Penalty function method for geometrically nonlinear buckling analysis of imperfect truss with multi-freedom constraints based on mixed FEMQuyen Vu Thi Bich, Tien Dao Ngoc and Van Tran Thi ThuyE3S Web of Conf., 410 (2023) 03028DOI: https://doi.org/10.1051/e3sconf/202341003028