Chapter 5
SUPERSTRUCTURES
5.1 Introduction
5.2 Reinforced Concrete
5.3 Steel Bridges
5.4 Special Bridges
5.1 Introduction
Efficient design of superstructure results in
overall economy
A clear understanding of superstructural
behavior under loads is essential for
efficient design
Superstructure structural elements: slab,
stringers (RC, PSC, steel, composite),
diaphragms, trusses, arches, etc
Introduction Contd. . .
Determining member forces in these elements is essential
Methods of analysis: static or dynamic
Static analysis: for structures insensitive to dynamic action
– IM is sufficient to account for dynamic effect
– SMALL DEFORMATION THEORY is used
Dynamic analysis: for those sensitive to dynamic effects
of vehicular live loads, wind and earthquake
– Susceptibility to vibration and large deformation
– IM is too small to take care of dynamic effects
– LARGE DEFORMATION THEORY should be used
OBJECTIVE: Structural analysis and section design with
emphasis given to RC bridges of small and medium span
5.2 Reinforced Concrete
Two principal materials used in superstructure
construction are steel and concrete
Advantages of RC bridges over steel
– Adaptability to variety structural shapes and forms, and
can be given desired aesthetic appearance
– Low cost of maintenance and Long life
– Better resistance to temporary over loads and dynamic
loads
– Cast in place RC are continuous and monolithic: easy
construction, low cost and good seismic resistance
5.2 Reinforced Concrete Contd. . .
Disadvantages of concrete
- Large dead weight, which require larger foundation
- Difficulty to widen or rebuild
- Longer construction time
- Requires formwork and false work, which are expensive
Advantages of steel
- high strength, homogeneous material: smaller depth
- faster construction
- light superstructure, smaller foundation
- easier and faster to repair
Disadvantages of steel
- Corrosion: high maintenance cost
- Fatigue problem
Bridge deck is the medium on which most loads act
and get transferred to other components (Fig.
shows load path)
Bridge Live Loads Distribution Contd.
Bridge Live Loads Distribution
Occupy random positions both longitudinally and transversely
ILs are used to get variation in response for different load
positions in longitudinal direction
Random transverse location of loads affect live load shared by
various beams
- Distribution factors are used
Section Types: Table 4.6.2.2.1-1
DF for moment for interior beams: Table 4.6.2.2.2b-1
DF for moment for exterior beams: Table 4.6.2.2.2d-1
DF for shear for interior beams: Table 4.6.2.2.3a-1
DF for shear for exterior beams: Table 4.6.2.2.2d-1
Section Types
Deck Analysis Methods
Various methods of deck analysis depending on complexity
of structural form and behavioral characteristics
Most common deck types
a) Slab bridges
b) Beam and slab deck bridges (Tee beams)
c) Cellular deck bridges
d) Frame bridges
5.2.1 Slab Bridges
- Used to span short up to 12m
- Load carrying mechanism is by plate action
Governing Equation of an elastic plate
Developed by S. D. Poisson and boundary condition
modified by G. R. Kirchhof
Difficult to get closed form solution to the partial
differential equation, approximate methods are
developed
– Method of influence surfaces
– Grillage method
– Line solution using finite difference
– Strip method, which AASHTO recommends
b) Wind Pressure on Vehicles: WL
It is possible for the wind to blow when the
vehicle is on the bridge
This is taken moving interruptible force with the
vehicle of 1.46kN/m acting normal and 1.8m
above roadway surface
c) Aeroelastic Instability
Aeroelastic force effects considered for wind
sensitive components( span/depth or width>30)
5.2.2 T-Girder Bridges
Used to span from about 10-25m
Consist of equally spaced beams (1.8-3.6m) with top
flange slab
Slab supports live loads
Diaphragms provided to ensure lateral distribution of live
loads
Load share of each beam depends on stiffness of
diaphragms relative to beams, method of connectivity,
stiffness of the slab, spacing and stiffness of beams-this
participatory action is called load distribution
Design of T-Girder
Consists of:
– Deck slab analysis and design: flexure
– Girder analysis and design: flexure and shear
Slab analysis and design
– 1m continuous strip taken for analysis
– Wheel loads distributed over strip widths
Overhang 1140 + 0.833X
Positive moment 660 + 0.55S
Negative moment 1220 + 0.25S
– In overhang slab design, collision forces on barriers
should be considered
Design of T-Girder Continued . . .
Girder analysis and design: for flexure and shear
– ILs used to get response to LL for variable longitudinal
position of LL
– Girder distribution factors used to get maximum live
load coming to a girder for transverse placement of LL
– Envelop equation written for shear and moment
– Section analysis follows that for T-beam
Design for flexure
– Mu
– using be and min
Design of T-Girder Continued . . .
