design principles of bridges - aphrdi.ap.gov.inaphrdi/aees/syllabus... · vertical clearences (cl...
TRANSCRIPT
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
19
WIND LOAD
21