Design Concept RCC Superstructure

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DESIGNOFSUPERSTRUCTURE

Er. Saroj Bhattarai2014

What is Superstructure?

The part of the bridge which directly bears and transfers loads from vehicles and pedestrians to the substructure.

Design of superstructure involves design of deck, girders, cross girders, railing posts, kerbs, footpath, drainage system and bearings.

Types of superstructureDesign of superstructure

Governing Standards and Codes: DOR Bridge Standards (2067) IRC:5 (General Features), IRC:6 (Loads & stresses), IRC:21 (Cement Concrete – plain and RCC),( p ), IRC:22 (Composite decks) IRC:24 (Steel Bridges) IRC:18 (Post-tensioned concrete bridges) IRC:112 (RCC and PC bridges on Limit State Method,

replaces IRC:18 and 21. To be used with IRC:6-2010) Indian Standard IS:456-2000 AASHTO, British Standards (BS:5400 series)

Design Loads: Permanent Dead Loads (Beams, Slabs) Applied loads: wearing course, railings, footpath

surface works

Design of superstructure

surface works Live Loads (pedestrians)Wind LoadMoving loads and impact Seismic load generally insignificant for short span

simply supported decks. Stoppers are provided to prevent slippage and falling out.

Design Procedure Determine carriageway width,

footpath and length of span. Arrange the positions of main

and cross girders Determine


and cross girders. Determine their dimensions (depths and widths)

Determine the critical arrangement of design vehicles on the carriageway and the eccentricity of its CG with respect to the bridge axis

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Moving load combination prescribed by IRC:6-2000

Design Procedure Determine the lateral load distribution factors for the main girders

Courbon’s method (based pure lever theory) Morrice Little method (Orthotropic plate theory) Hendry Jaeger method (Lever theory with variable transverse


parameters)

Grillage analogy (Two dimensional mesh of beams and slabs) Finite element method (Three dimensional analysis)

Workout the Influence Line diagrams for the girders. Many FEM software do not require IL diagram for design. They

directly give the critical bending moments, shear force etc at any section of the elements

Ri = Reaction factor of the beam under consideration P = Total live load Ii = Moment of Inertia of the beam under consideration e = Eccentricity of the load with respect to the bridge axis

Courbon’s method


e Eccentricity of the load with respect to the bridge axis di = Distance of the beam under consideration from the axis of the bridge n = Number of beams

When the Moment of Inertia of all the beams is equal and assuming the load as unit load then the equation is reduced as:


P

eCourbon’s method

1 2 3 4

d1 d3

d1, d2 Positived3, d4 Negative

P

The assumed deflection of the deck

Drawing Influence Lines (Support Reaction)

Variation of Reactions RA and RB as functions of load position

M 0

x 1

A B

C 10 ft

3 ft


MA =0 (RB)(10) – (1)(x) = 0 RB = x/10 RA = 1-RB = 1-(x/10)

x 1

A B C

RA=1-(x/10) RB = x/10

x

A C

RA=1-x/10 RB = x/10

Drawing Influence Lines (Support Reaction)

RA occurs only at A; RB occurs only at B

1 /10

1Influence


Influence line for RB

1-x/10line for RA

x 10-x

x 10-x

x/10 1.0

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Drawing Influence Lines (Shear force)

3 < x < 10 ft (unit load to the right of C)

C

x

3 ft A

RA = 1-(x/10) RB = x/10

B


Shear force at C is +ve = 1-x/10 Influence line for shear at C

C

1

1

-ve

+ve

0.3

0.7

RA = 1-(x/10)

Drawing Influence Lines (Bending Moment)

3 < x < 10 ft (Unit load to the right of C)

C

x ft

3 ft A

RA = 1-(x/10)

10 ft

1

RA = x/10

B


Moment at C is +ve Influence line for bending Moment at C

C

1-x/10

1-x/10

(1-(x/10))(3)

(1-(x/10))(3) 1-x/10

(1-(x/10))(3)

+ve

(1-7/10)(3)=2.1 kip-ft

Analysis by Computer Program based on Grillage A l


Analogy or Finite Element Method


Assigning Loads: Applied loads, live loads, moving load lanes


Bending Moment Diagrams (sample)


Shear force Diagrams (sample)

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Support Reactions and Frame Deformation (sample)

Design by working stress method Find the critical moments and shear at different locations of

the girder (1/2, 3/8, 3/4, 1/4 span and at supports) Follow any literature to design the “T” girder with working

stress method. Separate design for exterior and intermediate

Design of supersructure (RCC girder bridge)

girders Effective depth is taken from the CG of the tensile

reinforcement. Check for stresses at the lowest layer of the bars. Check for bond stress near supports.

Curtail the longitudinal reinforcement as per moment diagram. Check for shear at different locations and provide shear reinforcement

Analyze the critical forces for cross girders and design following the same procedure as for the main girders

Not necessary to curtail the bars since the cross girders are comparatively short

The cross girders at both supports should be designed also


for lifting the deck for maintenance by jacking up.

Analysis and design of deck slab by Pigeaud’s method or any other suitable theory for slab supported on all sides. (e.g. yield line theory or as a shell element in finite element analysis).

Minimum thickness of RCC slab 200 mm. Usually quite ffi i t f i f i d 5


sufficient for spacing of cross girders <5 m. Designed as both way reinforced slab. Normally light

reinforcement is sufficient. Arching Action – “Zero reinforcement”?

Deck slabs are generally safe on one way shear. But sometimes punching shear maybe critical.

Pigeaud’s method For slabs restrained on all sides and

patch load at the center. The bending moments both ways

are determined by using the

Design of superstructure (RCC girder bridge)

Piegeaud’s curves.

Real and dummy load positions for class 70R bogie and Track

Bogie

Track


Curves for M1 (K=0.4)Curves for M2 (K=0.4)

The graphs are read for the values of K, u/B and v/L which are ratios of dimensions of the load patch and the size of the slab within the girders.

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The Pigeaud’s curves are based on elastic analysis of isotropic plates.

Courtesey: R.R. Sharma, DOR

Thank you!Thank you!

Design Concept RCC Superstructure

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