Guidelines for monitoring of steel railway bridges Background document D5.2.S1

(1)

Guidelines for monitoring of steel railway bridges Background document D5.2.S1

Condition assessment (Inspection of the bridge condition )

YES NO

END Investigation of:

- Geometry - Material parameters - Macroscopical material condition - Modeling

Should the structure be analysed more

precisely?

Assessment NEGATIVE

Identification of critical components

Long-term monitoring not necessary

END

Existence of information about

the loading history ? ^NO

Control of thresholds by monitoring

(local strains, crack sizes) YES

Continuous monitoring + adaptive prognosis models

Life time prediction

Assumptions

- +

POSITIVE

PRIORITY 6

SUSTAINABLE DEVELOPMENT GLOBAL CHANGE & ECOSYSTEMS

INTEGRATED PROJECT

(2)

This report is one of the deliverables from the Integrated Research Project “Sustainable Bridges - Assessment for Future Traffic Demands and Longer Lives” funded by the European Commission within 6^th Framework Pro- gramme. The Project aims to help European railways to meet increasing transportation demands, which can only be accommodated on the existing railway network by allowing the passage of heavier freight trains and faster passenger trains. This requires that the existing bridges within the network have to be upgraded without causing unnecessary disruption to the carriage of goods and passengers, and without compromising the safety and econ- omy of the railways.

A consortium, consisting of 32 partners drawn from railway bridge owners, consultants, contractors, research institutes and universities, has carried out the Project, which has a gross budget of more than 10 million Euros.

The European Commission has provided substantial funding, with the balancing funding has been coming from the Project partners. Skanska Sverige AB has provided the overall co-ordination of the Project, whilst Luleå Tech- nical University has undertaken the scientific leadership.

The Project has developed improved procedures and methods for inspection, testing, monitoring and condition assessment, of railway bridges. Furthermore, it has developed advanced methodologies for assessing the safe carrying capacity of bridges and better engineering solutions for repair and strengthening of bridges that are found to be in need of attention.

The authors of this report have used their best endeavours to ensure that the information presented here is of the highest quality. However, no liability can be accepted by the authors for any loss caused by its use.

Figure on the front page: Flow diagram for life time prediction with the use of monitoring.

Project acronym: Sustainable Bridges

Project full title: Sustainable Bridges – Assessment for Future Traffic Demands and Longer Lives Contract number: TIP3-CT-2003-001653

Project start and end date: 2003-12-01 -- 2007-11-30 Duration 48 months Document number: Deliverable D5.2.S1 Abbreviation SB-5.2.S1 Author/s: Gerhard Sedlacek, Oliver Hechler, Andreas Lösche

(Lehrstuhl für Stahl- und Leichtmetallbau, RWTH Aachen, Germany)

Bertram Kühn (PSP, Planung und Entwicklung im Bauwesen GmbH, Aachen, Germany)

Date of original release: 2007-11-30 Revision date:

Project co-funded by the European Commission within the Sixth Framework Programme (2002-2006)

Dissemination Level

PU Public X

PP Restricted to other programme participants (including the Commission Services) RE Restricted to a group specified by the consortium (including the Commission Services) CO Confidential, only for members of the consortium (including the Commission Services)

(3)

Summary

The aim of the project “Sustainable Bridges” is to arm railway bridges to meet the demands of the 2020 scenario and to provide methods and tools to the bridge owners to up-grade their bridges if they fall short.

This project objectives agree with the general trend to well prepared rehabilitation to increase the service life and reduce the maintenance costs of steel railway bridges. Thus a precisely aimed assessment of steel railway bridges is required which can be realised by the combination of theoretical and experimental investigations by monitoring. An optimised condition assessment is achieved.

For the application of technical monitoring on steel railway bridges various monitoring systems to specific monitoring tasks are existing. The final choice, which system is most benefi- cial highly depends on the aim of the monitoring measure.

In this guideline hints, methods and tools for designing, implementing and maintaining a monitoring system of steel railway bridges are given.

The difference between inspection and monitoring is outlined and application aims of technical monitoring are defined. Further the design of a monitoring system is presented and demands on the system as well as physical quantities with a special regards to steel bridges are listed.

In this document the service life analysis is extended by the use of monitoring techniques to increase its accuracy respectively speed up or enable a continuous availability of the condition of the overall structure or critical parts. Monitoring with its testing methods are listed for a selection of parameters comprehending optional design limits.

Additionally the range of monitoring application is discussed. The predominantly motivation for the application of monitoring is to confirm analysis results and to optimise input values of the service life analysis using continuous monitoring. The use of monitoring in terms of a

„Red-alert system“ has to be assessed to be critical for steel railway bridges as backup capacity are required during an alarm, which are often not available. Monitoring as alternative respectively to support conventional normal inspections according to national guidelines is advisable. The possibility of a cost reduction for inspections is obvious.

Further on application scenarios of monitoring for steel railway bridges are discussed by case studies to show the potential and limitations of monitoring.

Supplementary information on practical cases are provided by WP7 and WP 8 comprehending experiences of monitoring addressing the design, implementation and maintenance is- sues.

(4)

Acknowledgments

This report has been drafted on the basis of Contract No. TIP3-CT-2003-001653 between the European Community represented by the Commission of the European Communities and the Skanska Teknic AB contractor acting as Coordinator of the Consortium. The authors ac- knowledge the Commission of the European Communities, RWTH Aachen and PSP, Planung und Entwicklung im Bauwesen GmbH for its financial support.

(5)

1 Introduction

1.1 Principle

Due to the age of existing steel bridges, increasing loads and changing requirements for use, maintenance and strengthening of steel bridges become more and more important. The high costs of building new bridges are only economically justifiable by proving a significantly longer service life together with smaller maintenance costs in relation to old structures.

Therefore, corrective maintenance of old structures is increasingly becoming a cost efficient alternative to the building of completely new structures. Simultaneously, sustainability aspects and a sensitive treatment of energy and material resources are, to a greater extent, in the focus of designers and financiers and may be an argument against a replacement of old structures. In addition, especially for steel bridges, historical and social aspects may lead to a cultural value which has to be preserved. As a result, the general trend goes towards rehabilitation efforts for significantly increasing the service life and reducing the maintenance costs of structures. This trend is depicted in Figure 1.1 showing the opposed developments of investment for new building and for maintenance of existing buildings in western countries.

