BRIDGES, BASIC CLASSIFICATION Types.pptx

Bridges are critical infrastructure elements that facilitate the movement of people and goods by spanning physical obstacles such as rivers, valleys, or roads. They come in various designs and constructions, each suited for specific functions, loads, and environmental conditions

1. BRIDGES SCHOOL OF CIVIL ENGINEERING Dr. SANJAY RAJ. A

2. Bridge - Definition What is a Bridge? A Structure built to span a Valley, Road, Body of water or other Physical obstacle for the Purpose of Passing over the Obstacle.

3. Historical Development of Bridges The simplest and earliest types of bridges were  stepping stones .  Neolithic  people also built a form of  boardwalk  across marshes; examples of such bridges include the  Sweet Track  and the  Post Track  in England, approximately 6000 years old.  Undoubtedly, ancient people would also have used  log bridges ; that is a  timber bridge  that fall naturally or are intentionally felled or placed across streams. Some of the first man-made bridges with significant span were probably intentionally felled trees

4. Historical Development of Bridges NATURAL FORMED ROCK BRIDGES

5. Historical Development of Bridges –

6. Historical Development of Bridges – The greatest bridge builders of antiquity were the  ancient Romans .  The Romans built arch bridges and  aqueducts  that could stand in conditions that would damage or destroy earlier designs. Some stand today.  An example is the  Alcántara Bridge , built over the river  Tagus , in  Spain . The Romans also used  cement , which reduced the variation of strength found in natural stone. One type of cement, called  pozzolana , consisted of water,  lime , sand, and  volcanic rock .  Brick  and  mortar  bridges were built after the Roman era, as the technology for cement was lost (then later rediscovered).

7. Historical Development of Bridges – With arches, everything is in compression all the time

8. Historical Development of Bridges –

9. Historical Development of Bridges – In India, the  Arthashastra  treatise by  Kautilya  mentions the construction of dams and bridges.  A  Mauryan  bridge near  Girnar  was surveyed by  James Princep . [10]  The bridge was swept away during a flood, and later repaired by Puspagupta, the chief architect of emperor  Chandragupta I  The use of stronger bridges using plaited bamboo and iron chain was visible in India by about the 4th century. A number of bridges, both for military and commercial purposes, were constructed by the  Mughal  administration in India.

10. Historical Development of Bridges – A major breakthrough in bridge technology came with the erection of the Iron Bridge in Coal brook dale, England in 1779. It used cast iron for the first time as arches to cross the river Severn. The Need to span increased distances continued to pressure bridge designers. With the New material of steel available, with its superior Performance in tension, New designs made bridges longer and stronger. Steel made the TRUSS BRIDGE possible.

11. Historical Development of Bridges – Once materials were available to Engineers which could be loaded in tension, a new type of arch bridge appeared:

12. Historical Development of Bridges – Suspension bridges are suspended from cables. The earliest suspension bridges were made of ropes or vines covered with pieces of bamboo. In modern bridges, the cables hang from towers that are attached to caissons or cofferdams. The caissons or cofferdams are implanted deep into the bed of the lake, river or sea. Sub-types include the  simple suspension bridge , the  stressed ribbon bridge , the  underspanned suspension bridge , the  suspended-deck suspension bridge , and the  self-anchored suspension bridge . There is also what is sometimes called a "semi-suspension" bridge, of which the  Ferry Bridge  in Burton-upon-Trent is the only one of its kind in Europe. The longest suspension bridge in the world is the 3,909 m (12,825 ft)  Akashi Kaikyō Bridge  in Japan.

13. Historical Development of Bridges – Cable-stayed bridges , like suspension bridges, are held up by cables. However, in a cable-stayed bridge, less cable is required and the towers holding the cables are proportionately higher. The first known cable-stayed bridge was designed in 1784 by C. T. (or C. J.) Löscher . The longest cable-stayed bridge since 2012 is the 1,104 m (3,622 ft)  Russky Bridge  in  Vladivostok ,  Russia . [31]

14. Historical Development of Bridges –

15. Historical Development of Bridges –

16. Historical Development of Bridges Famous Bridges around the World • Tower Bridge - London Famous Bridges around the World • Golden Gate Bridge – San Francisco Famous Bridges around the World • Harbour Bridge - Sydney Pamban Bridge  is a  railway bridge  which connects the town of Mandapam in mainland India with Pamban  Island , and Rameswaram. Opened on 24 February 1914, it was India's first sea bridge, and was the longest sea bridge in India until the  opening  of the  Bandra - Worli Sea Link  in 2010.

