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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
Design of superstructure
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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Design of superstructure
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
Design of superstructure
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
Design of superstructure
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:
Design of superstructure
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
Design of superstructure
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
Design of superstructure
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
Design of superstructure
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
Design of superstructure
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
Design of superstructure
Analogy or Finite Element Method
Design of superstructure
Assigning Loads: Applied loads, live loads, moving load lanes
Design of superstructure
Bending Moment Diagrams (sample)
Design of superstructure
Shear force Diagrams (sample)
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Design of superstructure
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
Design of supersructure (RCC girder bridge)
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
Design of supersructure (RCC girder bridge)
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
Design of superstructure (RCC girder bridge)
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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Design of superstructure (RCC girder bridge)
The Pigeaud’s curves are based on elastic analysis of isotropic plates.
Courtesey: R.R. Sharma, DOR
Thank you!Thank you!
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