Contents
  1. Arch bridge
  2. Arch bridge - Wikipedia
  3. Safety of historical stone arch bridges
  4. Arch Bridges.pdf

literature of arch stress analysis and design, it might be thought that little of value construction of the steel and concrete arch bridges used as examples in. Recent designs of arch bridges show many variations of arrangements for arches The nomenclature of the structural elements of an arch bridge is as follows. Types of Arch Bridges. Examples of Typical Arch Bridges. Analysis of Arch Bridges. Design Considerations for an Arch Bridge. Arch Bridge.

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Arch Bridges Pdf

the survey of arch bridges undertaken for the Virginia Transportation Research Council all arch bridges and culverts under VDOT's purview; in addition. PDF | An extended landslide cut the National Road Corinthos-Tripoli-Kalamata in Southern Greece and emerged the need for the construction. PDF | In this paper the procedure for establishing the funicular shape of a Conference: 6th International Conference on Arch Bridges, At Fuzhou, China.

Total length of the bridge is almost m, while the lengths of two main spans are To analyse behaviour of the bridge, spatial transformation model was developed using FEM software. The Eurocodes were applied for all verification procedures. Bridge global analysis and verification of arches against buckling is discu- ssed in more detail. The variation of upper bracing system and its influence on out-of-plane buckling of the arches is presented. Keywords: Arch bridges, bow-string girder, buckling, stability of arches, bracings. Introduction attractive location. One of such bridges, located in the line section between towns Puchov and Povazska Bystrica, will be bridging The main traffic lines in Slovakia, which have been included the Nosicky Canal at the Vah River. Experiences and remarks into European railway corridors, are under reconstruction and from design of this bridge are presented in this paper. The main modernization during last decade. In this context, our department has under buckling and the associated system of bracing arches. Given the nature of the line, majority of touched bridges are of small and medium spans [2]. But, there 2.

A demand of the State Navigation Administration Fig. Finally, the first proposed alternative introducing four to be necessarily satisfied.

In addition, because of the navigable single-span bridges consisting of stiff beams reinforced by arches waterway in the future, excavating the river bottom of the value of and combined with the bottom orthotropic plate decks was found 1. The small distance as the better one. Despite of any other arguments, the main from the end switches of the railway station together with request reason was clear. The second alternative was more expensive than for minimizing the piers in the canal induced the need of two- the first version proposed.

Nevertheless, the two middle spans of line bridge. Based on the adopted concept of arch bridges, two this chosen two-line railway bow-string arch bridge still belongs to alternatives for bridging the water canal were presented in the the unique structures in the design practise in our region. Four simply supported steel beams reinforced by arches have spans The total length of the bridge is Bottom steel orthotropic deck was designed to redistribute loads from ballast bed to the main girders.

Unfortunately, this solution required an 2 1 establishment of three piers in the canal-basin. Such solution would allow shipping on both sides of the middle pier. This solution had the ambition to bridge the entire main obstacle - the river, with the superstructure of a single bridge span. An interesting structure consists of two truss arches with a span of their lower chord of The total rise of upper arch chord has the value of Both arches were designed in inclined planes at a distance of Basic parameters of superstructures of the final Although expensive, it represents proposal a very courageous design without need for construction of a substructure in the bed of canal.

The steel superstructures of two internal fields 2 and 3 Fig. To choose the most favourable solution from the economic, 3 with theoretical spans of The eccentricities were considered in the numerical model, too.

Above the supports they are centrically were considered as a rigid, while the connections of hangers were connected to the beams of passable box-section with internal approximated by the hinge joints Fig. Stability of arches The steel superstructures of two edge fields 1 and 4 with theoretical spans of The Firstly, it was necessary to optimise the main dimensions, plate beams are also designed as the passable box-section with especially to find the optimal ratio of arc rise to its length.

