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MANJUL MATHUR Dy.CE/C/Stores, S.C Railway

S.VENKATA KUMAR

Dy.CE/TS, S.C Railway

KAILASH SINGH Dy.CE/West/CN/BNC, S.W Railway

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���� ! " � # $ � % � � &���� ! " � # $ � % � � &���� ! " � # $ � % � � &���� ! " � # $ � % � � & ���� The authors acknowledge their gratitude for the valuable guidance provided by Shri A.K. Yadav, Sr. Professor/Bridges/IRICEN/Pune, for without his valuable suggestions and inspiration this paper could not have been possible.

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CONTENTS 1. Introduction 2. What is assessment 3. Why assess 4. Types of analysis available 4.1 Empirical models, the prime one being MEXE 4.2 Two Dimensional Equilibrium based (Mechanism) 4.3 Elastic Methods 4.4 3D models using FE 5 Method presently used on Indian railways 6 Recent development in assessment 7 Limitations of Theoretical Analysis 8 Possible damages in masonry arch 8.1 Foundation damages 8.1.1 Damages due to the element degradation 8.1.2 Corrosion of steel elements used in foundation 8.1.3 Damages due to soil- foundation degradation 8.2 Superstructure damages 8.2.1 Damages resulting from a bad resistance performance 8.2.1.1 Gravitational actions 8.2.1.2 Imposed movements 8.2.1.3 Problems caused by abutment overturn by excessive earth pressure 8.2.1.4 Bulging of spandrels 8.2.1.5 Damages in wing walls 8.2.1.6 Stepped cracking 8.2.1.7 Transversal cracking resulting – arch mechanism failure 8.2.1.8 Loss or dislocation of pieces 8.2.2 Damages caused by deficient durability 9 Strengthening & retrofitting of arch bridges 9.1 Saddling 9.2 Sprayed Concrete 9.3 Jacketing below intrados 9.4 Surface Reinforcement 9.5 Insertion of Box inside the arch 9.6 Retrofitting 10 Conclusion 11 References

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1.0 INTRODUCTION In general bridges have been constructed from times immemorial even before their theoretical behaviour was known. Arch bridges were constructed in a large number for roadways and subsequently for the railroads. Bridge owners were empowered to restrict the maximum weight/axle load of the vehicles crossing any individual bridge on the basis of strength assessment available at that time. However the validity of these methods soon began to be questioned, as vehicles far in excess of the permitted weights/axle loads seem to be crossing some of the bridges, without any apparent ill effect on the structure. Further, with the passage of time, the need to permit heavier weight/axle loads also arose. This necessitated the need to assess the inherent load carrying capacity of these bridges, especially their residual strength and if need be their retrofitting/strengthening. This paper is an attempt in the strength assessment of arch bridges and their strengthening if warranted based on the current load carrying capacity. This is of particular relevance in the present Indian Railway context wherein it is being attempted to increase the speed and load carrying capacity of the trains over bridges, which were constructed long ago to the then prevailing standards.

2.0 WHAT IS ASSSESSMENT Assessment is the quantitative determination of the capacity of a bridge to carry static and dynamic loads. It is the combined effect of geometric form of structure, the materials used, the structural interaction of the parts and the condition of the structure. The objective of load assessment is to determine the load that the bridge can carry with a reasonable probability that it will not suffer serious damage incapacitating it to serve the intended use. The term assessment is carefully chosen. The capacity of an arch bridge cannot be determined, only assessed. The assessor will find it necessary to exercise judgment at many points in the process to arrive at a sensible result. The process of assessment may be regarded as one of developing confidence in the structure. Particular issues over which judgment must be exercised are the hidden details and the effects of existing damage. In many cases, important hidden details can reasonably be assessed from an external inspection of the bridge. 3.0 WHY ASSESS Assessment is necessary for a number of reasons. Perhaps most important is that bridges deteriorate with time so their capacity to withstand load also declines. It is therefore necessary, even in a stable environment, to check that bridges remain serviceable. The loading to which bridges are exposed also changes with time. The axle loads, numbers of axles and vehicle speeds increases with time, and in the process they might infringe upon the known or unknown safety margins.

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Masonry arch bridges form a vital part of the transport infrastructure, many of which are historic structures constructed over 100 years ago. Since construction, and particularly in recent years, traffic loads and speeds have increased drastically. The structural assessment of these bridges is therefore key to their continued service. A reliable assessment method is needed to ensure that strengthening is used only where necessary, and is as economical and efficient as possible. 4.0 TYPES OF ANALYSIS AVAILABLE First coordinated research in the modern sense into the behaviour of masonry arch bridges commenced in 1936, under Professor A.J.S. Pippard, at the request of the Building Research Station, on behalf of the Ministry of Transport, UK. Research. This involved development of theoretical and empirical methods accompanied by laboratory and field testing. Over the years, four distinct methodologies for load assessment have been developed:

1. Empirical models, the prime one being MEXE 2. Two Dimensional Equilibrium based (Mechanism) 3. Elastic Methods 4. 3D models using FE/ DE

4.1. EMPIRICAL METHODS MEXE Professor Pippard and his colleagues developed an elastic analysis method based on minimum strain energy principles, used earlier by Castigliano. The Pippard elastic method involved the calculation of stress in the cross section of the arch ring under working load conditions for different end conditions. The dead load of the fill and the self weight of the arch ring were represented by vertical applied loads. Analysis was carried out for a unit width and no action was taken for any contribution to strength from the spandrel walls or the surrounding fill.

Professor Pippard later used his method during the Second World War, to develop tables of allowable weights for wheeled and tracked vehicles for military use. In the years following, the war, the MEXE method (Military Engineering Experimental Establishment) was developed from the basic Pippard tables in the form of a readily usable nomogram. This method enabled military engineers to quickly assess the capacity of masonry arches.

The MEXE method is empirical, based on the work of Professor Pippard in 1930s and 40s and restructured by full-scale destructive tests carried out by Davey. The arch is first assumed to be parabolic in shape with span/ rise ratio of 4, soundly built in good quality brickwork/ stonework, with well pinned joints, to be free from cracks and to have abutments with adequate strength. For such an idealized arch, a Provisional assessment is obtained from a nomogram or the formula given below. This Provisional assessment is then modified by factors, which allow for the way in which the actual arch differs from the ideal.

