1. Department of Applied Mathematics, Technical University of Lublin, PL-20-618 Lublin, Poland
2. Department of Machine Construction, Technical University of Lublin, PL-20-618 Lublin, Poland
3. Department of Applied Mechanics, Technical University of Lublin, PL-20-618 Lublin, Poland; Department of Architecture, Buildings and Structures, Polytechnic University of Marche, Via Brecce Bianche, I-60131 Ancona, Italy