Theoretical cambers of the configuration of upper longitudinal bracings was varied from the circular curved arches are Above the supports common truss systems to frame ones. After the preliminary they are connected to the beams with the eccentricity of mm. The superstructures have a bottom steel plate orthotropic 3. Variation of upper bracings bridge deck with continuous ballast bed. The deck plate of thickness 16 mm is shaped into the profile of ballast bed channel In order to observe an influence of upper longitudinal bracing with vertical webs.

Flat longitudinal stiffeners located at distances on the stability of the arches, a parametric study was performed. Transverse Different types of the bracing system were incorporated into the stiffeners are arranged at distances of mm and they have spatial computational models of the both bridge superstructures.

The first comparative model was considered without any upper bracing system I. Then, two basic types of upper bracing systems were taken into account: the frame system II with 3.

Analysis of the bridge superstructure different number of cross-beams and the truss system III with various arrangements of diagonals. All considered bracing 3. Global analysis model systems are summarized in Fig. Besides, better understanding For the global analysis, the spatial transformation model of can be obtained from Fig.

In all cases, the bracing members the bridge structure given in Fig. In case of truss bracing based on Finite Element Method. Considering the size of the system, the slenderness of members did not exceed the value of structure, the member finite elements were used for modelling A steel plate of the deck was meshed by 2D shell finite elements.

The plate was stiffened by the ribs in the longitudinal and 3. Stability analysis transversal way, approximating in this manner the longitudinal and transversal stiffeners of the deck. The documents should be verified for correctness where ever possible. Thickness is more inside than at face. Use appropriate assessment methods. Fresh inspection for assessment should be carried out. Photo 3. Photographs should be taken to assist in the subsequent determination or investigation of the implications of observed defects and deterioration.

Above data is sufficient when only empirical methods discussed in subsequent paras are employed for assessment. Piers and Abutments The bridge should be inspected and the location and extent of any observed defects. Critical dimensions such as span at the springings. Where other mathematical modeling methods are used. Determination of Masonry. In order to determine the adequacy of a particular arch structure with the minimum degree of effort. Properties should be determined only to the extent required for a particular type of assessment.

The colour and nature leachates should be closely examined for brick or stone slurry that may indicate movement. If the structure is shown to be inadequate in relation to the required load carrying capacity at this level. Levels of assessment according to the methods of analysis are.

Arch bridge

Level 3 Same method as for level 2 but with reasonably accurate data in regard to materials strengths and structural details Level 4 Use of sophisticated 2-D. Judgment will be necessary at many points in the process to arrive at a sensible result. Level 2 Use of more refined analysis and better structural idealisation with approximate material properties. The rigorousness of further analysis must depend on the importance and complexity of the structure.

For other higher level assessment methods. There is no requirement to determine parameters Ring and Archie methods.

Fill and masonry are considered to be of equal density. The long term strength of a brick or masonry arch is almost impossible to calculate accurately and recourse The strength of the bridge may be affected by the strength of the spandrel walls.

The ring arch is considered as elastic but with pinned supports. The equations involved in the MEXE method do not represent the behavior of a real arch.

The MEXE method is approximate and should only be used where: This method assumes arch as parabolic with a spanrise ratio of 4.

Assessment of single and multi-span span structures may be carried out using the MEXE method. The method should not be used where the arch is flat or appreciably deformed. For such an idealised arch. Dimensions of arch L m in the case of skew spans. The provisional axle load obtained is then modified by the various modifying factors and the condition factor. For a particular ring thickness at crown.

Note should be taken of any evidence of separation of the arch rings. If there are a number of voussoirs displaced. Flat arches are not as strong under a given loading as those of steeper profile.

Fs should be obtained from graph in Fig.

The ideal profile has been taken to be parabolic and for this shape the rise at the quarter points. A quantitative estimate of the arch barrel condition factor should be made by the engineer. The material factor Fm is obtained from the following Material Fm Soft brick and soft stone 1. A low factor should be taken for a bridge in poor condition while 1. Ranges of condition factors for defects affecting the stability and load carrying capacity of the Arch Barrel are given below for crack patterns resulting from specific causes.

