Abstract To improve the dynamic behaviour of some structural elements, a well-adapted setup must be developed. The split Hopkinson pressure bar device is well-known for its application in dynamic tests on materials. To test structural elements, the energy of impact must be increased and direct impact may be used. That is the principle of the ‘block-bar’ device, which allows reconstruction of the crushing force on a sample. Nevertheless, the duration of the experimental impact is so long that it is not an obvious task to recover the force well. Lundberg has proposed equations for the reconstruction. Unfortunately, Lundberg’s method has some limitations. In this paper, the limitations are highlighted and new methods are proposed to overcome the problems. More precisely, new equations are proposed to recover the force directly from strain measurements at three given sections of the measuring bar. This method avoids numerical problems. Moreover, the problems and the methods are always viewed from the frequency and the time perspectives. The duality between time and frequency is present throughout this article. In this approach, a problem (or a method) in time domain has an equivalent in frequency domain and vice versa.