Risk Based Inspection of Highway Structures - … Good Practice Guide_Final.pdf · Risk Based...

Transport for London

Surface Transport

TfL Good Practice Guide

Risk Based Inspection of Highway Structures December 2011

TfL Good Practice Guide: Risk Based Inspections for Highway Structures

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DOCUMENT DETAILS:

NAME: TfL Good Practice Guide: Risk Based Inspection for Highway Structures

VERSION: 1.0

OWNER: Transport for London

PREPARED BY: Atkins

DATE OF ISSUE: 09 December 2011 /

VERSION HISTORY:

Version Purpose Description Date

0.1 First issue for review 03/06/11

0.2 Second review issue 01/09/11

1.0 Final 09/12/11


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Table of Contents

Glossary ..................................................................................................................... iii

Abbreviations ............................................................................................................. iv

1 Introduction .......................................................................................................... 1

1.1 General .......................................................................................................................... 1

1.2 Purpose ......................................................................................................................... 1

1.3 The Need for Risk Based Inspection Planning .............................................................. 1

1.4 Layout of the Good Practice Guide ............................................................................... 1

2 The Evolution of Risk Based Asset Management .............................................. 2

2.1 Overview ........................................................................................................................ 2

2.2 Why use a risk based approach? .................................................................................. 2

2.3 Risk Based Inspection ................................................................................................... 3

2.4 Risk Based Maintenance ............................................................................................... 3

2.5 Prioritisation and Value Management ........................................................................... 4

3 Framework for Risk Based Inspections .............................................................. 5

3.1 Overview ........................................................................................................................ 5

3.2 Assumptions .................................................................................................................. 5

3.3 Steps in Determining the Risk based Inspection Intervals ............................................ 6

3.4 Identification of Structure Type and Inspection Type .................................................... 9

3.5 Probability of Rapid Deterioration, Damage or Failure Score ..................................... 11

3.6 Consequence of Failure .............................................................................................. 19

3.7 Risk Score and Risk based Inspection Intervals ......................................................... 21

3.8 Develop Unsmoothed PI Programme.......................................................................... 23

3.9 Determine Target Annual Cost of Inspections ............................................................ 23

3.10 Develop Provisional Smoothed PI Programme ........................................................... 23

3.11 Develop Final Smoothed RBI Programme .................................................................. 23

3.12 Risk Profiling ................................................................................................................ 24

4 References .......................................................................................................... 27

Appendix A ................................................................................................................ 28


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Glossary Asset[1] Physical highway infrastructure and other items that have a distinct

value to a highway authority, e.g. carriageway, footway, structures, tunnels, lighting, etc.

General Inspection[2] The purpose of a General Inspection is to provide information on the physical condition of all visible elements on a highway structure. A General Inspection comprises a visual inspection of all parts of the structure that can be inspected without the need for special access equipment or traffic management arrangements

Lifecycle Plan[3] A considered strategy for managing an asset, or group of similar assets, from construction to disposal. A lifecycle plan should give due consideration to minimising costs and providing the required performance.

Maintenance[3] Activities and operations undertaken to manage and maintain an asset, e.g. inspection, assessment, renewal, upgrade, etc.

Principal Inspection[2] The purpose of a Principal Inspection is to provide information on the physical condition of all inspectable parts of a highway structure. A Principal Inspection is more comprehensive and provides more detailed information than a General Inspection. A Principal Inspection comprises a close examination, within touching distance, of all inspectable parts of a structure.

Prioritisation See ‘Value Management’.

Risk[4] An event or hazard that has the potential to hinder the achievement of business objectives.

Special Inspection[2] The purpose of a Special Inspection is to provide detailed information on a particular part, area or defect that is causing concern, or inspection of which is beyond the requirements of the General/Principal Inspection regime. A Special Inspection may comprise a close visual inspection, testing and/or monitoring and may involve a one-off inspection, a series of inspections or an ongoing programme of inspections.

Value Management[4] A systematic approach for identifying, assessing, prioritising and optimising a portfolio of projects, based on an agreed set of Value Criteria, which maximises contribution to the business objectives for a defined budget.

Whole Life Cost[1] The cost of all items/activities that need to be considered in a whole life costing analysis, such as the costs of acquiring (includes design and construction costs), operating and maintaining an asset over its whole life through to its eventual disposal.


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Abbreviations

CCS Current Condition Score

CF Consequence of Failure

CPI Condition Performance Indicator

CSS County Surveyors Society (currently known as ADEPT)

DfT Department for Transport

DLI Date of Last Inspection

DSI Date of Scheduled Inspection

FMS Failure Mode Score

FPII Principal Inspection Interval Adjustment Factor

GI General Inspection

GPG Good Practice Guide

IS Inspectability Score

MOFS Magnitude of Failure Score

PI Principal Inspection

PPII Proposed Principal Inspection Interval

PrddfS Probability of rapid deterioration, damage or failure score

RBI Risk Based Inspection

RODS Rate of Deterioration Score

SEIS Socio-Economic Importance

TfL Transport for London


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1 Introduction 1.1 General

This Good Practice Guide has been prepared for Transport for London (hereafter referred to as TfL) by Atkins. This Good Practice Guide describes the methodology developed for the risk based inspection planning for Highway Structures.

1.2 Purpose The purpose of this guide is to provide a step-by-step guide to risk based inspection planning for Highway Structures on the Transport for London road network.

1.3 The Need for Risk Based Inspection Planning Highway structures are subject to a regime of General Inspections (GIs) and Principal Inspections (PIs) every two and six years respectively (and to Special Inspections as and when required). These inspection regimes are clearly defined in:

• Management of Highway Structures: A Code of Practice (2005)[3];

• Inspection Manual for Highway Structures (2007)[5];

• BD 63: Inspection of Highway Structures (2007)[2].

However, the above documents also allow structures owners to increase or decrease the inspection intervals based on a robust and fully documented risk assessment, i.e. Risk Based Inspection. The aforementioned documents do not describe the specific details of the risk assessment that should be undertaken, as such, the majority of highway structures owners have tended to retain the standard two and six year cycle.

Previous work has shown that whilst there is a general consensus that the time-based two year General Inspection regime is effective, some organisations feel that the six year Principal Inspection regime could be tailored using a risk assessment in order to improve the risk profile, minimise the liability to the structures owner and utilise inspection resources more effectively.