Arch bridge - Wikipedia

No spalling 1. The choice of factor is made from a critical examination of the size. It will not necessarily be derived by multiplying the factors for several separate defects together: The overall figure representing several defects should be based on the relative importance of the worst type of defect present.

These frequently indicate flexibility of the arch barrel over the centre half of the span. The unfavorable defects which do not affect the stability of the arch barrel but may affect the stability of the track structure are indicated below. Extensive diagonal cracks indicate that the barrel is in a dangerous state. These faults can be accompanied by a dip in the parapet which may be more easily observed. They are probably due to subsidence at the sides of the abutment.

This is a frequent source of weakness in old arch bridges and the proximity of the carriageway to the parapet should be taken into account. These normally start near the sides of the arch at the springings and spread up towards the centre of the barrel at the crown. Bridge can be considered safe for multiple axles of above calculated MAL. Fc This is permissible axle load on bridge excluding dynamic effect. Following values of Fc can be used normally.

Dynamic effect should be estimated as per bridge rules. Ring 2. This is effected by constraining the thrust line. Where the stiffness of the arch can be regarded as infinite. This is the traditional 17th century approach.

This approach to arch behavior is also the simplest. It can be used for single and multi-span bridges. If line of thrust exists within the arch which can sustain the applied loads. Step 5. Partial Factors Insert the partial factors of safety Step 4.

General Project Settings Firstly. For details. Software program can be downloaded from www. Geometric detail once fed can be modified. Geometry This includes data about the abutments. Step 2. Materials Detailed properties for masonry. MBG loading is already available in database.

Data is filled in following steps. M method This method is also based on line of thrust. Program can be then run. Add vehicles Using the vehicle database.

One more advantage of this program is that user can analyse effect of various parameters like fill depth. If all properties are not known. Step 1. Step 3. Archie-M is graphical response which encourages users to explore potential behavior rather than expect to determine actual behavior. The basic minimum data required to be input into the program is span.

To assess a distorted arch ring. The program response is fast enough to allow a user to drag loads with a mouse to ascertain the critical position and load. The core working output is from the graphical screen where the user can immediately see the effect of loads and changes to the model.

Modelling of an arch is ticklish and user should know how boundary conditions are to be applied. Five basic ring shapes: The model is less sensitive to material strength and fill angle of friction than it is to self weight. Default values are also provided for fill properties and material self weights. Low strengths. These methods are not discussed in this book.

Load test can be static or dynamic.

Safety of historical stone arch bridges

The types of damage which can influence structural strength include: Some types of damage are however not important when considering the short -term structural strength. A load test should be carefully designed.

Load tests are expensive. In load test. Such type of approach can only be performed under expert advice. Once model is validated. Second approach can be to apply desired load and measure only deflection at crown and spread of abutments.

This approach is not normally preferred. If these two values are with in safe limit arch can be treated safe. For segmental and non-segmental arches of span 4.

Analysis has been made with the help of RING 1. To make a better appreciation. Effect of cushion i. For load test. For planning a test load. The effect of fill. Table 1 Fill Depth mm mm mm mm mm mm Load factor 1. Load factors in the above table are without considering impact.

The effect of cushion on different arches shall be different. If impact is also considered. For this arch. This is true for all shapes and sizes of arches. For analysis. For the above analysed arch. Again the arch as per details in para 3. Existing arches with bad looking brick just cannot be assumed to have significantly less strength.

Load Factor with various values of crushing strengths is given in Table 2. In some other cases. Take for example the following arch: Table 3 Backing Nil Load Factor 1. Load Factor with various values of backing is given in Table 3. It can be seen that with a backing of 1 m. The core diameter Whatever may be the extent but definitely backing increases load carrying capacity of arch bridge.

