BRIDGES AND ITS TYPES

:DIFFERENT TYPES OF BRIDGES AND ITS SUITABILITY:

Definition:-

 A bridge is a structure providing passage over an obstacle without closing the way beneath. The required passage may be for a road, a railway, pedestrians, a canal or a pipeline. The obstacle to be crossed may be a river, a road, railway or a valley. In other words, bridge is a structure for carrying the road traffic or other moving loads over a depression or obstruction such as channel, road or railway. A bridge is an arrangement made to cross an obstacle in the form of a low ground or a stream or a river without closing the way beneath.

Components of bridge

The bridge structure comprises of the following parts.   Superstructure or Decking This includes slab, girder, truss, etc. This bears the load passing over it and transmits the forces caused by the same to the substructures.   Bearings The bearings transmit the load received from the decking on to the substructure and are provided for distribution of the load evenly over the substructure material which may not have sufficient bearing strength to bear the superstructure load directly.

Substructure

This comprises piers and abutments, wing walls or returns and their foundation.

 • Piers and Abutments These are vertical structures supporting deck/bearing provided for transmitting the load down to the bed/earth through foundation.

 • Wing walls and Returns These are provided as extension of the abutments to retain the earth of approach bank which otherwise has a natural angle of repose.

• Foundation  This is provided to transmit the load from the piers or abutments and wings or returns to and evenly distribute the load on to the strata. This is to be provided sufficiently deep so that it is not affected by the scour caused by the flow in the river and does not get undermined. While the above mentioned are structurally operational parts, for safety hand rails or parapets, guard rails or curbs are provided over the decking in order to prevent vehicle or user from falling into the stream or for the separation of traffic streams.

Classification

Bridges may be classified in many ways, as below. 

 According to the flexibility of superstructure as fixed span bridges or movable bridges.

 • Fixed span superstructure

 In case of fixed span superstructure, the superstructure remains in a fixed position and most of the bridges are of this category. 

• Movable span bridges

 In case of movable span superstructure, the superstructure is lifted or moved with the help of some suitable arrangement.

  According to the position of bridge floor relative to the formation level and the highest flood discharge as deck bridges, through bridges or semi- through bridges.

• Deck bridges

Deck-type bridges refer to those in which the road deck is carried on the top flange or on top of the supporting girders. The deck slab or sleeper may cantilever out to some extent beyond the extreme longitudinal girder 

• Through bridges

 In the through type bridges, the decking is supported by the bottom flange of the main supporting girders provided on either side.

• Semi through bridges

 The semi-through bridge has its deck midway and the deck load is transmitted to the girder through the web of the girder. In this also, the main girders are on either side of deck.

According to the inter-span relations as simple, continuous or cantilever bridges.

• Simply supported

 Generally width of bridge is divided into number of individual spans. For each span, the load carrying member is simply supported at both ends. The plate girder and truss girders are used as this type of bridges. They are suitable at places where uneven settlements of foundations are likely to take place.

• Continuous 

In continuous bridges spans are continuous over two or more supports. They are statically indeterminate structures. They are useful when uneven settlement of supports does not take place. In continuous bridges the bending moment anywhere in the span is considerably less than that in case of simply supported span. Such reduction of bending moment ultimately results in the economic section for the bridge.  In continuous bridges the stresses are reduced due to negative moments developed at pier or supports. Thus continuous span bridges have considerable saving compared to simply supported bridge construction.

 Following are the advantages of RCC continuous girder bridges over simply supported girder bridges.

v As the bearings are placed on the centerline of piers, the reactions at piers are transmitted centrally.

v  It is found that the continuous girder bridge suffers less vibration and deflection.

v  The continuous girder bridge requires only one bearing at each pier as against two bearing for simply supported girder bridge.

v   The depth of decking at mid span is reduced and it may prove to be useful for over bridges where headroom is of prime consideration.

v  The expansion joints required will be less.

v  There is reduction in cost as less quantity of concrete and steel are required..

 Following are the disadvantages of RCC continuous girder bridges over simply supported girder bridges.

v  The design is more complicated as it is a statically indeterminate structure.

v  The detailing and placing of reinforcements are to be carried out with extreme care.

v  The placing of concrete and removal of formwork are to be executed carefully in proper sequence.

• Cantilever

  A cantilever bridge is formed of cantilevers projecting from supporting piers. The ends of a cantilever bridge are treated as fixed. A cantilever bridge combines the advantages of a simply supported span and a continuous span. For long spans and deep valleys and at places where it will not be practicable to use centering, cantilever bridges are more suitable. They are suitable in case of uneven settlement of foundation. The construction of a cantilever bridge may either be of simple type or of balanced type.

     In case of cantilever bridge with balanced type of construction, hinges are provided at the points of contra-flexure of a continuous span and an intermediate simply supported span is suspended between two hinges.

According to the form or type of superstructure as arch, beam, truss, slab, rigid frame or suspension bridges.

