Last reviewed 30 Sept 2026 · 6 min read
Where RCC bridges are used
Reinforced concrete is economical for short and medium spans because it needs little skilled labour, no special plant, and it is durable when detailed well. Typical use:
| Type | Span range |
|---|---|
| Box culvert / slab culvert | 1–6 m |
| Solid slab bridge | up to about 10 m |
| RCC T-beam and slab bridge | about 10–25 m |
| Hollow (voided) slab | up to about 25–30 m |
Beyond that, deadweight and cracking make prestressed concrete more economical.
Solid slab bridges
A solid slab bridge is a one-way slab spanning between abutments or piers. It is simple, has a low depth, and is easy to shutter — good for low clearance.
Slab under concentrated wheel loads — the load of a wheel spreads over an effective width of slab, not the width of the wheel. For a slab spanning in one direction the effective width of dispersion (IRC method, based on Pigeaud's / Westergaard's work) is
= effective span; = distance of the load centre from the nearer support; = width of the tyre contact area plus twice the wearing-coat thickness (the load spreads at 45° through the wearing coat); = a constant depending on the ratio of slab width to span (larger for wider slabs, about 2.6 for a simply supported slab with around 1).
The bending moment per metre width is the moment of the wheel load divided by . Load from several wheels is superposed while checking the overlap of effective widths. A minimum thickness is fixed by deflection (span/depth ratio) and cover requirements.
- Fix span, width and thickness ( to for simple spans).
- Dead load: self-weight + wearing coat + kerbs and railings.
- Live load: worst position of Class 70R / A wheels, using the effective width; add impact.
- Bending moment and shear, design as a singly reinforced section.
- Distribution steel; check shear, cover and crack width.
T-beam and slab bridges
A T-beam bridge has longitudinal girders (ribs) joined monolithically with a deck slab, which acts as the top flange of each girder. Cross-girders (diaphragms) at the supports and often at intervals tie the girders together and help the load to spread.
The deck slab
The slab spans between the girders (one way in the transverse direction, or two ways where cross-girders are close). It is designed for the wheel loads placed to give the worst moment and shear, with an effective width for the concentrated load and a moment reduction for continuity with the ribs.
Longitudinal girders — load distribution
A wheel load on one girder is shared with the neighbouring girders through the deck slab and cross-girders. Common analysis methods:
- Courbon's method — used when cross-girders are stiff and the deck is treated as rigid, so the deck rotates as a whole. The reaction on girder due to an eccentric load at eccentricity is
with = number of girders, = distance of the th girder from the axis of the bridge, = sum of squares of the girder distances.
- Morice–Little (Guyon–Massonnet) method — treats the deck as an orthotropic plate with a distribution coefficient; used for the more accurate design of grillage-type decks.
- Grillage or FEM analysis — used for skew, curved or wide decks.
A deck has four girders spaced at 2.5 m. A 100 kN axle load acts at 2.0 m from the centre line of the deck. Find the load on the outer girder (girder 1, at 3.75 m from the axis).
m².
(about 49 % of the axle load on the outer girder).
Design of the rib
The T-beam is designed as a flanged section with an effective flange width taken from the code (a function of the span, the web width, the slab thickness and the spacing of girders). Shear is critical near the supports, where the effective depth is often limited by clearance; stirrups are provided for the whole shear and shear-friction across the web–flange junction is checked.
Cross-girder design follows the same principle; the loads from the wheels are transferred through the deck slab and the diaphragms carry them to the main girders in torsion and bending.