Tests for compressive strength of brickwork should be performed on cylinders not smaller than mm in diameter loaded on the lateral surface. The cylinder should be drilled centred in the middle of a vertical joint.

A tomm thick joint is embedded inbetween two layers of gypsum mortar joint needs to be wet when building the gypsum layer. Punching test on existing mortar i ii since friction is developed between the mortar joint and the gypsum layer. The punch is to mm in diameter. For existing mortars. The load carrying capacity of an arch bridge can be strongly influenced by the nature of the infill material extending over and beyond the arch and by the stability of the abutment and pier foundations. To assess such scenarios the saturated unit weight and permeability of the backfill should be determined.

Tests can be performed as per relevant IS codes. The possibility of saturation of the fill by for example flooding should be considered.

The most commonly required backfill parameters are self weight and shear strength. Values of parameters of soil which can be roughly used in an assessment of a masonry arch are given below: It is likely to have capacity to carry more load how much? In particular. All the methods discussed above are only inputs to mind. Despite use of various assessment methods available. An arch with poor drainage should always be seen with suspicion.

Such a historical study could allow cause of damage to be identified and the historical damage which had taken place long time before is not active now is a lesser cause for concern. Monitoring of the damage for a reasonably long period is of great help as it shows whether cause of damage is i still active or ii exhausted. All string course lines should be horizontal. Even if no damage is seen in the bridge.

This is important only in case of arches where assessment of load carrying capacity is to be carried out. Deterioration in most cases is caused by. Since only ADEN goes for field inspections normally. Boroscope can be used for this purpose. Photographs can also be used for comparing condition of structure in successive inspections. A crack. Water washing down through the bridge may erode mortar and saturate porous masonry.

It is particularly necessary that the inspecting official should avoid a process of pattern matching. In regions where deep frosts develop. Measure geometry. Water usually finds its way through the arch at points where it is forced to gather. This might be at the top of solid masonry backing.

If drains are not provided, the water will make its own way and do damage at will. Lime runs are common. Severe erosion of joints and masonry units are less, but still frequent. Dropped stones may result where loss of mortar is severe. Dropped bricks can also occur, but the contorted water paths through the ring usually lead to more distributed erosion.

Spandrel walls also suffer water damage and again it may concentrate at the level of solid backing masonry. The walls are more exposed than the arch barrel and the damp wall masonry weathers markedly more.

Highest level at which waterproofing can be applied is at the top of the masonry. Many bridges were built with no effective waterproof layer and no designed drainage. If water is allowed to collect, the damage it can do is considerable. Damage to arches caused solely by loading is very rare.

The damage is usually a result of compounding other forms of deterioration.

Arch Bridges.pdf

Live load from traffic is inherently dynamic, though it may be possible to treat it as pseudo static. That is to use a magnifying factor to account for dynamic effects. In general, the result of load movement is greater than the effect of a single application of a load in the critical position.

Modest overloads which are not frequently repeated typically result in modest cracks which may heal with time. There is no doubt that bridges respond less well to faster and the more frequently repeated over loads. It is reasonable to assume that there is a threshold below which no damage accrues, but some railway bridges may be at or near that threshold if modern loads are applied at high frequency. The mechanisms of deterioration are not well understood.

If a crack is repeatedly opened and closed, it will grow through jacking effects of loosened particles and through steady Impact on arch bridges gets magnified with imperfections in rail or train wheels, therefore track on arch bridges should be maintained in best condition.

Joints in track increase impact. This is due to the fact that at this location both bending moment and shear force are less. But above statement is not true for arch bridges; in case of arch bridges, joints if unavoidable should be provided just above the pier for the simple reason that cushion at this place is maximum which absorbs impact and saves the arch.

