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RT/CE/C/025

Issue: 1

Date: February 2001

RAILTRACK LINE CODE OFPRACTICE

The Structural Assessment of Underbridges

Copyright 2001 Railtrack PLCAll rights reserved. No part of this publication may be reproduced, stored in a

retrieval system, or transmitted, in any form or by any means, electronic,

mechanical, photocopying, recording or otherwise, without the prior written

permission of Railtrack PLC.

Endorsement and Authorisation

Endorsed by:

Kim Teager, Professional Head Of Structures Engineering

Accepted for Issue by:

Graham Morris, Head Of Corporate Standards

Authorised by

This publication, including the dataand information relating thereto, is

not to be used, disseminated,

stored in a retrieval system,

reproduced, copied or adapted

either in whole or in part without

the express written permission of

RAILTRACK p l c .

Published & Issued by

Railtrack plc

Railtrack House

Euston Square

L O N D O NNW1 2 E E

2001 RAILTRACK PLC


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RAILTRACK LINE CODE OF PRACTICE RT/CE/C/025

Issue: 1

Date: February 2001

The Structural Assessment of Underbridges Page 2 of 11

SUMMARY

This Code of Practice provides recommendations for the parameters and methods to

be used for the assessment of underbridges owned by Railtrack.

ISSUE RECORD

This Code of Practice will be updated when necessary by distribution of a complete

replacement or revised sections. Amended or additional parts of revised pages will

be marked by a vertical black line in the margin. Due to the extensive number of

revisions compared with Issue 1 such changes have not be marked in this Issue.

ISSUE 1 DATE February 2001 COMMENTS: New Code of Practice

to provide a limit state code for

assessment of underbridges in respect ofsteel, wrought iron, and concrete and

composite bridges, and to codify

permissible assessment parameters and

methods for under bridges formed from

other materials of construction.

RESPONSIBILITIES AND DISTRIBUTION

This Code of Practice should be used by persons undertaking the assessment of

underbridges and by those responsible for managing the process of bridge assessment

carried out by others.

IMPLEMENTATION

This Code of Practice should be complied with from April 2001.

DISCLAIMER

Railtrack PLC has used its best endeavours to ensure that the content, layout and

text of this document are accurate, complete and suitable for its stated purpose. It

makes no warranties, express or implied, that compliance with the contents of this

document shall be sufficient to ensure safe systems of work or operation. Railtrack

PLC will not be liable to pay compensation in respect of the content or subsequent

use of this document for any purpose other than its stated purpose or for any

purpose other than that for which it was prepared except where it can be shown to

have acted in bad faith or there has been wilful default.

SUPPLY

Paper copies of this document will be available to Railtrack staff on request to the Document

Controller. Copies of this document will be available to other organisations from Technical

Indexes (01334 404409).


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The Structural Assessment of Underbridges Page 3 of 11

CONTENTS

SECTION 1 INTRODUCTION

SECTION 2 ASSESSMENT PHILOSOPHY

SECTION 3 INSPECTION FOR ASSESSMENT

SECTION 4 LOADING FOR ASSESSMENT

SECTION 5 STEEL AND WROUGHT IRON STRUCTURES

SECTION 6 MASONRY ARCHES

SECTION 7 CONCRETE STRUCTURES

SECTION 8 COMPOSITE STRUCTURES

SECTION 9 CAST IRON STRUCTURES

SECTION 10 TIMBER STRUCTURES

SECTION 11 SUBSTRUCTURES

SECTION 12 BEARINGS

APPENDIX A ASSESSMENT OF STEEL AND WROUGHT

IRON

APPENDIX B ASSESSMENT OF CONCRETE STRUCTURES

APPENDIX C ASSESSMENT OF COMPOSITE STRUCTURES

APPENDIX D FATIGUE ASSESSMENT OF STEEL AND

WROUGHT IRON

APPENDIX E MODEL BRIDGE ASSESSMENT REPORT

APPENDIX F INFORMATIVE ANNEX


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Section 1 - Introduction Page 4 of 11

CONTENTS

1. INTRODUCTION...................................................................................................................4

1.1 Purpose................................................................................................................................4

1.2 Scope....................................................................................................................................4

1.3 Units .....................................................................................................................................5

1.4 Definitions and Abbreviations.........................................................................................6

1.5 Competency .......................................................................................................................6

1.6 Procedures for Quantitative Assessment.....................................................................7

1.7 Qualitative Assessment Procedures............................................................................10

1.8 Railtrack s Technical Approval Procedures ...............................................................10

1.9 Reporting...........................................................................................................................101.10 Informative Annex.........................................................................................................11

1. INTRODUCTION

1.1 Purpose

The purpose of this Code of Practice is to recommend applicable standards and

analytical methods which may be used to determine the load carrying capacity of

existing Railtrack underbridges, in terms of British Standard Units of Type RA1

loading. The load carrying capacity is determined in the context of the performance

requirements of an underbridge. The requirements are that the bridge meets safety

and serviceability criteria whilst regularly carrying rail traffic up to a level of traffic

load and speed in accordance with operational system requirements.

1.2 Scope

This Code of Practice may be used for the assessment of all Railtrack owned

underbridges and is applicable for permissible speeds up to a maximum 125 mph.

This Code of Practice provides recommendations for the assessment of underbridges

constructed from steel, wrought iron, cast iron, concrete, timber, or composite

steel/concrete construction. Recommendations for masonry arches, substructures

and bearings are also included. Limit state principles are used for underbridges of

steel, wrought iron, concrete and steel/concrete composite construction.

Permissible stresses or allowable loads are used for other materials and forms of

construction.

Where appropriate, guidance on the use of simple and more rigorous methods of

analysis is given. Unusual forms of construction such as cable stayed, moveable or

combined road/rail bridges are not specifically covered, but the principles outlined

may be applied in checking the elements of such structures.


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Requirements for the assessment of superstructures and supports under accidental

loading conditions are excluded from this document.

1.3 Units

The S.I. system of units is adopted throughout this Code of Practice unless otherwise

stated.

In the course of assessment frequent reference may have to be made to existing

records which may be presented in Imperial Units. Great care should be exercised in

the conversion between the two systems of units. The following table gives

conversion factors for some of the most commonly occurring units.

PROPERTY IMPERIAL UNIT

METRIC

equivalent of

IMPERIAL UNIT

METRIC

UNIT

Length inch 2.5400 cm

foot 0.3048 m

yard 0.9144 m

mile 1.6093 km

chain 20.1168 m

Area inch 645.1600 mm

inch 6.4516 cm

foot 0.0929 m

yard 0.8361 m

Volume inch 16.387 cm

foot 0.0283 m

yard 0.7646 m

Mass lb 0.4536 kg

ton 1016 kg

ton 1.0160 tonnes

Modulus inch 16387 mm

inch 16.387 cm

Inertia inch4 416200 mm4

inch4 41.62 cm4

Speed mph 1.6093 kph


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Table 1.1

Conversion Factors

1.4 Definitions and Abbreviations

For the purpose of this Code of Practice the following definitions apply:

Bridgemeans a structure of one or more spans whose prime purpose is to carry

traffic or services over an obstruction or gap.

PSRmeans Permanent Speed Restriction.

Provisionally Sub-standard Bridgemeans a Bridge that has been assessed at the

Level 1 assessment stage of the Bridge Assessment process to have a safe load

capacity less than the RA Capacity of the route. The Bridge remains Provisionally

Sub-standard until it is confirmed on completion of the Bridge Assessment that the

safe load capacity is not less than the RA Capacity of the route or the Bridge is

classified as a Sub-standard Bridge.