1977 1985

1995 2005

0 10 20 30 40 50 60 70 80 90 100

%

Year

Building investments [%]

Maintenance New Structure

Figure 1.1: Development of building investments in % [1]

Therefore, the maintenance of existing structures which are subjected to fatigue by variable loading, as steel and composite railway bridges, is becoming an item of major concern. A well prepared rehabilitation of existing structures allows for an efficient allocation of money.

The fundamental knowledge required for decision making is gained by investigating the structure which should be supported by technical monitoring to a certain extent.

(8)

Accordingly the main scope of this guideline is to provide hints, methods and tools for designing, implementing and maintaining a monitoring system for fatigue assessment of railway bridges.

1.2 Introduction (Application of Monitoring)

Monitoring has to be divided into two categories. The first category of monitoring comprises regular visual inspections; the second category includes technical monitoring. This guideline focuses on technical monitoring. The main intention of the guideline is to provide hints, methods and tools for designing and implementing technical monitoring systems. In this connection monitoring techniques which contribute to estimate the effects of higher loads and speeds on railway bridges with respect to safety, serviceability, reliability and life time are examined and presented.

Civil engineering structures are mainly large and complex physical systems consisting of many heterogeneous components. A monitoring of all components with sensors is economically and technologically not feasible. Therefore, the monitoring process has to be restricted to a limited number of components. As a result the guideline should also provide solutions to select the decisive structural members to be analysed.

1.2.1 Inspections

Bridge structures have to be inspected according to the specifications of the particular national standard in pre-defined time periods, e.g. Richtlinie 804 of the DB Netz AG (RiL 804) in Germany [56]. For the inspections the following procedures are differentiated:

- Normal inspection

- Investigation

- Seeking expert opinion

- Special inspection

In general the normal inspection of steel bridges is visual and limited to visible defects and damages.

The investigations are usually restricted to structural members which are easily accessible.

The task of the investigation is the check of all members which could affect the safety, dura- bility and resistance of the bridge. In the scope of the investigation especially defects and damages previously identified should be monitored and changes should be analyzed.

The seeking of expert opinion should include a check of all relevant members. Therefore also members which are difficult to assess are included in the examination.

Special inspections might be required due to extraordinary regulations, events or impacts.

They include examinations of the bridge structure additionally to the regular inspection cycles with normal inspections.

For further information in respect to inspection and inspection techniques, see [60],[61] and [62].

1.2.2 Technical Monitoring

A technical monitoring system is a data acquisition and processing unit which provides continuously and autonomously real-time information about a structure or structural component.

The technical monitoring should contribute to an exact estimation of the bridge condition, which is impossible to achieve with inspections only. A technical monitoring can be applied in different ways and follow diverse aims, e.g.:

(9)

- ”Red-alert-systems”: monitoring of defined threshold values with alarm system

- Supporting application on normal inspections

- Monitoring of unapproachable areas

- Application to confirm analysis

- Determination of input values for residual service life estimation models

For a detailed listing of technical monitoring applications in the field of steel railway bridges see chapter 1.5.

Basic concepts of technical monitoring

The basic concepts of technical monitoring are: “Action monitoring”, “Performance monitoring”, “Stress monitoring” and “Health monitoring”. Depending on the analysis task one or even more of these concepts are applied, see chapter 1.6.1.

• Action monitoring

Action monitoring allows assessing the magnitude as well as the spatial and temporal distribution of specific forces acting on a structure or a structural component.

• Performance monitoring

Performance monitoring allows assessing whether a structure or a structural component meets the performance requirements under specific or any actions.

• Stress monitoring

Stress monitoring allows assessing the state of stress and strain in a structure or a structural component.

• Health monitoring

Health monitoring allows assessing the health condition of a structure or a structural component by means of health indicators. The scope of structural heath monitoring is to provide real-time information to assess the safety and serviceability of a structure or structural components. This implies that the behaviour of a structure under operating conditions is sufficiently well understood and that there exist a number of health indicators which can be measured with sensors and procedures which describe how to use these health indicators to assess the state of a structure.

Furthermore, the failure scenarios with regard to safety and serviceability, that have to be avoided during the life-span of a structure, have to be identified and character- ized by quantifiable health indicators that can be estimated through measurements of physical quantities.

Limitations

The goal of continuous monitoring is not to supersede periodical inspections but to improve the efficiency of the inspection process. This can be achieved by providing relevant information continuously. Thus the number of components subjected to a detailed inspection can be reduced. If a reduction of the time between inspections cycles is required technical monitoring can avoid additional special inspections. Further on monitoring could support the estimation of more accurate input parameters for the residual service life estimation.

1.3 International codes and guidelines

Information on international codes and guidelines concerning inspection and monitoring are available in Fel! Hittar inte referenskälla..

(10)

1.4 Design of a technical monitoring of steel railway bridges

This chapter provides hints and tools for the design of technical monitoring systems, in order to ensure the fulfilment of the respective monitoring task. Different possibilities of a technical design of a monitoring system are described and some systems already operating in practice are introduced.

1.4.1 Realisation of a health monitoring for railway bridges

A monitoring health system is designed to record, analyse and display the loading history of structures and the structural reactions while operating. All data should be recorded and provided if required.

The following environmental influences and basic conditions are relevant for the application of a monitoring system for steel railway bridges:

- High electromagnetic field (railway service) - Pollution e.g. with sand, soil, mud, etc.

- Splash water - Sabotage

- Temperature range –35°C…+50°C, with solar heating >50°C

The rough environmental conditions require a robust and long term monitoring system, see chapter 1.4.5.

The system can be subdivided into the following components:

- Sensors

- Communication network technology

- Central data acquisition and processing unit

Severe quality requirements are especially needed for the communication network, which is of outstanding significance for the entire monitoring system. The other components like sensors and central data acquisition and processing unit may be realised with already existing technology.

1.4.2 Physical quantities

Relevant physical data to be measured for health monitoring application

Depending on the measuring method and requirements in view of the damage model the following values have to be recorded for the evaluation of structural “health”.

- Force (e.g. wind or snow) - Acceleration

- Speed

- Deformation / path (direct measuring value or deduced quantity of acceleration) - Strain

- Temperature - Humidity

- Acoustic emission

(11)

The values have to be registered as static, quasi-static or dynamic on discrete points in real time and in service. In many applications the registration of one measuring value (see the list above) is sufficient.

In general, two alternative monitoring methods may be implemented:

- For measuring strains directly at the identified critical details (direct data acquisition) - For measuring global parameters (e.g. displacements) with which in a subsequent step all

relevant information are extrapolated (indirect data acquisition)

The direct data acquisition method is the most accurate. In general, this method requires a larger number of sensors than the indirect data acquisition method. Therefore, the costs for devices, installation, data management and maintenance are likely to be higher.

The accuracy of the indirect data acquisition method depends significantly on the structural model used for extrapolating the stresses of the critical components. Additionally the load introduction is hardly to be measured directly as well as the static and dynamic stress interaction cannot be properly separated.

Requirements on precision, sensitivity, reliability etc. of physical sensors have to be specified depending on the monitoring task.

Direct testing methods

Direct testing methods require a defined reference basis, from where the position of a measuring point on the object to be investigated is estimated, see Figure 1.2. As measured quantity, the distance to a reference basis is mostly used. The realisation of a defined reference basis for steel bridges with small measurement distances between sensor and basis sometimes results into an enormous additional effort, e.g. by positioning an additional girder. If large measurement distances (several metres) are sufficient, a reference basis independently from the structure (in the area of bridge bearing) may be advantageous.

object

reference basis

Figure 1.2: Measuring object with defined reference basis for small measuring distances with optical sensors and position encoder

Available commercial sensors for direct measuring systems are inductive position encoders and optical sensors of various types and producers. Laser diodes with a few centimetres measuring distance either in the visible or invisible wave bands of light are frequently used as optical sensors. Optical sensors for long measuring distances (several metres) are also available. A disadvantage of the systems’ application on-site is the requirement that the signal transmission must be undisturbed from source to receiver. With polluted optical components and/or under bad weather conditions (e.g. heavy rain), these measuring methods are often susceptible to disturbances. Sometimes, additive reflection helps are necessary (on the measuring point), to allow for a secure reception of signals. Even in this case, the reflection handicaps produced by soiling have a disadvantageous effect on the measuring accuracy.

(12)

Indirect testing methods

For indirect testing methods, a defined reference basis is not required. The sensors are directly fixed on the component and embedded in the object. The most used sensors for indirect measuring methods are capacitive acceleration and resistive strain sensors (strain gauges [DMS]). Also optical fibres can be used. For adequate dynamic monitoring, a robust and cost-efficient implementation is possible. An argument against the method is often, that suitable acceleration sensors for static or quasi-static stress sequences are missing.

Alternatively to DMS or acceleration sensors optical fibres can be used also, see [51] and [52]. Temperature measurements are not included in the indirect measuring procedures. Indi- rect measuring procedures comprehend the temperature effects.

Conclusion

Indirect testing methods on the basis of strain gauges or acceleration sensors are reliable, robust and economically feasible. They display a compact design and can be fixed without an enormous assembling effort, e.g. by magnets, on relevant measuring points of the structure. This applies especially for acceleration sensors. In case of strain gauges, the effort depends on the application, because, in general, special bonding techniques with extensive preparatory work (grinding, if necessary burnishing, scouring) are necessary. Additionally the long-term behaviour of the adhesive sealing of the strain gauges has to be validated.

For continuous monitoring of steel railway bridges all optical transmission systems provide the potential to be used. The advantage of this technique is the possibility to embed the optical fibres directly in a new bridge structure.

Additional information on monitoring technology can be found in the following “Sustainable Bridges” documents:

- SB-5.1 Monitoring Instrumentation and Techniques [64]

- SB-5.3 Crack sensor [65]

- SB-5.4 Fibre optic Bragg grating [66]

- SB-5.5 MEMS sensors network [67]

- SB-5.6 Shaker for vibration tests [68]

- SB-5.7 Wireless Sensor Network [69]

- SB-5.9 Time of Flight Sensor [71]

1.4.3 Communication networks

Communication networks are distinguished in wired and wireless communication networks.

Wired communication network

Wired communication is defined as a physical link with wires between the central data acquisition and processing unit and the sensors. Today’s most common implementation is the star-shaped topology network (see Figure 1.3). Each sensor is connected to the central data acquisition and processing unit via a separate wire. Due to the large amount of wires involved, the risk of damage in such a network is high, in particular, for monitoring systems operating during retrofitting activities. An appropriate protection and marking of wires and instruction of the involved persons is essential for preventing wire damage and data loss.

Additionally, wiring represents an important element of the installation of a health monitoring system in terms of time and effort.

(13)

sensors

processing unit

Figure 1.3: Wiring network with star-shaped topology

Compared to a star-shaped network topology, a considerable reduction of installation effort is achieved by applying modern field bus systems in which all sensors are connected to a single wire, see Figure 1.4.

sensors

processing unit

Figure 1.4: Wiring network as field bus system

The energy supply of the sensors and all (digital) configuration commands as well as data communication run through this single bi-filar wire with a high safety and error redundancy. In general the safety and redundancy is achieved by the simplicity of and the short transmission lengths in the system. For small to medium data rates, several field bus systems are available, e.g. LON, CAN Interbus Loop, ASI etc..

The more components have to exchange data in one monitoring system the more attractive is a field bus system. Consequently for modern health monitoring systems using physical wiring, a field bus system should be applied to limit the installation effort. Because of the limited information flow-rate of a field bus system a filtering of the parameters to be measured is essential. Additional information on wired bus systems is found in [59].

