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Abutment
These are first and last supports of a bridge and they retain earth
on their backside, which serves as an approach to the bridge.
Breast Walls (Stem)
Wing WallBack (Dirt) Wall
Footing
Abutment Cap
1
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Types of Abutment
Balancing Type
Gravity Type
Buried Type
2
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Abutment with wing wall
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The following measures often help in achieving economy in the design of abutments
• Provision of sliding bearings or roller cum rocker bearings or
elastomeric bearing without pin on abutment reduces
horizontal force on the abutment.
• Eccentric abutment towards the backfill increases stabilizingmoment.
• For 5 to 6 m height and spans up to 20m usually solid plain
mass concrete or masonry abutments are economical.
• For heights above 6m and spans beyond 20m RC abutments
are suitable.
Some considerations in preliminary planning of abutment
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Plan of abutment
b
0.4 to 0.6m clear distance
Preliminary Sizing of Abutment
Gravity (wall) type abutment
h
0.3h
150mm× 2 +
bearing width
1/6 to 1/3 slopeH
0.35H to 0.45H
1 to 1.5m
300mm to 450mm thick with
75 to 200mm projection
Max. scouring depth
HFL
N
N = 305 +2.5L + 10H mm
L – span in m
H- Ht of support in m
Reinforced concrete abutment
1 to 1.5m
300mm to 450mm thick with
75 to 200mm projection150mm× 2 +
bearing width
H H/12 to H/8
H/12 to H/8
Max. Scouring
depth
2/5 H to 3/4 H H/10 to H/8
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Materials for Piers and Abutments[Minimum grade of material]
• Mass Concrete - M10 grade
(With mix proportions of 1:3:6 with 40-mm maximum size aggregates.)
• Reinforced Concrete - M20 grade
(With mix proportions of 1:2:4)
• Coarse Rubble Masonry(With Cement mortar of proportions 1:4)
• Brick Masonry(With Cement mortar of proportions 1:4)
• Prestressed Concrete - M35
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1. Vertical loads• Self wt. Of abutment
• Dead & Superimposed Dead Load from Superstructure
• Live Load
• Earthquake load (vertical component)
• Wind load (vertical component)
• Uplift by braking effort
• Load due to soil mass
2. Horizontal loads• Force due to Braking Effort
• Force due to Frictional Resistance of Bearing
• Wind Load• Force due to Earthquake
• Force due to Earth Pressure
• Force induced by creep, shrinkage and temperature variation
• Force due to surcharge
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Load Combination(Refer IRC 6)
For working stress design method, there are nine
combinations of loads to be considered in design
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In Limit State Design Method, there are three combinations
of loads to be considered in design. These three
combinations are
•
Basic combination• Seismic combination
• Accidental combination
These combinations are given for stability check, limit
state of strength, limit state of serviceability andfoundation design.
Partial safety factors for loads for different combinations
and for different works are not similar. They are chosenon the basis of nature of work carrying out.
Refer IRC 6 – 2010, Table 3.1, 3.2, 3.3 and 3.4 for
combination of loads
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RC Abutment
A
Transverse Section of
Abutment
Longitudinal Section of
Abutment
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Dead load from deck
(vertical)
• Find Self wt of railing, kerb/footpath, wearing course, slab , cross
beam and main beam per unit length of abutment
Weight / length of abutment
Live load from deck
(vertical)
• Find maximum live load per unit length of abutment
Live Load on Abutment / Length of Abutment
Load due to
temperature
variation from
deck (horizontal)
Loads on abutment from deck
• Find temperature variation range T
• Find movement of deck at free end of deck
T× Coefficient of Thermal Expansion × Span of Deck
• Find shear stiffness of bearing from manufacturer’s list
Horizontal load requires for unit deformation
• Find horizontal load on each bearing H
H = Shear Stiffness × Movement of Deck Or H = A×G×Movement of deck/Thickness of bearing
• Find total horizontal load per unit length of abutment
(Horizontal Load on a Bearing × No. of Bearings) / Length of Abutment
• Find force due to earthquake Feq from superstructure and substructure per unit
length of abutment in longitudinal direction of bridge and find force due to
earthquake Feq from superstructure and substructure in transverse direction of
bridge
F eq = αβγ W or Z/2× I/R× Sa /g
Load due to
earthquake in
longitudinal and
transverse direction of bridge (horizontal)
Load due to wind in
longitudinal and
transverse direction of
bridge (horizontal)
• Find force due to wind Fw from superstructure and substructure per unit length
of abutment in longitudinal and transverse direction of bridge
F T w = pAC D G
F L w = fraction of F T w
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Stability Check
1. Find overturning and restoring moment about toe of abutment for differentload combination
• Backfill + DL+ LL+ temperature load/braking load
• Backfill + DL+ Surcharge due to compacting equipment/LL
• Backfill + DL+ par. LL + seismic load
Check overturning effectM restoring /M overturning ≥ 2 for basic combination
≥ 1.5 for seismic combination
2. Find shear and resisting shear at the base of footing
Shear = sum of horizontal forces at base
Resisting shear = sum of vertical load at base × tanø
Loads at rear of abutment
• Find force due to earth pressure Fb per unit length of abutment
F b = ½× k a×γ ×H× H
• Find force due to Surcharge Fs per unit length of abutment
1.2 m earth fill on the road level is taken as surcharge load
F s = k a×w×H
H
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Design Of Abutment Cap, Main Stem, Back Wall and Slab Base
• Design abutment cap
When bearing stress in cap does not exceed the permissible value of bearing stress in concrete, providereinforcement according to IRC78
• Design main stem of abutment as a RC slab and check the stem as a RC column
When design axial load on abutment ≤ 0.1f ck A, abutment is designed as RC cantilever slab
•
Design back wall as a RC cantilever slabBack wall is designed for earth pressure and surcharge and check for its self wt. and wt of approach
slab
• Design slab base as a spread footing.
Footing is designed for maximum BM and maximum one way shear at the critical sections of footing.
3. Check bearing pressure at base of footing
Pressure = P/A ± Pe /Z ≤ bearing capacity of soil
Check sliding effect
V resisting / V sliding ≥ 1.5 for basic combination
≥ 1.25 for seismic combination
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• Carry out detailing of reinforcement [Refer cl. 16.3, IRC 112}
Vertical Reinforcement
Dia. of bar≥ 12mm
Total area steel of vertical bar 0.0024 to 0.04 of area of concrete
area of bar in one face ≥ 0.0012
Spacing of vertical bars ≤ 200 mm
Horizontal Reinforcement
Area of horizontal reinforcement ≥ 2.5% of total area of vertical bars
≥ 0.001 of concrete area
Spacing of horizontal bars ≤ 300 mm
Dia of bar≥ 8mm or one fourth of vertical bars
Transverse Reinforcement
If the area of load carrying vertical bar in two faces > 0.02 × area ofconcrete theses bars should be enclosed by stirrups
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…
. .
… … …
Reinforcement of Abutment
Cross Section Longitudinal Section
Section at A-A
AA
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