Table 2.5.2.6.3-1 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.

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:

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.