Wired networks can be implemented using conventional copper wires, possibly shielded, or optical fibres. The typical advantages of optical fibres compared to copper wires are the following:

- Very high transmission band range respectively the possibility of huge data rates

- Immunity against disturbing influences of external signal sources, high EMC-stability also without covering

- Undisturbed by-pass of high electric potential differences, therefore the use in high volt- age systems is possible

- Installation of suitable optical fibres in high temperature areas, little thermal sensitiveness of special optical crystal fibres

- Little sensitivity to environmental influences, suitable for outdoor use

(14)

- Long term stability

- Good suitability for signal transmission

The disadvantages of optical fibres compared to copper wires are:

- High device-related effort and price for plug sockets between optical transmitter and receiver

- Sensitivity of plug sockets against pollution

- Mechanical sensitivity of optical fibres leading to a high effort in encasing - Difficult to repair a cracked or broken optical fibre

- Impossible to use for energy supply of peripheral equipment

Concerning the implementation of a practical health monitoring system for steel bridges for railway service, optical fibre sensors are very attractive because of their high immunity to electromagnetic fields, their long term stability and their robustness to environmental influences. However, the power supply of sensors has to be achieved by other means.

Wireless communication network

General

With respect to installation and operation, wireless health monitoring systems are less susceptible than wired systems against harsh environment. The particular disadvantage of these systems is that more than one central energy supply is required. The energy consumption of the individual sensors can be reduced significantly, if the connection of the sensors to the central processing unit is active only in case of a necessary data exchange. The triggering will be realised by a suitable impulse or rather a measuring signal. In other time periods, the measuring line will be in the “stand by” mode with a minimal energy consumption.

Wireless communication media are:

- Electromagnetic waves in radio frequency and little power - Acoustic carrier waves in massive metallic structures

Radio frequency transmission

A well known, inexpensive, digitally coded and fail safe radio network with little power and small range is the Digital Enhanced Cordless Telephone (DECT), see Figure 1.5. The system represents the basis for cordless tap-proofed telephone systems. As the network topology is star-shaped an exchange of data of each single station is only possible via the central processing unit.

Figure 1.5: Communication via digital little power radio network

Range, transmittable data rate and interference resistance of DECT are considered to be sufficient for monitoring task. Nevertheless, the possibility of external parasitic induction is a

(15)

weak point of the system. In certain circumstances, a break down of the system may occur which could limit the application of this system for monitoring.

Additional information on wireless monitoring systems is found in [69].

Transmission by acoustic carrier waves

Massive and stretched steel structures appear to be a very good transmission media for lon- gitudinal acoustic carrier waves. An application of a digital communication protocol based on high frequency acoustic carrier waves is imaginable, see Figure 1.6. Such a communication system provides a sufficient data transmission rate with high reliability and safety. At present, acoustic communication systems have no been installed. Nevertheless a realisation is prom- ising.

Figure 1.6: Acoustic signal transmission in the structure

1.4.4 Data acquisition and processing

The most important elements of the central processing unit (master) are:

- Memory

- Storage of complete loading history of a defined time range

- If relevant derivation of parameters from measured values (e.g. order of turning back points)

- A/D-conversion

- Saving of general parameters (e.g. temperature pattern, charge state of battery) - Interfaces

- To local communication network / sensors

- To analysis modules next to or far away from the monitored structure - Energy supply

- Taken from conventional network for lights - By battery

- By accumulators with solar reloading

1.4.5 Requirements

A smart and economical concept for performance or health monitoring is characterised by the following requirements:

- Applicability on structures with large sizes and on single component with small sizes - Measurements of mechanical values as a result of static and dynamic loading, e.g. defor-

mation, velocities, accelerations, strains etc.

(16)

- Storage of relevant data acquired at various measurement points - Time-stability of the overall measurement chain

- Resistance against weather conditions - Robustness

- Energetically self-sustaining and minimal energy consumption - Easy handling

The sensoric sub system should also be easily integrated in all kinds of data transfer systems available nowadays (e.g. wired or wireless transmission or transmission by internet). In addition, a monitoring concept has to be identified which is suitable to provide the necessary stress or strain information used as input e.g. for the residual service life analysis or fracture mechanics approach. The outcome of a monitoring concept for the service life analysis should support a reliable quantification of the remaining service life and the residual load- carrying capacity.

1.4.6 Health monitoring systems

The world market already offers a variety of different health monitoring systems. A selection of available systems is given as follows :

- Strain Monitor Systems / USA [47], [48] and [49]

- SMS (Structure Monitoring System) 2001® / Germany [50]

- OSMOS safety system (fibre-optic system) [52]

- SOFO - structural monitoring by optical fibres [51]

- 3DeMoN (3 Dimensional Deformation Monitoring Network) Autonomous GPS Re- ceiver [51]

- Wireless systems with MEMS sensors [53] and [54]

The decision on which health monitoring system should be applied highly depends on the task and requirements (see chapter 1.4.5) of the specified monitoring application.

(17)

1.5 Technical monitoring for steel railway bridges

In the following chapter options and aims of technical monitoring of steel railway bridges are described. A summary of potential monitoring options is given in Table 1.1. The options are differentiated in view of the duration, kind and application options as well as the aim of the appliance of technical monitoring.

