DESIGN PRINCIPLES OF BRIDGES

By J. Gunakar Rao, M.E.

Dy. E.E. CENTRAL DESIGNS ORGANISATION,

I&CAD Dept., HYDERABAD.

BRIDGE DEFINITION

A BRIDGE IS A STRUCTURE THAT IS BUILT OVER A

CANAL, DRAIN, RIVER, VALLEY, ROAD, RAILWAY/

RAIL ROAD TRAFFIC ETC SO THAT PEOPLE OR

VEHICLES CAN CROSS FROM ONE SIDE TO THE

OTHER SIDE.

CARRIAGE WAY WIDTHS

VERTICAL CLEARENCES (Cl 106.2.1 0F IRC: 5-1998)DISCHARGE IN CUMECS MIN. VERTICAL CLEARENCE

UPTO 0.30 150 MM> 0.30 AND < 3.00 450 MM> 3.00 AND < 30 600 MM> 30 AND < 300 900 MM> 300 AND < 3000 1200 MM > 3000 1500 MM

SCOUR DEPTH CALCULATIONSNORMAL SCOUR DEPTH = dsm = 1.34 { q2 / f }1/3

where f = silt factor = 1.76 ( m ) ½

m = weighted mean diameter in mm MAXIMUM DEPTH OF SCOUR Near Piers = 2.00 dsm

Near Abutments = 1.27 dsm (when approaches retained )

= 2.00 dsm (when scouring allround ) For Rafts/ shallow footings = 1.27 dsm (In straight reach )

= 1.50 dsm (At moderate bend )= 1.75 dsm (At severe bend)= 2.00 dsm (At right angled bend)

Open Foundations as per cl. 705.2 IRC:78-2000➢ In soil

The minimum depth of open foundations shall be uptostratum having safe bearing capacity but not less than 2.0 mbelow the scour level or the protected bed level.

➢ On rocks

(a) For hard rocks, with an ultimate crushing strengthof 1.0 MPa or above arrived at after consideringthe overall characteristics of the rock, such as,fissures, bedding planes, etc. : 0.6 m(b) All other cases : 1.5 m

AFFLUXAfflux is the rise/ heading up of water over the normal water level on u/s of a bridge caused by constriction of water way at the bridge site. Afflux as per Molesworth formula:

h = { (V2 / 17.85 ) + 0.015 } { ( A2 / a2) - 1 }Where

V = Average velocity prior to obstruction in m/sec

A =Un obstructed sectional area of canal/ drain in Sq.m

a = sectional area of canal/ drain at obstruction in Sq.m

LIMITATIONS OF SPAN LENGTH

Loads, Forces and Stresses to be considered in the design

➢ 1. Dead Load➢ 2. Live Load➢ 3. Impact of Live load➢ 4. Water currents➢ 5. Longitudinal forces:➢ a. Tractive or Braking

force➢ b. frictional resistance to ➢ bearings➢ 6. Centrifugal forces➢ 7. Buoyancy

➢ 8. Earth pressure incl. Live load surcharge

➢ 9. Temperature effects➢ 10. Deformation effects➢ 11. Secondary effects➢ 12. Wind loads➢ 13. Wave pressures➢ 14. Impact due to floating

bodies➢ 15. Errection effects➢ 16. Sesmic forces

DEAD LOADDEAD LOAD CONSISTS OF LOADS OF ALL COMPONENTS OF SUPER

STRUCTURE:

➢ DECK SLAB AND GIRDERS

➢ FOOT PATHS

➢ WEARING COAT

➢ KERB

➢ HAND RAILING

LIVE LOADALL BRIDGES SHALL BE DESIGNED AS PER THE

FOLLOWING INDIAN ROAD CONGRESS LOADINGS:

IRC CLASS A

IRC 70 R LOADING

IMPACT

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WIND LOAD

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