# Reinforced concrete slab bridge — Initial sizing

### AASHTO 8th

**Table 2.5.2.6.3-1** of *AASHTO LRFD Bridge Design Specification 8th Edition* recommends minimum depths for reinforced concrete highway bridge slabs:

Simple |
\(\frac{1.2(S+10)}{30}\) |

Continuous |
\(\frac{S+10}{30} \geq 0.54\textrm{ ft}\) |

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

### WSDOT

**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:

< 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.*

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

\(w\) |
Imposed loads in \(kN\ m^{-2}\) |

\(l\) |
Span length in \(m\) |