 • Slab 

• Beam 

• Girder

• Truss

 The girder/beam as well as the truss can be made up of timber, steel or concrete, or can be made up of combination of steel and concrete.

• Arch 

• Suspension

Suspension Bridge Suspension bridges are made up of high tensile steel cables strung in form of catenaries to which the deck is attached by steel suspenders, which are mainly made up of steel rods/members/cables. The decking can be of timber, concrete or steel spanning across the stiffening girders transmitting load to the suspenders.

 • Cable stayed

 Cable-stayed bridges are similar to the suspension bridges excepting that there will be no suspenders in the cable-stayed bridges and a number of these can be of masonry, concrete or steel.

Various economical span ranges for these types generally adopted are:

Arch       : For small spans of 3 to 15m in

            

 masonry, steel arch up to 519m and concrete arches up to 305m spans.

 Slabs : Up to 9m.

Girders  and beams : 10 to 60m (exception up to 250m in continuous construction).

 Trusses : 30 to 375m simply supported and up to 550m with cantilevered combination.

 Suspension  Bridges : Over 500m up to 1400m.

Cable  Stayed : 300 to 600m.

  According to the materials of construction used for superstructure as cement concrete, prestressed concrete, steel, masonry, iron, timber or composite bridges.

 The earliest form of materials used for construction of bridges was first stone and later brick. The masonry bridges are used for short spans and according to availability of material and skilled labor. They are mainly of arch type of bridges. The next form of construction was Timber Bridge in which timber was used for spanning the gap and also for supporting the beams. Timber bridges are used for short spans, light loads and for use as temporary and unimportant bridges. With the invention and development of concrete, bridges are being built entirely with concrete, either reinforced or pre-stressed or a combination of both for superstructure. Many combination of above types are also possible. The common examples of composite construction are:

v  Concrete beams reinforced with steel bars.

v  Precast pre-stressed concrete girder with cast-in-situ RCC slab.

v  Rolled steel joists topped by a cast-in-situ RCC slab.

 Following are the advantages of composite bridges.

v  It leads to reduction in deflection and vibrations. o It leads to speed in construction.

v  It proves to be economical.

v  It results in better quality control.

v  The cost of formwork is reduced. o The cost of foundations for abutments is reduced.

v  The cost of transportation is minimized.

v  The overall depth of beam for a composite construction is reduced and it leads to savings in lengths of approaches.

• Movable – bascule bridges

 In bascule bridges, the main girders are lifted together with deck about the hinge provided on one end of the span. Depending upon the width of channel, the bascule bridge may either be single or double.

• Movable – swing bridges

 In swing bridges, the girders and deck can be swung about its middle over the middle pier, clearing the span on either side for passage ship.

• Movable – lift bridges 

In this case, gantries are provided at the piers at either end of the span and the entire girder and the floor system is lifted up by a hydraulic arrangement to the extent required for free passage of the ship.

• Transporter.

 In case of transporter bridges, a moving cage is suspended from an overhead truss with the help of cable or wire ropes. The overhead truss rests on two towers and it contains rails for cage to roll. This type of bridge is used within a harbor area to provide an arrangement for shifting of men and materials across a channel.

  According to the expected utility period of service as temporary, military or permanent bridges.

 • Temporary bridges

.

 The temporary bridges are defined as structures, which are constructed to cross a river or a stream in place of permanent works. Such bridges are useful when it is not possible to construct a permanent bridge due to shortage of money, time, good materials or skilled labour. The temporary bridges are often constructed on diversions during the reconstruction of an existing permanent bridge. The useful life of a temporary bridge is estimated as about 10 years.

• Military (pontoon, bailey)

 Pontoon bridges are constructed on roads on which traffic is minor and seasonal and the river itself is subjected to floods during only short periods, not exceeding three months of the year when the traffic on the load can safely be suspended floating type bridges, pontoons are provided. These are made up of floating cylinders or barges/flats which are kept afloat in a row and are connected to each other by hinged beams over which decking is provided. Such bridges can also be used for movement of army, which movement may be of short duration, across a waterway or on roads used in fair weather only. Bailey refers to bridge superstructures normally made up of assemblage units, which can be carried in units, assembled and launched, in a short duration over a gap. They may be provided for the short-term movement of an army or also be used in case of damage to a permanent bridge due to the approaches or even a part of the bridge having been damaged leaving a wide gap, or if the traffic has been suspended and permanent repairs to the bridge are likely to take a long time

• Permanent bridges

 The permanent bridges are defined as structures, which are constructed to cross a river or a stream permanently.

  According to the function as road, railway, road-cum-railway or pipeline bridges.   

 According to the method of connections adopted in steel bridges as riveted, welded or pin-connected bridges.

Majority of steel bridges are of riveted type, welded type is recently used.

• Pin-connected

• Riveted

 Riveted connections are proved to be more rigid and responsible for secondary stresses.