If arch bridge has no visible damage and there is no proposal of increasing load carrying capacity of the arch bridge, arch can be certified as good and minor repairs as may be required can be carried out. In case, damage is noticed in arch bridge, it can be studied in following steps. If load carrying capacity of bridge works out to be adequate with handsome safety margin, then it is O. If strength of bridge does not work out to be adequate with handsome safety margin, recalculate the strength using methods like Thrust Line Analysis, RING-2 software as described in Chapter 3.

Prepare 3D diagrams of the arch and mark these cracks on the diagram with exaggerated scale as shown in Fig. The cracks have been marked in reference to observed cracks as per photo 4. Note that crack has been marked in such a way so as to clearly show its width. By drawing crack pattern at exaggerated scale, it can be clearly visualized that damage on arch vaults and spandrels is due to rotation of abutment against the backfill and formation of three hinges. All types of such combinations can not be thought of and covered in the book.

Local erosion of foundation Photo 4. Howarh Div.. Bridge no. In all cases crack patterns may not be due to a single cause and it is sharp observation and experience of an engineer that is important. Some common types of crack patterns normally observed along with likely cause and action required to be taken is given in the following pages.

Determination of the longitudinal profile of the river bed. Determination of the cross section of the river bed down and upstream Mechanical failure of masonry Photo 4. Determination of soil type. Observations required: Structural Importance: Local erosion does not affect bridge integrity within a short time however it may turn into serious damage in long run. Determination of the typology and dimensions of the foundation.

Where as undermining of foundation may affect bridge integrity within a short time. Mechanical failure of masonry Long term effects: Collapse failure due to masonry collapse under compression orthogonal to the joints.

Usual locations Localized near to the springings and haunches of the vaults. Internal forces are caused by the live loads or indirect actions due to foundation problems.

There are three collapse criteria: Where d is thickness of ring. It may appear in any kind of pattern. R is rise at centrepoint and L is span. Collapse failure due to masonry collapse under interaction between axial bending and shear.

Such damages affects the strength behavior of the bridge. It announces high risk of collapse. It may appear in all kinds of typologies. Damage that affects the strength behaviour of the bridge. Small longitudinal cracks are not serious and can be stabilised by grouting. It affects bridge integrity within a short time if damage is not stabilized. It usually appears on deep bridges with saturated backing. It affects bridge integrity within a short time in case damage is not stabilized.

Effect can be aggrevated if the traffic runs near the spandrel. Damage that affects the strength behavior of the bridge. High risk of failure if other damages due to mechanical failure of masonry appear. Three-Hinges Three-Hinges Fig. It may appear in any kind of typologies. Transversal cracking on Vault. Mono-Arch Usual locations: Transversal cracking pattern characterized by cracks that open alternatively to intrados and extrados.

Failure and overturn of abutment due to horizontal pressure of the vaults Structural Importance: It announces the imminent collapse of the structure. Mono-Arch Mechanism Cause: Failure due to lack of bearing capacity of the vault under application of live load Structural Importance: Photo 4. Loss or dislocation of pieces Fig. Where track on it has poorly maintained joints. The arch becomes specially vulnerable if drainage is poor.

Dislocation at number of places affects the strength behavior of the bridge. It announces long term collapse of the structure. Drainage Long terms effects: Very poor drainage Structural Importance: Isolated dislocation of elements can be re-fixed without much long term effect.

Foundation failure rotations and settlements on piers and abutments producing hinges that cause local decompressions. Vertical cracking on pier Stair cracking on pier Usual locations: Localized on the central zone of the piers.

It is common on bridges with undermining problems that produce differential settlements on the pier. Diagonal cracking on vault Stair cracking on pier Vertical cracking on abutment Horizontal cracking on abutment 4.

Horizontal cracking on abutments. Long terms effects: Localized on central part of the abutments. Localized on the wing walls and side walls. Three-hinges may form. It may appear on any bridge typology with wing walls of a certain height. Lack of bearing capacity of wing and side walls to resist the thrust due to the backfill Thrust due to the water contained on the backfill caused by a deficient performance of the drainage system Growth of vegetation on the wing and sidewalls.

