Railway

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  • Topic: Bridge, Dead and live loads, Bridges
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  • Published : May 2, 2013
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Design of bridges

Syllabus
• IRC Code • Types of bridges and their components • Basic concepts of bridge design • Design of slab bridge • Design of T-beam bridge (Courbon’s method)

IRC Code
• IRC – Indian Roads Congress • Three categories of Live loads • Types of vehicles • Placement of vehicles • Description of three category loadings • Permissible stresses and design coeff.

Categories of Live loads as per IRC
a.

IRC Class AA loading
i. For bridges subjected to very heavy loading ii. Used only in certain specified areas and municipal limits iii.Bridges designed for classAA loading should be checked for class A loading also

b.

IRC Class A loading
i. For permanent bridges and culverts

c.

IRC Class B loading
i. For temporary structures ii. Used only in specific areas

Types of vehicles
a. b.

c.

Tracked vehicles Wheeled vehicles Trains of vehicles

Placement of vehicles
a. b.

c.

d.

e.

f.

Extent: Within the kerb to kerb width of the roadway. NOT on the footpath, etc Orientation: travelling along the bridge. NOT across the bridge Multiple vehicles: with minimum clearance between vehicles. NOT touching each other Position: Any possible position which produces max. stresses. Standard vehicle: All the axles shall be acting simultaneously and at the prescribed distances No other live load shall be assumed on the carriage way besides the IRC loading

Class AA loading

Class AA loading (contd.)
a.

b.

c.

The nose to tail spacing between two successive vehicles shall not be less than 90 m. fig For multilane bridges and culverts, only ONE train of Class AA (tracked or wheeled vehicles) shall be considered for every two traffic lane widths. No other live load on the 2-lane width The maximum loads of the wheeled vehicles shall be 20 tonnes for a single axle, or 40 tonnes for a bogie of two axles spaced not more than 1.2 m centres.

Minimum clearance between the road face of the kerb and the outer edge of the wheel or track, C shall be taken as follows:

Number of lanes
Single lane bridge Multilane bridges

Carriage way width 3.8 m and above
Less than 5.5 m 5.5 m or above

Max. value of C
0.3 m 0.6 m 1.2 m

Class A loading

Nose-to-tail distance + Axle loads and their spacing

Class A loading
Axle Ground contact load area (tonnes) B (mm) W (mm) 11. 4 250 500 6.8 200 380 2.7 150 200

Ground contact of various axles

Class A loading
Clear carriage way width 5.5 to 7.5 m Above 7.5 m Clearances g f

0.4 to 1.2 m 1.2 m

150 mm
150 mm

Clearances

Class A loading - Summary
– The nose to tail distance between two successive vehicles shall not be less than 18.4 m – No other live load shall cover any part of the carriage way when a train of vehicles (or trains of vehicles in multilane bridges) is crossing the bridge – The ground contact area of the wheel shall be taken as shown in the table – The clearances shall be as shown in the table

Impact Loading

Impact Loading

Permissible stresses
In concrete
Property Modulus of elasticity Permissible direct compressive stress Ec (GPa) Grade of concrete M 15 20 M 20 25 M 25 28 M30 M35 M40 31 33 36

σcc (MPa)
σcbc (MPa)

3.8

5.0

6.2

7.2

8.5

8.5

Permissible flexure compressive stress

5.0

6.7

8.3

10.0

11.5

11.5

Permissible stresses
In steel

In concrete in shear M15 Τco 0.28 (MPa) M20 0.34 M25 0.40 M30 0.45 M35 0.50 M40 0.50

Working Stress Design coefficients

Neutral Axis coeff. Lever arm Coeff. Moment Coeff.

Types of bridges and components
– Slab bridge or culvert
• • • • • • • • • Deck slab Wearing coat, kerb, hand rails, etc Abutments and wing walls Foundations Deck slab Wearing coat, kerb, hand rails, foot paths if provided Cantilever portion Longitudinal girders Cross- girders

– T- beam bridge

Basic concepts of bridge design
– Analysis of slabs carrying wheel loads
• Effective width method • Pigeaud’s...
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