en Optimal design Optimal design پژوهشي Research <font size="2">NURBS-based isogeometric analysis (IGA) has currently been applied as a new numerical method in a considerable range of engineering problems. Due to non-interpolatory characteristic of NURBS basis functions, the properties of Kronecker Delta are not satisfied in IGA, and as a consequence, the imposition of essential boundary condition needs special treatment. The main contribution of this study is to use the well-known Lagrange multiplier method to impose essential boundary conditions for improving the accuracy of the isogeometric solution. Unlike the direct and transformation methods which are based on separation of control points, this method is capable of modeling incomplete Dirichlet boundaries. The solution accuracy and convergence rates of proposed method are compared with direct and transformation methods through various numerical examples. </font> isogeometric analysis; NURBS, essential boundary conditions; lagrange multiplier 247 271 http://ijoce.iust.ac.ir/browse.php?a_code=A-10-1-65&slc_lang=en&sid=1 S. Shojaee 18003194753284600312 18003194753284600312 Yes E. Izadpenah 18003194753284600313 18003194753284600313 No A. Haeri 18003194753284600314 18003194753284600314 No