Effective width for small box culvert at ULS

[kww2008]

AS 1597.2-2013 Clause 3.5.3 effective width for traffic wheel loads for ULS is 1.22m which seems reasonable for a large box culvert with a span length between legs of at least 1.2m. However, small box culvert standard AS 1592.1-2010 allows design of small boxes to use AS 1577.2-2013. Since small box culverts may have the smallest span between legs of 300 mm, using an effective width of 1.22m seems excessive. What are your views on this as the basis for using 1.22m in AS 1592.2-2013 is not known?

 

[apsix]

As AS 1597.2-2013 is for spans exceeding 1200mm, the effective width of 1.22m would not be appropriate for spans less than this.
The serviceability limit state e[sub]w[/sub] = 0.5 + 0.3L would be more appropriate.