project report on truss bridge


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project report on truss bridge

  1. 1. Department of Civil Engineering Page 1 1. SYNOPSIS Since inception of Indian Railways in 1853 the Railway Engineers has history of more than 250 years of construction and maintenance of railway bridges. During the long journey they had achieved several heights and continuing to excellence. Recently Indian Railways has either constructed several worldclass bridges or they are in the process of construction. A technical review of design and construction of recent bridges (viz. Bogibeel, Chenab and New Jubilee Bridges) through light on the recent technological advancements attained by the Indian Railways in the field of bridge engineering.
  2. 2. Department of Civil Engineering Page 2 2. ABSTRACT The joints of Riveted Steel Truss Railway Bridge consist of gusset plates which lose their rigidity due to repeated passages of train loads; therefore the loss of rotational rigidity is to be taken into account in analysis of Bridge. This joint flexibility tends to alter the vibration characteristics of the Bridge system and each component of the bridge responds dynamically to the rapidly varying loads and thus the time history obtained is a function of load variation and dynamics of the structure, which consequently affects fatigue life of the bridge components. In past, effect of semirigid joints has been studied in case of building frames. So here the knowledge of semirigid joints on building frame has been extended to Steel Truss Railway Bridge. This present article tries to study the influence of joint flexibility on the fatigue life of 76.2 m Truss bridge due to moving load at different speeds. The joint rotational stiffness is reduced by 5%, 10%, 25% and 50%. The result of preliminary studies conducted on Steel Truss Bridge is presented. It is prime facia that upto 50% reduction in rotational Stiffness of the joints does not affect the stability of the bridge.
  3. 3. Department of Civil Engineering Page 3 3. INTRODUCTION In India, Economic progress mainly depends on the railway and is considered as the Life line of the Nation. India has the second largest rail network in the world, transporting over four billion people annually and the total figure of existing railway bridges are approx. 1,20,000. Out of these, 731 are long span open girders, 19014 are rolled steel joist or plate girders. So it can be seen that more than 20% are Steel girder bridges. Due to continuous movement trains, the members and their connections are subjected to repeated loadings due to which the stiffness of the joint gets reduced, which are more prone to fatigue damage. The conventional static, dynamic or stability analysis of Steel Trusses bridges assumes that their members are connected at rigid or hinged joints. However in reality Steel Trusses are reinforced at their joints by Gusset plates, which possess rotational flexibility. The presence of this gusset plates has an appreciable effect on the stiffness of the members of the Bridge and consequently on its behavior to Static and Dynamic loading. However, the behavior of connections is neither rigid nor pinned. Structures having such flexible Joints in which Joint flexibility becomes important are called as semirigid frame members. In fatigue assessment of the bridge components the joints are assumed to be rigid as per RDSO, where joint flexibility is neglected which may affect the dynamic behavior of the bridge component, consequently its fatigue life. Therefore it is necessary to evaluate the bridge components for semirigid connections.
  4. 4. Department of Civil Engineering Page 4 4. HISTORY Basic bridge designs are developed from natural bridges- a tree trunk has fallen across a stream, vines hanging over a river, or stones that make a stepping-stone path across a shallow stream. These natural bridges were probably built upon by ancient bridge builders. For example, someone may have built up the steeping stones, placed flat stone slabs or logs on top of them, and connected the stones to make a low bridge. This type of bridge was called a “clapper bridge.” It is one of the earliest bridge constructions. Such simple bridges are probably still built today in many places. In general, though, bridge construction has changed greatly. The ancient Romans refined bridge building with two important contributions. Nearly all of their bridges used the arch design- a structure that can support more weight than a flat surface can. Also, the Roman’s discovery of natural cement allowed them to build strong, long-standing bridges. Many of these ancient Roman bridges are still standing today. There were excellent bridge builders in Asia, too. Some early bridges in Asia used a cantilever design. This design enabled the builder to make simple, long-span bridges across fairly wide rivers. One famous bridge in China, built about 1300 years ago, is the Great Stone Bridge. Its graceful arch shape is not the same type of arch shape used by the Romans. Instead, this bridge is quite low, and the arch is very shallow. The Renaissance brought new scientific ideas to bridge building. Leonardo da Vinci and Galileo developed theories about the strength of building materials. Their theories have helped architects understand how to make strong structures from lightweight materials. Bridge building became more exact as people began to use more mathematical theories about it. Another new development that changed bridge building was the development of metal. About 200 years ago, the first cast-iron bridge was built. This was the Iron Bridge at Coal brookdale in England. Before that time, bridges were made of stone, brick, clay, or timber. Eventually, wrought iron was used instead of cast iron. Much later, steel was
  5. 5. Department of Civil Engineering Page 5 used. Many new bridges were created and tested during this time. The Britannia Tubular Bridge, completed in 1850, showed one such new development. It was built from rectangular tubes of wrought iron. Similar bridges are often used today. Other important developments came with the truss bridge and the suspension bridge designs. The truss is an old design, but it was improved when scientists and engineers knew enough about science and mathematics to work out the mechanics of the design. Covered bridges were usually built on the truss design. Truss bridges were improved even more when metal was used. The suspension bridge was another basic design that was changed by the use of metal. The Brooklyn Bridge is one famous suspension bridge built during that time. It uses steel wires for the suspension cables. About a hundred years ago, engineers began using concrete for bridges. A new method called “prestressing” helps prevent concrete from cracking after a structure is built. Today, most new bridges are made of prestressed concrete and steel. 5. GENERAL INFORMATION ABOUT TRUSSES.  A truss is a structure comprising one or more triangular units constructed with straight members whose ends are connected at joints or nodes.  If all the bars lie in a plane, the structure is a planar truss  The main parts of a planar truss.  In other words, Trusses are designed to form a stable structure. 6. TYPES OF TRUSSES.  Kingpost and Queenpost  Howe truss bridge  Pratt truss bridge  Warren Truss Bridges  K-truss bridges  Continuous truss bridges
  6. 6. Department of Civil Engineering Page 6 6.1 Kingpost and Queenpost Kingpost: • It is used for simple short-span bridges: 40 feet. (probably, it was the first used with small open-work bridges). • Fewest number off truss members.- two diagonal members, kingpost braces, that meet at the apex of the truss, one horizontal beam and the king post which connect the apex to the horizontal beam below. • Kingpost braces are in compression, and the Kingpost, in compression. CHECAR Figure1 KINGPOST Queenpost: • It has two vertical post. • Very strong and stable. • It´s more stable and can support a wider span than a kingpost Figure2 QUEENPOST 6.2Howe Truss Bridge • William Howe, 1840.
