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