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To summarize the method involves calculation of a provisional axle load based on the geometry of the arch. The geometric data required are:

(i) span L in metres (ii) the rise ‘rc’ of the arch barrel at the crown (in metres). (iii) the rise ‘rq’ of the arch barrel at the quarter points (in metres). (iv) the thickness ‘d’ of the arch barrel adjacent to the key stone

(in meters) (v) The depth ‘h’ of fill at the arch crown (in meters).

These geometrical characteristics have been shown in Fig. (1).

Fig.1 A provisional axle load (PAL) is calculated either from a nomogram given in BA 16/97 or from the following expression.

PAL = L

hd3.1

2)(740 + tonnes.

The following factors are then applied to the calculated value of PAL to take into account variations from the idealized arch.

(1) Span rise factor Fst: It is assumed that flat arches are not as

strong as deep arches. A span rise ratio of 4 or less is given a factor of 1.0 and this is reduced for span rise ratios greater than 4.

(2) Profile factor Fp: The ideal arch profile is assumed to be

parabolic and for this shape the rise at the quarter points is given by rq = 0.75 rc.

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The profile factor for c

q

r

r ≥ 0.75 is taken to be unity and is less than

unity for c

q

r

r ≤ 0.75.

(3) Material factor Fm: The material factor is a combined factor which

takes into account the estimated strength of the arch material and strength of the fill over the barrel.

(4) Joint factor Fj: The strength and stability of the arch barrel depends

to a large extent on the size and condition of the joints. The joint factor is determined from the width and depth of the joints and the quality of the mortar.

(5) Condition factor Fc: The estimation of the preceding factors is based

upon quantitative information available from a close inspection of the structure. The factor however depends more on an assessment of the importance of the various cracks and deformations that may be present. The value of Fc varies between zero and unity.

The five modifying factors are then applied to the value of PAL to give a modified axle load as follows:

Modified axle load = Fst * Fp * Fm * Fj * Fc * P.A.L.

Pippard made the following assumptions in his elastic analysis:

(1) The arch was parabolic (2) The arch was two pinned (3) The span: rise ratio was 4.0 (4) The arch section increased from the crown to the abutments (5) The load was applied as a point load at midspan (6) Fill density was same as the masonry density. (7) Fill had no structural strength (8) A limiting compressive stress of 1.39 N/mm2 was assumed.

(9) The maximum tensile strain permitted was 0.69 N/mm2.

An obvious curiosity of the method is that the method evaluates the effect of total thickness of ring and fill (d +h) and this can lead to excessive compressive stress. However, this method was easy to use but now it is considered to be conservative particularly for long spans. It also has an additional shortcoming in that the spans are limited to 18m and distorted arches cannot be assessed.

A detailed review of the MEXE method is in hand. Until it becomes available, it should be known that recent experience at Network Rail suggests that the method may not be conservative, when applied to bridges to:

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(i) Small span (ii) Modest cover, and Suffice it is to say that the majority of old railway arch bridges, however, fall in the above categories. 4.2. TWO DIMENSIONAL EQUILIBRIUM BASED MECHANISM

METHODS The oldest approach to arch behaviour is also the simplest one - demonstrating that a line of thrust exists within the arch, which can sustain the applied loads. In this approach the load, which just transforms the arch into a hinged mechanism, is evaluated. The following assumptions are made in this method:

a). no tension b). Infinite compressive strength c). Infinite elastic modulus and d). No sliding between voussoir.

The mechanism approach to arch collapse looks for the minimum load, needed to introduce the number of hinges at the arch intrados and extrados large enough to transform the arch into a mechanism. The collapse load can be found by statics and it depends on the geometry of arch, the position of the hinges and the weight of the blocks into which the hinges have divided the arch. For a two-hinged arch, two more hinges will be required to be formed to convert the arch into a mechanism. The mechanism method involves lengthy iterative calculations and is gaining in popularity as more and more computer programs are developed. The method provides no information on stresses and deformations and as such the load test is of no value in this procedure.

However the important conclusion of above theory is that any isolated cracks appearing in the arch barrel need not be the cause for panic. While

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ascertaining their cause and finding appropriate remedial measure are necessary, nevertheless the arch will not fail until four hinges are formed. This is also confirmed from the results of experiment carried on actual arches, which have shown that the loads causing failure of arch are nearly 2 to 4 times the loads at which, first crack appeared. S.No. Bridge Condition Load for

First crack (Tonnes)

Load for Collapse (Tonnes)

Ratio

1 Arch 76 near Sonepur, NER

Bare arch with load at 1.68 m spacing over a BG sleeper

40.6 132 3.25

2. Arch 73 near Sonepur, NER

Arch intact with 0.91 m cushion

71 >233 3.28

3. Bridge 41C Kota- Bina Section, WR

Bare arch of 0.61 n stone masonry and 0.25 n gravel and 0.18 m water bound macadam pavement

71 >330 4.65

4. Bridge across Uppodai (Madras) &X 9.75 m span

Arch 7, bare (that is, without cushion parapets and spandrel walls)

24.4 61

2.57

5 Bridge across Uppodai (Madras) &X 9.75 m span

Arch 4, bare 30.5 91.4 2.50

4.3 ELASTIC METHODS Elastic models have been used for masonry arch analysis, since Castigliano solved the problem of analysing indeterminate structures. Castigliano developed an elastic analysis by assuming that the arch material could not take tension. This was a reasonable assumption particularly arches built in lime mortar which may have been in service for more than hundred years. He calculated that line of thrust by the method of minimum strain energy and if it fell outside the middle third of arch ring, the material under tension was removed and calculation repeated until no tension was found at any point. At this point the line of thrust is accepted and the stresses are calculated with normal methods like MEXE. The ultimate load carrying capacity is defined as the load, which produces the combined dead and live load axial and bending stresses at the critical section, equivalent to characteristic compressive strength of the masonry. This method was later developed into a computerized assessment.

