Composite Bridge

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Composite Bridge

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Composite Bridge:

Composite Bridge Prepared By : Jay Bhavsar Assistant Professor CSPIT, CHARUSAT, CHANGA

Steel Composite Bridge:

Steel Composite Bridge A composite bridge is one whose decking system consists of a concrete slab and which in conjunction with steel girders resists moving loads on bridge. This type of bridge found to be economical for spans of 10 to 20m. 2

Advantages:

Advantages For a given span and loading system a smaller depth of beam can be used than for a concrete beam solution, which leads to economies in the approach embankments. The cross-sectional area of the steel top flange can be reduced because the concrete can be considered as part of it. Transverse stiffening for the top compression flange of the steel beam can be reduced because the restraint against buckling is provided by the concrete deck. 3

Composite Action:

Composite Action In the composite girder, the top flange of the steel girder as well as the R. C. deck slab resist the compressive force, the bottom flange taking the tensile force as usual. As a result of having larger compression area, the steel girder possesses higher load carrying capacity when the area of the bottom flange of the steel girder is increased . 4

Composite Action:

Composite Action   5

Composite Action:

Composite Action Composite action is developed by the transfer of horizontal shear forces between the concrete deck and steel via shear studs which are welded to the steel girder. It is in the best interest of the designer to increase the section modulus as much as possible. Composite sections provide substantial section modulus with minimum material and it is here that the principal advantage of composite action comes into play 6

Shear Connector:

Shear Connector Shear connecter are part of parcel of composite deck system. The need for shear connectors can be understood by considering the interaction between the slab and the steel beam. If the slab simply rests on the steel beam, a phenomenon known as slippage occurs. To avoid slippage it is necessary that both unit behave like common unit and this can be achieved by shear connector. 7

Shear Connector:

Shear Connector It is generally a metal element of particular shape, which extends vertically from the top flange of the supporting beam and gets embedded into the slab. Depending upon the magnitude of the shear force at the interface of the beam and the slab, a number of shear connector can be placed along the length of the beam. 8

Types of Shear Connector:

Types of Shear Connector 9

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Design of Composite Bridge:

Design of Composite Bridge Analysis of Deck Slab DLB.M & S.F LL BM & S.F Design of Deck Slab Analysis of Girder Calculate B.M and S.F in Main and Cross Girders Design of Girders Flange Web Connection between flange and Web Stiffener Connection between stiffeners and web Design of Shear Connector 12

Shear Connectors:

Shear Connectors According to elastic theory of bending, the stresses and strains in a beam in bending vary linearly from the extreme tension fiber to the extreme compression fiber and the shear flow at any level (relative to the neutral axis) in the cross section is given by : t = VaY / I y V=is shear force at cross section a = is effective area further from neutral axis than the level considered Y= is the vertical distance from the neutral axis to the centroidal area a I y = is the second moment of area of the effective cross section of the member 13

Slide 14:

For both a and I, the area of concrete should be taken as its transformed area, i.e. its actual area divided by the modular ratio. Since the steel girder and the R.C.C. deck slab are made of materials having different modulus of elasticity, the area of the deck slab is required to be converted into equivalent steel area. For this purpose, the depth of the slab is kept unchanged and the effective flange width is reduced by dividing the effective width by the modular ratio, m, given by : m = E s / E c Where E s  = Modulus of elasticity of steel of girder . 14

Effective Flange Width :

Effective Flange Width The effective flange width of T or L beams shall be least of the following: a) In case of T-beams: One fourth the effective span of the beam . The distance between centre of web of the beam ii) The breadth of the web plus twelve times the thickness of the slab. b) In case of L-beams: i ) One-tenth the effective span of the beams. ii) The breadth of the web plus one-half the clear distance between the webs. iii) The breadth of the web plus six times the thickness of the slab 15

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A CE = [(2000 x 300)/13] =46154 mm 2 Consider modular ratio m = 13 Calculate Centroid of the composite section Y = S A Y/ S A =(A1Y1+A2Y2+A3Y3…..)/ (A1 + A2 + A3….) S A Y = [(46154 x 1210) + (500 x 30 x 1045) + (1000 x 10 x 530) + (500 x 30 x 15)] =77046340 mm 3 S A = [(46154) + (500 x 30) + (1000 x 10) + (500 x 30)] = 86154 mm 2 Y = 77046340 / 86154 = 894mm 17

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I comp = [(46154 x 316 2 ) + (500 x 1060 3 /12) - (460 x 1000 3 /12) + (( 86154 -46154) x 364 2 )] = 2.12 x 10 4 mm 4 t = VaY / I y = (548 x 1000 x 46154 x 316)/ 2.12 x 10 4 = 377 N / mm Total shear at junction = 377 x 500 = 188500N Use 20 mm diameter mild steel studs, capacity of one shear connector is given by Q = 196d 2 √ f ck Assume diameter of stud 20 mm and H = 5d = 20 x 5 = 100 mm f ck = 20 N/mm 2 18

Slide 19:

Q = 196d 2 √ f ck = 196 x 20 2 x √20 = 350615N Number of studs required in raw = 188500 / 350615 < 1 Hence provide minimum of two mild steel stud in a raw. Pitch of shear connector = p = (NQ/F t ) N = number of shear connector in a raw Q = Capacity of one shear connector F = Factor of safety =2 t = Horizontal shear per unit length 19

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P= [2 x 350615/(2 x 377 )] = 930 mm Maximum Permissible Pitch is least of Three times the thickness of slab = 900 mm 4 times the height of the stud =400 mm 600mm Hence adopt pitch of 400 mm in the longitudinal directions. Refer figure 20

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22 Thank you

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