1.4 Layout of the Good Practice Guide The layout of the Good Practice Guide is summarised in Table 1.

Section Description

2. The Evolution of Risk Based Asset Management

Describes how risk based methodologies are utilised to support objective and optimum management of Highway Structures.

3. Framework for Risk Based Inspections

Defines the approach which should be used to determine the Risk based Inspection intervals in a step-by-step manner.

4. References Relevant documents referred to for the purpose of this study.

Appendices Provide supporting information.

Table 1: Layout of the Good Practice Guide

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2 The Evolution of Risk Based Asset Management 2.1 Overview

Asset Management was first defined in the Framework for Highway Asset Management [6] as:

Asset management is a strategic approach that identifies the optimal allocation of resources for the management, operation, preservation and enhancement of the highway infrastructure to meet the needs of current and future customers.

There is a growing movement to develop methodologies that support formal asset management of highway structures. The publication of the Code of Practice[3] has given further impetus to this.

An important component of Risk-Based Asset Management is categorisation of assets by level of risk which enables asset owners to objectively assess asset condition, evaluate maintenance and inspection programs, study operating protocols, and estimate the remaining life of assets – in relation to or considering the likelihood and consequences of structural and/or other failure. This information is then used to modify and optimise inspection and maintenance programs, audit procedures, operating limits, and other risk mitigation measures.

2.2 Why use a risk based approach? Until recently, the structural safety of most highway structures has been assured by two factors:

• Design, assessment and maintenance in accordance with codes or rules incorporating empirical safety factors; and

• Regular inspections to provide assurance that no accidental damage or unanticipated deterioration has occurred.

However, it is prudent to accept that operational loads may vary beyond design levels, and that material degradation may be greater than anticipated. The safety factors used at the design stage may not, therefore, guarantee through-life structural safety. Hence periodic inspection is also carried out to determine the actual levels of deterioration, and to check the adequacy of the design loads (e.g. inspection for assessment) and identify appropriate maintenance actions.

Both inspection and maintenance strategies should take account of the risk of structural failure. Using traditional approaches to inspection planning, risk tends only to be considered implicitly and is not assessed in an auditable manner. There is thus a real concern that ‘high-risk’ and ’low-risk’ assets may not be clearly identified and distinguished for management purposes. This may then mean that ‘low-risk’ assets are inspected more frequently than is necessary which leads to needlessly high inspection costs, while ‘high-risk’ assets may not all be afforded sufficient attention and priority. Without the explicit consideration of risk, resources may not be appropriately targeted.

An inspection strategy based on risk overcomes the limitations of the traditional approach and ensures that resources are appropriately targeted. The concept of risk takes into account, not only the probability of failure, but also the consequences of failure. These may encompass consequences in terms of lost profits, repair and re-justification costs, human casualties and environmental costs. Such a strategy ensures that inspection effort is targeted appropriately to optimise costs and benefits, and provides an auditable demonstration that this has been done with due diligence.

The maintenance and inspection strategies have evolved, as shown in Figure 1.

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Figure 1: Evolution of Inspection and Maintenance Planning Strategies[7]

Clearly risk is an important consideration in the asset management of highway structures. Available guidance for three principal areas of application is reviewed below.

2.3 Risk Based Inspection The intervals for General and Principal Inspections are mandated in BD 63[2] and they are largely the same for all structures regardless of their strategic importance and risk of failure. There has been considerable interest in applying a risk based approach to setting inspection intervals or seeking departures from standards for low risk structures which are difficult to inspect, however, practical guidance is not yet available.

The Code of Practice for the Management of Highway Structures[3] provides some basic guidance for varying the Principal Inspection intervals based on risk considerations. Recently, Network Rail[16] and Welsh Assembly[17] introduced flexibility in their inspection regimes to alter the inspection intervals based on qualitative estimate of risk. Further work is needed in producing guidance for wider application in practice.

2.4 Risk Based Maintenance Risk based Maintenance is widely used in a number of sectors but its application to highway structures is rather limited. The Highways Agency has applied risk based methods in developing maintenance regimes for Midland Links Viaducts and for half joints and hinge joints.

The Highways Agency commissioned Atkins to develop optimum maintenance strategies and plans (i.e. lifecycle plans) based on risk and whole life cost approaches. The lifecycle plans will be utilised to develop a long term Asset Management Plan for the entire stock of the Highway Agency’s highway structures. It is intended that the project will also produce some guidance for use by the Highway Agency’s maintaining Agents (MAs & MACs) to apply risk and whole life cost approaches to individual bridges to determine optimal maintenance interventions.

Time-Based

Condition-Based

Risk-Based

• Calendar-based intervals (e.g. every 2 years, 6 years, etc.)

• Prescriptive Regulatory / Rule requirements

• Trending (likelihood)

• Inspection intervals based on condition

• Holistic risk-based approach

• Balance between risk and inspection effort

• Proactive approach


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2.5 Prioritisation and Value Management The Highway Agency’s Value Management process already takes account of risk in prioritising maintenance needs. A simple risk scoring procedure is given for scoring a range of maintenance and upgrade works. TfL has also developed suitable Value Management process for all asset types.

Following the Selby incident the DfT has issued a risk ranking procedure for ranking road/rail intersections. A risk based system has been used by the London Bridges Engineering Group (LoBEG) to prioritise bridges for strengthening. A similar system has recently been developed by the UK Bridges Board for prioritising road over rail bridges for strengthening.

Atkins is currently developing end-to-end maintenance prioritisation processes, which take full consideration of risk, for LoBEG and Transport Scotland.


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3 Framework for Risk Based Inspections 3.1 Overview

The aim of the framework is to provide TfL with a practical approach and guidance for readily and objectively determining Risk based Inspection intervals for its highway structures in order to identify and adopt an inspection regime that minimises liability. In developing the framework, a classic risk approach has been adopted whereby the risk of an event (e.g. defect occurring and/or progressively getting worse over a period of time) is assessed through Likelihood and Consequence of the event, i.e.:

Risk = f (Likelihood of Event, Consequence of Event)

Where the probability and consequence of event are defined as:

• Probability of Rapid Deterioration, Damage or Failure – given the current condition, i.e. the condition at the last recorded General or Principal Inspection, the rate of deterioration, the inspectability, etc., what is the likelihood that a structure or part of a structure will deteriorate further or even fail?