17. Historical Development of Bridges – The history of India can be traced through various monuments, artefacts and books. However, its architectural legacy can also be traced back to the bridges that were built to facilitate movement of goods and people across cities and, more often than not, borders. Standing the test of time, weather and natural calamities, some of them withered, while others are as sturdy as ever. Umshiang Double-Decker Root Bridge, Meghalaya

18. Bridge substructures : There are three main types Abutments Piers Wing walls

19. Bridge structures : The two basic parts are: Substructure - includes the piers, the abutments and the foundations. Superstructure - consists of the deck structure itself, which support the direct loads due to traffic and all the other permanent and variable leads to which the structure is subjected. The connection between the substructure and the superstructure is usually made through bearings. However, rigid connections between the piers (and sometimes the abutments) may be adopted, particularly in frame bridges with tall (flexible) piers.

20. Bridge structures : Wearing surface The wearing surface is that portion of deck, which resists traffic wear. In most instances this is a separate layer made of bituminous material. Deck The deck is the physical extension of the Roadway across the obstruction to be bridged. In most instances this is a Reinforced Concrete Slab. Primary members Primary members are those, which distribute bridge loads longitudinally. Primary members consist of beam, truss, arch or frame. Secondary members Secondary member are bracing between primary members help to distribute loads transversely.

21. Bridge Components :

22. Typical bridge components

23. Classification of Bridges Bridges are Classified - According to Material of Construction of superstructure • According to form or type of structure • According to function • According to inter span relations • According to span length Material of Construction - • Timber • Masonry • Steel • Reinforced Concrete • Pre-stressed Concrete • Composite Type of Structure - • Slab • Beam • Truss • Arch • Cable Stayed • Suspension

24. Classification of Bridges 3. Function • Aqueduct • Viaduct (road or railway over a valley) • Pedestrian • Highway • Railway 4. Inter-span Relations • Simple • Continuous • Cantilever 5. Span Length • Culvert ; L < 6m • Small Span Bridge ; 6m < L < 15m • Medium Span Bridge ; 15m < L < 50m • Large Span Bridge ; 50m < L < 150m • Extra Large Span Bridge ; L > 150m 6. Culverts

25. Classification of Bridges Beam bridges  – employ  the simplest of forms  – one or several horizontal beams that can either simply span the area between abutments or relieve some of the pressure on structural piers. The core force that impacts beam bridges is the transformation of vertical force into shear and flexural load that is transferred to the support structures (abutments or mid-bridge piers). Because of their simplicity, they were the  oldest bridges  known to man. Initially built by simply dropping wooden logs over short rivers or ditches, this type of bridge started being used extensively with the arrival of metal works, steel boxes, and pre-stressed construction concrete. Beam bridges today are separated into girder bridges, plate girder bridges, box girder bridges and simple beam bridges.

26. Classification of Bridges

27. Classification of Bridges- Typical beam bridge

28. Classification of Bridges- A SIMPLE SLAB BRIDGE and plate girder bridge

29. Classification of Bridges- BOX CULVERT BEAM BRIDGE

30. Classification of Bridges -Arch bridges Arch bridges  – use arch as a main structural component (arch is always located below the bridge, never above it). With the help of mid-span piers, they can be made with one or more arches, depending on what kind of load and stress forces they must endure. The core component of the bridge is its  abutments and pillars , which have to be built strong because they will carry the weight of the entire  bridge structure  and forces they convey.

31. Classification of Bridges- Arch bridge

32. Classification of Bridges- Truss bridges Truss bridges  – is a very popular bridge design that uses a diagonal mesh of most often triangle-shaped posts above the bridge to distribute forces across almost entire bridge structure. Individual elements of this structure (usually straight beams) can endure dynamic forces of tension and compression, but by distributing those loads across entire structure, entire bridge can handle much stronger forces and heavier loads than other types of bridges. The two most common  truss designs  are the king posts (two diagonal posts supported by single vertical post in the center ) and queen posts (two diagonal posts, two vertical posts and horizontal post that connect two vertical posts at the top).