• Welded

According to the length of span as culvert, minor bridges, major bridges or long span bridges

• Culverts (less than 6m)

Culvert is a bridge having a gross length of six meters or less between the faces of abutments or extreme vent way boundaries and measured at right angles thereto.

v  Minor bridge (6 to 30m)

v  Major bridge (above 30m)

v  Long span bridge (above 120m) 

 According to the degree of redundancy as determinate or indeterminate bridges.

• Determinate 

• Indeterminate 

  According to the level of crossing of highways and railways as over bridges or under bridges.

• Over bridge

 When the highway bridge is carried over or under the highway by means of bridge, the bridge is known as over bridge.

• Under bridge

When the highway is carried under the railway track by means of bridge is known as the under bridge

  According to the road level relative to the highest flood level of the river below, particularly for a highway bridge

 • Causeway

 It may be found that in some cases, the high flood in the stream is occasional and the normal flow is very little or the bed is dry most of the time. In such cases, causeways are provided. Causeways are a paved road formed between retaining walls on either side, taken sufficiently deep to withstand scour action.

• High-level

 A high level bridge is a bridge, which carries the roadway above the highest flood level of the channel. There may be cases were ships may have to pass through them requiring very high clearance. They will then need very high approaches or highly graded approaches even if the maximum clearance need be provided in the middle portion only. Where the traffic on the bridge is such that suspension of traffic for a short duration on the road or railway will not materially affect the overall traffic, they are built with decks with minimum clearance above high flood level for normal passage of water.

• Submersible

  A submersible bridge is a bridge designed to be overtopped in floods. Submersible bridge is something between the normal causeways and the high-level bridges. This is provided in such a way that the bridge openings provided with properly built piers and slabs can pass ordinary floods and rarely expected high floods spill and pass over the deck. The normal convention in India is to provide submersible bridges on roadways with less traffic and where the stoppage of traffic due to the passage of high floods is not likely to exceed 3 days at a time and not more than 18 days during the course of the year.

Selection of type of bridge

 In the selection of the proper type of concrete bridge for any particular case, cost is usually the determining factor. Occasionally, however, the problem is complicated by special requirements, such as appearance, restricted headroom, difficult foundations, limited time of construction, or difficulties in formwork caused either by the required height of supports or by the fact that it is necessary to maintain traffic under the bridge during construction.

For bridges having one span, the following types of structures may be used:

v  Simply supported deck or through girders.  

v  Right angle rigid frames. 

v  Right angle frames with concealed cantilevers with or without counterweights. 

v  Simply supported girders with concealed cantilevers, with or without counterweights.  

v  Two short concealed spans, one at each side of the opening, each provided with a cantilever extending into opening and supporting a short center span.

v  The simply supported structure with deck or through girder is statically determinate and is simplest to design, but its cost is the highest.

v  When unyielding foundation is attainable, the right angle rigid frame and right angle frames with concealed cantilevers with or without counterweights provide the most economical solution. 

v  Girders with cantilevers with simply supported girders with concealed cantilevers, with or without counterweights and two short concealed spans, one at each side of the opening, each provided with a cantilever extending into opening and supporting a short center span should be considered for long spans where small depth of girders is desired.

For a bridge with several spans, the following arrangements should be considered:

v  A number of simply supported girder spans.    

v  A combination of girders provided with cantilevers and short spans supported by these cantilevers.   Continuous girders supported by independent piers. 

v  Multi-span rigid frames in which the girders forming the superstructure are rigidly connected with elastic vertical supports.

v  The number of simply supported girder spans and a combination of girders provided with cantilevers and short spans supported by these cantilevers are statically determinate arrangements.

v  Continuous girders supported by independent piers and multi-span rigid frames in which the girders forming the superstructure are rigidly connected with elastic vertical supports are statically indeterminate.

v  The cost of the structure is highest for simply supported girder spans and lowest for rigid frames in which the girders forming the superstructure are rigidly connected with elastic vertical supports.

v  The number of simply supported girder spans and a combination of girders provided with cantilevers and short spans supported by these cantilevers should be used only where reasonably unyielding foundation is not easily obtainable.

v  Of these two types, the design of number of simply supported girder spans is simpler but more costly than a combination of girders provided with cantilevers and short spans supported by these cantilevers.  Where unyielding foundations are available, the continuous girders or rigid frames with continuous girders supported by independent piers or multi- span rigid frames in which the girders forming the superstructure are rigidly connected with elastic vertical supports should be used.

v  Where heavy piers are required, as in river crossings, or where the structure is to be supported on already existing piers, continuous girders supported by independent piers is recommended.

v  A rigid connection between heavy piers and more flexible superstructure should never be attempted. The rigid frame with multi-span rigid frames in which the girders forming the superstructure are rigidly connected with elastic vertical supports is preferable where vertical supports of the bridge are elastic, as in viaducts.

v  In options except number of simply supported girder spans having advantage of providing the end spans with cantilevers should be studied particularly with the view of reducing the cost of abutments.