Design of RC Abutment

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. Back (Dirt) Wall

Wing Wall

Abutment Cap

Breast Walls (Stem)

Footing 1

Types of Abutment

Gravity Type Balancing Type

Buried Type

2

Abutment with wing wall

Some considerations in preliminary planning of abutment 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 stabilizing moment.

• 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.

Preliminary Sizing of Abutment 150mm× 2 + bearing width h

300mm to 450mm thick with 75 to 200mm projection

300mm to 450mm thick with 75 to 200mm projection

150mm× 2 + bearing width

1 to 1.5m

0.3h

1 to 1.5m HFL

H

H

1/6 to 1/3 slope Max. scouring depth

H/12 to H/8

Max. Scouring depth

0.35H to 0.45H

2/5 H to 3/4 H

H/12 to H/8 H/10 to H/8

Reinforced concrete abutment

Gravity (wall) type abutment

N b N = 305 +2.5L + 10H mm L – span in m H- Ht of support in m 0.4 to 0.6m clear distance

Plan of abutment

Materials for Piers and Abutments [Minimum grade of material]

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Mass Concrete - M10 grade (With mix proportions of 1:3:6 with 40-mm maximum size aggregates.)

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Reinforced Concrete - M20 grade (With mix proportions of 1:2:4)

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Coarse Rubble Masonry (With Cement mortar of proportions 1:4)

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Brick Masonry (With Cement mortar of proportions 1:4)

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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

For working stress design method, there are nine combinations of loads to be considered in design Load Combination (Refer IRC 6)

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 and foundation design.  Partial safety factors for loads for different combinations and for different works are not similar. They are chosen on 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

Loads on abutment from deck Dead load from deck (vertical)

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Live load from deck (vertical)

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Load due to temperature variation from deck (horizontal)

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Find Self wt of railing, kerb/footpath, wearing course, slab , cross beam and main beam per unit length of abutment Weight / length of abutment Find maximum live load per unit length of abutment Live Load on Abutment / Length of Abutment 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

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Load due to earthquake in longitudinal and transverse direction of bridge (horizontal)

Load due to wind in longitudinal and transverse direction of bridge (horizontal)

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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 Feq = αβγW or Z/2× I/R× Sa/g

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Find force due to wind Fw from superstructure and substructure per unit length of abutment in longitudinal and transverse direction of bridge FT w = pACD G FL w = fraction of FT w

Loads at rear of abutment •

Find force due to earth pressure Fb per unit length of abutment Fb = ½× ka×γ×H× H

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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 Fs = ka×w×H

Stability Check 1.

Find overturning and restoring moment about toe of abutment for different load combination • Backfill + DL+ LL+ temperature load/braking load • Backfill + DL+ Surcharge due to compacting equipment/LL • Backfill + DL+ par. LL + seismic load

Check overturning effect M 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

Check sliding effect V resisting / V sliding 3.

≥ 1.5 for basic combination ≥ 1.25 for seismic combination Check bearing pressure at base of footing Pressure = P/A ± Pe/Z ≤ bearing capacity of soil

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, provide reinforcement according to IRC78

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Design main stem of abutment as a RC slab and check the stem as a RC column When design axial load on abutment ≤ 0.1fck A, abutment is designed as RC cantilever slab

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Design back wall as a RC cantilever slab Back wall is designed for earth pressure and surcharge and check for its self wt. and wt of approach slab

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Design slab base as a spread footing. Footing is designed for maximum BM and maximum one way shear at the critical sections of footing.

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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 of concrete theses bars should be enclosed by stirrups

Reinforcement of Abutment ..

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A

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A

Cross Section

Longitudinal Section

Section at A-A