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The following observations of RDSO during the Static and Dynamic tests conducted on arch bridges, published under various reports (C72- 76, C 79-80, C83-84) corroborates the elastic behaviour of masonry arches:

(i). The graph showing deflection versus load is almost straight up

to 325 tonnes. (ii). The deflection values were practically same under repetitive

loading. (iii). There were no residual deflections for applied loads not

exceeding 200 MT. (iv). For loads up to 244 tonnes, the residual spread was

insignificant. (v). Tensile strain occurs at crown level along the Arch Vault, where

as at all locations compressive strains are recorded in accordance with the elastic theory.

The elastic approach with a cracking model offers the advantage that the effect of deflection can be studied by analysing the deflected shape at each iteration. The method however, demands a high level of confidence in the material properties used, a fact, which is likely to be overlooked by many users. On Indian Railways as per the arch bridge code, for the analysis of new masonry or concrete arches, the elastic method of analysis should preferably be adopted. Either purely analytical methods or a combination of analytical and graphical methods may also be used. 4. 4 3D MODELS USING FE/ DE Conventional assessment of strength is either based on semi-empirical methods such as the MEXE method, elastic method or mechanism method While these methods are useful for initial assessments and quick to use, they can only be applied to certain arch configurations, for example single spans and give no information on displacements. Numerical simulation by the FE/ DE method permits a more comprehensive analysis for predicting the behaviour of masonry. The accurate representation of masonry for structural analysis is a complex problem. In addition to the materially and geometrically non-linear behaviour of the masonry blocks themselves, contact-gap-friction effects at the joints and the evolution of further joints due to fracture are important. The ability to model post failure behaviour in order to verify analysis against physical tests is also useful. Many of these features can be modelled to a degree using continuum finite element methods with sophisticated material models and arrays of gap elements. Such methods require pre-defined contact definition and post failure behaviour cannot be observed. A more natural approach to the problem is to use DE. The Distinct method was first developed in the late 1960’s to address geotechnical and granular flow applications. The basic concept is that separate

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parts are modelled, possibly connected at boundaries, with friction, cohesive and adhesive contact. Initially the interaction of rigid parts was considered but later developments led to what is now termed the DE method where deformation and fracture can be included. The computer program, have been developed to model the arches with the above concept. For application to masonry arch bridges, typically discrete parts using non-linear material properties where appropriate and allowing non-linear geometric deformations, are used to model the abutments, piers, fill, any backing, and representative vehicle axles. The masonry of the arch barrel is modelled by a number of individual blocks, each normally representing a number of real masonry parts. Multiple rings can be modelled where appropriate, allowing the effect of features such as existing ring separation to be taken into account. Multi-span structures can also be modelled and existing defects such as mortar loss and weathering can be included. The forces between all component parts are automatically calculated, both under initial dead loads and then required traversing live load conditions. In this manner the complex non-linear behaviour of the masonry is accurately represented at a fundamental level. Fig. Below shows DE and FE meshes used for the analysis of a typical two span, multiple ring arch bridge. Both the analysis technique and the efficacy of the strengthening system have been verified by full-scale tests.

Fig. : Discrete and finite element meshes for a two span, multiple ring bridge

5.0 METHOD PRESENTLY USED ON INDIAN RAILWAYS: On Indian Railways, the Load Carrying capacity of arch bridge is assessed by carrying out a Load test on the representative sound arch, where theoretical overstress exceeds 100% and on a distressed bridge, after pressure grouting of cracked masonry. The parameters observed during the load test are the deflection and spread of the arch. The Safe Load is stipulated to be one, which does not produce a deflection of more than 1.25 mm at the crown and spread of more than 0.45 mm for arches with span between 4.5 m and 15 m. The method derives its roots from the “Assessment by Load Testing” proposed by Building Research station, UK.. Building Research Station carried out load tests on typical actual arch and other older bridges duly applying the vehicle loads of different configurations for assessment purposes. Based on tests, the Building Research Station recommended the following criterion for the assessment of arch bridge:

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If, under a 20-ton single axle, the spread and deflection of the arch do not exceed 0.015 inch (0.38 mm) and 0.05 inch (1.27 mm) respectively for bridges upto 45 feet (13.71 m), it can be assumed that the bridge can safely carry a 40 ton two- axle bogie.

The above criterion was derived from the load deflection characteristics observed in a number of collapse tests on the basis that pins (hinges) did not form until such deflections were reached. Thus, the BRS criterion recommended by Load testing was primarily based upon the consideration of crack development, and hence, was a form of serviceability criterion. It did not provide any indication of the ultimate capacity of a bridge, which can be much more than the assessed load. The load tests have shown that significant tensile cracks appear long before the ultimate capacity of arch is reached. The Tests have also shown that deflections increase rapidly after the level of approximately half the ultimate load. Since repeated opening and closing of joints under moving loads may seriously affect the integrity of the bridge, a serviceability limits have been laid down for the deflections.

To determine the safe load, by the method used on Indian Railway, Influence Line Diagram of the bending moment at crown of arch is first drawn. The proposed loading is taken as EUDL. The arch is divided into convenient number of segments. The value of ordinates of influence line (Mc) is given by:

L/2 Σ MR x(S/I)- Hc x 2Σ (yxS/I)

Mc= 0_____________________ L/2 2 Σ S/I 0 The position of proposed loading to give the maximum bending moment at crown is determined ( point A and E in the Fig. below). Maximum Bending moment caused by the proposed loading will be product of EUDL and net area of ILD corresponding to the load position mentioned above. The equivalent Single load or the Test load is then determined by equating the bending moment at crown, produced by this single load (assuming 45% dispersion through arch fill) to that produced by the proposed load.