• Consequence of Failure – given that deterioration or damage remains undetected what are the likely consequences of damage or failure in terms of traffic disruption, casualties and other socio-economic impacts.

3.2 Assumptions The risk based inspection framework has been developed to aid TfL in establishing its current risk profile (i.e. while the time-based inspection regime is adopted) and to allow comparisons to be made with the risk profile that would result from the application of this framework. It is anticipated that by using the risk based inspections framework TfL will be reducing its overall level of risk exposure and hence minimise liability.

It is considered that the framework can be applied at any time to assist in the identification of the level of risk that may be posed by different assets including parts of a structure, e.g. very high importance elements (as defined by the CSS inspection procedure[8]), individual structures or a group of structures with similar characteristics. Subsequently, this can be used to determine appropriate Principal Inspection intervals, thus focussing effort and resources where these are most needed to ensure that any risks identified can be mitigated and/or eliminated effectively.

It has been assumed that as a baseline, TfL will hold suitable condition data, inventory and other relevant data, e.g. structure dimensions, exposure conditions, etc., that can be used to inform the risk based inspections framework. It is essential that all structures in TfL’s stock have undergone at least one General Inspection and that the data from this inspection has been recorded and is readily available.

It is noted that inspection alone is not sufficient to mitigate risk even when it is undertaken at the intervals identified under this framework. Appropriate action arising from inspection findings will also be required.

The following were excluded when formulating the risk based inspection framework for the reasons outlined below:

• Live loading capacity – The inspection intervals that will be identified under the proposed framework will enable significant deterioration to be identified at an optimum time. As such it is likely that the appropriate maintenance needs will be identified and the relevant maintenance activities undertaken so that the structural integrity and the live loading capacity of the structure will remain un-affected during the service life of a structure. Where a structure had undergone structural assessment and was found to be substandard, it is assumed that it would be managed in accordance with BD79[9], i.e. its


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risk is managed through weight/width restrictions or monitoring, as appropriate. Any reserve capacity above the assessment live load would clearly have an influence on risk and could, in principle, be taken into account in setting inspection intervals. However, information on reserve capacities of major structural elements is typically not recorded. Hence it was decided not to consider ‘capacity’ as a factor in developing the RBI methodology.

• Monitoring systems – Where a monitoring system is present on a structure, the level of monitoring applied is commensurate with the level of risk posed by the critical element in accordance with BD 79[9]. Where a monitoring system may be required for a structure and/or element of a structure but is not present and the structure is analysed under the risk based inspection framework, it should yield ‘very high’ likelihood and consequence values indicating ‘very high’ levels of risk which would suggest that Principal Inspection intervals should be reduced.

• Impact and/or vandalism damage – Safety inspections and General Inspections will identify obvious deficiencies which represent, or might lead to, a danger or may pose a safety risk to the public.

3.3 Steps in Determining the Risk based Inspection Intervals An overview of the process for determining the risk based inspection intervals is shown in Figure 2.


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Figure 2: Flowchart for Determining the Risk based Inspection Intervals

Step 1: Select Structure(s) Type

The risk based inspections framework can be adopted to determine the appropriate Principal Inspection intervals for parts of a structure, i.e. very high importance elements (as defined by the CSS inspection procedure[8]), individual structures or a group of structures with similar characteristics. In all three cases the structure type of the structure(s) under evaluation is

1. Select Structure Type (Section 3.4)

2. Identify Inspection Type (Section 3.4)

Risk-based PI? Schedule time-based GI

3. Evaluate the Probability of Rapid Deterioration, Damage or Failure (Section 3.5)

4. Evaluate the Consequence of Failure (Section 3.6)

5. Determine the Risk score and the Risk based Inspection Interval (Section 3.7)

6. Develop Unsmoothed RBI Programme (Section 3.8)

Repeat the analysis until an acceptable level of risk is determined for the proposed PI interval

No

Yes

7. Determine Target Annual Cost of PIs (Section 3.9)

8. Develop Provisional Smoothed RBI Programme (Section 3.10)

All structures within acceptable risk limit?

9. Develop Final Smoothed RBI Programme (Section 3.11)

Yes

No

Revise the programme until an acceptable level of risk is determined for all structures


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initially selected from Table 3 in Section 3.4.

Step 2: Identify Inspection Type

For the selected structure(s) type from Step 1, the appropriate inspection type, i.e. time-based or risk-based, is identified from Table 3 in Section 3.4. If the inspection type is identified as risk-based Principal Inspection (PI), proceed to Steps 3, 4 and 5 below. If the inspection type is identified as time-based General Inspection (GI), schedule a 2 yearly General Inspection. In the majority of instances both a risk-based PI and a time-based GI are required.

Step 3: Evaluate the Probability of Rapid Deterioration, Damage or Failure

The probability of rapid deterioration, damage or failure is a function of the current condition of a structure and/or element of a structure, the rate of deterioration, the potential failure mode, the inspectability, and the Principal Inspection interval, i.e. time-based every 6 years vs. risk based decreased/increased interval.

Step 4: Evaluate the Consequence of Failure

The consequence of failure is based on the magnitude of failure and the socio-economic importance of the structure(s).

Step 5: Determine the Risk Score and the Risk based Inspection Interval

The analysis is initially undertaken for the time-based 6 year Principal inspection Interval and the Risk score for this scenario is selected from the Risk Score Matrix in Section 3.7 (see Table 13) by combining the outcomes from Steps 3 and 4 above. The Principal Inspection interval is then altered, if deemed appropriate; by using the Risk based Inspection Intervals Matrix in Section 3.7 (see Table 14). The analysis is then undertaken for the proposed Principal Inspection Interval until an acceptable risk score and risk-based inspection interval is obtained. The acceptable risk score can be defined considering factors, such as accuracy of the analysis, level of refinement required, etc. In the current study acceptable risk is defined as equal to or better than the ‘moderate’ category, i.e. risk score < 60 (see Table 13).

Step 6: Develop Unsmoothed RBI Programme

The PI interval, derived from Step 5 above, is used in combination with the last known PI date to produce a PI programme. For a structure, this means adding the proposed inspection interval to the last inspection date in order to determine when the next PI should be conducted.