33. Truss bridges

34. Classification of Bridges Cantilever bridges  – are somewhat similar in appearance to arch bridges, but they support their load, not through a vertical bracing but trough  diagonal bracing with horizontal beams  that are being supported only on one end. The vast majority of cantilever bridges use one pair of continuous spans that are placed between two piers, with beams meeting on the center over the obstacle that bridge spans (river, uneven terrain, or others). Cantilever bridge can also use mid-bridge pears are their foundation from which they span in both directions toward other piers and abutments.

35. Classification of Bridges

36. Classification of Bridges Suspension bridges  – utilize spreading  ropes or cables  from the  vertical suspenders  to hold the weight of bridge deck and traffic. Able to suspend decking over large spans, this type of bridge is today  very popular  all around the world. Originally made even in ancient times with materials such as ropes or vines, with decking’s of wood planks or bamboo, the modern variants use a wide array of materials such as steel wire that is either braided into rope or forged or cast into chain links. Because only abutments and piers (one or more) are fixed to the ground, the majority of the bridge structure can be very flexible and can often  dramatically respond to the forces of wind, earthquake  or even  vibration of on-foot or vehicle traffic .

37. Classification of Bridges

38. Classification of Bridges Cable-stayed bridges  – use deck cables that are directly connected to one or more vertical columns (called towers or pylons) that can be erected near abutments or in the middle of the span of the bridge structure. Cables are usually connected to columns in two ways – harp design (each cable is attached to the different point of the column, creating the harp-like “strings” and “fan” designs (all cables connect to one point at the top of the column). This is a very different type of cable-driven suspension than in suspension bridges, where decking is held with vertical suspenders that go up to main support cable.

39. Classification of Bridges Originally constructed and  popularized in the 16th century , today cable-stayed bridges are a popular design that is often used for spanning medium to long distances that are longer than those of cantilever bridges but shorter than the longest suspension bridges. The most common build materials are steel or concrete pylons, post-tensioned concrete box girders and steel rope. These bridges can support almost every type of decking (only not including heavy rail) and are used extensively all around the world in several construction variations.

40. Classification of Bridges

41. Classification of Bridges- composite bridge

42. Classification of Bridges- CULVERTS

43. Classification of Bridges Movable Bridge – A Movable Bridge is a bridge which allows passage for Ships and barrages

44. Classification of Bridges Based on Connections Based on Purpose of Bridge Welded Bridges Road Bridge Bolted Bridges Railway Bridge Riveted Bridges Pedestrian Bridge Pinned Bridges

45. Abutments : 46

46. Abutment refers the substructure at the ends of bridge span where on the structures, superstructure rests. single span bridges have abutments had the each ends which provide vertical and lateral support for the bridge, as well as acting as retaining walls to resist lateral moment of the earthen fill of the bridge approach. Use of Abutments in Engineering To transfer load from super structure to its foundation elements To resist or transfer self weight, lateral loads and wind loads To support one end of an approach slab

47. Gravity abutments: Cantilever abutments Full height abutments Counter fort abutment Types of Abutments

48. 49 Balancing Type Gravity Type Buried Type

49. Piers :

50. A pier is an upright support for a structure between the abutments The cross section of the pier may be square or rectangular A pier resist the slab weight ,moving vehicular weight and horizontal force of the abutments

51. TYPES OF PIER Piers are important portion of bridge substructure. piers are transferring the dead load of the superstructure and the loads supported by superstructure such as live load and stresses induced by the change of superstructure length due to temperature change, creep and shrinkage to bridge foundation. piers can have a different cross-sectional shape. the photo below showing some typical cross-sections for piers. Solid pier Trestle Type pier Hammer- head Type pier Cellular Type pier Framed Type pier

52. REQUIREMENTS of BRIDGE PIERS It should effectively transfer loads from Superstructure to  foundation  without failure. It should withstand all force actions The material for the piers should be easily available It should have pleasant appearance. Its design should be simple. The piers should be durable against weathering, impacts and corrosion. The cost of construction should be cheap. It should have minimum repair and maintenance cost It should have stability against the lateral and longitudinal force actions, viz. Seismic, Wind, Ice, Currents, Impacts.

