1. Professor, Faculty of Civil and Environmental Engineering, Dept. of Structural Mechanics, Gdańsk Univ. of Technology, ul. Narutowicza 11/12, 80-233 Gdańsk, Poland (corresponding author).
2. Faculty of Civil and Environmental Engineering, Dept. of Structural Mechanics, Gdańsk Univ. of Technology, ul. Narutowicza 11/12, 80-233 Gdańsk, Poland.