  7. 7. Department of Civil Engineering Page 7 • It became very popular and was considered one of the best designs for railroad bridges back in the day. •Wooden beams for the diagonal members, which were in compression. It used iron (and later steel) for the vertical members, which were in tension. Figure3 HOWE TRUSS 6.3 Pratt Truss Bridge (1844) • Very common type but has many variations (Baltimore, Pennsylvania, and the Parker) • The basic identifying features are the diagonal web members which form a V-shape. (Howe truss bridge has a A-shape). • Maximum length of the this bridge can be 250 feet. • Commonly used for supporting railways. •The Pratt truss’s verticals functioned as compression members and diagonals functioned as tension members. • The Pratt truss required more iron than a Howe truss, and due to the increased cost and less rigid construction, builders did not extensively use it for wooden trusses
  8. 8. Department of Civil Engineering Page 8 Figure 4 PRATT TRUSS 6.4 Warren Truss Bridge (1848) • It uses equilateral triangles to spread out the loads on the bridges. The equilateral triangles minimize the forces to only compression and tension. The forces for a member switch form compression to tension, especially to the members near the center of the bridge. • This bridges are often used with verticals to reduce the panel size.Without vertical present aesthetically pleasing appearance. 6.5 K-truss Bridges • The length of members undergoing compression is reduced. •This reduction in length enables components of bridges to endure the compressional force.
  9. 9. Department of Civil Engineering Page 9 • The design is complicated and it is considered to be one of the hardest bridges to build. Howrah Bridge, Kolkata, India. Figure5 K-TRUSS 6.6 Continuous Truss Bridges. • Comparatively, it is more rigid and statically indeterminate structure. • Only suitable in case where the differential settlements of abutments and piers are not significant. • A continuous truss bridge may use less material than a series of simple trusses, because a continuous truss distributes live loads across all the spans; in series of simple trusses • This have been used in span ranges of 150m to 400m.
  10. 10. Department of Civil Engineering Page 10 Figure6 CONTINUOUS TRUSS 7. TRUSS BRIDGES Trusses are used in bridges to transfer the gravity load of moving vehicles to supporting piers. Depending upon the site conditions and the span length of the bridge, the truss may be either through type or deck type. In the through type, the carriage way is supported at the bottom chord of trusses. In the deck type bridge, the carriage way is supported at the top chord of trusses. Usually, the structural framing supporting the carriage way is designed such that the loads from the carriage way are transferred to the nodal points of the vertical bridge trusses.
  11. 11. Department of Civil Engineering Page 11 Figure 7 Some of the trusses that are used in steel bridges Truss Girders, lattice girders or open web girders are efficient and economical structural systems, since the members experience essentially axial forces and hence the material is fully utilised. Members of the truss girder bridges can be classified as chord members and web members. Generally, the chord members resist overall bending moment in the form of direct tension and compression and web members carry the shear force in the form of direct tension or compression. Due to their efficiency, truss bridges are built over wide range of spans. Truss bridges compete against plate girders for shorter spans, against box girders for medium spans and cable-stayed bridges for long spans. Some of the most commonly used trusses suitable for both road and rail bridges. For short and medium spans it is economical to use parallel chord trusses such as Warren truss, Pratt truss, Howe truss, etc. to minimize fabrication and erection costs. Especially for shorter spans the warren truss is more economical as it requires less material than either the Pratt or Howe trusses. However, for longer spans, a greater depth is required at the centre and variable depth trusses are adopted for economy. In case of truss bridges that are continuous over many supports, the depth of the truss is usually larger at the supports and smaller at midspan.
  12. 12. Department of Civil Engineering Page 12 As far as configuration of trusses is concerned, an even number of bays should be chosen in Pratt and modified Warren trusses to avoid a central bay with crossed diagonals. The diagonals should be at an angle between 50° and 60° to the horizontal. Secondary stresses can be avoided by ensuring that the centroidal axes of all intersecting members meet at a single point, in both vertical and horizontal planes. However, this is not always possible, for example when cross girders are deeper than the bottom chord then bracing members can be attached to only one flange of the chords. 8 GENERAL DESIGN PRINCIPLES 8.1 Optimum depth of truss girder The optimum value for span to depth ratio depends on the magnitude of the live load that has to be carried. The span to depth ratio of a truss girder bridge producing the greatest economy of material is that which makes the weight of chord members nearly equal to the weight of web members of truss. It will be in the region of 10, being greater for road traffic than for rail traffic. IS: 1915-1961, also prescribes same value for highway and railway bridges. As per bridge rules published by Railway board, the depth should not be greater than three times width between centres of main girders. The spacing between main truss depends upon the railway or road way clearances required. 8.2 Design of compression chord members Generally, the effective length for the buckling of compression chord member in the plane of truss is not same as that for buckling out-of- plane of the truss i.e. the member is weak in one plane compared to the other. The ideal compression chord will be one that has a section with radii of gyration such that the slenderness value is same in both planes. In other words, the member is just likely to buckle in plane or out of plane. These members should be kept as short as possible and consideration is given to additional bracing, if economical. The effective length factors for truss members in compression may be determined by stability analysis. In the absence of detailed analysis one can follow the recommendations given in respective codes. The
  13. 13. Department of Civil Engineering Page 13 depth of the member needs to be chosen so that the plate dimensions are reasonable. If they are too thick, the radius of gyration will be smaller than it would be if the same area of steel is used to form a larger member using thinner plates. The plates should be as thin as possible without losing too much area when the effective section is derived and without becoming vulnerable to local buckling. Common cross sections used for chord members are shown in Fig. 7. Trusses with spans up to 100 m often have open section compression chords. In such cases it is desirable to arrange for the vertical posts and struts to enter inside the top chord member, thereby providing a natural diaphragm and also achieving direct connection between member thus minimizing or avoiding the need for gussets. However, packing may be needed in this case. For trusses with spans greater than about 100 m, the chords will be usually the box shaped such that the ideal disposition of material to be made from both economic and maintenance view points. For shorter spans, rolled sections or rolled hollow sections may be used. For detailed design of compression chord members the reader is referred to the chapter on Design of axially compressed columns. 8.3 Design of tension chord members Tension members should be as compact as possible, but depths have to be large enough to provide adequate space for bolts at the gusset positions and easily attach cross beam. The width out-of-plane of the truss should be the same as that of the verticals and diagonals so that simple lapping gussets can be provided without the need for packing. It should be possible to achieve a net section about 85% of the gross section by careful arrangement of the bolts in the splices. This means that fracture at the net section will not govern for common steel grades. In this case also, box sections are preferable for ease of maintenance but open sections may well prove cheaper. For detailed design reader is referred to the chapter on Design of Tension members.