Its roots deteriorate the bearing capacity of the masonry. Determination of the constitution and state of the backfill at the abutment, wing and side wall Determination of the performance state of the drainage element at the abutment Bond between wing or side wall and abutment. Horizontal cracking on abutments Vertical cracking on abutments Bulging of spandrels Fig.

Usual locations: Localized on the spandrels over the pier, where the spandrel reaches its greatest height. It is common on bridges with deep and not very wide vaults. Generally located along the intrados of the vault and on the front elevation of the piers. They usually concentrate around elements with the highest level of humidity nearby the drainage pipes at spandrels and intrados of vaults. Damage that affects durability. Mechanical deterioration due to action of the roots and vegetation on the masonry.

The presence of efflorescence and crypto efflorescence warn that. This phenomenon is unleashed when soluble salts. It does not affect bridge integrity within a short time. Detection of possible salt sources coming from the masonry itself or from external agents or pollutants. These are the manifestations of salt crystallization. If the evaporation occurs on the surface. Sometimes the bearing behavior of the bridge have been altered without studying its consequences.

If it is of great extension and of advanced scope the roots cause the breaking of pieces and their loss it must be repaired. These damages may affect durability or the strength behavior of the bridge depending on their importance on the structural safety. May damage bridge in long term 4. Interventions made due to the upgrading of the railway electrification.

Generally located on the crown of the vault or piers of structures with traffic underneath. Scratches and abrasions caused by the impact of vehicles on masonry arch bridges. Sketch of the damage location.

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Research of the traffic underneath the structure. Causes loss of Observations required: Through vault or spandrel walls. It affects durability in long term. It is primarily the experience and objectivity of inspecting official. In case of masnory arch bridges. A fruitful inspection should result in: The two steps are not necessarily connected one to the other: The repair and retrofitting can be broadly divided into following groups 1.

Basic Repairs Once the cause of damage has been identified. Strengthening is needed when damages are so severe that the safety margin remains too low even after repairing of the damages.

If no cause is recognized. The common approach to retrofitting and strengthening of a masonry bridge assumes that any work will somehow enhance the bridge safety.

Arches in most cases will have much more strength than what is normally assumed. Whenever strengthening becomes necessary. Assuming that damage means necessarily that the bridge must be strengthened leads to unjustified retrofitting works.

Shotcreting 9. Pointing 4. Under ringing 8.

Reconstruction of lost of damaged pieces o Reconstruction of elements o Replacement of ellements 3. The position of its crystals inside the porous of mortars and pieces contributes to its cracking.

Content of salts falls asymptotically. The process progresses from the evaporation surface exterior. Details of each of above techniques and situations where these can be used is described in following paragraphs. This paste is soaked with distilled water and it is left.

Grouting 5. The repair work is carried out with the application of paper pulp dressing or any other absorbent material with clay. Transverse ties Items 1 to 4 above can be called repair and from 4 to 10 retrofitting. Sustaining walls Near surface reinforcement 7. Concrete saddling 6. The re-composition must preserve the properties that the brick had before.

Photo 5. The reconstruction will be done when the losses and missing section are less than 5 cm. Showing losses in brick elements The properties. More details can be seen from cleaning manual of UIC refer para 1.

Cleaning should also be done at appropriate intervals. When bigger than that. Chemicals such as CuSO4 may also be required to be used for certain type of deposits. Similarly any vegetation on the structure should be carefully removed along with roots. Problem is generally more in brick masonry than on stone masonry. Stones used on masonry structures include a large range of granites and sand stones. Once surface is clean. It has similar features that of masonry materials like lime stones.

Before application of mortar. Lime mortars are the most elastic and do not add soluble risky components. Heavy loss of mass of bricks Portland cement types are not suitable.

In case it is not possible to use lime. Repointing consists of: Shrinkage of re For the same reasons use of epoxy is dangerous. A wrong choice of re-pointing mortar.