Serviceability Limit State (SLS)means the condition at which the behaviour of a

Bridge becomes unsatisfactory to the extent that it can no longer satisfactorilyperform its function under service loads.

Sub-standard Bridgemeans a Bridge where, following completion of a Bridge

Assessment, action(s) is (are) required to protect the safety of the Bridge. A Bridge

remains classified as Sub-standard until actions are taken to remove the applied

controls, or the RA Capacity of the route is amended to not more than the safe load

capacity of the Bridge.

TSRmeans Temporary Speed Restriction.

Ultimate Limit State (ULS)means the condition at which the Bridge, or one of

its constituent parts, would fail due to loss of equilibrium, fatigue induced

deterioration, or exceedance of its collapse strength.

Railtrack Director s Nomineemeans the Structures Engineer with formally

delegated responsibility for the assessment of underbridges within the Railtrack Zone.

1.5 Competency

The skills, expertise and training of those persons responsible for, and carrying out,

the assessment should be appropriate to the nature and complexity of the structureunder consideration.


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1.6 Procedures for Quantitative Assessment

The assessment should commence with a definition of the assessment objective. A

clear statement of the required load carrying capacity should be made. In particular,

it should state specifically the existing RA capacity of the structure, the existing RA

capacity of the route and whether an increased structure capacity greater than that of

the route is required. The initial assessment (Level 1) should generally comprise

three distinct phases as follows:

1. Desk Study

All available information relevant to the structure, including record drawings,

inspection and maintenance records, details of past performance and previous

assessments, and any available ground investigation data should be collatedand examined. The documents should be verified for correctness and in

particular, whether they were updated after previous works on the structure.

2. Inspection for Assessment

A detailed examination of the structure is required to verify the form of

construction, its dimensions and the nature and condition of the structural

parts.

3. Analysis

Based on the information obtained from the first two phases of theassessment process, structural analysis to determine the distribution of forces

within the structure and the load capacity of the structural parts is required in

most cases.

In order to determine the adequacy of a particular structure with the minimum

degree of effort, the assessment should be carried out in levels of increasing

refinement and complexity, with the initial level (Level 1) being based on the most

conservative distributions of loads and analytical assumptions. If the structure is

shown to be inadequate in relation to the required load carrying capacity at this level,

assessment work should continue, with subsequent levels seeking to removeconservatism in the assessment where this can be justified. Subsequent more detailed

levels may use:

more refined structural analysis;

more precise estimates of loading based on real vehicles;

material properties based on testing of materials samples;

supplementary load testing.

As illustrated in Figure 1.1 the process is cyclical in nature, each cycle being at an

increasingly refined level until a decision on the adequacy of the bridge is reached.Conceptually it is useful to envisage levels of assessment as follows:


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Level 1 Simplest level using assumptions known to be conservative.

Level 2 Use of more refined analysis and better structural idealisation. This

level may also include use of data on materials strengths based on mill

test certificates or recent material tests on another similar structure.

Level 3 Use of a bridge specific live loading based on the known traffic and/or

the use of tests on materials samples or the use of worst credible

strengths or the use of load tests.

Where, by inspection, it is considered that greater benefit may be gained by theadoption if live loading based on real trains than from a more refined analysis, the

assessment may progress from Level 1 directly to level 3.

The conclusion from the assessment should be subjected to a plausibility check. In

particular, discrepancies between the results of structural analysis, indicating

inadequacy say, and the real structural condition, for example no sign of distress or

failure, should be explained.


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Analytical

assessment?

Qualitative assessment

Review structurePass

Further investigation

and review

Level 1 assessment

Analysis

Provisionallysub-standard structure

Urgent safety

measures?

Implement

measures

Level 2-3 assessment

Assessment report

Urgent safety

measures

Review assessment

objective

Implement

measures

Review assessmentobjective

Yes

No

No

YesYes

No

Sufficientcapacity/

adequate condition?

Operational restrictions

/repair/upgrade?

Review past performance and inspection data.

Decide assessment objective.

Bridge

management

programme

Periodic

inspection

Maintenance

Performancereview

Fail

Pass Pass

Pass

Fail

Fail

Fail

Yes

No

Review safety measures

Figure 1.1

Assessment Process Flow Diagram


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1.7 Qualitative Assessment Procedures

For some types of structure where no established method of quantitative theoretical

assessment exists and where increased capacity is not required, assessment may be

made qualitatively on the basis of satisfactory past performance. Structures for which

this procedure may be considered are spandrel and dry stone walls, retaining walls,

jack arches, substructures and foundations. The requirements for assessment on this

basis are:

the structure has demonstrated satisfactory performance over a long periodof time (over 5 years) since any significant repairs or alteration;

careful inspection does not reveal significant damage, distress ordeterioration;

review of the structure confirms its force transfer system;

predicted future deterioration will not jeopardise safety;

no significant changes in the loads and actions on the Bridge are anticipated.

Where the assessing engineer proposed a qualitative method of assessment, this shall

be justified and recorded in accordance with Railtrack s Technical Approval

Procedures.

1.8 Railtrack s Technical Approval ProceduresAll assessments shall be subject to Railtrack s Technical Approval Procedures for

assessment.

Irrespective of whether the assessment is to be carried out on a quantitative or

qualitative basis, the chosen method should be recorded and justified within the

Form AA. Where a qualitative method is proposed for the assessment of one of the

structure types identified in Clause 1.7, reference to this document may be deemed

to be sufficient justification for adoption of the method.

For the assessment of Bridges or structural elements which are outwith the scope ofthis document, the method of assessment should be agreed within the Technical

Approval Procedure by Railtrack s Professional Head of Structures Engineering.

1.9 Reporting

When the assessment has been completed, a report should be prepared detailing the

various stages of the process, together with the results. A suitable format for the

assessment report is given in Appendix E. Summary tables for reporting the

assessment results have been included in Appendix E for metallic structures, masonry

arches and concrete structures. These summary tables should be completed and

incorporated in the final report where applicable.


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1.10 Appendices

Additional notes and further information relating to the assessment of underbridges

are contained in Appendices A to F. Clauses within each appendix are numbered

sequentially from 1.0 and are followed by a letter denoting the appendix to which

they belong. For example Clause 4.1.1B indicates Clause 4.1.1 of Appendix B.

1.11 Informative Annex

Background information on the derivation of certain clauses of this code of practice

and guidance on its usage is contained in Appendix F. It should be noted that this

Appendix is not intended to give comprehensive guidance, and should not be assumed

to indicate all aspects of a structure that should be checked in the course of an

assessment.


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Section 2 - Assessment Philosophy Page 1 of 8

CONTENTS

2. QUANTITATIVE ASSESSMENT PHILOSOPHY...............................................................1

2.1 Applicability.........................................................................................................................1

2.2 Basis for Quantitative Assessment.................................................................................1

2.3 Assessment Situations ......................................................................................................1

2.4 Limit States..........................................................................................................................3

2.5 Assessment Load Values ..................................................................................................4

2.6 Load Factors .......................................................................................................................4

2.7 Assessment Load Effects ..................................................................................................6

2.8 Assessment Resistance.....................................................................................................7

2.9 Verification of Structural Adequacy...............................................................................8

2. QUANTITATIVE ASSESSMENT PHILOSOPHY

2.1 Applicability

The analytical procedures for quantitative assessment given in this Section are

applicable to most structural forms. They are not applicable to structures where

analysis is impractical and where the original design was based on good construction

practice of the time and no codes existed. In these cases assessment can be based on

qualitative judgement of satisfactory past performance and the information obtained

from assessment inspections. In all cases the purpose of assessment is to determine

whether the bridge meets relevant safety and serviceability criteria, see Clause 1.1.