Step 7: Determine the Target Annual Cost of PIs

The resulting programme from Step 6 can be expected to produce an uneven distribution of PI costs over time, i.e., more resources being required in some years than others. A smoothing process is introduced in order to smooth the PI cost profile with time. As the first step in the smoothing process, a desirable target annual cost of PIs needs to be identified. The average cost required to undertake PIs over a 30 year programme is set as the initial target annual cost of PIs for each financial year (see Section 3.9).

Step 8: Develop Provisional Smoothed RBI Programme

A provisional smoothed PI programme is developed by comparing the cost of PIs due in a financial year, including any moved forward from previous years, against the initial target cost of PIs.


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Step 9: Develop Final Smoothed RBI Programme

The revised risk score of each structure, resulting from the provisional smoothed programme, is checked against the acceptable risk level. If it falls within the acceptable range then the PI will be programmed according to the provisional smoothed PI programme. Otherwise, the inspection programme will be revised until all structures fall within the acceptable risk range.

3.4 Identification of Structure Type and Inspection Type A simplified risk assessment is undertaken using the criteria contained in Table 2 to identify the appropriate inspection type(s) for the generic structure types listed in Table 3, i.e. Time based GI only or Time based GI and Risk Based PI. This acts as a high-level ‘filter’ that enables the identification of structure types (i) that do not require more in-depth risk analysis; and (ii) that do require more in-depth risk analysis to be readily identified.

The simplified risk assessment criteria cover:

• Inspectability, i.e. ease of access, and whether or not a General Inspection provides all the necessary information for decision making; and

• Consequences, i.e. assessing whether there are any significant safety/service consequences (in particular to identify those asset types with low consequences that as such do not merit a Principal Inspection regime).

Inspectability *

Consequence ^ Yes No

Very Low Time Based GI Time Based GI

Low Time Based GI Time Based GI + Risk Based PI

Moderate Time Based GI Time Based GI + Risk Based PI

High Time Based GI + Risk Based PI Time Based GI + Risk Based PI

Very High Time Based GI + Risk Based PI Time Based GI + Risk Based PI

Criteria Definitions:

* Inspectability

Can the necessary information about the condition of the structure(s) and any significant safety concerns be readily obtained without any access difficulties?

^ Consequence

• Very Low – No fatalities, no injuries and no/minor disruption to service

• Low – No fatalities, fewer slight injuries and minor disruption to service

• Moderate – Fewer serious injuries, and some slight injuries and moderate disruption to service

• High – Possibility of a fatality, some serious injuries and a significant number of slight injuries, and major disruption to service

• Very High – One or more fatalities and widespread disruption to service

Table 2: Criteria for the Identification of the Inspection Type(s)


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No. Structure Type Inspection Type Comments

1. Bridge Time Based GI + Risk Based PI

2. Cat Lighting Time Based GI + Risk Based PI 25m in height

3. Chamber Time Based GI + Risk Based PI

4. Culvert

Time Based GI + Risk Based PI

For culverts with width > 2m.

Time-based, GI’s only For culverts width ≤ 2m and fill > 1.5m

5. Embankment Treat as Retaining Walls

These are actually retaining walls.

6. Footbridge Time Based GI + Risk Based PI Treat as a ‘Bridge’.

7. Gantry Time Based GI + Risk Based PI

8. Mast Time Based GI + Risk Based PI Less than 20m in height.

9. Pipe Subway Time Based GI + Risk Based PI Treat as ‘Subway’

10. Retaining Wall Time Based GI + Risk Based PI

11. River Wall Time Based GI + Risk Based PI Treat as ‘Retaining Wall’

12. Subway Time Based GI + Risk Based PI

13. Tunnel Time Based GI + Risk Based PI

14. Vault Time Based GI + Risk Based PI Treat as a ‘Bridge’.

Table 3: Identification of Structure Type and Inspection Type


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3.5 Probability of Rapid Deterioration, Damage or Failure Score 3.5.1 Overview of the Probability of Rapid Deterioration, Damage or Failure Evaluation

Process

The probability of rapid deterioration, damage or failure is a function of the following:

• Current condition of a structure and/or element of a structure, i.e. the condition of the structure and/or element of the structure at the last recorded General or Principal Inspection (Section 3.5.2).

• Rate of deterioration, which is dependent on the structure specific characteristics and the severity of exposure to external influences (Section 3.5.3).

• Potential failure mode, e.g. brittle or ductile failure (Section 3.5.4).

• Inspectability, i.e. the ability to adequately inspect all parts of the structure (Section 3.5.5)

• Principal Inspection Interval, e.g. time-based every 6 years or risk based decreased/increased interval (Section 3.5.6)

The probability of rapid deterioration, damage or failure score is calculated using Equation 1:

PrddfS = (CCS + RoDS + FMS + IS) x FPII but not < 1 or > 100

Equation 1

Where PrddfS = Probability of rapid deterioration, damage or failure score

CCS = Current Condition Score for the entire structure and/or the critical element,

whichever has a lower Condition Performance Indicator (CPI)

RoDS = Rate of Deterioration Score

FMS = Potential Failure Mode Score

IS = Inspectability Score

FPII = Principal Inspection Interval adjustment factor

3.5.2 Current Condition Score, CCS

The current condition score is selected from Table 4 below based on the Condition Performance Indicators (CPI) for an individual structure, i.e. CPIAv (All Structure Elements) or CPICrit (Worst Critical Element) whichever is lower, established during the most recent General Inspection. The analysis can also be undertaken for a group of structures with similar characteristics, for which the current condition score is based on the CPI of the structure in the worse condition in the group.

CPI values of less than 40 are not permissible under the RBI framework. In these cases it is recommended that the maintenance needs of the structure should be identified (if not already known) and duly addressed.

Unknown CPI values are not permissible under the RBI framework. In these cases it is recommended that a General Inspection is scheduled and undertaken so that the relevant information may be obtained prior to carrying out an analysis of any structure(s) with unknown CPI values.

The interpretation of the CPI Range shown in Table 4 was adopted from the Guidance Document for Performance Measurement of Highway Structures, Part B1: Condition Performance Indicator [8] and is contained in Appendix A.