Table 1.1: Application of technical monitoring for steel railway bridges

Kind of monitoring and application options

What will be monitored? Aim of technical monitoring

Short-term monitoring Present condition of the structure

Identification of existing damages respectively possible

damages occurring in the near future

1. Option:

Knowledge about occur- ance of a special load case

All relevant data (in particular peak values)

Accurate determination of the behaviour of the bridge in a

special load case 2.Option:

Supplementary measurements to the normal visual inspection

Data, which are relevant after an inspection (confirmation of

the inspection result)

Ascertainment of the bridge condition

3.Option:

After the assemblage of strengthening methods

Data to verify the calculation results

Direct information of the present behaviour of the structure (check of assumptions and information on the effect of the strengthening method)

Continuous online moni- toring (Red-Alert-effect)

Every change in the condition of the structure respectively any exceedance of specified

threshold values

prompt alert at the initiation or progression of a defect, con-

tinuous disposability of data

1.Option:

Weak point of the structure is known

Relevant data at the identified location

Detection of any failure at an early stage for immediate

reaction 2.Option:

Acute risk of failure of a previously strengthened weak point of the structure

Data at the weak points’ location which is relevant to identify

failure

Permanent information of the condition of the strengthened

member, possibility of a prompt reaction in case of

failure 3.Option:

Continuous information of relevant damages of the structure at any time

Data of essential members of the bridge structure

Risk minimization

(18)

Continuous monitoring (storage of the data and successive analysis)

Behaviour of the structure in service under existing condi-

tions

E.g. for the optimization of the input parameters in the ser-

vice life analysis 1.Option:

Results of a service life analysis would lead to the termination of the operation of the bridge

Data for the optimization of the input parameters in the service

life analysis

Improvement of the results of the service life analysis leading to the maintenance of the

bridge operation

2.Option:

Verification of strengthening measures, there is no failure expected in the near future (safeguarding)

Data at the location concerned as input parameters for the assessment of the strengthen-

ing measure over a long-time period

Effectiveness and behaviour of a strengthening measure,

affirmation of the own assumptions

3.Option:

Action to avoid shortening of the normal inspection periods

Relevant data for the assessment of the condition of the bridge, continuous monitoring

of the critical detail leading to the shortening of the normal

inspection periods

Check of the bridge condition over a long-time period

According to Table 1.1 the application of monitoring is variously motivated; all options have got the aim to assure the stability of the bridge and to guarantee a safe operation of the traffic.

1.6 Service life analysis with and without monitoring

Variable loading requires fatigue considerations which represent an important matter in the life time design. With the help of realistic residual life estimation models, a prolongation of the residual service life can be achieved.

Therefore the residual service life of a structure is estimated by a service life analysis.

If the result of the service life analysis leads to the conclusion, that the residual life expecta- tions are unsatisfactory, measures have to be considered. The following actions to keep the bridge in operation can be realised:

- Check of the stresses in the significant members by monitoring (real stress versus stress assumptions)

- Strengthening respectively exchange of the overstressed members - Slow speed of the trains during crossing

- Decrease of the of the train weight limitation

If no measures are initiated the bridges’ operation will be disturbed step by step due to the increase of deterioration until the bridge has finally to be closed. A cost-benefit analysis has to be performed to come to the final decision, if a measure or which measure should be carried out.

In the following the application of monitoring for the optimisation of the service life analysis of steel railway bridges will be described. As a result further expensive actions, e.g. strengthening might be avoided to enable a safe operation of the bridge.

(19)

1.6.1 „Classical“ und „adaptive“ approach

The classical approach to obtain realistic residual life propagation uses three models; a model for actions, a structural model and a damage model, see [2], [3], [4], [5], [6] and [7]. All three models have a significant degree of uncertainty. E.g. the model for actions is based on a stochastic approach, the structural model is generally based on significant simplifying assumptions and structural and geometrical imperfections are only theoretically verified. How- ever, the major uncertainty in the residual life analysis is related to the damage models for fatigue design, which could differ notably from reality, see [8] and [9].

As the results are obtained solely by theoretical models and are also used as input parameters for subsequent theoretical models, the reliability of the residual lifetime prediction is significantly decreased.

For this reason the application of monitoring is very expedient for the improvement of the input parameters for the different models.

(20)

Classical monitoring to improve the residual life time prediction

Action model

e.g. real traffic

Structural model

static, dynamic

Strains at critical details

Damage model

PREDICTION

Monitoring

measurements

Stochastics

analysis and interpretation

Classification

Figure 1.7: Classical application of monitoring to improve the residual life time prediction

Improved approach of residual life time estimation by avoiding models

In order to improve the reliability of the residual lifetime analysis, an approach has been developed by Mehdianpour [26], which reduces uncertainties of the classical models of residual lifetime estimation by modern monitoring technology.

The uncertainties resulting from the action and structural model are eliminated by continuously measuring the loads and strains (and/or stresses) of critical components of the structure. For existing structures (and the use of the S-N-curve concept) this is helpful only for the future, because assumptions of the already accumulated impact with the use of available knowledge from past up to present is necessary, comprehending also a further degree of uncertainty. The results of the measurements (load-time-function), determined by the help of monitoring, are classified into load collectives using statistic counting procedures (e.g. rainflow-method). Afterwards they are used as input parameters for linear or nonlinear methods of damage estimation.

The reliability of the residual life time prediction can be increased additionally by using damage models with an experimental life span determination [1].

The approach for the determination of the life span with the help of monitoring in general is described in chapter 1.6.5 in detail.

Health Monitoring Performance Monitoring Action Monitoring

Stress Monitoring

(21)

1.6.2 Action models

For estimating the remaining service life, realistic action models have to be applied. They are used as input parameters for fatigue design.

Not only the peak load but also the sequence of service loadings throughout the structure’s life governs the resistance to fatigue. The cycle sequence may be important because it may affect the stress range, particularly, if the structure is loaded by more than one independent action system.

A set of action parameters based on typical loading events are described by the positions of loads, their magnitudes, frequencies of occurrence, sequence and relative phasing. The fatigue actions can be found in [10] representing upper bound values based on evaluations of measurements of loading according to [10] - Annex A.

Eventually, every railway track section has its own traffic characteristic to be differentiated by:

- Main line track or light railway track

- The particular traffic composition of goods and passenger trains on the track and - Local and geographical influence

As it is impossible to take the different characteristics of each track into account, idealisations to traffic load models need to be made.

For convenience, actions are usually simplified into a set of loading, which defines a series of bands of constant load levels and the number of times that each band is experienced.

The anticipation of the sequence of service loading on a track is in the designer’s objective.

Typical loading sequences that represent a credible estimated collective of service load events expected during the fatigue design life should be determined using prior knowledge from similar structures.

The reliability of the loading history of the sequence of service loading throughout the structures life can be increased by taking the logbook into account which is available at the railway. Furthermore, uncertainties are significantly reduced if monitoring techniques for the acquisition of data are used. For further information, see the Guideline for Load and Resis- tance Assessment of Existing European Railway Bridges [11] or [12], [13], [14] and [15].

