Bridge Hydrology
Short Description
Bridge Hydrology...
Description
Bridge Hydrology Unit-VI Prof Nitesh P. Tantarpale Assistant Professor PRMCEAM, BADNERA
Contents
Estimation of flood discharge,
water way, scour depth, depth of foundation, Afflux, clearance and free board.
Loads, forces, stresses acting on bridges.
IRC Specification and code practices
Critical combination.
Rating and Maintenance – Methods and techniques of rating of existing bridges, repairs, maintenance
Corrosion – causes and prevention
Strengthning of bridge superstructure.
2 PROF NITESH TANTARPALE, PRMCEAM BADNERA
Estimation of flood discharge
One of the essential data for the bridge design is fair assessment of the
maximum flow which could be expected to occur at the bridge site during the design period of the bridge.
Following are the methods for determining Design
Discharge
By an empirical formula method.
By a rational method.
By the area velocity method.
By unit hydrograph method.
PROF NITESH TANTARPALE, PRMCEAM BADNERA
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Empirical Method
This is an indirect method of determining the maximum flood discharge, in this method maximum flood discharge is determined by an empirical formula in which the area catchment or basin is mainly considered.
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Rational Method
Indirect method: This method is applicable for determination of flood discharge for small culverts only.
The runoff, Q = 0.028 P.F.A.Ic Q = Discharge or runoff in m3/sec F = Co-efficient A = Catchment area in hectares Ic = Critical intensity of rainfall in cm/hour P = % coefficient of run-off
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Waterway
The area through which water flows under a bridge structure is known as waterway.
While fixing the waterway of a bridge, the following guiding principles must be kept in mind to ensure safety of the bridge structures:
The increased velocity due to obstructed waterway should not exceed the permissible velocity under the bridge.
The free board for high level bridges should not be less than 600 mm.
Sufficient clearance should be allowed according to the navigation requirements.
If ‘Q’ is maximum flood discharge (design discharge) and 'V' is the permissible velocity of under the bridge, then
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The maximum permissible velocity of flow (V) depends upon the nature of the river bed as in Table
The velocity of flow of stream or river water should not be more than the values mentioned in this table.
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Afflux
The phenomenon of heading up of water on the upstream site of the bridge is called afflux
When a bridge is constructed, its components like abutments and piers, cause the reduction of the natural waterway.
Due to this reduction in natural waterway, the velocity under bridge increases so as to carry the maximum flood discharge.
This increased velocity gives to a sudden heading up of water on the upstream side of the stream or river. The phenomenon of this heading up water is known as afflux.
Thus, greater the afflux greater will be velocity under down stream side of the bridge and greater will be the depth of scour consequently greater will be the depth of foundation required.
Hence, determination afflux is necessary for the safe design of the bridge.
PROF NITESH TANTARPALE, PRMCEAM BADNERA
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Determination of Afflux
Afflux is determined by using any one of the following two equations:
Marriman's equations.
Molesworth's equations
Marriman's equation: This equation is generally used for determining the values of afflux. According to this equations
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PROF NITESH TANTARPALE, PRMCEAM BADNERA
Molesworth's equations: According to this equation
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CLEARANCES
To avoid any possibility of traffic striking any structural part clearance are specified.
The horizontal clearance should be the clear width and the vertical clearance the clear height, available for the passage of vehicular traffic as shown in the clearance diagram
For a bridge constructed on a horizontal curve with superelevated road surface, the horizontal clearance should be increased on the side of the inner kerb by an
amount equal to 5 m multiplied by the superelevation.
The minimum vertical clearance should be measured from the superelevated level of the roadway.
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FREEBOARD
Free board is the vertical distance between the designed high flood level, allowing for afflux, if any, and the level of the crown of the bridge at its lowest point.
It is essential to provide the free board in all types of bridges for the following reasons:
Free board is required to allow floating debris, fallen tree trunks and approach
waves to pass under the bridge.
Free board is also required to allow for the afflux during the maximum flood discharge due to contraction of waterway.
Free board is required to allow the vessels to cross the bridge in case of navigable rivers. The value of the free-board depends upon the type of the bridge. 17
PROF NITESH TANTARPALE, PRMCEAM BADNERA
S. No.
Type of bridge
Free board
1
High level bridges
600 mm
2
Arch bridges
300 mm
3
Girder bridges
600 to 900 mm
4
Navigational streams
2400 to 3000 mm
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Determination of Length of Bridge:
After determining waterway and economic span the length of bridge can be determined by following relation L = Nl + (N - 1) b
where
L = Length of the bridge
N = Number of economic span
l = Length of each economic span
b = Thickness of each pier
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Scour Depth
The process of cutting or deeping of river bed due to action of water is called scouring.
When the velocity of stream water exceeds the limiting velocity it causes vertical cutting of the river bed, which is known as scouring.