  14. 14. Department of Civil Engineering Page 14 Figure 8 Typical cross-section for truss members 9. DESIGN OF VERTICAL AND DIAGONAL MEMBERS Diagonal and vertical members are often rolled sections, particularly for the lightly loaded members, but packing may be required for making up the rolling margins. This fact can make welded members more economical, particularly on the longer trusses where the packing operation might add significantly to the erection cost. Aesthetically, it is desirable to keep all diagonals at the same angle, even if the chords are not parallel. This arrangement prevents the truss looking over complex when viewed from an angle. In practice, however, this is usually overruled by the economies of the deck structure where a constant panel length is to be preferred. Typical cross sections used for members of the truss bridges are shown in Fig. 8 9.1 Lateral bracing for truss bridges Lateral bracing in truss bridges is provided for transmitting the longitudinal live loads and lateral loads to the bearings and also to prevent the compression chords from buckling. This is done by providing stringer bracing, braking girders and chord lateral bracing. In case of highway truss bridges, concrete deck, if provided, also acts as lateral bracing support system.
  15. 15. Department of Civil Engineering Page 15 Figure 9 Lateral bracing systems The nodes of the lateral system coincide with the nodes of the main trusses. Due to interaction between them the lateral system may cause as much as 6% of the total axial load in the chords. This should be taken into account. Fig.8 shows the two lateral systems in its original form and its distorted form after axial compressive loads are applied in the chords due to gravity loads. The rectangular panels deform as indicated by the dotted lines, causing compressive stresses in the diagonals and tensile stresses in the transverse members. The transverse bracing members are indispensable for the good performance of St. Andrew’s cross bracing system. In diamond type of lateral bracing system the nodes of the lateral system occur midway between the nodes of the main trusses [Fig.8]. They also significantly reduce the interaction with main trusses. With this arrangement, “scissors-action” occurs when the chords are stressed, and the chords deflect slightly laterally at the nodes of the lateral system. Hence, diamond system is more efficient than the St. Andrew’s cross bracing system. It is assumed that wind loading on diagonals and verticals of the trusses is equally shared between top and bottom lateral bracing systems. The end portals (either diagonals or verticals) will carry the load applied to the top chord down to the bottom chord. In cases, where only one lateral system exists (as in Semi through trusses), then the single bracing system must carry the entire wind load.
  16. 16. Department of Civil Engineering Page 16 10. HOW BRIDGES WORK? Every passing vehicle shakes the bridge up and down, making waves that can travel at hundreds of kilometers per hour. Luckily the bridge is designed to damp them out, just as it is designed to ignore the efforts of the wind to turn it into a giant harp. A bridge is not a dead mass of metal and concrete: it has a life of its own, and understanding its movements is as important as understanding the static forces. 11. CONNECTIONS Members of trusses can be joined by riveting, bolting or welding. Due to involved procedure and highly skilled labour requirement, riveting is not common these days, except in some railway bridges in India. In railway bridges riveting may be used due to fatigue considerations. Even in such bridges, due to recent developments, high strength friction grip (HSFG) bolting and welding have become more common. Shorter span trusses are usually fabricated in shops and can be completely welded and transported to site as one unit. Longer span trusses can be prefabricated in segments by welding in shop. These segments can be assembled by bolting or welding at site. This results in a much better quality of the fabricated structure. However, the higher cost of shop fabrication due to excise duty in contrast to lower field labour cost frequently favour field fabrication in India. If the rafter and tie members are T sections, angle diagonals can be directly connected to the web of T by welding or bolting. Frequently, the connections between the members of the truss cannot be made directly, due to inadequate space to accommodate the joint length. In such cases, gusset plates are used to accomplish such connections (Fig. 9). The size, shape and the thickness of the gusset plate depend upon the size of the member being joined, number and size of bolt or length of weld required, and the force to be transmitted. The thickness of the gusset is in the range of 8 mm to 12 mm in the case of roof trusses and it can be as high as 22 mm in the case of bridge trusses. The design of gussets is usually by rule of thumb. In short span (8 – 12 m) roof trusses, the member forces are smaller, hence the thickness
  17. 17. Department of Civil Engineering Page 17 of gussets are lesser (6 or 8 mm) and for longer span lengths (> 30 m) the thickness of gussets are larger (12 mm). Figure 9 12. LOADS ON BRIDGES The following are the various loads to be considered for the purpose of computing stresses, wherever they are applicable.  Dead load  Live load  Impact load  Longitudinal force  Thermal force  Wind load  Seismic load  Racking force  Forces due to curvature  Forces on parapets  Frictional resistance of expansion bearings  Erection forces
  18. 18. Department of Civil Engineering Page 18 12.1 DEAD LOAD The dead load on a bridge consists of the weight of all its structural parts and all the fixtures and services like deck surfacing, kerbs, parapets, lighting and signing devices, gas and water mains, electricity and telephone cables. The weight of the structural parts has to be guessed at the first instance and subsequently confirmed after the structural design is complete. 12.2 Live loads Bridge design standards of different countries specify the design loads which are meant to reflect the worst loading that can be caused on the bridge by traffic permitted and expected to pass over it. The relationship between bridge design loads and the regulations governing the weights and sizes of vehicles is thus obvious, but other factors like traffic volume and mixture of heavy and light vehicles are also relevant. Short spans, say up to 10m for bending moment and 6m for shear force, are governed by single axles or bogies with closely spaced multiple axles. The worst loading for spans over about 20m is often caused by more than three vehicles. The worst vehicles are often the medium-weight compact vehicles with two axles, and not the heaviest vehicles with four, five or six axles. The criteria thus change from axle loads to worst vehicles as the span increases, with the mixture of vehicles in the traffic being an important factor for the longer spans. When axles or single vehicles are the worst case, the effect of impact has to be allowed for, but several closely spaced vehicles represent a jam situation without significant impact. The adjacent lanes of short span bridges may all be loaded simultaneously with the worst axles or vehicles, but this is less likely for long spans. Apart from the design loading for normal traffic, many countries specify special bridge design loading for the passage of abnormal vehicles of the military type or carrying heavy indivisible industrial equipment like generators. The passage of such heavy vehicles on public roads usually involves special permits from the highway authorities and often supervision by the police. In addition to these legal heavy loads, there is the growing problem of illegal overweight vehicles weighing as much as 40% over their legal limits. In each country traffic regulations limit the wheel and axle loads and