2.2 Basis for Quantitative Assessment

Assessment of steel, wrought iron, concrete and steel/concrete composite Bridges

should be undertaken by the application of limit state principles. Bridges and

structural elements constructed from cast iron, timber or masonry should be

assessed on permissible stresses or loads.

Irrespective of the basis on which a Bridge is to be assessed, the bridge is required to

satisfy the Operational Safety Limit State requirements given in Clause 2.4(d).

2.3 Assessment Situations

The circumstances in which the Bridge is required to fulfil its function should be taken

into account by selecting relevant situations for assessment. The situations should

encompass all conditions that can reasonably be foreseen during use of the Bridge by

rail traffic. The situations should be determined by making a critical selection of

conditions arising due to dead and imposed load, live traffic loads and where relevant

temperature and wind effects. The situations chosen, characterised by a dominant

live load and one or more coexistent loads, should include the most adverse liveloads as follows:


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Situation (1) Maximum vertical live load with coexistent transverse and longitudinal

live loads;

Situation (2) Maximum longitudinal live loads with coexistent minimum vertical and

transverse live loads;

Situation (3) Maximum transverse loads with coexistent minimum vertical and

longitudinal live loads.

In the above situations other live loads where required by Section 4 such as those due

to wind and temperature should also be included where a more onerous loading may

result.

The values of maximum and minimum live loads for each situation are determined by

multiplying the nominal live loads given in Section 4 by the applicable factors given in

Table 2.1. The coexistent loads should be taken as zero if this results in a more

onerous loading of the Bridge.

SITUATION (1) SITUATION (2) SITUATION (3)

Railway Live Loading

Component

Maximum Vertical +

coexistent

Longitudinal and

Transverse

Maximum

Longitudinal +

coexistent

minimum Verticaland Transverse

Maximum

Transverse +

coexistent

minimum Verticaland Longitudinal

Vertical:

Type RA Loading 1.0 0.5 0.5

Longitudinal:

Traction & Braking 1.0 (0) 1.0 0.5 (0)

Transverse:

Nosing

Centrifugal

1.0 (0)

1.0 (0)

0.5 (0)

0.5 (0)

1.0

1.0

Table 2.1

Factors for Combinations of Components of Railway Live Loading

Partial factors for use in commonly occurring situations are given in Table 2.2. In

special cases, other situations may arise and govern the assessment.


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2.4 Limit States

Where Bridges are to be assessed under the selected situations using limit state

principles, the following should be considered:

(a) Ultimate Limit State (ULS)

The ULS is generally the governing condition for the assessment of

underbridge capacity.

This condition relates to the collapse strength of individual elements of the

Bridge, and to the stability of a part or the whole of the Bridge when

considered as a rigid body. To verify that an ultimate limit state is not

reached, it is necessary to demonstrate that the criteria in the relevantSection of this Code of Practice are not exceeded under the application of

ULS assessment loads.

(b) Serviceability Limit State (SLS)

Serviceability limit states are those situations where excessive deformations

or a deterioration in structural condition may lead to a loss in utility of the

Bridge such that remedial action may be required. Circumstances in which it

may be necessary to carry out checks against SLS criteria are defined in

Clauses 4.2.2A, 4.1.1B and 4.3.2C.

(c) Fatigue Limit State

The limit state for fatigue may be either an ULS or SLS. Where an assessment

situation exists requiring fatigue evaluation (see Clause 4.3.2A) it should be

checked taking the load factors fL and 3f equal to 1.0. For cast iron

Bridges, see Section 9.

(d) Operational Safety Limit State (OSLS)

These conditions are attained when specified limits which govern the safe

operation of the railway are reached. These limits will generally be related to

the changes in structural deformation that occur under the passage of a trainand which, if exceeded may lead directly to derailment, or to degradation of

the track which may, in time, have the same effect. They are limits of

serviceability beyond which a Bridge is operationally unserviceable. Further

information regarding OSLS requirements is given in Appendix F.

For the bridge structure as a whole, an Operational Safety check should be made

relating to track twist in accordance with Section 4. For some structures,

Serviceability Limit States, such as bridge deflections and rotations, may also need to

be checked. Appropriate criteria should be agreed in accordance with Railtrack s

Technical Approval Procedures.


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2.5 Assessment Load Values

The assessment loads, *AQ , are determined from the nominal loads, KQ according to

the equation:

KfL

*

A QQ = Equation 2.1

where:

fL is a partial factor for each type of loading as given in Table 2.2.

Nominal dead and superimposed dead loads may be determined using theinformation given in Section 4. Details of the nominal live loading and its application

are also given in Section 4.

2.6 Load Factors

Dead and superimposed dead loads should be taken together with live loads using the

factors given in Table 2.2 and in accordance with Section 4. Where it is necessary to

consider loads, such as those due to wind or temperature, which are not defined in

Section 4 of this Code of Practice, reference should be made to BD37/88: Loads for

Highway Bridgesin accordance with Clause 4.4.


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LOAD Limit

State

fL to be considered inCombination

1 2 3

Dead:

Steel, wrought iron ULS 1.05

SLS 1.0

Cast iron ULS 1.1

SLS 1.0

Concrete, masonry, timber ULS 1.15

SLS 1.0

Superimposed dead:Ballast *1, *2 ULS 1.75

SLS 1.2

Track *3 ULS 1.2

SLS 1.0

Fill ULS 1.2

SLS 1.0

Services ULS 1.25

SLS 1.0

Live:

The multiple components of LiveLoading should be considered to

act in accordance with Clause 2.3

ULSSLS

1.4 *4

1.11.2 *

4

1.01.2 *

4

1.0

Wind:

ULS

SLS

1.1

1.0

Temperature:

Restraint to movement or due to

frictional bearing restraint

ULS

SLS

1.3

1.0

Table 2.2Values of Partial Factors (fL) for Loads in Combinations

*1 A value of fL of 1.35 at ULS and 1.1 at SLS may be adopted provided the depth ofballast is controlled or dictated by the form of construction. Control measures may

include datum plates or a Plimsoll line.

*2 Ballast more than 300 mm below underside of sleepers may be considered as fill.

*3 Track includes rails, fixings and sleepers, but excludes ballast between sleepers.

*4 Subject to the approval of the Railtrack Director s Nominee a reduced value of 1.25

for combination 1 and 1.1 for combinations 2 and 3 may be adopted where theloading is of a controlled nature as follows:


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(a) There is reliable control over the trains that can enter the route in question,

and

(b) For vehicles which comprise any of the following:

Locomotives;

Locomotive hauled passenger and/ or mail trains;

Other passenger and/ or mail trains;

Cranes and track plant not able to carry loads whilst in travelling mode;

Freight wagons where loading is physically controlled, for example fluid fuel

tank wagons, closed grain or closed cement wagons;

Standard coal hopper or similar wagons where the load is weighed beforedispatch.

Reduced values of fL can only be assumed for other vehicles where every vehicleafter loading is weighed or is otherwise subject to proper assessment of weight,

before details are submitted and accepted for such vehicles to cross the Bridge.

These vehicles include freightliner container wagons, open top wagons for

aggregates, spoil or waste and wagons for track infrastructure maintenance or

renewal.

