This article presents a method for strength calculations of truck trailer frames with two central axles designed for the transport of swap bodies. The finite element method formalism was used to carry out the calculations. In the design of truck trailer frames, the commonly applied static finite element method does not reflect the actual operating conditions of the structure, as it does not account for dynamic loads occurring during road manoeuvres. Therefore, the authors decided to employ a calculation approach based on an analysis of the dynamics of the truck–trailer vehicle combination using the MSC Adams and NX Nastran/Femap interface. The Craig-Bampton method was used to perform the computations. For the purposes of the analysis, a diagram of the pneumatic suspension system of the trailer was also developed. The dynamic characteristics of the air springs were determined on the basis of a polytropic process resulting from changes in their volume during operation. Sample results of reduced stress calculations in the trailer frame during three selected road tests are presented, namely braking, a U-turn and a series of two S-turns. In the authors’ opinion, the method could be utilised in the design process of the trailers under analysis.