1.6.3 Structural modelling

The calculation model and basic assumptions for the calculations shall reflect the structural behaviour of the railway bridge to be investigated with appropriate accuracy and reflect the anticipated type of behaviour of the cross-sections, members, joints and bearings [16].

Hence, the model shall reflect the current state of the bridge structure.

The method used for the analysis shall be consistent with the design assumptions. For the structural modelling and basic assumptions for the components of the structure, see also [16]

and [17]. The effects of the behaviour of the joints on the distribution of internal forces and moments within a structure, and on the overall deformations of the structure, may generally be neglected, but where such effects are significant they should be taken into account, see [18]. The account of ground-structure interaction shall be taken into account for the deformation characteristics of the supports where relevant.

The internal forces and moments may generally be determined by using either first-order analysis considering the initial geometry of the structure, or second-order analysis taking into account the influence of the deformation of the structure. The effects of the deformed geometry (second-order effects) shall be considered if they increase the action effects significantly.

(22)

Further on methods of analysis considering material non-linearities for the determination of the internal forces and moments could be used.

The results determined from the calculation model could later be calibrated to measurements from the monitored bridge structure for further reliability.

1.6.4 Damage models

General

In the life of a bridge the question may arise, for how long it can be preserved for the future.

In terms of fatigue strength that means the remaining fatigue life up to the point when the probability of occurrence of cracks of vital structural members exceeds a given limit [19]. The estimation of the residual life of the structure is achieved by evaluating the residual fatigue strength of critical components. These critical components which are designed to survive a single load may fail if the load is repeated a large number of times. This failure behaviour is classified as fatigue failure.

Fatigue

Fatigue is defined as the process of initiation and propagation of cracks through a structural part due to action of fluctuating stress. For the fatigue design, see [20].

The fatigue process is not a single mechanism but the result of several mechanisms occurring in sequence during the life of a structure:

- Initiation of a microscopic defect - Slow incremental crack propagation - Stable crack growth

- Unstable crack growth (fracture)

In most welded steel structures, the initiation phase is not relevant for the fatigue process as initiative defects are very likely to exist due to fabrication in particular welding.

In general, the following concept for fatigue verifications are widely accepted : - Verification with particular fatigue tests using prototypes

- S-N-curve concept with detail classes based on nominal stresses - Geometrical stress method using reference S-N-curves

- Local notch stress method using local elastic stresses - Local notch strain method using local stresses and strains - Fracture mechanics analysis.

The verification with particular fatigue tests is supported by experimental investigations, the local notch strain method using local stresses and strains is based on the analysis of the crack initiation, the fracture mechanics concept depends on crack propagation analysis whereas the other 3 concepts are based on service life assessment, see [36] and [55].

In the following, the concepts for fatigue verification are briefly described to enable a pre- selection of the relevant concept to be applied. For further information, see [21] and [22]. For relevant information concerning material properties of steel railway bridges, see [62].

(23)

Verification using fatigue tests with prototypes

The verification of the fatigue resistance is performed with S-N-curves that are experimentally determined using original or representative components. The material, the geometry and the manufacturing of the component as well as the loading sequence and environmental conditions of the tests configuration have to be as close as possible to reality to achieve results of acceptable reliability.

The residual service life is determined by a damage accumulation model, e.g. Palmgren- Miner, on the basis of the S-N-curves derived experimentally. For further information, see [23] and [24].

S-N-curve concept with detail categories based on nominal stresses

The application of the S-N-curve concept is based on nominal stresses. The nominal stress is defined as the stress in the parent material or in a weld adjacent to a potential crack location calculated in accordance with elastic theory and excluding all stress concentration effects. The nominal stress as specified can be a direct stress, a shear stress, a principal stress or an equivalent stress. It is derived from the loads applied on the net sections of a structure or member. A stress increase due to geometry or the effects of other imperfections are not included in this nominal stress concept.

Furthermore, the investigated detail has to be classified as a detail category using the corresponding S-N curves. The detail category is defined by the numerical allocation given to a particular detail for a given direction of stress fluctuation, in order to indicate which fatigue strength curve is applicable for fatigue assessment (The detail category number indicates the reference fatigue strength ∆σ_C at 2·10⁶ load cycles in N/mm²). The S-N curve, resp. the fatigue strength curve, defines the quantitative relationship between the stress range and the number of stress cycles for fatigue failure and is used for fatigue assessment of a particular category of structural details. Consequently, macro-geometrical effects and areas of stress concentration are taken into account in the nominal stress approach by the fatigue resistance. The S-N-curve concept with detail categories is described in the literature, e.g. [20].

Geometrical stress method using reference S-N-curve

The concept of the geometrical stress method is very similar to the S-N-curve concept. It has mainly been developed for welded structures.

For complex structures, the nominal stresses cannot be determined using elementary structural models. The stresses need to be determined on the basis of more advanced structural models (slab theory, theory of sheets or shell theory). These stresses are calculated for critical locations where the crack may occur and are defined as structural stresses.

Stress concentrations as effects of weld notches are not comprised in the structural stresses.

They are taken into account on the resistance side of the verification by S-N-curves which are experimentally determined or empirically estimated. The fatigue life verification is carried out identically to the S-N-curve concept. The geometrical stress method has been taken up in the codes of practise and literature, e.g. [25], [27], [28], [29] and [30] to verify the fatigue life of welded connections.

Local notch stress method using local elastic stresses

The local notch stress method is based on the elastic calculation of local stresses at the tip of a crack that is assumed to occur during the fabrication process, e.g. welding. The amplification of stresses by notch effects has to be considered. The determination of stresses is typically carried out by FE-analysis (Finite Element Analysis), BE-analysis (Boundary Element Analysis) or by applying Kt-values taken from literature.