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A

B

C

D

E

F

G

EUDL FROM BRIDGE RULESCORRESPONDING TO SPAN

EFFECTIVE SPAN

TEST LOAD

DISPERSION LENGTH

CUSHION

FOR MAX. BMPOSITION OF LOAD

INFLUENCE LINE DIAGRAMFOR BM AT CROWN

POINTS OFCONTRAFLEXURE

45°

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6.0 RECENT DEVELOPMENT IN ASSESSMENT:

Recently UIC has recommended the use of an software application page called RING for analysis and assessment of load carrying capacity of masonry arch bridges. RING idealises a masonry arch structure as an assemblage of rigid blocks and uses computational limit analysis methods to analyse the collapse state only. Although limit analysis, or ‘plastic’/ ‘mechanism’ analysis techniques were originally developed for steel components and structures, it has since been shown that these can be applied to masonry gravity structures, such as piers and arches.

To help understand why limit analysis theory is applicable, compare and contrast the response of a steel column with uniform plastic cross-section and a weakly mortared masonry pier, both subject to a horizontal load F, as shown below

Laterally loaded (a) steel column, (b) masonry pier, and (c) idealised response curves

It can be deduced that:

• while the tensile and compressive strength of the steel column endow it with a finite plastic moment of resistance, Mp, the absence of tensile strength means that the masonry pier does not possess a comparable (i.e. strength derived) moment capacity;

• however, the thickness and self weight of the pier mean that there is some resistance against overturning and the masonry pier could conceptually be considered as possessing a moment capacity, albeit one that varies with height (equal in magnitude to the normal force at a given cross-section multiplied by half the pier thickness);

• furthermore, provided pier displacements do not become large, the resistance of the masonry pier against overturning at a given cross-section will remain broadly constant;

• hence the response of the pier can be considered ‘ductile’, an important requirement for a limit analysis theory to be applicable.

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The behaviour of arch bridges is more complex than the one shown in the above example. RING uses rigorous mathematical programming techniques to identify the most critical of numerous possible failure modes.

6.1 Validation

In Bolton, UK, in the early 1990's a number of 3m and 5m span bridges were tested in the laboratory. A key advantage of these tests over field tests was that the internal constructional details and material properties were known. RING was originally developed to assist with the interpretation of the results from these laboratory tests.

In the Table shown below, sample RING 1.5 analysis results are presented along with the experimental test results (only bridges with detached spandrel walls are included since these behave in a two dimensional manner).

*Approx. of the full classical passive pressure coefficient indicated by measured angle of friction of fill (60°)

+The experimental collapse load of this bridge was reduced by the sudden onset of partial ring separation

In 2001 TRL were commissioned to independently validate RING 1.1 and other available masonry arch bridge analysis software. As part of the validation process it was decided that the programs would be used to predict the carrying capacities of 5 of the field bridges load tested more than a decade previously. Details taken from the TRL report relating to RING for 4 of

Bridge Description Expt. Collapse Load (Kn)

Limiting load dispersion angle (degrees)

Effective classical passive earth pressure coefficient

Theoretical collapse load

(kN)

Theoretical/ Experimental Collapse Load

3-1 3m single span

540 45 4.5* 550 102%

3-2 3m single span; debonded arch rings

360 45 4.5* 245 68%

5-1 5m single span

1720+ 45 4.5* 2238 130%+

5-2 5m single span; debonded arch rings

500 45 4.5* 463 93%

Multi-2

3m triple span

320 45 4.5* 358 112%

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the bridges are provided in Table 2 below (Strathmashie bridge was also modelled but was in poor condition and, because ‘none of [the] defects were modelled during the analysis, all the programs returned non-conservative results’).

Bridge Theoretical /experimental collapse load

Torksey 81% Bridgemill 100% Barlae 92% Preston 90%

Correlation between TRL field bridge test and RING collapse loads (independently produced by TRL) It is evident that agreement between the RING predictions and the full-scale test results was found to be reasonably good. Thus the TRL report concluded that RING ‘gives good results’ and that ‘RING, with some investment in an improved solver, would be a very effective tool for most assessment engineers…………”

Based on this evidence Network Rail have confirmed that RING is a suitable program for use to assess masonry arch bridges on the UK rail network.

An advanced version of this software, RING 2.0 is under development. 7.0 LIMITATIONS OF THEORETICAL ANALYSIS 7.1 Boundary conditions All structural analysis is critically dependent on boundary conditions. Arch bridges have more boundaries than most structures and the resulting complex relationships present a major challenge to the analyst. (i). If an arch alone is to be analysed, the behaviour of the springings must

be represented in a reasonable way. With the simplest models, rigid abutments can be assumed. For an elastic model of any sort, very small movements of the springings will have a dramatic effect on the stresses in the arch.

(ii). Representing the real behaviour of the abutments is, however, extremely

difficult, even in a situation where the regime is effectively two dimensional (eg near the center of a long tunnel). It is well known that loads approaching a bridge cause stresses in the fill, which push abutments together and compress the arch. When the load is on the bridge, this compression is reversed and the abutments spread slightly. The soil pressure may be similar in these two cases, but is likely to be differently distributed.

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(iii). The edges of arches are usually bounded by spandrel walls. It is normal

to assume that these make no contribution to structural behaviour. That assumption is likely to be conservative, but may play a major part in the gross underestimation of capacity, which is evident in some bridges.

7.2 COMPLEX NATURE OF TRACK/ VEHICLE LOADS: The following forces contributed by track and live load, which are difficult to quantify, adds to complexity of assessment: (i) Track maintenance, involving adding, removing, or just moving ballast, is

a routine operation on modern railways. The sleepers cannot be regarded as fixed points of application.

(ii) The force exerted on the rail from a stationary vehicle is simply derived.

In most assessments, all moving loads must be treated as pseudo static. That is their effect is viewed as static though the load may be moved to find its worst effect

(iii). At the rail level, there are Longitudinal forces generated as a result of

acceleration or braking. The latter is usually the greater force. The longitudinal stiffness of the track is so much greater than the stiffness of its connection to the ballast that the force will distribute over a considerable length and most will be carried off the bridge. There may be some need to consider lateral effects from these forces on tall piers of curved viaducts.

(iv). The impact, or shock loading effect of loads on an arch in good condition

is likely to be small, because the response of the foundation to track loads will not differ greatly from that of the general surroundings.

