Reinforced concrete slab bridge — Initial sizing


Table of AASHTO LRFD Bridge Design Specification 8th Edition recommends minimum depths for reinforced concrete highway bridge slabs:

Form Minimum depth
Simple \(\frac{1.2(S+10)}{30}\)
Continuous \(\frac{S+10}{30} \geq 0.54\textrm{ ft}\)

where \(S\) is the slab span length.


Clause 2.4.1 of WSDOT Bridge Design Manual recommends a span/depth ratio of 22 for simple spans and 25 for continuous spans.

Network Rail

NR/CIV/SD/1710/B recommends the following span/depth ratios:

Span Range Span/Depth Ratio
< 2.5m Min 250mm slab
2.5m - 5.0m 1:10 to 1:12.5
5m - 10m 1:12.5 to 1:15
10m - 15m 1:15 to 1:18
> 15m 1:18

Concrete Design Guide No.8

The following span to effective depth ratio is a quick and dirty curve fit to Table 2 of Concrete Design Guide No. 8: Deflection – the span-to-effective-depth method and EC2, The Structural Engineer, Aug 2015, pp30-34.

\[ \frac{l}{d} = 42.5 - \frac{46.8}{\sqrt[3]{w}} + \frac{48.2}{\sqrt{w}} - 1.21 l - 0.46w \]

Symbol Description
\(w\) Imposed loads in \(kN\ m^{-2}\)
\(l\) Span length in \(m\)