2.7 Assessment Load EffectsThe assessment load effects, *AS , are obtained from the assessment loads by the

relation:

( )*Af*

A QS ofeffects3= Equation 2.2A

( )KfLf*

A QS = ofeffects3 Equation 2.2B

Note: For steel and wrought iron only (Section 5), 3f is applied within the

resistance *R (see Clause 2.8) such that:

*

A

*

A QS ofeffects= Equation 2.3A

KfL

*

A QS = ofeffects Equation 2.3B

where:

3f is a factor that takes account of inaccurate assessment of the effects of

loading, such as unforeseen stress distribution in the structure, inherent

inaccuracies in the calculation model, and variations in the dimensional

accuracy from measured values. The effects of the assessment loads should


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be obtained by the use of analytical procedures applicable to the form of

construction.

The factor 3f should normally be taken as 1.1 for ULS and 1.0 for SLS. 3f

may be taken as 1.0 for the ultimate limit state for members where the

following conditions (a), (b) and (c) are all met:

(a) members are either:

(i) Rail bearers or cross girders of steel, wrought iron or

composite construction that are assumed to be simply

supported, or;

(ii) Main girders of steel, wrought iron or steel/concrete

composite bridges with skew not greater than 25 (If main

girders are continuous, any splices should be welded or made

with HSFG bolts or rivets, and have cover plates to both

flanges) or;

(iii) Main beams of reinforced or prestressed concrete bridges with

skew not greater than 25 that are assumed to be simply

supported.

(b) load effects are based upon static distribution within the structure;

(c) geometric dimensions of the members are verified during inspection.

2.8 Assessment Resistance

The assessment resistance, *AR , of any structural element is the calculated resistance,*R , of that element, making appropriate allowance for any deterioration identified.

The calculated resistance, *R , determined from material strengths and measuredsection properties should be calculated from the following equation:

( )mk* fR = function Equation 2.4

Except for steel and wrought iron structures only where (Section 5):

( )( )3function fmk* fR = Equation 2.5

where:


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kf is the characteristic (or nominal) strength of the material;

m is a partial factor for material strength.

Values ofk

f andm

are given in Sections 5, 7 and 8 according to the material ofconstruction.

For those materials where the calculated resistance is determined on a permissible

stress basis, the following may be applied:

( )p* fR function= where pf is the material permissible stress.

2.9 Verification of Structural Adequacy

Structures should be deemed to be capable of carrying a specified level of assessment

loading when the following relationship is satisfied:

*

A

*

A SR >


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Section 3 - Inspection for Assessment Page 1 of 18

CONTENTS

3. INSPECTION FOR ASSESSMENT .......................................................................................1

3.1 General ................................................................................................................................1

3.2 Requirements prior to Inspection..................................................................................2

3.3 Inspection for Loading ......................................................................................................2

3.4 Inspection for Resistance.................................................................................................3

3.4.1 General ........................................................................................................................3

3.4.2 Metal Bridges ..............................................................................................................4

3.4.3 Masonry Arch Bridges ..............................................................................................7

3.4.4 Reinforced and Prestressed Concrete Bridges.................................................13

3.4.5 Composite Bridges..................................................................................................153.4.6 Timber Bridges.........................................................................................................15

3.4.7 Substructures............................................................................................................16

3.4.8 Bearings......................................................................................................................17

3.5 Report on Inspection......................................................................................................18

3. INSPECTION FOR ASSESSMENT

3.1 General

This Section gives recommendations for the inspection of underbridges, following the

desk study of existing information. The purpose of the inspection is to obtain

information required for the structural assessment and determination of safe load

carrying capacities. The principles outlined below may be applied to all types of

underbridge, and all materials of construction referred to in this Code of Practice.

Inspection for assessment is necessary to verify the form of construction, the

dimensions of the structure and the nature and condition of the structural

components. Inspection should cover not only the condition of individual

components but also the condition of the structure as an entity, noting especially any

signs of distress and possible causes.

Should the inspection reveal a defect which is believed to seriously compromise the

structures ability to carry load safely, the Railtrack Directors Nominee is required to

be advised urgently in order that consideration may be given to the appropriate

emergency action to be instructed. Examples of defects that may require urgent

action to maintain the safety of the Bridge would include cracks in metallic structures,

or in the case of a masonry arch bridge if part of the arch is sagging.

Where practicable, advantage should be taken of the presence of scaffolding forrepairs/painting, the removal of ballast, longitudinal timbers, walkway boarding,


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periods of low water etc. which may improve access for inspection of concealed and

otherwise inaccessible parts.

Where reasonably practicable Bridges should be observed under rail traffic and any

signs of abnormal movement such as excessive deflection, working of connections,

vibration or movement should be noted and considered as part of the assessment.

Where possible, these observations should be made under the passage of the heaviest

rail traffic using the Bridge.

When inspection is to be carried out in the hours of darkness the Bridge should first

be observed in daylight.

The skill, expertise and training of the person carrying out the inspection should be

appropriate to the complexity of the structure being assessed. This person should be

involved in the subsequent assessment process.

Where the taking of samples is considered necessary to confirm material parameters

or condition, the number, position and size of samples to be taken and any

consequential making good is required to be agreed by the Railtrack Directors

Nominee. With regard to metallic structures, material testing should generally only

be used to confirm the material types, allowing the adoption of typical material

properties form Table A2 for assessment. Only in circumstances where this process

shows the material to be untypical should additional testing be undertaken to confirm

the yield stress and other appropriate material properties. Guidance on material

identification, sampling and testing is included in Appendix F.

3.2 Requirements prior to Inspection

Prior to undertaking an inspection of a Bridge all existing information pertaining to

the Bridge should be examined, including as-built drawings, soils data, past assessment

and examination reports and details of mineral extraction, as appropriate. This

examination may be useful in determining what further information should be

obtained from the inspection and which items require special attention. Special

attention should be paid to checking whether previously identified defects have

worsened.

Emergency reporting arrangements should be established and inspection personnel

advised of these in advance of all site activities.

3.3 Inspection for Loading

The inspection should enable the material type and all dimensions necessary to

calculate an accurate estimate of the dead and superimposed dead loads to be

determined.


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The position of tracks, rail joints (e.g. fish plated, welded and expansion) or switchesand crossings (within 18 metres of the bridge bearings) relative to the Bridge and

whether timber or concrete sleepers are installed should be recorded.

Track cant, radii, permissible speeds and any PSR or TSR should also be recorded

where appropriate.

The presence of longitudinal timbers, methods of fastening and positions of joints and

notches in timbers should be recorded.

Where the Bridge carries ballasted tracks, the overall ballast depth and depth to

underside of sleepers should be determined. The extent and height of any ballastheaped on the bridge should also be noted, and the level relative to any control

marks recorded.

The location, number, size and type of services and service troughs should be

recorded.

3.4 Inspection for Resistance

3.4.1General

The Bridge should be inspected to record all the parameters needed to determine:

the strength of elements and joints, including any observed defects, such ascracks, loss of section due to corrosion, settlement, defective materials,

damage etc.;

the form of the structure to enable, in particular, assessment of dynamiceffects (see Section 4).

This inspection should be carried out within touching distance.

The inspection should supplement and provide confirmation of any informationobtained from existing records, particularly:

dimensions of internal sections that may not be related to external features;

strengthening and repairs that may not appear on record drawings, as theseelements may limit the load carrying capacity of the Bridge.