53. TYPES OF PIER Solid Type Trestle Type Hammer Head Type Cellular Type Framed Type Types of Pier

54. TYPES OF PIER

55. The wing walls which is constructed adjacent to the abutment It acts as a retaining wall They are generally constructed of the same material as those of abutment Wing walls is provided at the both ends of abutment It resist the lateral moment of the earthen fill of the bridge. Wing walls : 56

56. Wing walls are classified based on the position of the banks and abutments 1) Straight wing walls : It is used for smaller bridges, on drains and railway bridges 2) Splayed wing wall: It is provided across the river the splay is usually 45 degree as it provide smooth entry and exit for the water 3) Return wing wall: When the banks are high and hard the returning wing walls are provided Classification of wing walls: 57

57. Loads and Forces : The Various Loads, Forces and Stresses to be Considered for Design of Bridges and Culverts are as Follows: Dead Load Live Load Snow Load Impact factor on Vehicular Live Load

58. Loads and Forces : Impact due to floating bodies or vessels as the case may be Vehicle Collision Load Wind Load Water current Longitudinal forces caused by tractive effort of vehicles or by braking of vehicles and/or those caused by restraint of movement of free bearings by friction or deformation.

59. Loads and Forces : Centrifugal force Buoyancy Earth Pressure including Live Load surcharge, if any Temperature Effects Deformation Effects ( for steel bridges only)

60. Loads and Forces : Secondary Effects Erection Effects Seismic force Wave pressure ( liquefaction) Grade Effect

61. Loads and Forces : Dead load The Dead Load carried by a girder or member shall consist of the portion of the weight of the superstructure (and the fixed loads carried there on) which is supported wholly or in part by the girder or member including its own weight. Unit weights of materials shall be used in determining loads, unless the unit weights have been determined by actual weighing of representative samples of the materials in question, in which case the actual weights as thus determined shall be used.

62. Loads and Forces : live load Live Load The Live Load on the bridge, is moving load on the bridge throughout its length. The moving loads are vehicles, Pedestrians etc. but it is difficult to select one vehicle or a group of vehicles to design a safe bridge. So, IRC recommended some imaginary vehicles as live loads which will give safe results against the any type of vehicle moving on the bridge. The vehicle loadings are categorized in to three types and they are IRC class AA loading IRC class A loading IRC class B loading

63. Loads and Forces : live load IRC Class AA Loading This type of loading is considered for the design of new bridge especially heavy loading bridges like bridges on highways, in cities, industrial areas etc. In class AA loading generally two types of vehicles considered, and they are Tracked Type Wheeled Type

64. Loads and Forces :

65. Loads and Forces :

66. Loads and Forces :

67. Combination of Live Load As per clause 204.3 shall be read in conjunction with Clause 104.3 of IRC:5. The carriageway live load combination shall be considered for the design as shown in Table 6

68. Combination of Live Load

69. Combination of Live Load

70. Combination of Live Load

71. Combination of Live Load

72. Combination of Live Load

73. Combination of Live Load

74. Combination of Live Load

75. Combination of load for LIMIT state design Loads to be considered while arriving at the appropriate combination for carrying out the necessary checks for the design of road bridges and culverts are as follows : Dead Load Snow load Superimposed dead load such as hand rail, crash barrier, foot path and service loads. Surfacing or wearing coat Back Fill Weight Earth Pressure

76. Combination of load for LIMIT state design Primary and secondary effect of Pre-stress Secondary effects such as creep, shrinkage and settlement. Temperature effects including restraint and bearing forces. Carriageway live load, footpath live load, construction live loads. Associated carriageway live load such as braking, tractive and centrifugal forces. Accidental forces such as vehicle collision load, barge impact due to floating bodies and accidental wheel load on mountable footway

77. Combination of load for LIMIT state design Wind Seismic Effect Construction dead loads such as weight of launching girder, truss or cantilever construction equipments Water Current Forces Wave Pressure Buoyancy ** LOAD COMBINATIONS AND PERMISSIBLE STRESSES ARE GIVEN IN CLAUSE 202.3 OF IRC 6-2016

78. Combination of load for LIMIT state design

79. SELECTION OF BRIDGE SITE The choice of the Right site is a crucial decision in the planning and designing of Bridge. The characteristics of ideal site for Bridge across a River are: A straight reach of the River. Steady river Flow without serious whirls and cross currents A narrow channel with firm Banks Suitable High Banks above High Flood Level on each side Rock or other hard in erodible strata close to the river bed level