  19. 19. Department of Civil Engineering Page 19 gross vehicle weights, and impose dimensional limits on axle spacing and size of vehicles. Goods vehicles may be of the following types:  vehicles with two axles  rigid vehicles with three or more axles  articulated vehicles with two or three axles under the tractor and one or more axles under the trailer  road trains comprising a vehicle and trailer. 12.3 Longitudinal forces Longitudinal forces are set up between vehicles and the bridge deck when the former accelerate or brake. The magnitude of the force is given by where W is the weight of the vehicle, g is the acceleration due to gravity (¼9.81 m/s2) and V is the change in speed in time t. Usually the change in speed is faster during braking than while accelerating. Railway bridges: Railway bridges including combined rail and road bridges are to be designed for railway standard loading given in bridge rules. The standards of loading are given for: · Broad gauge - Main line and branch line · Metre gauge - Main line, branch line and Standard C · Narrow gauge - H class, A class main line and B class branch line The actual loads consist of axle load from engine and bogies. The actual standard loads have been expressed in bridge rules as equivalent uniformly distributed loads (EUDL) in tables to simplify the analysis. These equivalent UDL values depend upon the span length. However, in case of rigid frame, cantilever and suspension bridges, it is necessary for the designer to proceed from the basic wheel loads. In order to have a uniform gauge throughout the country,
  20. 20. Department of Civil Engineering Page 20 it is advantageous to design railway bridges to Broad gauge main line standard loading. The EUDLs for bending moment and shear force for broad gauge main line loading can be obtained by the following formulae, which have been obtained from regression analysis: For bending moment: EUDL in kN = 317.97 + 70.83l + 0.0188l2 ≥ 449.2 kN (7.1) For shear force: EUDL in kN = 435.58 + 75.15l + 0.0002l2 ≥ 449.2 kN (7.2) Note that, l is the effective span for bending moment and the loaded length for the maximum effect in the member under consideration for shear. 'l ' should be in metres. The formulae given here are not applicable for spans less than or equal to 8 m with ballast cushion. For the other standard design loading the reader can refer to Bridge rules. 12.4 Impact load Figure 10 Impact percentage curve for highway bridges for IRC class A and IRC class B loadings The dynamic effect caused due to vertical oscillation and periodical shifting of the live load from one wheel to another when the locomotive is moving is known as impact load. The impact load is determined as a product of impact factor, I, and the live load. The impact factors are specified by different authorities for different types of bridges. The impact factors for different bridge for different types
  21. 21. Department of Civil Engineering Page 21 of moving loads are given in the table 1. Fig.10 shows impact percentage curve for highway bridges for class AA loading. Note that, in the above table l is loaded length in m and B is spacing of main girders in m. TABLE 1 BRIDGE LOADING LOADING IMPACT FACTOR (I) Railway bridges according to bridge rules Broad gauge and Meter gauge (a) Single track (b) Main girder of double track with two girders .72 (c) Intermediate main girder of multiple track spans .60 (d) Outside main girders of multiple track spans Specified in (a) or (b) whichever applies (e) Cross girders carrying two or more tracks 72 Broad gauge Rails with ordinary fish plate joints and supported directly on sleepers or transverse steel troughing Meter gauge Narrow gauge 12.5 Thermal forces – The free expansion or contraction of a structure due to changes in temperature may be restrained by its form of construction. Where any portion of the structure is not free to expand or contract under the variation of temperature, allowance should be made for the stresses resulting from this condition. The
  22. 22. Department of Civil Engineering Page 22 coefficient of thermal expansion or contraction for steel is 11.7 x 10-6 / . 12.6 Wind load – Wind load on a bridge may act · Horizontally, transverse to the direction of span · Horizontally, along the direction of span · Vertically upwards, causing uplift · Wind load on vehicles Wind load effect is not generally significant in short-span bridges; for medium spans, the design of sub-structure is affected by wind loading; the super structure design is affected by wind only in long spans. For the purpose of the design, wind loadings are adopted from the maps and tables given in IS: 875 (Part III). A wind load of 2.40 kN/m2 is adopted for the unloaded span of the railway, highway and footbridges. In case of structures with opening the effect of drag around edges of members has to be considered. 12.7 Racking force – This is a lateral force produced due to the lateral movement of rolling stocks in railway bridges. Lateral bracing of the loaded deck of railway spans shall be designed to resist, in addition to the wind and centrifugal loads, a lateral load due to racking force of 6.0 kN/m treated as moving load. This lateral load need not be taken into account when calculating stresses in chords or flanges of main girders. 12.8 Forces on parapets - Railings or parapets shall have a minimum height above the adjacent roadway or footway surface of 1.0 m less one half the horizontal width of the top rail or top of the parapet. They shall be designed to resist a lateral horizontal force and a vertical force each of 1.50 kN/m applied simultaneously at the top of the railing or parapet. 12.9 Seismic load – If a bridge is situated in an earthquake prone region, the earthquake or seismic forces are given due
