T-Beam and Slab bridge: General Configuration
RC T-beam bridges are economic
choice for 10-25 m span
Possible types: Girder and slab type:
• Deck slab is supported on and cast monolithically with longitudinal girders
• No cross beams provided
• Deck slab designed as one-way slab
• The system lacks in torsional rigidity and the longitudinal girders can spread laterally at the
bottom level
• Not adopted in recent designs
Girder, slab and diaphragm type:
• Slab supported on and monolithically cast with longitudinal girders
• Diaphragms connect the longitudinal girders-- provided at supports and at one or more
intermediate locations within the span.
• Diaphragms do not extend up to the deck slab. Hence deck slab behaves as one-way slab
• Possesses a greater torsional rigidity
Girder, slab and cross beam type:
• At least three cross beams extending up to the deck slab
• Slab panels supported along the four edges. Hence designed as a two-way slab.
• Due to two-way action- More efficient use of the reinforcing steel and reduced slab thickness
reduced dead load on the longitudinal girders.
• Cross beams stiffens the structure better distribution of concentrated loads among the
longitudinal girders.
• With two-way slab and cross beams, the spacing of longitudinal girders can be increased, resulting
in less number of girders and reduced cost of formwork.
Proportioning
• Smaller S will increase the
number of girders.
• The required thickness of
deck slab will be decreased.
• Usually results in lower cost
of materials.
• But the cost of formwork will increase due to larger number of
girder forms, as also the cost of vertical supports and bearings.
For conditions in India, a three-girder system is usually
• The aim of the design should be to adopt a system which will call more economical than a four girder system for a bridge
for the minimum total cost. width of 8.7 m catering to two-lane carriageway
BM computation in the cantilever portion of the slab due to LL: Effective width method (IRC-112)
Pigeaud’s Method of BM computation in centrally loaded 2-way simply supported slab
• Moment in the shorter (transverse)
direction per meter width
= W (m1 + µm2)
• Moment in the longer (longitudinal)
direction per meter width
= W (m2 + µm1)
where, µ (Poisson’s ratio)= 0.15 for
concrete, W the total load, m1 the shot-
span moment coefficient and m1 the
long-span moment coefficient.
Considering continuity at support 80% of this moment may be used for
maximum span moment.
Pigeaud's method is applicable to rectangular slabs supported
freely on all four sides subjected to a symmetrically placed load
at center.
Approximate way to deal with eccentrically loaded slabs:
Ref: D J Victor Chapter 7
Apply Pigeaud's method: Dead load BM in deck slab panels supported on all 4 sides
Clear span
Apply Pigeaud's method: LL BM -- due to IRC Class AA Tracked vehicle
Position the tracks/ wheels such a way that maximum
Span moment develops in the panel.
Use the charts for K=0.6 and K=0.7 to get these estimates
Note on impact %
Apply Pigeaud's method: LL BM -- due to IRC Class AA Wheeled vehicle
0.3 0.3
Class AA wheeled vehicle
300 x 150 wheel
Position of wheels such a way that maximum
contact area
Span moment develops in the panel.
Load is centrally placed,
Direct application of
Pigeaud's method
414.2
For wheel 5 and 6, ideally adopt the following scheme
For wheel 5 and 6, Victor adopts simpler/ reasonable
approximation-
Unfactored total BM (DL + LL)
including continuity effect
Compare the wheeled load
induced moment with those
from tracked load
Analysis of the
cantilever slab
portion: effective
width method for LL
BM due to DL
BM due to LL: Class A wheeled load
Impact fraction = 0.5 for span
less than 3 m
Distribution of load to the main girders- Courbon’s method
When the deck system along with cross-girders is stiff enough, under
application of wheel loads– it may be assumed to be deforming such a way
that it remains plane after deformation.
(a) The ratio of span to width of deck is greater than 2 but less than 4.
(b) The longitudinal girders are interconnected by at least five
symmetrically spaced cross girders.
(c) The cross girder extends to a depth of at least 0.75 times the depth of
the longitudinal girders.
Deformation is resisted by spring actions of the main girders. The stiffness
of these springs are proportional to EI values of main girders.
Bending moment and shear force in longitudinal girders
Girder effective span: 14.5 m;
Total length of girder: 15.1 m
Overall depth: 1575 mm
DL per meter run
The full vehicle could not be accommodated within
Span.