All constituent parts of the structure should be inspected in sufficient detail so that

their respective strengths can be determined. In some cases sampling of materials

may be required. Those parts not inspected should be recorded clearly and reasons

given.


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For buried members and those with hidden parts, excavation of trial holes etc.,

should be considered where there is doubt about the above parameters, especiallywhere such parameters could be critical. Care should be exercised to ensure that

there is no permanent damage caused to the structure by such excavations.

3.4.2Metal Bridges

3.4.2.1General

Prior to the inspection, a preliminary review of the structure, should be undertaken

to identify and assess potentially fatigue prone components and details.

The location, extent and remaining section of members where corrosion or otherforms of deterioration has occurred should be recorded accurately (preferably in

sketch form) to enable calculations to be made of section properties. The extent of

corrosion should also be established where metal sections are in contact with timber

decking or longitudinal timbers.

The location, nature and extent of distortion of structural elements resulting from

bridge strikes should be recorded.

Samples should be taken where required for testing to determine yield stress or

other material properties. Signs of poor quality and inferior metal should be notedand further tests carried out if appropriate.

All cast iron members should be checked for the presence of cracks and blow holes

especially in tensile areas. The location and extent of such defects should be

recorded.

Where suspension bolts support a live load carrying member, particularly where their

failure could directly lead to collapse of the member, consideration should be given to

removal of a bolt or plate for inspection purposes. The stability of the structure must

be maintained after removal of these components.

Evidence of water seepage which may have contributed to corrosion of parts that are

not directly amenable to inspection should be noted. Exploration to establish the

extent of any corrosion should be considered.

Loose or missing bolts or rivets, rivets with severely corroded heads and any

working or rust staining of any connections should be recorded.

The dimensions and condition of free spanning longitudinal timbers should be

recorded.


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The verticality, and magnitude and direction of horizontal bow of top flanges of maingirders as required by Clause 9.8.2A plus details of the end restraints including those

for vehicle restraint should be recorded. Out of flatness of web panels should also be

recorded.

For half through type bridges with solid web or truss girders the presence of and

condition of features which may be contributory to compression flange stability

should be noted such as:

(i) cross girder to main girder connections including the relative locations of

vertical stiffeners;(ii) signs of loose or working elements such as rivets, bolts or packings;

(iii) presence of concrete or other haunching or infilling to the main girders;

(iv) other connections between floor and main girders such as troughing, plate or

timber floor, resting onto the bottom flange etc.;

(v) trimmers or end cross girders and any infilling at or adjacent to the bearings;

(vi) type of bearings and whether they or any infilling or haunching is providing

torsional restraint to the main girders. A note should be made of any wear,

cracking or spalling of bedstones;(vii) details and location of bearing stiffeners, end plates and other stiffening local

to the bearings;

(viii) verticality of the main girders at the bearings. Magnitude, shape and direction

of horizontal bow of the main girder top flanges. A note should be made of

any additional movements of the main girders under live loading.

3.4.2.2Fatigue

Members particularly susceptible to fatigue should be closely examined for visible

cracks so far as reasonably practicable. In particular close attention should be paid tothe details shown in Figure 3.1 which are known to be fatigue susceptible. In addition

to these, areas of severe and/or pitted corrosion around areas which have been

subjected to mechanical damage and distortions, such as may arise from vehicle

impact should be closely examined.

Where visible cracks are found, their extent should be measured and recorded.


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NOTCHED RAILBEARERS

LIABLE TOFATIGUECRACK

WELDED DOUBLER ENDS

LIABLE TOWELDUNDERCUT

FATIGUE CRACK

RIVET HOLES INTENSION AREAS

CRACK

LIABLE TOFATIGUECRACK

NOTCHED CROSS GIRDER END

WELDED ATTACHMENTSAT FLANGE EDGES

STRESSRAISER

STRESSRAISER

ENDS OF TRUSS MEMBERS

TENSIO

N COMPRESSION

CRACK

WELDED REPAIRSOR

ATTACHMENTSTO

RIVETED MEMBERS

ATTACHMENTSWELDED

WELDED REPAIRPATCH

Figure 3.1

Fatigue Susceptible details


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3.4.3Masonry Arch Bridges

3.4.3.1General

The external fabric should be inspected. The arch barrel should be inspected to

ascertain all the information needed to determine the loading and resistance in

accordance with Section 6. This information should be recorded on the arch data

sheets on Figures 3.2 and 3.3. In particular the following information should be

determined:

(i) thickness of the arch ring carrying rail traffic (this may not be the same as the

number of rings visible on the face) and its shape;

(ii) nature and condition of the brickwork, stonework and mortar, including thelocation and extent of any crushing, and the direction of bonding in the case of

skew bridges;

(iii) thickness of the joints and the depth of any mortar loss;

(iv) presence of cracks, their width, length, position and number;

(v) location and extent of any loss of section due to spalling or damage by vehicles

from bridge strikes;

(vi) location of any displaced voussoirs and displacement across cracks;

(vii) deformation of the arch barrel from its original shape;

(viii) the presence and effectiveness of any previous strengthening such as saddling,

stitching, grouting or strengthening rings;

(ix) the presence and extent of any ring separation, which may be deemed to have

occurred if the engineer has any reasons to believe that the ring is not acting

integrally with the rest of the arch;

(x) haunching over abutments and piers of multispan structures.

On site measurements should be made in imperial units and then converted to metricprior to commencement of assessment analysis.

If part of the arch exhibits a significant change in profile from that described in

previous reports, the bridge should not be assessed but the condition of the bridge

reported to Railtrack immediately.

Where there is uncertainty about the above information a site investigation should be

considered, including trial holes where necessary. Probing into the construction

should be carried out where the strength of the bridge is in doubt or if internal scour

and leaching of the fill is suspected.


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The extent and location of water seepage should be recorded. The colour and

nature of any leachates should be closely examined for signs of brick or stone slurrythat may indicate internal movement.

Parapets and spandrel walls should be inspected for evidence of any defects and their

extent recorded on Figures 3.2 and 3.3, including, but not limited to:

tilting, bulging or sagging;

lateral movement of parapet or spandrel wall relative to the face of the archbarrel;

lateral movement of parapet or spandrel wall accompanied by longitudinalcracking of the arch barrel;

weathering and lack of pointing;

cracking, splitting and spalling;

loosening of any coping stones;

presence, location and details of ties, straps and patress plates.


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LINE OR

BRANCH

MAP REFERENCE

NEAREST

STATION

BRIDGE No. M C LINE REFERENCE

ARCH PROFILE SEMICIRCULAR

SPAN DIMENSION

(SQUARE)

SEGMENTAL

SPAN DIMENSION

(SKEW)

ELLIPTICAL

NUMBER OF RINGS PARABOLIC

POINTED

C ARCH

A

A

= ===

SPAN

ELEVATION LOOKING :

SECTION A-A

The following information should be recorded above:

A. SKETCH PROFILE OF SURFACE BALLAST AND TRACKS.

B. DIMENSION FROM TOP OF PARAPET TO SOFFIT OF

ARCH.

