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SECTION-B

UNIT-ISTANDARD

SPECIFICATION FOR ROAD

BRIDGE

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• IRC bridge code• Determination of dead loads and live

loads• Wind loads• Longitudinal forces• Centrifugal forces• Horizontal forces due to water current and

buoyancy effect• Earth pressure• Temp. effect

contents

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• Deformation stresses• secondary stresses• Erection stresses• Seismic forces

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IRC BRIDGE CODE:

The Indian roads congress has formulated standard specification and codes of practice for road bridges with a view to establish a common procedure for the design and construction of road bridges in India. The specification are collectively known as the bridge code.

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Determination of loads

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DEAD LOAD: 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.

It is carried by bridge member consists of its own weight.

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There are 4 types of standard loadings for which road bridges are designed:

1. IRC class AA loading: It consists of either a tracked vehicle 700 kN or a wheeled vehicle of 400 kN. E.g.: combat tank used by army .

2. IRC class 70R loading: It consists of tracked vehicles of 700kN or a wheeled vehicle of total load of 1000kN . The tracked vehicle is similar to that of class AA.

3. IRC class A loading: It consists of a wheel load train composed of a driving vehicle and trailers of specified axle spacing and loads. This loading adopted for permanent bridges and culverts.

4. IRC class B loading: This loading is similar to class B loading but adopted for temporary structures.

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WIND LOADS:The horizontal load used in the design of a

structure to account for the effects of wind.

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Longitudinal forces: longitudinal forces are caused in road bridges due to:•Tractive effort caused through acceleration of the driving wheels•Braking effect due to application of brakes to the wheels

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Centrifugal forces: where a road bridge is located on

a curve , the effects of centrifugal forces due to movement of vehicles should be taken into account .Centrifugal forces is given by eq. C= (wv^2)/12.95R

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HORIZONTAL FORCES DUE TO WATER CURRENTS:

Any part of a bridge structure which may be submerged in running water should be designed to sustain safely the horizontal pressure due to the force of the current.

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•Buoyancy effect:

upward forces exerted by fluids on submerged objects.Buoyancy=density of fluid *vol. of object * gravity

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Lets us take an example of buoyancy effect

This beach ball floats.Why?

Because the Buoyant Force is greaterthan the weight of the ball.

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Earth pressure:

The pressure exerted on a structure, such as a wall, by the earth which it retains.There are 3 states of lateral earth pressure:I. At rest(Ko)II. Active earth

pressure(wall move away from soil)=Ka

III. Passive earth pressure(wall move into soil)=kp

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Temperature effect: When steel structure is not free to expand or contrast under variation of temperature, the stresses due a variation of 30 C. From local main must be considered. The coefficient of expansion for steel and concrete is 0.00001. If we consider unequal variation of temperature, in some structures which are not affected by equal changes, we allowed only for 15 C.

In two hinged arches and suspension bridges the equal change of temperature has an effect on the internal forces. In continuous bridges the equal change of temperature has no effect because the girders are free to expand, but the unequal change has an effect.

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DEFORMATION STRESSES: Deformation stresses are considered for steel bridges only. It is defined as the bending stress in any member of an open –web girder caused by the vertical deflection of the girder combined with the rigidity of the joints.

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Stress stain curve

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Secondary stresses:

In steel structure, secondary forces are those which caused due to eccentricity of connections, floor beam loads, cross girder, lateral wind loads, and the movement of support.

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Erection stresses:

The stresses that are likely to be induced in member during erection should be considered in design.

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Seismic forces:•Seismic force is generated due to earthquake.•It is a natural disaster.•It is a violent shaking of earth.

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Effect of seismic forces:

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Unit-II

culverts

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Introduction:

A culvert is a closed conduit used to convey water from one area to another, usually from one side of a road to the other side. 

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Classification of culverts:

RCC slab culvertPipe culvertBox culvertStone arch culvertSteel girder culvert for railways.

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RCC slab culvert:

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Pipe culvert:

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Box culvert:

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Stone arch culvert:

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Steel girder culvert for railways:

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Design of RCC slab culvert:

EXAMPLE FOR R.C. SLAB CULVERT:• GIVEN DATA CULVERT TO BE STATE HIGHWAY. WIDTH OF BRIDGE = (GIVEN) NO FOOTPATH PROVIDED CONDITION OF EXPOSURE: ’MODERATE’ MATERIALS: CONCRETE GRADE M25

STEEL-DEFORMED BARS TO IS : 1786 (GRADE fe415) CLEAR SPAN =GIVEN HT. OF VENT = GIVEN DEPTH OF FOUNDATION = GIVEN WEARING COURSE = GIVEN

STEP-1 : DESIGN OF DECK SLABI. PRELIMINARY DIMENSIONSII. DEAD LOAD BENDING MOMEMNT PER METRE WIDTH OF SLABIII. LIVE LOAD BENDING MOMENT

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STEP-2 : DESIGN BENDING MOMENT STEP-3 : STRUCTURAL DESIGN OF DECK SLABSTEP-4 : CHECK FOR SHEAR STEP-5: DESIGN OF KERB

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DESIGN OF BOX CULVERT:GIVEN DATA:Clear span =given Concrete grade M25 = 25 MpaClear height = given Steel grade Fe 415 = 415 MpaTop slab thickness =given БSc (Concrete)= 8.33 MpaBottom slab thickness= given БSt (Steel) =200 MpaSide wall thickness=given Modular ratio=10Unit weight of concrete =24 kN/m3 n (for depth of neutral axis) =givenUnit weight of earth= 18 kN/m3 j (for effective depth)=givenUnit weight of water =10 kN/m3 k (for moment of resistance) =1.105 MpaCo-efficient of earth pressure at rest =0.5.

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Total cushion on top 0.0m.Thickness of wearing coat=given

load calculation1. Top slaba. Dead load b. Live load

2. Bottom slab• Dead load • Live load• Total load(DL+LL)

3.Side wall4. Base pressure• Dead load• Live load

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Moment calculation•top slab•Bottom slab•Side wall

Distribution factor:Moment distribution:Braking force:Design of section:• design moment•Top slab•Bottom slab•Side walls

Check for shear: