Here in the US, flexible pavements are empirically designed based on equivalent 18 kip axle loads. The design tables have equivalency factors for use with various axle loadings, including single axle, tandem axles and tridem axles. There are equations that can be used for equivalence as well.
When using elastic layer analysis for pavement design, equivalence is not an issue as actual loadings are used.
For your reference, try downloading a flexible pavement design manual from one of the state departments of transportation in the US, such as Washington or Florida. Both have their manuals online and they are free. Both states follow the AASHTO method of flexible pavement design for most designs, though Washington state has an elastic layer design as well.
If you can obtain a copy of BISAR, a computer program for pavement analysis/design, it will help as well. I do not have a copy and have found it to be difficult to obtain.
Good luck.