C. DIMENSIONS FROM TOP OF PARAPET TO RAIL LEVEL.

D. DIMENSIONS BETWEEN PARAPETS.

E. POSITIONS OF TRACK ON STRUCTURE.

F. TYPE OF SLEEPER AND TRACK

Figure 3.2Arch Data Sheet 1


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Figure 3.3

Arch Data Sheet 2

GENERAL FAULTS

MAP REFERENCE

BRIDGE No. M C LINE REFERENCENEARESTSTATION

ARCHRINGMATERIAL

HARD STONE

MEDIUM STONE

ENGINEERING BRICKS

BUILDING BRICKS

CONCRETE

OTHER (STATE) :

ARCH RING JOINTS

LIMESTONE

MORTAR ARCH RINGMATERIAL

GOODSOUND OR FRIABLE

TYPE OF LAYINGRANDOMSQUAREDCOURSED

CORRECT BONDINGREGULAR JOINTS

YES NO

WIDTH OF JOINTS

UP TO 6mm6mm TO

12mmOVER 12mm

DEPTH OF JOINTS

0mm (FLUSH TO FACE)

UP TO 12mm

12mm TO 0.1 OF RINGTHICKNESS

OVER 0.1 OF RING THICKNESS

YES NO IF YES' GIVE DETAILSDIAGONAL CRACKS FROMSPRINGING TO CENTRE

ARCHRING

LONGITUDINAL CRACKS INSOFFITTRANSVERSE CRACKS INSOFFITRADIAL DISPLACEMENT OFINDIVIDUAL STONE OR BRICKSPERMANENTDEFORMATIONCONSTANTLY WET

OR DAMPDIFFERENTIAL SETTLEMENTSPREADCRACKS AT QUARTER POINTSBULGINGCRACKSMOVEMENTSCONCRETE SLAB OR SADDLEGROUTED MATERIALWELL COMPACTED MATERIALS

ABUTMENTS&/OR PIERSSPANDRELWALLS

WINGWALLS

FILLINGNOT KNOWN

WEAK MATERIALS EVIDENCED BY`TRACKING' OF SURFACE

LINE ORBRANCH

SOFT STONE


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3.4.3.2

Cracking in Masonry ArchesThe inspection should investigate all cracks in an effort to establish their size and

depth, any associated displacement and their age. Old cracks, which probably

occurred soon after the bridge was built, and are no longer propagating, may usually

be ignored. Recent cracks, on the other hand, usually show clean faces, with possibly

small and loose fragments of masonry. Although appearing as shear of the bricks or

masonry, cracks normally follow an irregular line through the mortar. For this

reason, care should be taken in checking that the defects are cracks and not

deficiencies of the pointing material.

Cracks in abutments may generally be ignored unless they are new or growing. Ifcracks in abutments are caused by subsidence they may have affected the arch ring.

The possible causes of cracks in the arch are noted below:

longitudinal cracks outside the centre third of the arch between the spandrelsand the arch ring may be caused by shear stresses generated by the spanwise

deformation of the arch relative to the spandrels under the passage of live

load (see Figure 3.4);

longitudinal cracks within the centre third of the bridge emanating from theabutments may be due to varying amounts of subsidence in different places

along the length of the abutment, and are dangerous if large, because suchcracks tend to indicate secondary breaking up;

longitudinal cracks along the centre of a twin track bridge, spreading outwardsfrom the midspan area, may be caused by the stresses generated by the arrival

on the bridge of trains travelling in opposite directions;

transverse cracks, usually found near the quarter points, due to permanentdeformation of the arch, may be caused by partial collapse of the arch or

movement at the abutments;

Diagonal cracks normally start near the sides of the arch at the springing andspread up towards the centre of the bridge at the crown may be due to a

subsidence at the sides of the abutment. Diagonal cracks indicate that the

bridge could be in a dangerous state. Where diagonal cracks meet or cross,

there is a possibility that a portion at the joint could be punched out, as shown

in Figure 3.5 below, and therefore, action should be taken as soon as possible

to prevent this happening;

cracks in the corners and abutments of skewed arch bridges may be due tothe differential resistance provided by the backfill.


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(a)

(c) (b)

Spandrel wallsFace of abutment

Figure 3.4

Plan on Arch showing Longitudinal Cracks

a) Between arch ring and spandrels out with middle third

b) From abutment within middle third

c) Along centreline

Could be punched out

Diagonal cracks from arch springing

Figure 3.5

Diagonal Cracking in Arch


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3.4.3.3 I nspection of Jack Arches

The inspection of a jack arch deck should record the following information:

(i) the geometric configuration of the jack arches and their supporting members;

(ii) the presence of arch ties, details of their size, spacing, condition and position

within the height of the arch;

(iii) rotations or horizontal displacement of a supporting member;

(iv) transversely braced bottom flange of a supporting member;

(v) inadequate support to springings, for example, corrosion of the bottom flange

of supporting beam over a horizontal length or loss of bedding mortar;

(vi) cracking at the crown of the arch due to spreading of springings;

(vii) distortion and any associated cracking of the jack arch barrel;

(viii) arch cracking associated with substructure cracking or distress.

3.4.4Reinforced and Prestressed Concrete Bridges

A covermeter survey should be undertaken to check the cover and the location of

reinforcing bars and prestressing tendons particularly in critical areas. If there are no

drawings, if the available drawings do not give sufficient detail for assessment, or if

there is evidence that the bridge is not as shown in the drawings, further investigationwill be required. Other evidence may arise from records, from the covermeter

survey, or from other findings of the inspection. Further investigation usually consists

of a more comprehensive covermeter survey supplemented by local exposures of

reinforcement to determine its size and confirm the position of critical bars. It will

not normally be practical or desirable to expose sufficient reinforcing or prestressing

steel to fully determine, its position, cross-sectional area and condition. When it is

considered necessary to locally expose reinforcement, the extent and depth to be

removed and method of making good is required to be agreed by the Railtrack

Directors Nominee.

The worst credible strength of concrete should generally be derived from tests

carried out on cores in accordance with BS 6089. Cores are destructive and cannot

generally be taken at the critical locations of an element; hence interpretation or

extrapolation is necessary to arrive at worst credible strengths in these locations.

To assist in interpreting or extrapolating the results of core tests, an integrated

programme of testing which may include destructive, semi-destructive (e.g. near

surface) and non-destructive tests is necessary for each element. Care and

judgement is required in selecting the locations and numbers of samples for such

tests. The non-destructive tests can be used to give an indication of whether the area


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of concrete from which the cores are taken is representative of the concrete in the

critical areas.

For reinforcement or prestressing tendons and bars, a worst credible strength should

be obtained by testing samples taken from the element being assessed. It should be

noted that bars of different sizes are likely to have significantly different yield

strengths. Removal of prestressing steel for sampling will alter the stress distribution

in the concrete section and the change should be allowed for in the assessment

calculations.

The extent and nature of spalling, corrosion of reinforcement, rust staining, crazing

or soft or friable concrete should be recorded. Where cracking is present thefollowing information should be obtained:

details of position, extent and widths of significant or unusual cracks;

details of any cracks showing evidence of rust staining;

all cracks over approximately 0.2 mm wide;

all flexural cracks in prestressed elements.

Consideration should be given to undertaking additional tests to determine

constituents and condition of the concrete. The tests may include tests for chloride,

half cell potential, sulphates, carbonation, alkali silica reaction or ettringite formation

and cement content. However, in general these tests are not required unless there is

other evidence of the associated forms of deterioration.

For post-tensioned concrete structures the fundamental design and construction

details should be established by a desk study, prior to the inspection for assessment,

as outlined in BA 50/93: Post-tensioned Concrete Bridges. Planning, Organisation and

Methods for Carrying Out Special Inspections. The inspection should follow the

procedures for Special Inspections as described in BA 50/93, if the bridge may be at

risk of sudden failure following tendon corrosion or if the integrity of transverse

prestressing is to be assumed in the subsequent analysis of the adequacy of the

structure.