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Current Condition Score

CPI Range CPIAv (All structure elements)

CPICrit (Worst Critical Element)

90 ≤ x ≤ 100 5 5

80 ≤ x < 90 15 10

65 ≤ x < 80 30 20

40 ≤ x < 65 50 30

0 ≤ x < 40 Not Permissible: Appropriate maintenance is required

Table 4: Current Condition Score, CCS

3.5.3 Rate of Deterioration Score, RoDS

Many defects are known to take many years to develop to the point where they require maintenance or present a risk to structural integrity or public safety. However, the structure specific characteristics, i.e. construction form, details susceptible to deterioration and the overall construction material are significant considerations in determining the rate of deterioration for observed deterioration mechanism(s). Table 5 provides the anticipated deterioration rates for typical construction forms for bridges, culverts, gantries, retaining walls, subways and masts and their respective construction material types. Three classifications were used to distinguish between the different deterioration rates as follows:

• Slow (S) – Given mild exposure circumstances structural forms and materials with a slow rate of deterioration would require over 120 years to reach a condition score of 40 or below.

• Medium (M) – Given mild exposure circumstances structural forms and materials with a medium rate of deterioration would require between 60 – 120 years to reach condition score 40 or below.

• Fast (F) – Given mild exposure circumstances structural forms and materials with a fast rate of deterioration would require less than 60 years to reach condition score 40 or below.

Note: The list presented in Table 5 is not exhaustive. Where a particular combination of material and structural form is not available, a close approximate combination can be selected or engineering judgement can be used in estimating the rate of deterioration.

In addition to the structure specific characteristics the rate of deterioration is significantly influenced by the exposure severity, i.e. external influences that may cause rapid or progressive deterioration, damage or failure. Four classifications are used to distinguish between the different severities of exposure, i.e. mild, moderate, severe and very severe, and a detailed description of these is provided in Table 6.

Structures that are not currently included in Table 5 will be treated as follows:

• ‘Cat Lighting’ will be treated as ‘Mast’,

• ‘Chamber’, ‘Pipe Subway’ and ‘Tunnel’ will be treated as ‘Subway’,

• ‘River Wall’ will be treated as ‘Retaining Wall’, and

• ‘Vault’, ‘Footbridge’ will be treated as ‘Bridge’.


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The Rate of Deterioration Score, RoDS, takes account of the above considerations and can be selected from Table 7 which is implicitly based on a 6 year PI interval and distinguishes between the different deterioration rates and exposure severities.


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Table 5: Rate of Deterioration Depending on Structure Characteristics

Structure Type Bridge Culvert Gantry Retaining Wall Subway Mast

Structural Form Arc

h

Bea

m &

Sla

b

Slab

Trus

s

Arc

h

Pipe

Box

Port

al

Slab

Trus

s

Bea

m

Can

tilev

er T

russ

Can

tilev

er B

eam

Gra

vity

Can

tilev

er

Embe

dded

Box

Wal

ls

U-b

ase

& S

lab

N/A

Mat

eria

l Typ

es

Masonry S N/A N/A N/A S N/A N/A N/A N/A N/A N/A N/A N/A S M N/A N/A S N/A N/A

Mass Concrete S N/A N/A N/A S N/A N/A N/A N/A N/A N/A N/A N/A S N/A N/A N/A S N/A N/A

Reinforced Concrete S M M N/A S M M M M N/A M N/A M M M M M M M N/A

Prestressed Concrete (Pre-Tensioned)

N/A S S N/A N/A N/A N/A N/A N/A N/A S N/A S N/A N/A S N/A N/A N/A N/A

Prestressed Concrete (Post-Tensioned) N/A S S N/A N/A N/A N/A N/A N/A N/A N/A N/A N/A N/A N/A S N/A N/A N/A N/A

Steel (regularly painted and maintained)

S S S S S S N/A N/A N/A S N/A S N/A S N/A S N/A N/A N/A F

Weathering Steel N/A S S N/A N/A N/A N/A N/A N/A N/A N/A N/A N/A N/A N/A N/A N/A N/A N/A N/A

Cast Iron / Wrought Iron (regularly painted and maintained)

N/A S S N/A N/A N/A N/A N/A N/A N/A N/A N/A N/A N/A N/A N/A N/A N/A N/A N/A


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Exposure Severity

Exposure Description Examples

Mild

Structure and/or elements of a structure:

Generally exposed to mild weather conditions, i.e. may be sheltered or in an environment that results in little or no exposure to severe weather conditions; and

Not exposed to any aggressive agents, e.g. no exposure to road de-icing salts or 10 m away from traffic spray, not exposed to or buried in aggressive soil agents, no exposure to contaminated water, etc.; and

With no ventilation or condensation problems or where poor ventilation or the level of condensation are unlikely to increase the rate of deterioration.

Elements protected from salt spray with cladding or by a protective enclosure.

Deck soffit and piers of integral bridges where the obstacle crossed is not a road, i.e. elements are not subjected to spray from salted road.

Tenanted arch bridges.

Half-joints or hinge joints overlaid with functional expansion joints

Moderate

Structure and/or elements of a structure exposed to:

Moderate (normal) weather conditions, e.g. direct rain, moderate humidity or condensation, some freeze-thaw action etc.; and/or

Moderate de-icing salt spray and airborne chlorides; e.g. within 3 to 10m of traffic spray on routes with de-icing salts; and/or

Low to moderate river flow.

But elements are not exposed to or buried in aggressive soils that are contaminated with acidic water or water containing sulfates.

Top of roadside bridge pier or abutment subject to light vehicle spray from salted road.

Bridge deck soffit subject to light vehicle spray from salted road.

Structural elements, e.g. piers, subjected to abrasion/erosion.

Severe


Continuous or regular severe/extreme weather conditions, e.g. hot and cold extremes, high freeze-thaw action, severe humidity or condensation, etc.; and/or

Severe de-icing salt spray, e.g. within 3m of traffic spray on routes with de-icing salts; and/or

Run-off and/or ponding on routes with de-icing salts; and/or

Aggressive soils, i.e. completely or partially buried in aggressive soils that are contaminated with acidic water or water containing sulfates.

Medium to rapid river flow and flooding.

Roadside bridge abutment, Parapet upstand or deck edge beam subject to heavy vehicle spray from salted road.

Section of bridge deck near a leaking expansion joint or gutter, e.g. deck end or crosshead.

Half joints or hinge joints overlaid with non-functional expansion joints.

Top surfaces of unwaterproofed bridge decks.

Areas where corrosion or spalling of surface concrete is evident.