(v). Centrifugal forces create two effects. Transfer of load from one wheel to

another on an axle and a direct horizontal component. In most circumstances, track will be canted sufficiently to minimise the former effect. Nosing and sway produce similar effects.

If a bridge is carrying loads without apparent distress and the analysis says it shouldn’t, it is incumbent on the engineer to express a rational view as to why the analysis falls short. On the other hand, if loads substantially below the assessed capacity distress a bridge, the whole analysis becomes questionable. Therefore an analytical approach needs to be validated with physical symptoms. From the above theories of analysis, it is evident that the load assessment of existing masonry arch bridges is dependent on their physical state. The Physical state has to take in account the possible damages in the masonry arch. The possible damages in the arch bridges are discussed as under.

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8.0 POSSIBLE DAMAGES IN MASONRY ARCH: The appearance of damages in structures, is as inevitable as inexorable is the passage of time, partly due to action of nature and aging of the masonry materials and partly due in increased traffic loads, with the passage of time. In masonry arches, it is not strange that some cracks appearing due to normal structural behaviour may be classified as serious defects whereas the defects, which can trigger the collapse, are overlooked or not given the due consideration. Further the defect appearing in a particular component of arch may not necessarily have its root in the component itself. It may be the manifestation of defect in the other part. For example a crack in arch barrel may have its origin in deficient foundation behaviour. It is therefore essential to understand the different degradation mechanisms that take part on the bridges to establish an accurate diagnosis and prescribe an efficient therapy. The possible defects in the Arch bridges can be broadly classified into two categories:

1. Foundation Damages 2. Structural Damages

8.1 FOUNDATION DAMAGES: The main problem of ascertaining the damages in foundations is the difficulty to inspect them. If this inspection is made in summer, it would be easier to detect degradation problems of the structural element, foundations and even problems related with soil foundation degradation provided the riverbed is dry during the summers. For the bridge where standing water exists throughout the year it is difficult to inspect, the as sub-aquatic inspection even using underwater equipment, is a difficult task to perform. Therefore, in practice, the first useful stage to detect problems of bad behaviour of foundations is to observe and analyze symptoms that eventually are shown on the super- structure as a consequence of rotation, or differential movement on foundations. The following are the damages in foundations, which are usually observed on Arch Bridges: 8.1.1 DAMAGES DUE TO THE ELEMENT DEGRADATION The above category of damages is not only because of the degradation of the foundation due to weathering but also due to damages in the elements supposed to protect them. Most of the old masonry arches were constructed with lime mortar. Lime dissolution leads to mortar and old lime concrete elements disintegration, very usual on pile caps and masonry bridges foundation piles. River water tends to dissolve the lime, free the mortar, bringing them to loose sand condition. The resultant effect is the formation of cavities or even the complete disintegration of the foundation section.

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Mortar is a material perfectly plastic when it is placed. It hardens few hours after being placed and imparts rigidity with time. Mortar has two main functions inside the masonry:

- Regularize the setting among the blocks and distribute the loads uniformly.

- Transfer the horizontal trusts from individual blocks to abutments. It is therefore imperative that the repairs in case of loss of mortar should preferably be carried out with originally used mortar or with mortar having composition nearest to above. In case of arches constructed with lime mortar, the repairs should be carried out using lime mortar and in case of non- availability cement mortar can be used. Use of epoxy as is usually being done, will alter the load transfer mechanism and hence should be avoided.

Fig. Masonry arch bridge footing degradation example.

Dragging of gravel and sandstone in aggrading rivers results in damages in foundations, footings and plinths elements, together with erosive action and degradation of mortar. These processes may also be due to due to the increase of speed caused by the narrowing of the riverbed or by the longitudinal profile modification. The final result is loss of material, affecting its strength. Figure below shows the case where the foundation truss is exposed as a consequence of the above-mentioned effect.

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Fig. Damages caused by lime dissolution on footing and socket. 8.1.2 Corrosion of steel elements used in foundation During the latest third half of the 19 th century and first half of the 20 th century metallic caissons have been profusely used to execute pile foundation on river beds, especially if the river bed was deep. On other occasions, sheet piling has been executed with metallic elements that were suitable as protection against the stream. On these structures with the passage of time and change in humidity, damages due to corrosion, with important material loss, appeared. The importance of above factor lies in the fact that the material, which was protected earlier, is now exposed to erosive action of water and wind. Corrosion has resulted in serious damages to stability of structure and therefore needs to be given a serious consideration. 8.1.3 Damages due to soil- foundation degradation Riverbeds excavations were notably accelerated in recent times as a consequence of the increase of the sand extraction on the river shores. Extraction of sand on the upstream of the river is dangerous. River tries to fill up these man made depressions, by dropping the sediments it carries. When the solid contents of the river depletes, the flowing current digs the riverbed to make up the loss of sediments, resulting in scouring around the piers and abutments.

Fig. General and Local River bed undermining.

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Local undermining next to the supports – piers and abutments – is basically an erosion of the bottom of the river bed as a consequence of the formation of horizontal axis whirlwinds that are developed with ringlets forms around these elements. Riverbed materials get scoured from the bank river by the vertical flow component, lifted and propelled by the current water. Thus, it is forms a conic shaped hole in case of soils without consistency where the deepest point is upstream of the pier.

Fig. Local action of the current water on foundation

If any of the above mentioned damages are observed following action should be taken for:

- Visual inspection of the foundation - Estimation of the dimensions and typology of the foundations - Inspection of the soil around the foundation - Estimation of the longitudinal profile of the river - Estimation of the transversal section of riverbed - Determination of the cleaning and general state of the river

8.2 SUPERSTRUCTURE DAMAGES Structural damages can be broadly classified in two categories those resulting from bad resistance performance and those resulting due to durability problems which have potential of reducing the resistant capacity of the main structural elements. 8.2.1 Damages resulting from a bad resistance performance The structural damages can be better understood by knowing the processes of deterioration and its associate damages in the bridge in general rather than knowing the variety of damages that an element of arch, can possible suffer during its service life. 8.2.1.1 Gravitational actions These actions are due to the weight of the structure (piles, abutments, vaults, spandrel, backfill) and due to the correspondent dead loads (ballast, rail, parapets, etc).

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In case of brick masonry bridges, where spandrel walls are constructed monolithically with the arch barrel, longitudinal cracks sometimes appear under the inside edge of spandrel wall on the intrados. If such cracks are very fine and do not widen with time then they are mostly attributable to the difference in stiffness between the spandrel wall, which acts like a deep beam, and the flexible arch barrel (which results in incompatibility of deflections at their junction). Such cracks are not considered serious, but they must be kept under observation.

Many times, track level on the arch is raised bit by bit and new masonry courses are added on the spandrel wall without giving thought to the adequacy of spandrel wall cross section. This is also a cause for such cracks. 8.2.1.2 Imposed movements Imposed movements are the result of foundations movements in abutments and pile and their consequences, for example, the undermining. This action is the most important and is the responsible for most of the structural damages. Unequal settlements of the soil underneath the foundations of piers and abutments cause differential movements among different areas in the structure resulting in development tension forces on foundations and arch barrels which ultimately result in formation of cracks the masonry elements. Depending on the location and magnitude of the differential movements of the foundations and on the type of monolithic masonry, the cracking will have a vertical or inclined pattern. The structural importance of this defect will depend on its stage of development whether or not it is stabilized and on the structural characteristics of the elements.

Fig. Settlement of foundation and induced damages.

Fig. below shows the example of the transversal rotation of a foundation about its longitudinal axis, resulting in an inclined and stepped cracking, due

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to differential settlement of the same footing. Such type of cracks are also observed due to local undermining of foundations.

Fig. Differential settlement and transversal rotation of longitudinal axis of pier base. The above figure also shows the bottom view (intrados) of the crack in the arch. Together with this type of cracking it is also possible to find joints opening on the arch, or even relative movement between arch blocks, particularly on the ring course that is on the same side of the settled area of the foundation. It is also possible to detect damages on the spandrel that depends on their rigidity and on the existing bond between spandrel and arch ring. In extreme case of transverse bending coupled with differential settlement, a partial collapse of arch, as shown below can result.

Fig. Relative movement among longitudinal elevation of the bridge.

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Relative movement between edges and central part of foundations particularly when the barrel is wide – or it has been widened- is also observed. This differential settlement causes transverse bending of the foundation and therefore, vertical cracking, among joints which can also extend into arch barrel. On the contrary, if the edges of the foundations near the cutwaters settle more than the central part, the damage is revealed in the opposite way, the width of the crack is very small or inexistent in or near foundations and is wider on the arch barrel.

Fig. Vertical cracking on pile because of differential settlement between the edges

and the central part of foundation. 8.2.1.3 Problems caused by abutment overturn by excessive earth

pressure These types of problems are infrequent, as not many cases of collapse due to this reason have been reported. Nevertheless, it is true that, over exploitation of these structures (increase of loads and, speeds), the progressive deterioration of the drainage systems of the embankments and backfills, among other factors, contribute to increase the pressure that soil and water have on the abutments and walls. Figure below shows a sketch with the above-mentioned failure. This problem is more frequent in structures with short deep spans and high abutments. However the cases of the increase of the horizontal trust due to improper drainage in backfill resulting in generation of pore water pressure and ultimately the leaning of abutment and wing walls are many.

Fig. Damages caused due to inward rotation of the abutment.

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8.2.1.4 Bulging of spandrels This problem is connected with an excessive earth pressure of the fill and water retained in it, and also to the horizontal component of live loads. This damage is generally seen in bridges with deep and not very wide barrels and with big depth fill over the crown.

Fig. Bulging of spandrels

Blockage of drainage and excessive surcharge may also, sometimes, lead to sliding forward of the spandrel wall, particularly in case of bridges where spandrel wall and the arch barrel are not monolithically connected. The damage happens because the external forces due to reasons mentioned above are more than the stabilizing force of the weight of the spandrel multiplied by the friction coefficient. It is very important to know the bond employed to connect the spandrel and the arch barrel and the actual width of spandrel at the bottom. This damage occurs in bridges of deep and not very wide barrels and with a high depth fill over crown.

Fig. Sliding of spandrels

In extreme cases, the whole of the spandrel wall can be overturned, if the destabilizing forces due to choked fill, surcharge and increased live loads are very large.

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Fig. Overturning of the spandrels 8.2.1.5 Damages in Wing walls 8.2.1.5.1. Overturning and bulging The same considerations for spandrels are also for valid for wing walls and sidewalls. As it has been already mentioned the origin of these damages can be a deficient drainage in the backfill, caused by weep-holes obstruction resulting in increased horizontal pressure on the walls causing a dangerous destabilizing conditions.

Fig. Overturning and bulging of the wing walls and walls.

8.2.1.5.2 Vertical cracking in the joint between the abutment and

wing Walls: This damage is due to the different kinematics of the elements movements. The abutment is joined to the spandrels and the arch barrel, while the wing walls and sidewalls have free horizontal movement, particularly on the top.

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Almost vertical cracking, with width increasing with height, happens in the joint between the abutment and walls.

Fig. Vertical cracking in the joint between the abutment and the wing walls and side

walls. 8.2.1.6 Stepped cracking This failure is due to differential settlements in the plane of the wing wall.

Fig. Cracking in stair pattern on wing walls or on side walls.

In all of the above cases following action need be taken: - Inspection of the foundation and check for possible movements - Check the type of bond employed between the abutment and the wall - Inspection of the backfill of the walls - Inspection of the drainage system of the wall and improve drainage 8.2.1.7 Transversal cracking resulting – arch mechanism failure This is the most serious type of damage shows up in form of transverse cracks in the intrados of arch barrel. It is generally accepted that in masonry arch bridges, the compressive strength of stone - or even of the brick is greater than, the level of stress in the arch. So it is said that, generally, the collapse of a single masonry vault will happen because of the formation of mechanism. Thus, it can be said generally that the arch must have four alternative hinges to collapse, under the application of live load. The

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transverse cracks in intrados indicate the presence of tensile stresses, which if neglected, will result in formation of hinges. The damage therefore implies the imminent collapse of the structure so urgent measures must be taken.