(24)

Due to the fact that the linear elastic material law offers only an approximate estimation of the stresses for constant amplitude fatigue limit under a pure alternating stress range, the concept is only applicable for the alternating endurance limit of the member (R = -1). In this case, the local stresses calculated are compared with the alternating material properties. The influence of roughness and the size effect are considered in the analysis. A generalisation of the concept is realised referring to the proportionality of local elastic stresses to nominal stresses which enable a transfer of S-N-curves of the nominal stress concept to local elastic stresses. Therefore, the average stress relation and the service life can be estimated using the S-N-curve method.

This concept can be applied for members with and without notches and is documented for the verification of the fatigue life of members in the literature, e.g. [31], [32], [33], [34] and [35].

Local notch strain method using local stresses and strains

The local notch strain method refers to a characteristic function and to characteristic values of a material which is subjected to elastic-plastic deformations due to cyclic loads along a single axis. The results are cyclic stress-strain curves and strain-life-curves. The curves are determined experimentally or estimated empirically and described analytically. They comprehend material characteristics as well as effects from fabrication, size and environment. The characteristic loads as input in the local notch strain method are local stresses and strains. A correlation between loads on the structure and local elastic-plastic strains is guaranteed by load-strain curves. They are derived by numerical calculations, e.g. FE-analysis, or formulae for their approximation. The outcomes are local elastic-plastic stress-strain-curves calculated for a specific loading event. They are closed σ-ε-hysteresis curves which are evaluated with respect to their damage effect. The results are S-N-curves or Gaßner-curves for crack initiation. The magnitude of the initial crack length can be taken from literature, e.g. [36].

The local notch strain method can be applied for notched or un-notched as well as welded or non-welded members.

Fracture mechanics analysis

The fracture mechanics analysis is used to determine the remaining fatigue life after a macroscopic crack has been developed. The relevant information for this analysis approach are:

- Under which circumstances could crack propagation occur?

- Is it possible to intercept the growing of the crack?

- When will instable fracture occur?

The fatigue life assessment using fracture mechanics is based on the relationship observed between the range of the stress intensity factor, ΔK, and the crack growth rate of fatigue cracks, da dN. A simplified approach to estimate crack propagation is the equation of Paris and Erdogan:

K m

dN C

da = (Δ )

where:

- C, m are material properties respectively crack growth constants

-

dN

da is the crack propagation (da) per load cycle (dN) respectively the crack extension

(25)

- ΔK the range of the stress intensity factor

min

max K

K

K= −

Δ

Kmax is the maximum stress intensity in each cycle Kmin is the minimum stress intensity in each cycle

Since the crack growth rate is related to (ΔK)^m raised to an exponent, the exponent m having typically a magnitude of 3-4, it is important that ΔK should be known accurately if an acceptable reliability of prediction of crack propagation has to be achieved.

The input parameters for the materials properties C and m have to be obtained by representative material tests of the member being investigated. In general, these values can be taken from [63] or published data in literature, see e.g. [21], [22] or [72].

If da dN versus ΔK is displayed for an actual crack in a double-logarithmic diagram, an approximate sigmoidal curve results as shown in Figure 1.8. Below a certain threshold range of the stress intensity factor, ΔK_th, no crack growth occurs. For intermediate values of ΔK, the growth rate is idealised by a straight line.

Figure 1.8: Crack propagation rate da dN in dependence on the range of the stress intensity ΔK (schematic)

For a crack in a certain structural member, the following equation for ΔK can be derived:

Mk

Y a K =Δ ⋅ ⋅

Δ

σ π

where:

- a crack length derived from the cracks’ geometry

(26)

- Y stress intensity correcting function depending on crack configuration

- M_k stress intensity correcting function of welded components

- Δσ stress range

As Δσ, Y and M_k are not constant the estimation of service life has to be carried out incre- mentally using the following equation which is applicable for cracks with a crack depth greater than 0,1 mm to 1 mm :

M da Y a da C

K dN C

N da

e

e e a

a

m k a

a

m

∫

⁼

∫

_⋅_Δ ⁼ _Δ _⋅ _⋅

=

0

0 0 ( )

1 1

) (

1

σ π

a₀(initial crack depth) and a_e (final crack depth corresponding to failure) are used in order to predict the fatigue strength of a cracked member, assuming that the remaining life depends on crack growth starting from a pre-existing initial crack of known size. This crack has to be detected during an inspection or equivalent measures first and the size of the initial crack has to be determined.

Independently from the assessment concept, the local sequence of loading during service life mainly influences the fatigue strength.

Further information is documented in the codes and literature, e.g. [37], [38], [39], [40], [41], [42], [43], [44], [45] and [46].

Monitoring requirements based on the damage concept for service life analysis

The damage of the overall structure is represented by the damage of the critical detail. In general the number of necessary locations of measurements per critical detail depends on the experience. The ideal case is only a single measurement point for the investigation of each single critical detail.

The choice of the appropriate damage concept is essential for a reliable service life estimation. It should be based on chapter 1.6.4 and the following evaluations.

The analysis based on the verification with particular fatigue tests with prototypes is suitable for the life-time assessment of components with any geometry and material. The damage accumulation is derived very accurately and interactively for each structural component. The change of loading sequences can be directly considered in the analysis. Nevertheless, the verification with particular fatigue tests using prototypes depends on the possibility to design a representative prototype for each component to be analysed. Additionally the test set-up and the prototypes have to be available over the complete life time of a bridge structure. If it is impossible to identify only one, single critical detail various prototypes have to be investigated during the use of the bridge. Further on fabrication and erection as well as environmental conditions always include an uncertainty with a relevance on the fatigue behaviour of the critical detail. Detailed information on the practical application of this concept can be found in [24].

For structures with standardized components, the application of the S-N-concept with detail classes is very appropriate for estimating the service life. The damage of the overall structure is determined using the damage of each single component. One stress measuring point per detail is generally sufficient for monitoring input data for the S-N-concept. The S-N-concept is leading to a conservative service life estimation which is affirmed by various test results.

However, the reliability of an analysis carried out by the S-N-concept with detail classes highly depends on the accuracy of the stresses used as input factors.