80. SELECTION OF BRIDGE SITE Economical approaches which should not be very high or long or liable to flank attacks to the river during floods: the approaches should be free from obstacles such as hills, frequent drainage crossings, sacred places, graveyards, or built up areas or troublesome land acquisition Proximity to a direct alignment of the Road to be connected Absence of sharp curves in the approaches Absences of expensive river training works Avoidance of excessive underwater construction

81. Preliminary data to be collected The Engineer in charge of the investigation should collect the following :information: Name of the stream, road or sea route and the identification mark allotted to the crossing and location in km to centre of crossing. Location of the nearest G.T.S (Great Trigonometric Survey) bench mark with its reduced level. Present and anticipated future volume and nature of traffic on the road at the bridge site. Hydraulic data pertaining to the sea, including the highest tide level and lowest tide level. The level of the high tides and low tides should also be investigated and the suitable height of the bridge above thus established

82. Preliminary data to be collected Soil profile along the probable bridge sites over the length of the bridge and approaches. Navigational requirements of the site. Liability of the site to earthquake disturbances. Availability, quality and location of the nearest quarries for stones for masonry and concrete aggregates. Means of transport for materials. Availability of both skilled and unskilled labour. Facilities for housing labour during construction

83. Preliminary data to be collected Availability of electric power. Details of any utilities and services to be provided for example telephone cables, water supply pipes.

84. ECONOMICAL SPAN What does span have to do with bridges? A span is  the distance between two bridge supports , whether they are columns, towers or the wall of a canyon. A modern beam bridge, for instance, is likely to span a distance of up to 200 feet (60 meters), while a modern arch can safely span up to 800 or 1,000 feet (240 to 300 m). The span of the bridge for which the total cost of the bridge is minimum is called economic span. ... In other words, Economical span is defined as that  span for which the total cost of the substructure is equal to the total cost of superstructure . Economies of span refer to  the efficient coordination or sequenced utilization of assets and through the decreased transaction costs between the stages of production , the unit cost declines. Control and logistical systems are examples.

85. Important definitions Design discharge Q :The estimated discharge for the design of the bridge and its appurtenances. Afflux(h) : The rise in water level upstream of bridge as a result of obstruction to the natural flow caused by the construction of the bridge & its approaches. Free board (F): The vertical distance between the water level corresponding to design discharge (Q) including afflux(h) and the formation level of its approach bank / top level of guide bank

86. Important definitions 4. Clearance ( C ) : The vertical distance between the water level corresponding to design discharge Q including afflux and the point on the bridge super structure where the clearance is required to be measured. 5. Depth of Scour : The depth of eroded bed of river measured from the water level for the discharge considered. 6. Highest flood level (HFL): Highest water level known to have occurred. 7. Low Water level(LWL) : water level generally obtained during dry weather

87. Relevance of Design discharge It is required for proper and economical design, construction and maintenance of Bridge water way Foundations Protection works Fixing / deciding Other parameters Afflux Free board & Vertical clearances

88. Method of Estimation of Design discharge The Maximum Discharge Which a Bridge across a Natural Stream is to be Designed to pass can be Estimated by the Following methods By Using one of the empirical formula applicable for the region By Using rational method involving the Rainfall Data and Other characteristics for the area By the area velocity method, Using the hydraulic characteristics of the stream such as cross sectional area and slope of the stream. By Unit Hydrograph Method From any other available records of Flood Discharge observed along the bridge site.

89. Empirical Formula Empirical Formula for Flood discharge from a catchment area have been proposed of the Form: Q= CA ^n = CA^(2/3) Where Q= Maximum Flood Discharge in m3/Sec A= Catchment Area C= Constant depending upon the catchment and location n= constant The Value of C is 6.8 for Flat tracts within 25km of the coast and 8.5 for the between 25 and 160km of the coast and 10 near hills

90. Rational method A rational method for flood discharge should be taken into account the intensity, distribution, duration of the rainfall as well the slope of the terrain, permeability and initial wetness of the catchment (drainage basin) Many complicated formula are available. Atypical rational Formula is Q= Al o α Where Q= Maximum Flood Discharge in m3/Sec A= Catchment Area L o= Peak Intensity of rainfall in mm/ Hr α =a function depending on the characteristics of the catchment area = 056pf/[tc+1]

91. Rational method tc = Concentration time in Hrs= [0.87* L 3 /H] 0.385 L=Distance From the critical point to the bridge site H= Difference in Elevation between the critical point and the Bridge site m P= Coeff of Runoff for the catchment areas as shown in Table 1 f= factor of correct for the Variation of Intensity of Rainfall over the catchment area.