Any evidence of distress should be recorded, especially evidence of rust staining,

spalling, cracking or water penetration at anchorage or tendon positions. In

particular unexpected cracking and unexpected or changing deflection should be

recorded. Further investigation is required whenever there is evidence that suggests

tendon corrosion. Corrosion of tendons in post-tensioned members may not be

visually manifest during inspection. For pretensioned concrete members, significant

tendon corrosion usually causes visible rust staining and cracking of the cover

concrete.


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3.4.5Composite Bridges

The steel and concrete elements of composite bridges or composite members known

to have been designed for composite behaviour should be inspected in accordance

with Clauses 3.4.2 and 3.4.4 as appropriate. For other bridges or members where

concrete (or brick jack arches) is in contact or surrounds steel or wrought iron

members then inspection should be used to decide whether composite behaviour can

be assumed using Appendix A or Appendix C. It should be noted (see Clause 8.3.1C)

that composite behaviour of cross girders is not to be assumed in Type A or other

filler beam type decks less than 300 mm deep where there is no encasement above

the top flange or below the bottom flange.

For concrete slabs supported on steel or wrought iron beams the steel/ metal

interfaces should be examined especially near the supports for signs of:

corrosion;

fretting;

relative longitudinal slip;

vertical separation;

cracking or spalling of concrete.

Any relative movement should be recorded including any under live loading.

For cased beams the soffit (and other surfaces where practicable) should be examined

for signs of:

rust forcing or leakage;

separation of or hollowness of the casing concrete.

For filler beams or concrete or brick arch decks the soffit (and other surfaces where

practicable) should be examined for signs of:

corrosion; relative longitudinal slip;

separation;

cracking or spalling of concrete.

Where the infill consists of unreinforced concrete or brickwork or is unknown then

probing should be undertaken to prove the presence of dense material in contact

with the beam before composite behaviour can be assumed in assessment.

For concrete infilled troughs, probing or other inspection should be undertaken to

determine the depth and condition of concrete above the crests before composite

behaviour can be assumed in assessment.


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3.4.6Timber Bridges

The timber should be inspected, noting the presence of any protective coatings and

searching for the presence of rot or infestation (especially around metal fixings and

where there is standing water round piers in estuary waters). Heart rot may only be

detected by probing. The use of pressure sensitive, non-destructive drilling

techniques should be considered. Inspection for splitting and rot should be carried

out especially in areas of notching. Care should be taken not to transfer fungal spores

to sound timber by the use of contaminated tools. Any holes drilled should be made

good with sound timber dowels. Particular attention should be paid to timber in

contact with metal.

If site inspection indicates modifications to the structure, especially to primary

members, and weak timber is suspected, or if the species is unknown, samples should

be taken for identification of the timber.

3.4.7Substructures

It is not normally possible to inspect the foundations, but where they are exposed, for

example in tidal waters, their condition should be checked. Any defects present and

their extent should be noted; defects may include cracking, erosion, disintegration or

corrosion of reinforcement.

Dimensional checks are required for preparing sketches for analysis or for

confirmation of record drawings. The dimensional checks may require excavation or

probing to determine depth and the extent of foundations. Care should be taken to

ensure that exploratory work does not impair stability or damage underground

services.

In river beds and banks the removal of material by scouring, from around the base of

piers or abutments may lead to undermining of the foundations, especially during

flooding. Whilst assessment of the susceptibility of a substructure to scour is outside

the scope of this standard, evidence of scour holes and approximate dimensionswhere possible should be recorded. The presence and type of scour protection

should be recorded.

Foundation deficiencies usually appear as movements which may be sufficiently large

to cause tilting, cracking or excessive movements at joints or bearings. In arch bridge

foundations movement or arch spreading is generally apparent from cracks showing

distress in the arch rings and spandrels; diagonal cracking may be indicative of

differential settlement of the foundations.

All accessible parts of the substructures should be examined and any defects,including extent, and possible causes recorded.


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Some typical substructure defects are:

tilting and rotation, in any direction;

rocking;

cracking, splitting and spalling;

erosion beneath water level;

weathering and other material deterioration, including lack of pointing formasonry and brickwork;

vegetation intrusion; lack of effective drainage;

internal scour and leaking of fill;

settlement of structure;

settlement of fill.

Movement of substructures is likely to be caused by foundation movements.

Differential foundation movements may be evident on abutment or pier walls in the

form of vertical or inclined cracks.

The effects that any observed substructure movement may have on the

superstructure or deck should be investigated. For example, differential settlement

will cause a twist in the deck; inspection may reveal dislocated bearings. Where

continuous decks are encountered, substructure movements may be evident from

signs of distortion or distress consistent with a sag over the settling support or

hogging over intermediate adjacent supports.

Movement of substructure may be related to the support of spans of unequal length

or character.

In arch bridges, predominantly horizontal cracks in piers or abutments may be the

result of the arch spreading.

3.4.8Bearings

Bearings if present, should be inspected so that the general condition and efficiency or

operation of the bearings can be established. The following should be noted:

general condition of bearings and their type and articulation;

any binding or jamming, looseness, or reaching limits of rotational or

translational movement, or vertical movement under live load; condition of seating bedding and plinth;


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whether correct operation of the bearings is prevented or impaired, such asby structural members built into abutment or pier;

for metal girder bridges, where applicable, whether the bearings are fulfillingtheir function of providing end torsional restraint.

In bridges without bearings or where the bearings have failed to function correctly,

there may be local crushing or cracking, especially where supports are stone or

brickwork.

3.5 Report on Inspection

A report containing all the relevant information obtained from the inspection should

be produced. The report should include:

a description of the structure including details of any services carried;

a description of the condition of the structure including any repairs and adiscussion of the effect of any significant defects on the operational safety and

assessment of the structure;

sketches, drawings or photographs identifying the nature, location and extentof any defects;

sketches giving as measured dimensions;

other photographs, including general views and specific details.


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Section 4 - Loading for Assessment Page 1 of 33

CONTENTS

4. LOADING FOR ASSESSMENT.............................................................................................1

4.1 Dead Loads .........................................................................................................................1

4.2 Superimposed Dead Loads..............................................................................................2

4.2.1 Ballast ...........................................................................................................................2

4.2.2 Track ............................................................................................................................2

4.2.3 Services ........................................................................................................................2

4.2.4 Miscellaneous..............................................................................................................2

4.3 Railway Live Load ..............................................................................................................3

4.3.1 Vertical Static Loading ..............................................................................................3

4.3.2 Dynamic Effects..........................................................................................................74.3.3 Dispersal of Railway Live Loading onto the Structure.....................................25

4.3.4 Nosing ........................................................................................................................28

4.3.5 Centrifugal Load.......................................................................................................29

4.3.6 Longitudinal Loads...................................................................................................30

4.3.7 Load Combinations .................................................................................................31

4.3.8 Elements Supporting More Than One Track.....................................................31

4.3.9 Structures Carrying Light Rail Systems...............................................................32

4.4 Other Live Loads.............................................................................................................32

4.4.1 Wind Loads...............................................................................................................32

4.4.2 Temperature.............................................................................................................32

4.5 Operational Safety Requirements................................................................................33

4.5.1 Track Twist ...............................................................................................................33

4.6 Accidental Loads from Vehicles....................................................................................33

4.6.1 Bridges over Highways ...........................................................................................33

4.6.2 Intersection Bridges ................................................................................................33

4.6.3 Train Derailments on Bridges ...............................................................................33

4. LOADING FOR ASSESSMENT

4.1 Dead Loads

The dead loads should, where possible, be based on dimensions verified during the

inspection. For assessment Level 1 analysis the applicable values of unit weight given

in Table 4.2 should be used. Where, however, the initial assessment shows

inadequacies, or there is doubt about the nature of a particular material, tests should

be carried out to determine actual densities.