  23. 23. Department of Civil Engineering Page 23 consideration in structural design. Earthquakes cause vertical and horizontal forces in the structure that will be proportional to the weight of the structure. Both horizontal and vertical components have to be taken into account for design of bridge structures. IS:1893 – 1984 may be referred to for the actual design loads. 12.10 Forces due to curvature - When a track or traffic lane on a bridge is curved allowance for centrifugal action of the moving load should be made in designing the members of the bridge. All the tracks and lanes on the structure being considered are assumed as occupied by the moving load. 12.11 Erection forces – There are different techniques that are used for construction of railway bridges, such as launching, pushing, cantilever method, lift and place. In composite construction the composite action is mobilised only after concrete hardens and prior to that steel section has to carry dead and construction live loads. Depending upon the technique adopted the stresses in the members of the bridge structure would vary. Such erection stresses should be accounted for in design. This may be critical, especially in the case of erection technologies used in large span bridges. 13. ECONOMY OF TRUSSES Trusses consume a lot less material compared to beams to span the same length and transfer moderate to heavy loads. However, the labour requirement for fabrication and erection of trusses is higher and hence the relative economy is dictated by different factors. In India these considerations are likely to favour the trusses even more because of the lower labour cost. In order to fully utilize the economy of the trusses the designers should ascertain the following:
  24. 24. Department of Civil Engineering Page 24  Method of fabrication and erection to be followed, facility for shop fabrication available, transportation restrictions, field assembly facilities.  Preferred practices and past experience.  Availability of materials and sections to be used in fabrication.  Erection technique to be followed and erection stresses.  Method of connection preferred by the contractor and client (bolting, welding or riveting).  Choice of as rolled or fabricated sections.  Simple design with maximum repetition and minimum inventory of material. 14. ADVANTAGE OF STEEL IN BRIDGE CONSTRUCTRION The steel is a very versatile material having many advantages over the other material. Presently, the mega bridge projects being undertaken by the Railways involves steel super structures. For longer spans, the railway has shown more confidence in steel compared with PSC. This is basically due to the fact that in case of steel bridges, rehabilitation procedures are easier and involve lesser delays, inspections are easier as it allows the deformations to be seen and easily evaluated/measured besides the basic fact that Railway engineers feel comfortable in constructing and maintaining steel bridges. Generally speaking, steel bridges may have the following advantages when compared to concrete/PSC bridges: -_Reduced dead loads. -_More economic foundations. -_Simpler erection procedures. -_Shorter execution time. -_Faster and easier rehabilitation. When constructed in insurgency affected areas like North-East and J&K and in high seismicity areas where damage to the bridges is more likely, steel bridges provides easier and faster options for rehabilitation. More over, structural redundancies can be easily inbuilt in steel bridges. A disadvantage of steel when compared to concrete is
  25. 25. Department of Civil Engineering Page 25 the maintenance cost for the prevention of corrosion. However, it is now recognized that concrete bridges also have problems relating to maintenance i.e. relating to the effects of corrosion of steel reinforcement on the durability of the structure. In addition to the various points cited above, structural steel as the basic bridge construction material involves following advantages which have also played an important part in this shift of Railway engineer’s ideology from concrete/PSC bridge construction to steel bridge construction: - 14.1 HIGH STRENGTH TO WEIGHT RATIO High strength to weight ratio of steel minimizes substructure costs, which is particularly beneficial in poor ground conditions. Minimum self weight is also an important factor in transporting and handling of bridge components specially in hilly areas like North-East and Jammu & Kashmir. In addition, it facilitates very shallow construction depth for girders, which over come problems with headroom and flood clearances and minimizing the length of approach ramps. The low self weight also minimizes foundation work in case of bridges being constructed near existing rail lines. 14.2 HIGH QUALITY MATERIAL Steel is a high quality material, which is readily available world wide in various certified grades, shapes and sizes. The testing regime carried out at the steel mills imparts confidence to the engineers for the bridge projects. Prefabrication in controlled shop condition leads to high quality work at minimum cost. The quality control extends from the material itself and follows on through the processes of cutting, drilling, welding and fit-up. The total weight of steel constructions is a fraction of the total weight of concrete bridges. Therefore steel bridges can be used with long spans, even in earthquake-prone areas. 14.3 SPEED OF CONSTRUCTION The prefabrication of the components means that construction time on site in hostile environment is minimized. The light weight nature of steel permits the erection of large components. Besides this, resource,
  26. 26. Department of Civil Engineering Page 26 such as water, aggregates etc may sometimes not be easily available at sites on this project, for the purpose of production of concrete. 14.4 VERSATILITY The prefabrication of the components means that construction time on site in hostile environment is minimized. The light weight nature of steel permits the erection of large components. Besides this, resource, such as water, aggregates etc may sometimes not be easily available at sites on this project, for the purpose of production of concrete. 14.5 RECYCLING Steel suits a wide range of construction methods and sequences. Installation may be by cranes, launching, slide-in-techniques or transporters. For example, in Jammu & Kashmir area on Katra- Quazigund section, the erection of the main steel arch of Chenab and Anjikhad bridge is being planned by mechanized rope way. Steel gives the engineer flexibility in terms of erection sequence and programme. Components can be sized to suit access restriction at site, and once erected the steel girders provide a platform for subsequent operations. 14.6 REPAIR & REHABILITATION Steel is a ‘sustainable’ material. When a steel bridge reaches the end of its useful life, the girders can be cut into manageable sizes to facilitate demolition, and returned to steelworks for recycling. The increased emphasis of the green techniques for construction, steel is lot ‘Greener’ than concrete for bridges. 14.7 AESTHETICS Steel bridges can readily be repaired after accidental damages. In case of damage to the bridge due to derailment/accident, damage due to a terrorist activity or damage due to natural causes such as earthquakes, floods etc. complete steel spans can be replaced without much delay which is not the case with PSC super structures. This aspect is very important in the case of Railways where longer disruption to rail traffic can not be afforded.