92. Values of p S. No. Description of the Catchment Value of P 1. Sandy Soil/Sandy loam/Arid areas 0.249 2. Alluvium/silt loam/coastal areas 0.332 3. Red soil/clayey loam/cultivated plains/tall crops/wooded areas 0.415 4. Black cotton clayey soil/lightly covered/plain & barren 0.456 5. Hilly soil/plateau and barren 0.498

93. Values of factor f in rational formula Area in Km2 Value of f Area in Km2 Value of f 1.0 80 0.760 10 0.95 90 0.745 20 0.90 100 0.730 30 0.875 150 0.675 40 0.845 200 0.645 50 0.820 300 0.625 60 0.800 400 0.620 70 0.775 2000 0600

94. Area velocity method The area velocity method based on the hydraulic characteristics of the stream is the probably the most reliable among the methods for determining the Flood Discharge The Discharge Q= A.V Where Q= Maximum Flood Discharge in m3/Sec A= wetted area in m 3 V= Velocity of Flow in m/sec = 1/n.R^0.67 * S^ 0.5 n = Roughness coeff R= Hydraulic mean depth

95. unit hydrograph method The design discharge for desired recurrence interval is computed using above unit hydrograph developed and applying appropriate design storm Basic principles of UH Storms of equal duration will produce runoff hydrographs with equivalent time bases regardless the intensity of rain. Inst. discharge will be proportional to volume of surface runoff produced by storms of equal duration. Time distribution of runoff from a given storm period is independent of precipitation from antecedent or subsequent storm periods

96. Examples   Determine the Design Discharge at a Bridge by ( i ) Empirical Formula (ii) Rational Method. Using the Following Data Catchment Area = 170 Km 2 , Distance from Coast= 12 Km, Distance from Critical Point to Bridge Site= 16Km, Difference in Elevation Between the critical point and Bridge Site= 96m, Peak intensity of Rainfall= 60mm/H, Surface of Catchment area is Loam P= 0.3, C/s area at Stream MFL= 120m 2 Perimeter of Stream= 1/500. Assume f= 0.669, and n= 0.3

97. Solution Step 1: By Empirical Method Q= CA n C = 6.8 ; Q= 6.78* (170) 2/3 = 209 m 3 /s   Step 2: By Rational method Given Data : A= 170km 2 ; lo= 60mm/H; P=0.3: f = 0.669 L= 16km T c = [0.87* L 3 /H] 0.385 = 0.87*16*0/96 ^ (0.385) = 4.04 hours α= 0.56pf/[tc+1] = 0.56*0.3*0.669/(4.04+ 1)= 0.0223 Q= 170*60*0.0223= 227m 3 /s.

98. SOLUTION Step 3: Area Velocity method The Discharge Q= A.V Where Q= Maximum Flood Discharge in m3/Sec A = C/s area of stream wetted area in m 3 = 120 S = 1/500, V= Velocity of Flow in m/sec = 1/n.R^0.67 * S^ 0.5 n = Roughness Co- eff =0.3 R = Hydraulic mean depth= Wetted Area in Km 2 / Perimeter of Wetted area = 120/90= 1.33 V = 1/0.3* 1.33 0.67 *(1/500) 0.5 = 1.80m/s Q = A.V = 120*1.80= 216m 3 /s Adopt maximum value

99. Types of Rivers Aggrading : Rivers in this reach are prone to raise their beds by sediment deposition, due to reduction in velocity. Degrading : lowering of bed by erosion due to higher velocity Stable : No perceptible rise of lowering of river bed occurring over long periods Virgin : They have no outfall in the sea nor do they join any other stream. Such rivers after traversing some distance loose all their water by percolation & evaporation.

