Analysis of Prestressed Concrete Box Girder Bridge

 

A box girder bridge is a special type of bridge in that beams have to compromise girders in the shape of empty box. Bridge construction has got a paramount importance worldwide. Box girder is achieving popularity in bridge engineering because of its stability, economy, structural efficiency.

Calculation of Ultimate Strength

(a) Failure by yield of steel (under-reinforced section)

Mult = 0.9dbAsFp

Here,
As = the area of high tensile steel

Fp = the ultimate tensile strength for steel without definite yield point or yield stress or stress at 4 per cent elongation whichever is higher for steel with a definite yield point.
db = the depth of the beam from the maximum compression edge to the centre of gravity of the steel tendons.

(b) Failure by crushing concrete

Mult = 0.176 bdb2fck

Where,

1. b = the width of rectangular section or web of beam
2. fck= characteristics strength of concrete
3. G = Calculation of Section un- cracked in flexure
4. b = width in the case of rectangular member and width of the rib in the case of T, I and L beams
5. d = overall depth of the member
6. fcp = compressive stress at centroidal axis due to prestress taken as positive.

(c) Analysis and Design of Post-Tensioned Deck type Box-Girder Bridge

A post- tensioned deck type Box ? Girder

Bridges of clear span 30m and width of roadway is 7.5m. Assume Live Load as per IRC: 6-2000 vehicle is passing over deck given in the chapter 4. The Bridge analysis for different L/d ratio starting from 15 to 20 and different L/d ratio considered are as follows:

Case 1 L/d= 19, d = 1.6
Case 2 L/d =18, d = 1.7
Case3 L/d = 17, d = 1.8
Case4 L/d= 16, d= 1.9
Case5 L/d= 15, d=2.0

Preliminary Data

1. Clear span = 30m
2. Width of roadway = 7.5 m
3. Overhang from face of girder = 1.2m
4. Deck thickness = 0.2 m
5. Bottom slab thickness = 0.2 m
6. Girder thickness = 0.3 m
7. The tendon profile is considered as parabolic in nature.

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