Firstly a case forming must be executed by drilling the affected elements. It is very important to eliminate rest of the mortar in the sides of the case formed. The joint should be clean and wet. Since the load carrying capacity of an arch-type structure depends approximately linearly on the arch thickness.

Under these conditions. IR Bridge Manual allows use of cement mortar or epoxy grouting for arches. These more stiff areas may attract more force and may collapse. Secondly it changes the stiffness of arch barrel at certain locations where grout percolates and load distribution gets affected.

For similar reasons. Epoxy grouting shall never be done. Lime mortar with properties similar to the original construction allows the masonry to breathe and for water to pass through it rather than the masonry.

It can be done in four different situations: Portland cement based mortars should not be used unless the structure was originally constructed with this material as further damage can be caused. This kind of saddle does not directly act on the arch but rather restrains the vertical displacement of the fill.

Bearing in mind this approach. Related issues Effects of concrete saddles. In this way collapse is caused by compressive crushing in some sections but a four hinge mechanism is no longer active. Effects of concrete saddles.

The over ringing r. Steps for execution of works are. Steps for execution include fill removal. The retrofitted bridge is expected to exhibit highly increased natural frequencies. Apart from the dead loads of the masonry arch. If the arch thickness is increased. Careful attention in filling the rebates is required. Transverse reinforcement is placed on the arch before longitudinal reinforcement.

The bridge brickwork should be grouted if needed and the mortar joints re-pointed before any other work is performed. The bridge retrofitting and strengthening is completed by insertion of steel bars in the mortar joints of abutments and spandrels. Then rebates on arch intrados are made to place reinforcement. Longitudinal bars are placed on the arch intrados and the rebates are all filled with mortar of resigns.

This work needs to be performed on brickwork in the best possible conditions. Problem arises in case of concrete under ringing if In this strengthening technique.

This is a quick and widely used method. Since both the original masonry arch and the new one are active when the trains pass on the bridge. Under ringing can be done in complete barrel or partly. It should be ensured that there is no gap between old and new addition.

Recent experimental research shows that the bond provided by the epoxy resins to the bars and to the brickwork is enough for preventing the bars from detaching. Water leakage problem is original arch should be addressed before going in for concrete under ringing. Water leakage problem can be tolerated in steel under ringing.

Also grouting should be done if there is any sign of hollowness in original arch. Usually under-rings are connected to the original piers of the bridge, since new supports would make underrings much more expensive.

The supports may be: The latter solution is preferable since it does not need holes to be perforated in the original pier. The efficiency of the supports is crucial for the effectiveness of underringing. The second, and probably the main crucial issue for under-ringing is the connection of the new steel ring with the masonry structure.

If the material filling the gap in-between the two arches is deformable or the gap is not properly filled, the two arches do not cooperate and the original masonry arch remains the only load bearing structure.

An alternative to discontinuous steel under-ringing, is continuous under-ringing by means of r. Problem in this method is proper filling of gap between new r. Use of shotcreting may overcome the difficulty in making a perfect and stiff connection between the two arches. The continuous under-ringing is not suitable if there is water leakage problem in original arch.

In this design filling of gap between old and new elements is not important. This alternative is being used widely in Indian Railways. Shotcreting consists of spraying a layer of concrete shotcrete on to the intrados of the arch Photo 5. Shotcrete is a mixture of cement, aggregate and water projected pneumatically at high velocity onto the surface.

It It can be impacted onto any type or shape of surface, including vertical or overhead areas. This solution undoubtedly improves the load carrying capacity of the bridge but, however, a number of questions arise: What is the most effective form of detail at the springings so as to ensure the transmission of horizontal thrust to the abutments?

Two systems of strengthening can be adopted with regard to the thickness and rigidity of lining: A mm thick shotcrete layer is sprayed on the intrados of the arch usually with double reinforcement.

The connection between the existing arch and the shotcrete layer is provided by steel bolts.

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