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4.2 Superimposed Dead Loads

4.2.1Ballast

The superimposed dead load due to ballast should be based on the measured depth

with unit weight 1800 kg/m.

4.2.2Track

Where applicable, the superimposed dead loads due to track components given in

Table 4.1 may be used. Where a different configuration of sleepers and rails has been

identified during the inspection, the self weight to be used should be determined by

measurement of dimensions of the configuration and by reference to data on weights

of components produced by the manufacturer.

Component Mass

Single Bullhead Rail 47.07 kg/m

Single Type 113A Rail 56.22 kg/m

Single UIC 60 Rail 60.22 kg/m

Conductor Rail 75.2 kg/m

Concrete Sleeper (Type F40 for use with 113A Rail)*11 300 kg

Concrete Sleeper (Type G44 for use with UIC 60 Rail)*11

315 kgTimber Sleeper 94 kg

Chair for Bullhead Rail 21 kg

*1 Includes shoulder, clips and rail pads.

Table 4.1

Permanent Way Component Weights

4.2.3Services

The superimposed dead load resulting from service cables and ducting should bedetermined, where possible, from examination and measurement during the

inspection or from information provided by the service owner. Where this is not

possible, any assumptions made regarding such equipment should be clearly stated in

the assessment calculations.

4.2.4Miscellaneous

Miscellaneous items such as walkways which are not deemed to be part of the

structure should be considered as superimposed dead load. The nature and

dimensions of such items should be established during the inspection, and the partial

factor fL for dead load applicable to the material (given in Table 2.2) should be used.


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Material11

Unit Weights kg/m

Metal AluminiumCast Iron

Wrought Iron

Steel

27507200

7700

7850

Concrete Reinforced or Prestressed

Plain

Breeze Block

2400

2300

1400

Masonry Engineering Brickwork

Other Brickwork

Granite

Sandstone

2400

2100

2600 to 2930

2200 to 2400

Timber2 Softwood

Hardwoods generally

Jarrah

Greenheart

640 typical (480 to 720)

640 to 1200

840 to 960

1040 to 1200

Fill Sand (dry)

Sand (saturated)

Hardcore

Crushed Slag

Packed Stone RubbleEarth (dry, compact)

Earth (moist, compact)

Puddled Clay

1600

2000

1920

1440

22401600

1800

1920

Asphalt

Macadam

2300

2560

1 Reference may also be made to BS 648 and BS 5268: Part 2: 1996.

2 Wide range of unit weights because of the variability of timber. For densities

of specific timber types refer to BS 5268: Part 2: 1996

Table 4.2

Density of Materials used in Bridge Construction

4.3 Railway Live Load

4.3.1Vertical Static Loading

4.3.1.1Route Availabil ity (RA) Number

The assessment of a Bridge should be determined in terms of its Route Availability(RA) number, that is its safe traffic load capacity. Route Availability numbers generally


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range from the lowest capacity RA0 to the highest at RA15 represented by 25 British

Standard Units (BSUs) of Type RA1 loading respectively as shown by Table 4.3.R.A. NUMBER RANGE OF BSUs

IN GROUP

RANGE OF SINGLE AXLE

WEIGHTS IN GROUP

RA0 Up to 10.99 units Under 13.96 tonnes

RA1 11.00 to 11.99 units 13.97 to 15.23 tonnes






RA7 17.00 to 17.99 units 21.59 to 22.85 tonnesRA8 18.00 to 18.99 units 22.86 to 24.12 tonnes







RA15 25.00 units and over 31.75 tonnes and over

Table 4.3

Route Availability Classification for Bridges

Type RA1 loading excludes dynamic effects which should be added in accordance

with Clause 4.3.2 and are dependent upon train speed. RA numbers should therefore

be determined according to a given train speed. In some cases it may be necessary to

determine more than one RA number for a given Bridge, for example RA6 at

100 mph representing passenger trains (normally the permissible speed) and RA10 at

60 mph for freight trains.

The number of units of Type RA1 loading that the Bridge can carry should be

determined by calculating the live load capacity factor, C, as defined below:

loadingRA1Typeofunits20ofEffects

CapacityLoadLive=C Equation 4.1

Capacity in terms of units of Type RA1 loading = C20

The RA number of the Bridge should be obtained from Table 4.3.


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Where the assessed RA number is below the RA of the line, the effects under static

EUDLs for the real (actual) permitted vehicles and combinations, together withdynamic factors for their respective permitted speeds, may be considered acceptable.

It should be noted that the RA effect of vehicles on a specific span (loaded length) is

often less than the RA classification for the vehicle which has to allow for a full range

of Bridge spans.

4.3.1.2RA1 Loading

The static loading used to determine the RA number is shown in Figure 4.1 for

20 units of Type RA1 loading. The Short Lengths configuration should be used when

it produces more onerous effects than the axle and uniformly distributed load model.

65kN/m

4x200kN 4x150kN 4x200kN 4x150kN

1.51.81.82.71.51.51.5 1.51.51.54.01.81.8 1.81.82.7

2x250kN

1.8

SHORT LENGTHS

2.4

Figure 4.120 Units of Type RA 1 Loading

Note 1: 20 units of Type RA1 loading is equivalent to Route Availability RA10without allowance for dynamic effects.

4.3.1.3Equivalent Unif ormly Di stributed Loading

For simply supported spans (with the exception of Masonry Arches, see Section 6),Type RA1 loading may be represented by an Equivalent Uniformly Distributed Load

(EUDL). Table 4.4 gives EUDL and maximum end shear values for simply supportedspans for 20 units of Type RA1 loading. The EUDL values equate with the maximumbending moment within the span that occurs under RA1 loading.

For continuous spans the values in Table 4.4 are not strictly applicable, and loadingshould be as shown in Figure 4.1. This loading should be considered as a whole, butany parts of the loading that reduce the effects on the part of the element beingconsidered should be omitted.

4.3.1.4Application of Loads

Type RA1 loading should be applied to each track and such as to produce the

maximum effect in the part of the element being considered.


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(SPAN)

(m)

EUDL

(kN

END

SHEAR(kN)

SPAN

(m)

EUDL

(kN)

END

SHEAR(kN)

1.2 500 250 9.4 1105 640

1.4 500 250 9.6 1121 650

1.6 500 250 9.8 1137 659

1.8 500 250 10.0 1152 668

2.0 500 269 11.0 1219 707

2.2 500 291 12.0 1282 752

2.4 500 308 13.0 1351 792

2.6 500 322 14.0 1411 835

2.8 500 335 15.0 1475 873

3.0 500 346 16.0 1547 9073.2 513 356 17.0 1620 947

3.4 532 364 18.0 1687 983

3.6 554 372 19.0 1760 1017

3.8 574 378 20.0 1837 1055

4.0 594 384 22.0 1983 1146

4.2 618 390 24.0 2126 1233

4.4 643 395 26.0 2265 1319

4.6 667 401 28.0 2415 1405

4.8 689 417 30.0 2547 1488

5.0 709

cec025 - rail track code

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