E.V., and Davis, S.R., 2001, Evaluating scour at bridges, fourth edition:

Federal Highway Administration Hydraulic Engineering Circular No. 18, FHWA-IP-90-

017.

7. Highways Agency. Assesment of Scour at Highway Bridges. Department for Transport

UK. [Online][Cited:4 August 2011.]

http://www.dft.gov.uk/ha/standards/dmrb/vol3/section4/ba7406.pdf .

8. Transportation Research Board of the National Academies, Risk-Based Management

Guidelines for Scour at Bridges with Unknown Foundations., NCHP Project 24-25,

October 2006.

http://www.dft.gov.uk/ha/standards/dmrb/vol3/section4/ba7406.pdf



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A Self Informed Bridge Assessment, Maintenance Management Tool for Bridge Monitorng