Fig. Configuration of collapse by enough number of hinges. (4 hinges –arch mechanism) 8.2.1.8 Loss or dislocation of pieces The origin of this type of damages can be either due to strength or to durability, or due to combination of them. If the origin of the damage is due to a bad resistance behaviour, it is usually a symptom of separation movements among the springing lines of the arch barrel, or rarely, by loss of axial force in vault, or because of problems of heavy local loading, affected by impact near the crown of the arch barrel when there is little depth of fill over the crown.

Fig. Falling of pieces on masonry vaults.

Following need to be looked into for the above damage: - Inspection of the track (looking for track devices that can generate

vertical impacts on the barrel) - Determination of the speed and the maximum axial load that the bridge

is carrying. - Estimation of the type of masonry employed (limestone, sandstone,

granite, etc.) - Inspection of the drainage system of the bridge, specially that of barrel.

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8.2.2 Damages caused by deficient durability Besides the above damages the other damages are damages due to climatic conditions, weathering of constituent materials with age and damages due to neglected maintenance. The failures caused due to climatic conditions e.g rain, ingress, disruptive forces due to freeing and thawing of frost, solar radiation and abrasive actions of floating particles carried by wind, are demonstrated by erosion of surface of arch spandrel walls, abutments, joints infilling etc. The emissions have unfavorable impact on arch bridges and both surface of elements and joints infillings are chemically eroded. The degree and speed of erosion will depend on the type of material used in construction, quality of infill joint and on the emissions concentration. As a consequence of degradation action of the weather conditions with time (i.e ageing of structures), the load carrying capacity of the bridge deceases due to material and structural deterioration. The failure results due to action of traffic coupled with the material and structural deterioration. The common neglected maintenance, which may become the contributory reasons for the failure, are growth of vegetation and improper maintenance of drainage. The growth of vegetation results in deterioration of joint infill (mortar) and consequent lossening of masonry units. When drainage is blocked either behind spandrel walls, abutments and wing walls, the fill material at these locations becomes water bearing exerting excessive earth pressures which may result in sliding/ bulging and cracks in spandrels, cracks in abutments and wings and other defects.

Fig. Presence of plants on the bridges and disorders caused by them. The knowledge above damages is necessary for their incorporation in the models used for assessment of load carrying capacity. Before any retrofitting is attempted the minor damages like loss in mortar, cracks originating due to reasons other than structural reasons etc. need to be attended.

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9.0 STRENGHTENING & RETROFITTING OF ARCH BRIDGES Based on the assessment of the strength of arch bridges a variety of strengthening methods are available. These vary in effectiveness and each has advantages and disadvantages. Some of the methods are patented and proprietary. 9.1 Saddling One of the simplest and most popular of the traditional methods is saddling. This involves removal of the fill to expose the extrados of the barrel. A reinforced or mass concrete flat or curved slab is subsequently cast in place over the original barrel. While saddling will undoubtedly increase the capacity of the bridge, with minimal change to the external appearance of the bridge, it is expensive and will cause considerable disruption to traffic and buried services like communication cables, pipe lines etc.,. The bridge is also in a temporarily vulnerable state once the fill has been removed, unless the barrel is supported, which can be a costly process. As per Para 5.3.5.2 of IRS arch bridge code, in the case of strengthening over extrados of arch, the new arch ring should be designed to take the entire load, viz .dead and live loads. 9.2 Sprayed Concrete Another traditional method is the use of sprayed concrete applied to the intrados of the arch. This may be used in conjunction with a reinforcing mesh. Whilst this negates the need for drilling, the intrados is the part of the barrel exposed to weathering, resulting in friability. Applying sprayed concrete can cause moisture to be locked into the barrel. Other problems include poor composite behaviour and incompatible of materials. Sprayed concrete has been applied in conjunction with a corrugated metal lining.

Fig. Bridge strengthened with sprayed concrete

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9.3 Jacketing below intrados For strengthening weak/distressed arches, method of jacketing at the intrados is preferable, if the resultant reduction in the waterway is permissible as per the guidelines of IRS arch bridge code. The new arch ring should be designed to take the entire load by itself where the existing arch has transverse crack(s) by ensuring that a proper bond is established between the existing masonry and new material by suitable means such as dowels and post grouting through grout holes to be left while casting the jacket. Also it should be ensured that cracked masonry, whether in arches or in abutments and piers, should be grouted under pressure to plug all the cracks before the additional material is provided. 9.4 Surface Reinforcement There are proprietary systems available using a network of steel bars located in slots cut into the intrados and bonded using special adhesives. Such systems have been shown to increase the strength of the bridge. However, access to the arch intrados is not always easy or possible. More importantly, the application of sprayed concrete and/or externally bonded or slotted reinforcement will have a detrimental effect on the appearance of the intrados. This is un-desirable in many situations and unacceptable for many structures of historic importance. 9.5 Insertion of Box inside the arch On Indian Railways we have strengthened weak/distressed arches by constructing boxes inside the arch, either single or multiple spans. The space above the box upto the intrados of the arch is filled with lean concrete under pressure. Again the effectiveness of load transfer is questionable as a small gap left between the intrados and top of box may result in arch deflection and consequently it’s carrying the load. This is only possible where adequate waterway is available. This method is again not acceptable in many situations especially for many structures of historic importance. Also strictly speaking, this method cannot be counted as one of the means of strengthening or retrofitting of arches.

9.6 Retrofitting The method of retrofit no-prestressed reinforcement enables strengthening masonry arch bridges without significant intervention into the whole structure. The principle is to stabilize newly originated tensile stresses, to restrict/reduce further development of existing cracks and to constrain/prevent an onset of the new ones. From the static viewpoint, unreinforced masonry structure is unable to transfer tensile forces that can originate on existing structure from following reasons.

• Higher imposed load against the designed one

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o The load-bearing ability of a structure or its parts weakens due to the degradation process,

o The static action/loading changes either due to the appearance of significant cracks or due to the abutments settlement.