(27)

The use of the geometrical stress method using reference S-N-curves is proper as damage model for the service life estimation of a structure with many components having a unified but complex geometry not regulated in codes. The structural stresses are estimated by FE- analysis for each detail. An extrapolation of the structural stresses is necessary to enable a comparison with the appropriate S-N-curve of the detail. An advantage of the service life analysis by the geometrical stress method using reference S-N-curves is the exclusion of the influence of real material properties and exact geometry of the investigated detail. As for the S-N-concept, one stress measuring point per investigated detail is sufficient for monitoring input data for the reference S-N-concept.

The local notch stress method using local elastic stresses is applicable for components with any geometry. Because the geometry of the analysed component has to be modelled very accurately, the concept is very expensive for extended use. Consequently, the concept should only be applied for the exact investigation of single components with specific high accuracy requirements.

The local notch strain method using local stresses and strains is used to estimate crack ini- tiations for a certain detail. Due to the fact that it is not possible to detect crack initiation by monitoring the local notch concept has to be rejected for this purpose. Nevertheless, it can be used for the pre-design of a monitoring system to identify critical components of a structure going to be investigated.

The fracture mechanics analysis is appropriate to assess the residual service life for a member respectively segment containing a crack. Therefore, the concept has to be used for evaluation after a crack has been detected. In general this is possible by non-destructive testing from a crack length of 1mm to 2 mm on only.

The estimation of the residual service life by fracture mechanics analysis methods can be additionally verified by measuring crack growth using monitoring techniques. The potential position of cracks and crack sizes need continuously to be investigated as well as the stresses for each detail. These measurements are very complex and expensive unless it is possible to reduce the number of critical details.

As alternative the fracture mechanics analysis could be applied assuming a fictive crack in the member. The concept would predict a residual operation reliability if a crack has not been detected during an inspection. The application of technical monitoring could be used to check fictive crack initiation respectively to precise the assumptions used in the concept (e.g. for the loading: ΔK).

1.6.5 General approach for the service life analysis with the help of monitoring in the following the general approach for the service life analysis with the help of monitoring is roughly described and pictured, see Figure 1.9.

1. Step:

The first step in a service life analysis of a structure should be the recording of the structures’

condition and its damage development (condition assessment) where required.

A detailed inspection of the entire structure is necessary. The inspection should identify all relevant defects and damages.

The main focus is on damage indicators such as crack sizes, deformations, increasing strains, chemical thresholds, humidity or other limit states. Frequently even colour changes or flakings are first indicators of damages.

Decision:

Structure should be analysed more precisely (yes ⇒ 2. step / no ⇒ end)

(28)

2. Step:

More precise analysis of the structure by: recording of geometry; determination of material parameters and condition; determination of the macroscopic material condition; modelling of the structure.

The geometry of the structure (global geometry of system, local geometry of cross section) is recorded by measuring sticks, photogrammetry, tachymetry or laser scanning. The material parameters and condition can be estimated by destructive or non-destructive testing, see 1.8.2. The macroscopic material condition in the range of critical details is also analysed by non-destructive testing methods. Depending on damage type the identification of internal imperfections is e.g. implemented by ultrasonic or radiography and the identification of sur- face cracks by using magnetic powder or dye penetrant. The modelling (e.g. with FEM) shows the static and dynamic behaviour of the structure. An imprecise modelling can hide or pretend critical components of a structure.

By means of the more precise analysis an identification of the critical components is possible.

Assessment:

A positive estimation leads to the conclusion, that a continuous long term monitoring of the structure is not necessary and as a result there are no additional actions to be implement.

If the estimation is negative regarding the expected life span, a continuous monitoring in combination with adaptive prognosis models has to be accomplished. However that is only expedient if sufficient information about the history and load conditions of the structure exist.

Otherwise the accuracy of input parameters is questionable . If there does not exist any use- ful information, monitoring can still be used to check thresholds like local strains, crack sizes, etc, defined by an assessment of the operating time interval.

3. Step:

The advantage of the service life analysis on the basis of adaptive models is the continuous adaptation to the constantly changing structure condition by local measurements. The uncertainties of the action and structural models are avoided by strain measurements at the critical details.

At this point a damage calculation (linear or non-linear) could be implemented. This an be achieved by a modelling of the past loading history, a monitoring of the current loads and the resultant by classifying methods (e.g. rainflow method) determined loading collectives. How- ever the damage model with its significant uncertainties is still necessary.

In order to eliminate these uncertainties, an experimental service life analysis with a digitally regulated testing machine is possible. Therefore a model of the detail is required.

⇒ life time prediction

As a result of the analysis the residual life time of a structure is predicted which can be compared with the expected operation time of structure.

(29)

Condition assessment (Inspection of the bridge condition )

YES NO

END Investigation of:

- Geometry

- Material parameters

- Macroscopical material condition - Modeling

Should the structure be analysed more

precisely?

Assessment NEGATIVE

Identification of critical components

Long-term monitoring not necessary

END

Existence of information about

the loading history ? ^NO

Control of thresholds by monitoring

(local strains, crack sizes)

YES

Continuous monitoring + adaptive prognosis models

Life time prediction

Assumptions

- +

POSITIVE

Figure 1.9: Life time prediction with the use of monitoring

1.6.6 Alternative approach using operation time interval

A further possibility to secure the operation of a railway bridge using technical monitoring is the operation time interval approach according to [3]. Already detected defects of the bridge structure are under surveillance. If a threshold value specified by the operation time interval is exceeded an alert will be given, see 1.5 “Continuous online monitoring (Red-Alert-effect)”.

The major advantage of the operation time approach is, that the loading history is not required to guaranty the stableness of the bridge in the time interval.

1.7 Identification of critical members

Modern monitoring techniques allow to acquire a large quantity of information concerning the performance and health of structures. To design a monitoring system economically, it is essential to filter the relevant information needed, e.g. for a service life analysis.

Consequently, one major item in the design of a monitoring system is the identification of the critical details of the steel bridge structure to be monitored. Critical details of the bridge are details of structural components, so called neuralgic components, having an outstanding damage risk (e.g. stress amplification).

Guidelines for monitoring of steel railway bridges Background document D5.2.S1