It differs from erosions which causes horizontal widening of the river bed.
Determination of Normal Scour Depth:
The normal scour depth is the depth of water in the middle of stream when it is carrying the maximum flood discharge.
Scour depth of alluvial streams:
Case -1: When linear waterway of the bridge is equal to the regime width: In this case, the normal scour depth is equal to the regime depth given by the following Lacey regime equation.
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Case -2: When linear waterway of the bridge is less than regime width
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Determination of Maximum Scour Depth :
Maximum scour depth is the depth of water at the round obstruction to the flow of water when the river carries maximum flood discharge.
It usually occurs at bends, pier noses and on the under stream noses of guide banks provided for a bridge.
Therefore, for the safety of the bridge foundations it becomes essential to estimate the maximum scour depth correctly and design the bridge foundations accordingly.
As per recommendations, the maximum depth of scour may be taken as follows:
In case of a bridge on a straight reach of the stream having single span, the maximum depth of scour should be taken as 1.5 times the normal scour depth of water.
For bridge sites on curves or where cross current exists or when the bridge is a multi-span structure, the maximum depth of scour should be taken as 2 times the normal depth of scour.
In case of bridge causing construction, the maximum scour depth should not be less than the value obtained by the following equations 22
PROF NITESH TANTARPALE, PRMCEAM BADNERA
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Prevention of Scouring
The site of the bridge should have stream line flow.
At the site of bridge, the river bed soil should be such as to resist the maximum velocity of water.
Sufficient waterway should be provided under the bridge so that velocity of water may not exceed the limit after which scouring occurs.
The shape of the piers should be designed in such a way that it may not cause eddies and currents in water.
The river bed on upstream side, downstream side and the portion under the bridge should be properly pitched with beams and long stones.
In the case of sandy beds, sheet piling may be done on under stream and downstream sides of the bridge to prevent scouring.
Piles may also be driven in river bed, where scouring is likely to occur.
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Types of Loading in Road Bridges
For bridges and culverts, the following loads, forces and stresses should be considered where applicable. The loads and forces to be considered in designing road bridge and culverts are listed below: 1.
Dead loads
2.
Live loads
3.
Impact effect of live loads
4.
Wind loads
5.
Lateral loads
6.
Longitudinal forces
7.
Centrifugal forces due to curvature
8.
Earthquake forces Additional loads for substructure design:
9.
Forces due to water structures/currents
10.
Earth pressure
11.
Buoyancy
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In addition to the stress caused by the above loads and forces the following stresses should be taken into account:
Temperature stresses
Deformation stresses
Secondary stresses
Erection stresses
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Dead loads: The dead load carried by a bridge member consists of its own weight and the portions of the weight of the superstructure and any fixed loads supported by the member.
Live loads: Live loads are those caused by vehicles which pass over the bridge and are transient in nature. These loads cannot be estimated precisely, and the designer has very little control over them once the bridge is opened to traffic. Classifications of load are:
IRC class AA loading,
IRC class A loading,
IRC class B loading.
Impact effect of live load: The impact is caused due to fact that moving heavy vehicles over rough or uneven surfaces. The provision for impact effect should be made by an increment of live load. The magnitude of the impact depends not only on the span but also on the type of smoothness of the road surface, the speed of the vehicle and the type of its tyres.
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Wind load: Bridge structures are designed for the lateral wind forces, forces should be considered to act horizontally and in such a direction that the resultant stresses in the member under consideration are the maximum. The wind force on a structure should be assumed as a horizontal force of the intensity specified below and acting on an area calculated.
Lateral loading/loads:
(a) Force on railings and parapets : the railings and parapet should be designed to resist a lateral force and vertical force each of 150 kg/m applied simultaneously at the top of the railing or parapet.
(b) Force on kerbs: Kerbs should be designed for lateral loading of 750 kg/m run of kerb applied horizontally at top of the kerb.
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Longitudinal forces: In all road bridges, provision should be made for Iongidudinal forces arising from any one or more of the following causes:
Tractive efforts caused through acceleration of the driving wheels.
Braking effects resulting from the application of the brakes to braked wheels. Braking force is invariably greater than tractive efforts.
Frictional resistance offered to the movement of free bearing due to change in temperature or any other cause.
Centrifugal force: When a road bridge is situated on a curve, all portions of the structure affected by centrifugal action of moving vehicles are designed to carry safely the stress induced by this action in addition to all other stresses to which they may be subjected to:
The centrifugal force should be determined from the following formula:
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Seismic force: If a bridge is situated in a region subjected to earthquakes allowance should be made in the design for the seismic force.
As per IS 1893-1970 the seismic force to be used in the design of a structure is dependent on may variable factors and therefore it is extremely difficult to determine its correct value.