Structural elements, e.g. piers, susceptible to scour.


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Exposure Severity

Exposure Description Examples

Very Severe


Marine environment and/or abrasive action of sea water or completely immersed in sea water; and/or

Tidal splash and spray zone; and/or

Airborne salt but not in direct contact with sea water; and/or

Corrosive fumes in industrial areas

Surfaces directly affected by sea water spray, e.g. surfaces adjacent to the sea

Surfaces directly affected by airborne salts, e.g. deck, walls, parapet edge beams, etc.

Completely/partially submerged marine structures

Structures near to or on coastal areas

Structures in industrial areas with high humidity and aggressive atmosphere

Table 6: Exposure Severity Descriptions

Exposure Severity

Mild Moderate Severe Very Severe

Rat

e of

D

eter

iora

tion Slow (S) 5 15 30 40

Medium (M) 15 30 45 60

Fast (F) 20 45 60 80

Table 7: Rate of Deterioration Score, RoDS

3.5.4 Potential Failure Mode Score, FMS

The potential failure mode score, FMS, takes account of the potential mode of failure e.g. brittle or ductile failure. The scores shown in Table 8 should only used when the CPICrit is selected from Table 4. If CPIAv is used then FMS = 0.

Failure Mode Score

Brittle 10

Ductile 0

Table 8: Potential Failure Mode Score, FMS

Brittle, or non-ductile, materials will not undergo significant plastic deformation before breaking. Failure of a brittle material occurs suddenly, with little or no warning. Structural materials that are generally brittle include concrete, cast iron, stone and timber.

Ductility is the measure of plastic (permanent) strain that a material can endure. A ductile material will undergo a large amount of plastic deformation before breaking. It will also have a greatly reduced cross-sectional area before breaking. Structural materials that are generally ductile include steel and aluminium.


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3.5.5 Inspectability Score, IS

The Inspectability Score, IS, takes account of the ability of the inspector to adequately inspect all the elements that are significant to structural integrity during a General Inspection. Table 9 simply distinguishes between structures where elements that are significant to structural integrity can be adequately inspected and those that cannot be adequately inspected.

Table 10 provides a list of elements that are significant to the structural integrity of different structure types.

Inspectability Considerations Score

All elements significant to structural integrity (except foundations) are visible (not hidden) and can be adequately inspected during a General Inspection 0

One or more element(s) significant to structural integrity (except foundations) are not visible or hidden and/or cannot be adequately inspected during a General Inspection

20

Table 9: Inspectability Score, IS


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Structure Type Elements Significant to Structural Integrity (excludes foundations)

Bridge

• Primary deck element • Transverse Beams • Secondary deck element • Half joints • Tie beam/rod • Parapet beam or cantilever • Deck bracing • Abutments (incl. arch springing) • Spandrel wall/head wall • Pier/column • Cross-head/capping beam • Bearings

Cat Lighting See ‘Mast’ Group below

Chamber See ‘Bridge’ Group above

Culvert See ‘Bridge’ Group above

Footbridge See ‘Bridge’ Group above

Gantry

• Truss/beams/cantilever • Transverse/horiz. bracing elements • Columns/supports/legs • Base connections • Support to longitudinal connection

Mast Mast

Base Connection

Pipe Subway See ‘Bridge’ Group above

Retaining Wall • Retaining wall (Primary/Secondary) • Parapet beam/plinth • Anchoring system

River Wall See ‘Retaining Wall’ Group above

Subway See ‘Bridge’ Group above

Tunnel See ‘Bridge’ Group above

Vault See ‘Bridge’ Group above

Table 10: Elements Significant to Structural Integrity


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3.5.6 Principal Inspection Interval adjustment factor, FPII

The Principal Inspection Interval adjustment factor, FPII, takes account of the current and/or proposed Principal Inspection Interval. The Principal Inspection Interval adjustment factor can be calculated using Equation 2:

years6PPIIFPII =

Equation 2

Where FPII = Principal Inspection Interval adjustment factor

PPII = Proposed Principal Inspection Interval in years (between 1 – 12 years)

The analysis should be initially undertaken for the time-based 6 year inspection interval to determine the current level of risk. Based on the outcome of the initial analysis the inspection interval should be increased/decreased or maintained as appropriate and the resulting level of risk for the second run of the analysis be calculated accordingly. This should enable risk profiling to be undertaken that compares the time-based inspection intervals to the risk-based inspection intervals (also see Section 3.12).

3.6 Consequence of Failure 3.6.1 Overview of Consequence of Failure

The consequence of failure is evaluated as a combination of the:

• Magnitude of Failure of the structure or its elements (Section 3.6.2); and

• Socio-Economic importance based on the features importance, i.e. the route carried/obstacle crossed and the structure importance, i.e. which parts of the community it services (Section 3.6.3).

The scores for each of these criteria are combined using Equation 3 below:

CF = 0.6 x MoFS + 0.40 x SEIS but not < 1 or > 100

Equation 3

Where CF = Consequence of Failure

MoFS = Magnitude of Failure Score

SEIS = Socio-Economic Importance

3.6.2 Magnitude of Failure Score, MoFS

The consequence of failure of a structure and/or its element(s) depends on a wide range of factors; significant criteria where data is readily available are the condition and dimensions of the structure and/or the elements of a structure.

Failure is not necessarily the collapse of the element/structure, but may be failure of drainage elements or expansions joints which affect the function and durability of the structure.

As such, the Failure Score of a structure is based on:

• The size of the structure, i.e. span length for a bridge, footbridge, culvert, gantry, chamber, pipe subway, subway, tunnel and vault; and the height for a retaining wall river wall, mast and catenary lighting.

• The potential extent of failure, e.g. local damage or global collapse.


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The Failure Scores are shown in Table 11 below.

Span Length or Height

Extent of Failure ≤ 3m > 3 to ≤ 10m > 10 to ≤ 25m > 25m

Local damage 10 10 10 10

Structural collapse 40 45 65 85

Table 11: Magnitude of Failure Score, MoFS

When the analysis is undertaken for groups of structures with similar characteristics, the structure size and potential extent of failure of the structure that has the worse condition in the group should be used.

3.6.3 Socio-Economic Importance Score, SEIS

If deterioration or damage remains undetected and subsequent appropriate maintenance activities are not identified and undertaken, it has potential to impact on social and economic factors relating to the local and wider community, e.g. accessibility to community services, business deliveries, access to leisure facilities, etc. These are important considerations and can lead to adverse public opinion and “bad press” if not managed accordingly.