  27. 27. Department of Civil Engineering Page 27 14.8 DURABILITY Steel bridges now have a proven life span extending to well over 100 years. In fact, old steel girders of vintage 1854 etc are also in use on branch lines. Steel has a predictable life, as the structural elements are visible and accessible. Any signs of deterioration are readily apparent, without the need for extensive investigations. Direct measurements of stresses are possible as all the parts / members are accessible and thickness of members is less. Corrosion is a problem, which can, however, be addressed by timely painting. In addition, the latest coatings are anticipated to last well beyond 30 years before requiring major maintenance. The potential durability of steel may be summarized in the following quote by a Mr. J.A.Waddell in 1921: “The life of a steel bridge that is scientifically designed, honestly and carefully built, and not seriously overloaded, if properly maintained, is indefinitely long.” 15. COMPONENTS OF SUPERSTRUCTURE OF A STEEL TRUSS BRIDGE Figure 11 15.1 Top chords: • The most highly stressed compression members. • Need special attention while proportioning and detailing.
  28. 28. Department of Civil Engineering Page 28 Figure 12 Common cross-sections of top chords 15.2 Bottom chords: • The most highly stressed tension members. • Connections may be welded, bolted or riveted. Figure 13 Common cross-sections of bottom chords 15.3 Web members. • These members could be diagonals and verticals and may be subjected to tension and some to compression. ( it depends on the type of the truss) • Vertical members working at compression are termed “post” and those in tension are called “hangers”
  29. 29. Department of Civil Engineering Page 29 Figure 14 Typical crosssections of web members . 15.4 End posts or rakers: • These are located at the ends of a truss to carry the lateral forces from the top chord level to the bridge bearings. • For this purpose portal bracings are fixed onto them at the upper level. Figure15 15.5 Bracing system: • Considered as secondary members, but in fact, vital for the successful performance of the primary members.
  30. 30. Department of Civil Engineering Page 30 Bracings are designed to resist two types of forces: • Lateral forces: those acting transverse to the axis of the bridge. • Longitudinal forces: Those acting along the axis of the bridge. Lateral bracing: • Placed between the top chords and bottom chords of a pair of trusses. • In a road bridge, the deck slab can act as a stiffening member between the trusses. Figure 16 Lateral bracing systems 15.6 Sway bracing or cross bracing: • Placed between trusses. • Are provided for distributing the transverse loads to the lateral system. • Also for providing torsional rigidity to the truss frame Figure 17 Sway bracing 15.7 Portal bracings : • Located at the end posts or rakers. • Provide end supports to the top lateral bracing system.
  31. 31. Department of Civil Engineering Page 31 Figure 18 Portal bracing 16. ANALYSIS AND DESIGN OF TRUSSES  Stability and Determinacy • A stable and statically determinate plane truss should have at least three members, three joints and three reaction components. • To form a stable and determinate plane truss of “n” joints: • The number of members (m) = 2n-3 • If the stable, simple, plane truss has more than three reactions components, the structure is externally indeterminate. • If it has more than (m>2n-3) members, the structure is internally indeterminate and hence all of the member forces cannot be determined form the 2n available equations of static method of joints. • Truss analysis gives the bar forces in a truss, for a statically determined truss, these bar forces can be found by employing the laws of statics to assure internal equilibrium of the structure. • Method of Joints. • Method of Sections. For indeterminate truss: • Virtual work. Another methods: • Finite Element Method • Computer Analysis.
  32. 32. Department of Civil Engineering Page 32 17. METHOD OF JOINTS. • Chose a node whose number of unknown forces doesn’t exceed two, then study its equilibrium using static equilibrium equations to determine these forces: • ΣFx = 0, ΣFy = 0 and ΣM = 0 • Go to next node and study its equilibrium using the evaluated forces from the previous node then go to the next node and so one. Figure 19 Method of Joints. 18 METHOD OF SECTIONS • If only a few member forces of a truss are needed, the quickest way to find these forces is by this method. • In this method, an imaginary cut (section) is drawn through a stable and determinate truss. Thus, a section subdivides the truss into two separates parts. Since the entire truss is in equilibrium, any part of it must also be in equilibrium. •ΣFx = 0, ΣFy = 0 and ΣM = 0.
  33. 33. Department of Civil Engineering Page 33 Figure 20 19. DEFLECTION OF A TRUSS • The virtual work method can be used to determine the deflection of trusses. • with n equal to the virtual force in the member and equal to the change in length of the member. • A deflection is caused by three ways: • Applied loads acting on each member. • Temperature changes • Fabrication errors. • Axial deformation: • When the force of all the members are known we can determine the axial deformation of each member by using the equation: • If we modified the value of into the equation for the deflection. m= number of members. n= the force in the member due to the virtual load. N= the force in the member due to applied load. L= length. A= area. E= Young´s Modulus of Elasticity.