100. Linear Water ways In the case of a river which flows between stable high banks and which has the whole of the bank-to-bank width functioning actively in a flood of magnitude Q The waterway provided shall be practically equal to the width of water spread between the stable banks for such discharge . If, however, a river spills over its banks and the depth of spill is appreciable the waterway shall be suitably increased beyond the bank-to-bank width in order to carry the spill discharge as well.

101. Linear Water ways In the case of a river having a comparatively wide and shallow section, with the active channel in flood confined only to a portion of the full width from bank to bank, constriction of the natural waterway would normally be desirable from both hydraulic and cost considerations . A thorough study of both these factors shall be made before determining the waterway for such a bridge

102. Linear Water ways For River with Alluvial beds and sustained floods the waterway shall normally be equal to width given by Lacey’s formula : Pw = 1.811 C √Q  4.83 (Q) 0.5 Pw = Wetted Perimeter in metres which can be taken as the effective width of waterway in case of large streams Q = design discharge in cum/sec C = a coefficient normally equal to 2.67 , but which may vary from 2.5 to 3.5 according to local conditions depending upon bed slope and bed material.

103. Linear Water ways If the river is of a flashy nature i.e. the rise and fall of flood is sudden or the bed material is not alluvial and does not submit readily to the scouring effect of the flood , Lacey’s regime width formula as given will not apply In case of rivers in sub- montane stage, where the bed slopes are steep and the bed material may range from heavy boulders to gravel, it is not possible to lay down rigid rules regarding constriction of water way. Any constriction in such cases shall be governed largely by The configuration of active channels

104. Linear Water ways The Cost involved in diversion & training of these channels The cost of guide bunds which will need much heavier protection than the guide bunds of alluvial rivers . Each case shall be examined on merits from both hydraulic & economic consideration and best possible

105. AFFLUX AFFLUX- is the heading up of water over the flood level caused by construction of Water way at bridge site. The rise in water level upstream of bridge as a result of obstruction to the natural flow caused by the construction of the bridge & its approaches For streams with non-erodible beds, the afflux may be worked out by Moles worth formula given below : h = {V 2 /17.88 + 0.01524} x {(A/a) 2 - 1}

106. AFFLUX Where , h = afflux in metres V = Velocity in un-obstructed stream in m/sec A = Un obstructed sectional area of the river in m 2 a= Sectional area of the river at obstruction in m 2 In case of rivers with erodible beds, full afflux as calculated by the formula may not occur

107. FIG- AFFLUX At a Bridge

108. EXAMPLE Determine the Span length for a water way of a Bridge across a stream with a Flood discharge of 230m 3 /s and Velocity of flow= 1.5m/s, width of Flow at high Flood level is 55m, assume allowable velocity of Flow under the Bridge is 1.8m/c. Flow of Water Natural Velocity A= Q/V= 230/1.5 = 153.33 m 2 . Mean Depth of Flow= 153.33/ 55 = 2.78m Allowable Velocity = 2.78*0.9= 2.5 m/s , A= 143.125m 2 Normal Velocity of Flow= V= 150/133.125= 1.13m/s, Area of Artificial Waterway , a=230/2.5= 92 m 2 . Using Moles Worth Formula X = [[V 2 /17.9] +0.015]*(A 2 /a 2 -1) =[1.5 2 /[17.9+0.015]*[153.33 2 /92 2 -1]= 0.25 Span Length = L= [a/( d+X ) = 92/[2.78+0.25] = 30.36m

109. 111 EXAMPLE A two Span Plate girder bridge is to be Provided across a river having the Following Data: Flood Discharge 150m 3 /s : Bed Width= 52m, Side Slope 1:1, Bed Level=50m and HFL = 52.5m. Maximum Allowable Afflux = 20cm, Calculate the Span of the Bridge. Solution : Assuming Free Flow of 0.5m Area of Flow of Water A = Bed Width [HFL-B.L] = 30*[52.5-50]+2[0.5*2.5*2.5]= 81.25 m 2 Normal Velocity of Flow= V= 150/81.25= 1.85m/s

110. 112 EXAMPLE Using Moles worth Formula: Given X= 20cm X= [V 2 /17.9 +0.015]*(A 2 /a 2 -1) =20= [ 1.85 2 /17.9]+0.015 ]*[81.25 2 /a 2 -1]= a= 8.2 Span Length = L= [a/ ( d+X )= 8.2/[2.5+0.20]= 3.03m .

111. 113 THANK YOU