The Socio-Economic Importance Score is used to evaluate consequence in terms of:

• Inconvenience to the community due to diversions, delays and restrictions.

• Inconvenience to businesses due to diversions, delays and restrictions.

These effects can be difficult to quantify but it is considered that there is a close relationship between these and the route supported by or crossed by the structure.

The Socio- Economic Importance Score is therefore selected from Table 12 based on the route supported or adjacent to and the obstacle crossed by a structure.

When the analysis is undertaken for groups of structures with similar characteristics, the route supported by and obstacle crossed by the structure that has the worse condition in the group should be used.


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Route supported by or adjacent to the structure

Disused Unclassified, Cyclist and Pedestrian

B and C Class Road

A Class / Principal Road

Obs

tacl

e cr

osse

d by

the

stru

ctur

e

Waste Ground/disused/non-navigable watercourse

0 20 30 50

Unclassified, Cyclist and Pedestrian 10 30 50 70

B and C Class (local access / distributor) Road and Business Premises

30 50 70 90

Navigable watercourse and A Class / Principal Road

50 70 90 110

Railway 70 90 110 120

Table 12: Socio-Economic Importance Score, SEIS

3.7 Risk Score and Risk based Inspection Intervals The risk posed by a group of structures, an individual structure and/or an element of very high importance (as defined by the CSS inspection procedure[8]) due to deterioration or damage is evaluated as a function of the probability and consequence of rapid deterioration, damage or failure.

The risk score and associated rating is obtained from Table 13 below while the risk based inspection intervals are derived based on the risk rating and Table 14 below.


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Consequence of Failure

Probability of Rapid Deterioration, Damage or Failure

Very Low 1 – < 30

Low 30 – < 50

Moderate 50 – < 70

High 70 – < 90

Very High 90 – 100

Very Low 1 – < 10 Very Low

1 Very Low

15

Low

25

Moderate

40

Moderate

60

Low 10 – < 30 Very Low

15

Low

25

Moderate

40

Moderate

50

High

70

Moderate 30 – < 50 Low

25

Low

30

Moderate

50

High

70

High

80

High 50 – < 70 Low

30

Moderate

40

Moderate

60

High

80

Very High

90

Very High 70 – 100 Moderate

40

Moderate

60

High

80

Very High

90

Very High

100

Table 13: Risk Score and Ratings Matrix

Risk Rating Alterations to Principal Inspection Interval

Very Low Increase the Principal Inspection interval

Low Increase the Principal Inspection Interval

Moderate Maintain the Principal Inspection Interval

High Reduce the Principal Inspection Interval

Very High Reduce the Principal Inspection Interval

Table 14: Risk based Inspection Matrix for Amending Principal Inspections Intervals

A risk rating of ‘Very Low’, ‘Low’ and ‘Moderate’ is considered to represent structures that have inspection intervals which are broadly acceptable or negligible in terms of risk mitigation. This does not preclude the analysis being repeated for structures with ‘Very Low’ and ‘Low’ risk ratings with an appropriately increased inspection interval so that the associated resulting risk rating would be ‘Moderate’. A risk rating of ‘High’ and ‘Very High’ represents structures that pose critical risk(s) and are considered to have inappropriate inspection intervals. For these structures the analysis should be repeated with an appropriately decreased inspection interval so that the associated resulting risk score would be ‘Moderate’.


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Note: The RBI process described through Sections 3.4 to 3.7 can result in an uneven distribution of the PI costs over time, i.e., more resources being required in some years than others. This leads to difficulties in resource management and securing budget for inspections. A smoothing process is introduced in order to smooth the PI cost profile over the programme and hence ensure that there is not wide variation between inspection resource requirements between different years.

3.8 Develop Unsmoothed PI Programme The PI interval, derived from the RBI approach, is used in combination with the last known structure PI date to produce an unsmooth PI programme, i.e., by adding the proposed inspection interval to the last inspection date in order to determine when the next PI should be conducted (see Equation 4).

DSI = DLI + PII

Equation 4

Where, DLI = Date of Last Inspection

PII = Proposed Inspection Interval

DSI = Date of Scheduled Inspection

Note: If a structure’s Date of Last Inspection is unknown, then the Date of Scheduled Inspection is set as the next financial year. In addition, if the DSI calculated from the above equation indicates a date in the past, then also the structure is set to undergo a PI in the next financial year.

3.9 Determine Target Annual Cost of Inspections The average cost required to undertake PIs over a 30 year programme is set as the initial target annual cost of PIs for each financial year.

Target Annual Cost of PIs = Total cost of PIs required for the stock over 30 years

30

Equation 5

A 30 year period is considered to get a reasonable estimate for the annual average cost of inspections. However, the PI planning can be done for a shorter time frame (e.g. 12 years) using this target value, if preferred.

3.10 Develop Provisional Smoothed PI Programme A provisional smoothed PI programme is developed by considering the initial smoothing requirements for the financial year. These are determined by comparing the cost of PIs due in that financial year, including any PIs moved forward from previous years, against the initial target cost of PIs. If the unsmoothed cost of PIs is higher than the target annual cost of PIs, then the structures with lower risk scores (calculated from Step 5 in Figure 2) are moved forward to the following financial year. If the unsmoothed cost of PIs is lower than the target annual cost of PIs, then structures with the higher risk scores in future financial years are moved back into the current financial year.

3.11 Develop Final Smoothed RBI Programme For each structure, the interval between the last PI year and the proposed PI year from the provisional PI programme is taken as the revised PI interval and the risk score is recalculated. If it falls within the acceptable range, i.e. in very low, low or moderate risk category in Table 14, then the PI will be programmed according to the provisional


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smoothed PI programme. If the risk score is outside the acceptable range, i.e. in high or very high risk category in Table 14, then structures will undergo Step 8 in Figure 2 (i.e. provisional RBI programme). This process will be repeated until all structures fall within the acceptable risk range.

Note: If it is not possible to maintain the target annual average cost without exceeding the acceptable risk level in a particular year, then all the structures, which are identified as requiring a PI in that year, need to be inspected in that year (or in earlier years). This ensures that the methodology gives the higher priority for the safety over the cost.