  34. 34. Department of Civil Engineering Page 34 • Temperatures Changes= is the coefficient of thermal expansion L = length of the member AT= Change in temperature. • Fabrication Errors: • The original equation for deflection of a truss can be modified K= number of members undergoing fabrication errors. n= force in the member due to the virtual load = the change in length of the member due to fabrication errors. Total deflection of a Truss (Hibbeler, Structural Analysis) 20. DETERMINACY OF COPLANAR TRUSSES  Since all the elements of a truss are two-force members, the moment equilibrium is automatically satisfied.  Therefore there are two equations of equilibrium for each joint, j, in a truss. If r is the number of reactions and b is the number of bar members in the truss, determinacy is obtained by b + r = 2j Determinate b + r > 2j Indeterminate 21. STABILITY OF COPLANAR TRUSSES  If b + r < 2j, a truss will be unstable, which means the structure will collapse since there are not enough reactions to constrain all the joints.  A truss may also be unstable if b + r ≥2j. In this case, stability will be determined by inspection
  35. 35. Department of Civil Engineering Page 35 b + r < 2j Unstable b + r 2j Unstable if reactions are concurrent, parallel, or collapsible mechanics 21.1 External stability - a structure (truss) is externally unstable if its reactions are concurrent or parallel Figure21 21.2 Internal stability - may be determined by inspection of the arrangement of the truss members.  A simple truss will always be internally stable  The stability of a compound truss is determined by examining how the simple trusses are connected  The stability of a complex truss can often be difficult to determine by inspection.  In general, the stability of any truss may be checked by performing a complete analysis of the structure. If a unique solution can be found for the set of equilibrium equations, then the truss is stable.  Internal stability Externally stable Internally stable Figure22
  36. 36. Department of Civil Engineering Page 36 Collapsible mechanism Externally stable Internally unstable Figure23 Collapsible mechanism Figure24 Externally stable Internally unstable 22. TRUSS MEMBERS ARE CONNECTED BY SMOOTH PINS  The stress produced in these elements is called the primary stress.  The pin assumption is valid for bolted or welded connections if the members are concurrent.  However, since the connection does provide some rigidity, the bending introduced in the members is called secondary stress.  Secondary stress analysis is not commonly performed. Figure25
  37. 37. Department of Civil Engineering Page 37 23. ALL LOADING IS APPLIED AT THE JOINTS OF THE TRUSS  Since the weight of each members is small compared to the member force, the member weight is often neglected.  However, when the member weight is considered, it is applied at the end of each member.  Because of these two assumptions, each truss member is a two- force member with either a compressive (C) or a tensile (T) axial force.  In general, compression members are bigger to help with instability due to buckling. 24. CLASSIFICATION OF COPLANAR TRUSSES 24.1Simple Truss  The simplest framework that is rigid or stable is a triangle.  Therefore, a simple truss is constructed starting with a basic triangular element and connecting two members to form additional elements.  As each additional element of two members is placed on a truss, the number of joints is increased by one. Figure26 SIMPLE TRUSS 24.2 Compound Truss  This truss is formed by connecting two or more simple trusses together.
  38. 38. Department of Civil Engineering Page 38  This type of truss is often used for large spans.  There are three ways in which simple trusses may be connected to form a compound truss: 1. Trusses may be connected by a common joint and bar. 2. Trusses may be joined by three bars. 3. Trusses may be joined where bars of a large simple truss, called the main truss, have been substituted by simple trusses, called secondary trusses Figure27 COMPOUND TRUSS 25. FORCE ANALYSIS (TRUSS) • Loads members in tension and compression. • Members are pinned at joints (Moment = 0). • Triangles provide stability and strength. • Top members in Compression. • Bottom members in Tension.
  39. 39. Department of Civil Engineering Page 39 26. APPLICABLE SPAN OF TRUSS BRIDGE Truss brides are generally applied within the following range. 1. Simple truss bridge is in the range of 55 meter to 85 meter span. 2. Continuous truss bridge is in the range of 60 meter to 300 meter span. 3. Cantilever truss bridge is in the range of 300 meter to 510 meter span (in Japan, only one bridge has longer span than 200 meter, and that is Minato Ohashi, with 510 meter span.) Among the 3 types of bridges, the simple truss bridge or the continuous truss bridge, either with approximate 60 meter to 100 meter span is usually applied. 27. ADVANTAGES OF TRUSS BRIDGE 27.1 Mountain Region 1. When the members are difficult to be transported to the site and when the conditions of construction is restricted. 2. When a bridge in a curve alignment is required, a horizontal bent continuous bridge or a deck truss bridge with brackets can selected. 27.2 Open Field Region 1. Assurance of space below the bridge soffit due to adoption of through bridge 2. Long span bridge over the mouth of a river or the coast. 3. Double-deck bridge having upper and lower 2-layer road surface. 28. DISADVANTAGES OF TRUSS BRIDGE 1. As it is composed by many members, maintenance requirements such as repainting is needed. 2. The sight view from the driver in case of through bridges.
  40. 40. Department of Civil Engineering Page 40 29. OVER ALL HEIGHT OF TRUSS AND LENGTH BETWEEN PANELS 1. For simple truss, the truss height is 1/7-1/8 time the span length and the length between panels is 1/6-1/8 time the span length. 2. For continuous truss, the truss height is 1/9- 1/10 time the span length and the length between panels is 1/8-1/10 time the span length Figure28
  41. 41. Department of Civil Engineering Page 41 30. CONCLUSIONS 1. Bridge components are having substantial fatigue life even after considering the Joint Flexibility. 2. Joint flexibility tends to alter the vibration characteristics of each component to loading environment in presence of realistic damping of 2%, thus the damage potential of each member which depends upon the stress range and cycle counts is also got affected ,however the change was only 40%(max). 3. In most of members fatigue life got increased, however life of some component got decreased, the maximum decrease observed is about 40% in one of Bottom Chords and Verticals. 4. The variation in passage to failure exhibited by each component with the different speeds makes it difficult to find a particular trend, however the trend is similar at different flexibilities with change only in magnitudes. 5. It can be concluded that the reduction of joint rotational stiffness up to 50% has less effect on structural stability of Steel Truss Railway Bridge.