To give broad indications of reasonable values of seismic coefficient for different regions of Indian Standards (IS) has divided the country into five zones designated as zones I to V.
Force due to water currents: Any part of a bridge which may be submerged in running water should be designed to sustain safely the horizontal pressure due to the force of the current.
In case of piers parallel to the direction of water current, the intensity of pressure should be calculated from the following formula
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Earth pressure: I.R.C. recommends coulomb's theory of earth pressure with the modification that the height of the centre of pressure above bottom as 0.42 of the height of the height of wall above the base instead of 0.33 of that height.
Temperature stresses: All structures tend to change in length with variations in temperature. Temperature stresses are likely to develop if this change in length is fully or partly restrained by fixing the ends.
IRC has recommended the following range of temperature in the design of bridge structures.
(a) Steel structures: Moderate climate from minus 18°C to 50°C.
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Maintenance of Bridges
The maintenance details vary with the materials of construction.
Steel must be painted at regular intervals.
R.C.C. works must be inspected for the cracks and if any cracks are found, they should be sealed as soon as possible.
Masonry works must be kept well-plastered or pointed.
The regular inspection of bridges is a matter of great importance, since the early detection of trouble and the prompt attention may well prevent costly repairs which may be needed, if defects are allowed to develop too far.
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The matters required regular attention are as follows :
The proper functioning of weep holes and other drainage devices.
The free action of expansion joints and drainage.
Examination of bridge superstructures and sub-structures.
Clearing of obstructions in channels tending to cause scour.
Detection and tracing of water leakage through decks.
Maintenance of water-proofing coats.
Signs of movement of foundations, especially on clay, as evidenced by cracks in the structure or the road surface over it.
The careful examination of steel structures for corrosion, especially in parts where moist or polluted air may be partially trapped.
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Strengthening of Bridges
In the past, live load carried by a bridge was very light as compared to the dead load. There has been a tremendous increase in the carrying capacities of our transport vehicles. The road system however, has not sufficiently developed to cater for such increase in payloads. There is a terrible need for improvement of the old and out of service bridges on our roads.
Strengthening of Bridge Substructure: The substructure is strengthened in following different ways:
Masonry substructure: In case of old masonry substructure showing signs of disintegrations, all the loose material all around the structure is removed to find out the defect. If it is found that masonry has large cavities, then they should be filled with cement concrete. Later a wire netting should be stretched around the entire masonry and fastened there to with spikes. Finally a coat of cement mortar is forced by a gun.
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Strengthening of bridge pier:
In order to strengthen an old pier a cofferdam is constructed around the piers. A thick concrete casing is provided all around the pier after pumping out the water.
In case of foundation showing unequal settlements it is necessary to underpin the base of pier in deep and running water. This is done by sinking a pneumatic caisson near the pier.
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Strengthening of Bridge Superstructure
Method of strengthening a bridge superstructure depends upon the type of the bridge.
The following methods for different types of bridge are in vogue:
To fill large voids in honey combed concrete by cement grouting so that quality of concrete is improved and cover to reinforcement is obtained.
Sealing of cracks and voids by epoxy grouting.
To impact extra shear strength to girders when shear cracks appear on the girder, by providing shear plates.
Providing I-beams on sides.
Grouting: High pressure grouts are not useful for strengthening of R.C.C. bridges. Therefore grouting should be done by hand operated pump with low pressure operations for cracks more than 0.25 mm in any bad concrete, we may employ solid suspension grout such as cement water or cement sand water with water cement ratio 0.47 to 0.52. For finer cracks, we may employ chemical grout as epoxy grout.
For thin cracks and pressure, epoxy should be used under pressure. 36
PROF NITESH TANTARPALE, PRMCEAM BADNERA
I-Beams: R.C.C. beam and slab bridge - In this type, the beams are strengthened by providing steel I-beam on each side of the existing beams. In case of longer spans the load from the existing beams are directly taken away by steel cross girders supported on steel built-up girders.
If an existing bridge in sound condition is proposed to be widened by adding additional beams. It is preferable to do the entire widening on one side, from considerations of lateral stability provided the geometries of the road alignments permits it.
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Masonry Arch Bridges: They are usually strengthened by first removing the filling above the arch and then casting R.C.C. arch slab on the top of the roughened extrudes, the slab is securely keyed into the abutments.
When the arch is too weak to hear loud more than its own weight, then a new R.C.C. arch must be built several centimeters above existing extrudes with the help of removable shuttering.
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(d) Continuous Bridge: They are strengthened by methods similar to that of single span care should be taken that when a span is being strengthened the adjacent span is not weakened.
(e) Steel Bridge: They are strengthened by providing extra steel plates or angles or concrete encasements. (f) Suspension Bridge: They are usually strengthened by providing additional
cables with fasteners.
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