3.12 Risk Profiling It is anticipated that by using the risk based inspections framework the overall level of risk and hence the overall level of liability faced by the organisation can be minimised.

The risk profile for the time-based and the risk-based inspection regimes can be evaluated as follows:

• Determine the risk score for each individual structure and/or groups of structures with similar characteristics (a histogram such as that shown below could then be plotted);

• Calculate the Cumulative Risk Score = Sum(Risk Scorei x Number of Structures) with risk scorei) for both the time-based and the risk-based inspection regimes

• Evaluate the “Risk Change” = Cumulative Risk Score(Time-based – Risk-based)

The risk profiles of a sample data set are compared in Figure 3.


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Figure 3 : Example showing risk profiles for time based, unsmoothed RBI and smoothed RBI approaches

Figure 4 shows an example for comparison of the PI programmes, i.e. Time Based, Unsmoothed RBI and Smoothed RBI, over a 30 year period.

0

50

100

150

200

250

300

1 15 25 30 40 50 60 70 80 90 100

Num

ber o

f Str

uctu

res

Risk Score

Risk ProfileTime Based PI Regime Unsmoothed Risk Based PI Regime Smoothed Risk Based PI Regime

Risk Band 1: • Acceptable level of risk • Acceptable PI Interval

but may be inefficient • Increase PI Interval so

that Risk Band 2 is achieved

Risk Band 2: • Optimum level of risk • Suitable PI Interval

Risk Band 3: • Critical level of risk • Unsuitable PI Interval • Decrease PI Interval

so that Risk Band 2 is achieved


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Figure 4 : Example showing PI profiles over 30 years for time based, unsmoothed RBI and smoothed RBI approaches

£k

£100k

£200k

£300k

£400k

£500k

£600k

£700k

£800k

£900k

£1000k

12/1

3

13/1

4

14/1

5

15/1

6

16/1

7

17/1

8

18/1

9

19/2

0

20/2

1

21/2

2

22/2

3

23/2

4

24/2

5

25/2

6

26/2

7

27/2

8

28/2

9

29/3

0

30/3

1

31/3

2

32/3

3

33/3

4

34/3

5

35/3

6

36/3

7

37/3

8

38/3

9

39/4

0

40/4

1

41/4

2

Ann

ual C

ost (

£)

Financial Year

30 Year PI Programme

Time Based PI Regime Unsmoothed Risk Based PI Regime Smoothed Risk Based PI Regime


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4 References 1. CIRIA C677: Whole-life infrastructure asset management: good practice guide for civil

infrastructure, Construction Industry Research and Information Association, 2009.

2. BD 63 Inspection of Highway Structures, DMRB 3.1.4, TSO.

3. Management of Highway Structures: A code of Practice, TSI, 2005

4. Value Management of the Capital Renewals Programme 2010/11, Part 1: Overview, TfL, 2010.

5. Inspection Manual for Highway Structures, TSO, 2007.

6. Framework for Highways Asset Management, County Surveyors Society, April 2004.

7. A.K. Lee, C. Serratella, G. Wang & R. Basu, ABS a Flexible Approaches to Risk-Based Inspection of FPSOs, Offshore Technology Conference (OTC 18364), Houston, Texas, U.S.A., 1–4 May 2006.

8. Highways Agency & CSS Bridges Group, Guidance Document for Performance Measurement of Highway Structures, Part B1: Condition Performance Indicator, Atkins, 2007.

9. BD 79: The Management of Substandard Highway Structures, DMRB.

10. Manual of Contract Documents for Highway Works, Volume 1: Specification for Highway Works, Series 1900: Protection of Steelwork Against Corrosion, TSO, 2005.

11. ISO 9223 Corrosion of Metals and Alloys - Corrosivity of Atmospheres - Classification First Edition, International Organization for Standardization, 1992.

12. BS EN ISO 12944, Part 2: Paints and varnishes - Corrosion protection of steel structures by protective paint systems - Classification of environments, British Standards Institution, 1998.

13. BS 5400, Part 4: Steel, concrete and composite bridges - Code of practice for design of concrete bridges, British Standards Institution, 1990.

14. BS EN 206, Part 1: Concrete. Specification, performance, production and conformity, British Standards Institution, 2000.

15. BS 8500, Part 1: Concrete - Complementary British Standard to BS EN 206-1 - Part 1: Method of Specifying and Guidance for the Specifier, British Standards Institution, 2006

16. Handbook for the Examination of Structures – NR/L3/CIV/006, Network Rail, 2010.

17. Risk Based Inspections – Guidance Note, Welsh Assembly Government, February 2010.

18. Risk Based Bridge Evaluations – A Texas Perspective, Kowalik, A., Department of Transportation, ASCE Structures Congress’ 09, Austin, Texas, 2009.

19. Bridge Asset Management – Reduce Maintenance Costs and Improve Reliability with Risk-Based Management, Lloyd’s Register.


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Appendix A Interpretation of the Condition Performance Indicator for Individual Structures[8]

Range CPIAv (All Structure Elements)

CPICrit (Worst Critical Element)

90 ≤ x ≤ 100

▪ Likely to be no significant defects in any elements

▪ Structure is in a "Very Good" condition overall

▪ Insignificant defects/damage

▪ Capacity unaffected

80 ≤ x < 90 ▪ Mostly minor defects/damage, but may also

be some moderate defects

▪ Structure is in a "Good" condition overall

▪ Minor defects/damage

▪ Capacity unlikely to be unaffected

65 ≤ x < 80

▪ Minor-to-Moderate defects/damage

▪ Structure is in a “Fair” condition overall

▪ One or more functions of the structure may be significantly affected

▪ Minor to moderate defects/damage

▪ Capacity may be slightly affected

40 ≤ x < 65

▪ Moderate-to-Severe defects/damage

▪ Structure is in a "Poor" condition overall

▪ One or more functions of the bridge may be severely affected

▪ Moderate to severe defects/damage

▪ Capacity may be significantly affected

0 ≤ x < 40

▪ Severe defects/damage on a number of elements

▪ One or more elements may have failed

▪ Structure is in a "Very Poor" condition overall

▪ Structure may be unserviceable

▪ Severe defects/damage

▪ Failure or possible failure of critical element

▪ Capacity may be severely affected

▪ Structure may need to be weight restricted or closed to traffic

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