  42. 42. Department of Civil Engineering Page 42 31. SUGGESTION In comparison to the developed countries, the steel being used in Indian railways is of inferior quality. Following suggestions recommendations are given for early adoption of High Performance steel over Indian Railways:- 1. Furthermore studies should be conducted for the adoption of HPS or any other type of steel which suits Indian conditions and economy. 2. The Railway Board in consultation with RDSO may jointly discuss the issue for convincing the steel industry including SAIL for producing the special type of steel for Indian bridges. 3. A pilot project should be given to each railways for applying the High Performance steel in at least one bridge so that experience in the same can be gained. 4. The major supplier of HPS in US and Europe is Arcelor-Mittal steel and Corus Group (recently tied up with Tata steel) and they have a very good Indian connection. These groups may be approached for their help and guidance.
  43. 43. Department of Civil Engineering Page 43 32.Marvels of truss bridge in India: Howrah Bridge – The Bridge without Nuts & Bolts!: How about visiting a vintage bridge which has no nuts & bolts in its construction but still standing tall for the last 66 years? Hard to believe? The Bridge in concern - one of the busiest in the world - is located at Howrah in West Bengal. The Howrah bridge, the sixth longest of its type, has been an emblem of the city of Kolkata from its inception. So much so that the world knows Kolkata by its trams, the Victoria Memorial, and of course the Howrah Bridge. Opened to traffic in 1943, the construction of the bridge was started in 1937. The bridge has remained one of the most renowned landmarks of Kolkata. More than 150,000 vehicles and 4,000,000 pedestrians cross over the bridge every day. Technically speaking, Howrah Bridge is a "Cantilever Truss" bridge, constructed entirely by riveting, without nuts or bolts!
  44. 44. Department of Civil Engineering Page 44 Notable features of howrah bridge:-  705 meters in length, 97 feet in width, 82 meters in height  26,500 plus mega tonne of high-tensile steel was used  Suspension type Balanced Cantilever  325 ft, length of each anchor arm  468 ft, length of each Cantilever arm  564 ft, suspended span  Deck width 71 ft, footpath 15 feet on either side  No nuts & bolts  Total 8 articulation joints, 3 at each of the cantilever arms, and 2 in the suspended portions  Carriageway Minimum headroom is 5.8 m  River traffic freeboard is 8.8 m  Ranks sixth in World’s top 10 longest Cantilever bridges What is a “Cantilever Truss” bridge? A Cantilever bridge is a type of bridge constructed using cantilevers only. Cantilevers are constructions that protrude horizontally into space, secured on one end of the structure! In case of small footbridges the technique is quite simple however, for huge bridges the volume of work is enormous. Steel truss cantilever (STC) was one of the newer technologies in the 1930s which was used in building Howrah Bridge. The advantage of STC was the lack of complexity in designing and implementation that included little or no falsework. Falsework, in layman’s term, is the temporary construction, provided externally to a structure, till the time it needs no extraneous support to stand on its own architecture. For lovers of vintage architecture, the finest example of the above mentioned architecture lies en route Kolkata.
  45. 45. Department of Civil Engineering Page 45 The Godavari Bridge or Kovvur-Rajahmundry Bridge The godavri bridge is a truss bridge spanning Godavari river in Rajahmundry, India. It is Asia's second longest road-cum-rail bridge crossing a water body, after the Sky Gate Bridge in Kansai International Airport, Osaka. It is second of the three bridges that span the Godavari River at Rajahmundry. The Havelock Bridge being the earliest, was built in 1897, and having served its full utility, was decommissioned in 1997. The latest bridge is the Godavari Arch Bridge, a bowstring-girder bridge, built in 1997 and presently in service. The bridge is 2.7 kilometers long consisting of 27 spans of 91.4 m and 7 spans of 45.72 m of which 6 spans of 45.72m are in 6 deg. curve at long Rajahmundry end to make up for the built up area. The bridge has a road deck over the single track rail deck, similar to the Grafton Bridge in New South Wales, Australia. This bridge, in addition to Godavari Arch Bridge, has been widely used to represent Rajahmundry in arts, media, andculture. It is one of the recognised symbols of Rajahmundry.
  46. 46. Department of Civil Engineering Page 46 33. IMAGES
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  50. 50. Department of Civil Engineering Page 50 33. REFERENCE 1. “Use and Application of High-Performance steels for steel structures” Structural Engineering documents No 8 Published by IABSE Oct 2005. 2. NBSA White paper, “Advances in High performance steels for Highway bridges” by Alexander D Wilson, manager customer technical service, Mittal Steel USA 3. “Prospects of High-Performance welded steel bridge” , Advances in bridge Engineering, Mar 24-25, 2006 by P.K.Ghosh,Professor,Department of Metallurgical and Material Engineering, IIT , Roorkee. 4. “Improvements to High Performance steel “ by A. D. Wilson , Mittal Steel USA 5. IS 2062:1999 “Steel for general structural purposes”- specification, BIS ,N.Delhi 6. IS 8500 1977 “Specification for Weld able structural steel” ( Medium and High strength qualities ) , BIS, N. Delhi 7. “Steel Structures, Design and Behaviour” by Charles G. Salmon & John E.Johnson , Harper & Row 8. 9. 10. 11. 12. 13. Anon, “Design of Composite Trusses”, Steel Construction Institute”, Ascot, 1992. 14. Anon “Constructional Steel Design: An International Guide”, Elsevier, London, 1993. 15. S.Venkateswara Rao, and M.Prabhakar, Prestressingfor economical Bridge construction”, Indian Highways (IRC), 1990. 16. Vallenilla, C.R., and Bjorhovde, R., “Effective Width Criteria for Composite Beams”, Engineering Journal, AISC, Fourth Quarter, 1985, pp. 169-175.