Another consequence of the retrofit reinforcement application into masonry structures is the rigidity improvement. The effect is evident especially on the structures cracked due to higher live loads. For reinforcement it is possible to use

• Conventional reinforcing bars with rustproof coating, plating, etc., • Reinforcing bars of different special shapes either with anticorrosive

treatment or manufactured from stainless steel, • Other advanced materials (carbon, glass and aramid rods).

These can be applied for retrofit of masonry walls, arched structure, flat lintels, arch cords and piers. A disadvantage of conventional reinforcement application is insufficient protection against corrosion. The reinforcement with anticorrosive treatment (rustproof coating, plating, etc.,) needs careful handling as it is prohibited to use any bar with corroded surface. The problem may also arise at the ends of bars, which are normally not protected. Some applicable reinforcing products are presented in the below table. Product (system) Type of

product Description

Stainless reinforcement of special shapes

Helifix HeliBar Stainless austenitic cold rolled steel of special helical shape and high strength in tension

Brutt saver Brutt Saver Profile

Stainless austenitic cold rolled steel of special helical shape and high strength in tension

Cintec Cintec CHS Thor Helical Thor Helical

Bars

Fiber Reinforced Polymer (FRP)

Structural reinforcing bar made from filaments or fibers held in a polymeric resin matrix binder. The FRP bar can be made from various types of fibers such as Glass (GFRP), Carbon (CFRP) or Aramid (AFRP)

Huges Brothers Asian 100 Vinyl Ester Matrix Glass GFRP Rebar

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Asian 101 Polyester Matrix Glass GFRP Rebar

Asian 200 Carbon Fibers Reinforced Polymer (CFRP)

Fibes-Bond system FRP 101 Glass, carbon or aramid Fiber Reinforced Polymer

Wabo Mbrace Glass, carbon or aramid Fiber Reinforced Polymer

Preswitt Polyplast Pre-mixed, fiber-reinforced cement based plaster with high tensile strength fiberglass reinforcing mesh and specially formulated liquid resin.

As binding (transferring) medium between reinforcement and origin masonry has to be used special mortar (grouting substance) having following characteristics:

• Long-terms life cycle and durability, • Zero (or almost Zero) volume and temperature changes, • Good bond to brick/stone/combined masonry and to reinforcing

member Embedding of the reinforcing bars in the drilled chases is mostly realized either with cement mortar or with mortar based resins. The disadvantage of cement mortar is relatively long-term hardening and necessity of watering (mortar in thin joints dries quickly). The properties of resulting composites “reinforcement-mortar-origin masonry” are highly dependent on the physical-mechanical properties of mortar Its strength, cohesion with the base-masonry. The properties are significantly changeable as they have dependence on the temperature and moisture at setting and hardening of mortar. The performance of the existing structure and the strength enhancement provided by reinforcement is predicted by the use of numerical simulation using the FE/ DE method. These methods calculate the strength deficiency and target strength that the internal reinforcement was required to achieve. Typically, retrofitting with reinforcement can be broadly used to repair the existing arches in cases of:

1. Soffit – barrel arch beaming and pinning 2. Pier – crack pinning 3. Abutment – crack stitching and pinning of coping stones 4. Beaming and Pinning 5. Pier – crack stitching 6. Spandrel Pinning 7. Replacement and Pinning of spalled bricks 8. Ring Separation – barrel arch pinning

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Fig. Retro-fitting with reinforcement Some types of retrofitting using are shown below:

Fig. Spandrel Pinning

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A proprietary system known as the Cintec anchor is used to enable reinforcement to be accurately retrofitted. The reinforcement anchor consists of three main components, a stainless steel bar, a fabric sock and a cementitiuos grout. The stainless steel bar provides increased tensile and compressive capacity. The woven polyester sock permits sufficient leakage of grout during inflation to develop a chemical and mechanical bond with the surrounding masonry whilst protecting the masonry from being displaced or otherwise damaged by the pressurised grouting and limits the volume of grout escaping into the surrounding masonry. The grout used is similar to Portland cement based products. The main steel body of the anchor is completely surrounded by a fabric sock. The anchor is then located in an oversized drill hole joining the materials to be anchored together. Fluid grout is then injected under pressure through the middle of the anchor, until it reaches the remote end. There, it passes through a series of grout flood holes into the fabric sock. The entire assembly inflates like a balloon under the pressure. The excess milk of the grout and bonding agent passes through the fabric sock, both fixing and providing a mechanical and chemical bond to the parent material. Variation in the size and shape of the individual components enable the basic method to be extended to meet the designer's requirements.

Plan

Section

Fig. Schematic arrangement of Arch strengthening

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10.0 CONCLUSION The main objective of this paper was to bring out the various assessment techniques available for masonry arch bridges and the techniques for their repair and retrofitting. This is important since arch bridges are structures, which have enormous load carrying capacity, and thus require their proper assessment. Assessment of the strength of these bridges on Indian Railways has become necessary in view of large-scale gauge conversion works (meter gauge to broad gauge) and higher axle loads on BG with the introduction of high-powered locomotives and other higher load carrying freight stocks. Also, the various damages that an arch bridge undergoes have been described in some detail, as most of these bridges are more than a century old. This is because; the load carrying capacity of an existing bridge prior to and consequent to repair/strengthening cannot be assessed realistically without incorporating the damages in the models for strength assessment. The damages are manifestation of inherent problems that are prevailing, which are first required to be addressed, before any repair or strengthening is attempted. Repair and retrofitting solutions for each arch bridge have to be decided for each bridge on an individual basis taking into account the damages it has suffered and the load carrying capacity it has to be restored to. 11.0 REFERENCES (1) Catalogue of damages on masonry arch bridges-UIC Draft report,

October 2.005 (2) Assessment, strengthening and preservation of masonry structures

for continued use in today’s infrastructure- Lynne Mabon, Structural Analyst, Gifford and Partners, Southampton, UK

(3) http://www.helifix.com (4) http://www.cintec.com/ (5) IRS- Code of practice for the design and construction of masonry and

plain concrete arch bridges-1941 (6) Proposal for the methodology of retrofit reinforcement-UIC Report (7) Arch Bridges- Edited by Professor C. Melbourne