design of rc abutment

7/21/2019 Design of RC Abutment

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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



Types of Abutment

Balancing Type

Gravity Type

Buried Type

2



Abutment with wing wall



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



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



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



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



Load Combination(Refer IRC 6)

For working stress design method, there are nine

combinations of loads to be considered in design



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



RC Abutment

A

Transverse Section of

Abutment

Longitudinal Section of

Abutment



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



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



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



• 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



…

. .

… … …

Reinforcement of Abutment

Cross Section Longitudinal Section

Section at A-A

AA

design of rc abutment

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