Ultimate Strength Of Box Girder Based On Nonlinear Finite Element Method

It is very important to evaluate the ultimate strength of a ship and many methods have been developed to fulfill this requirement. Among all these methods, the finite element method is the most desirable one because there is a lot highly developed business software and the FEM can make an accurate estimation of the ultimate strength, though FEM is a little time consuming. However, it is not an easy task to use the FEM software correctly, especially for the ultimate strength problem. The aim of this research is to establish a database for evaluating the ultimate strength of a box girder based on nonlinear finite element used Ansys software.This paper contains a short historic about the development of ultimate strength research and finite element employment and also depicts the existing methods used for calculation of ultimate strength and gives a direct comparison between progressive collapse analysis (Smith's method, FEM and ISUM).For design and strength assessment of various types of structures such as ships, offshore platforms, land-based structures, and aerospace structures, it has been realized that ultimate limit states (or ultimate strength) are much better basis than the allowable working stresses. Within the framework of ultimate limit state design and strength assessment, the primary task is to determine the level of imposed actions which causes the ultimate limit states.The methods to evaluate the ultimate strength are divided into direct methods and progressive collapses analysis and can also be divided into:Empirical and analytical methodsNumerical methodsExperimental methodsIn numerical methods there is Finite Element Method (FEM) which is universally recognized as the most important technological breakthrough in the field of engineering analysis of structures. The development of computer has caused the finite element method to become one of the most popular techniques for solving engineering problems. For analyzing a complicated structure like a ship hull, the finite element method is the only tool which is giving satisfactory results. Many software are developed for that field like Abaqus, ADINA, LS-DYNA..., Ansys is one of those commercial software which is a general purpose finite element modeling package for numerically solving a wide variety of mechanical problems. These problems include: static/dynamic structural analysis (both linear and non-linear), heat transfer and fluid problems, as well as acoustic and electro-magnetic problems.For evaluate the ultimate strength, nonlinear analysis gives very close results using nonlinear material and geometry both.Ship and offshore structures consist of continuous panels stiffened by stiffeners and support members. The plate panels between stiffeners and frames are the most fundamental structural components and are subjected to lateral actions due to direct action of water or cargo pressure, as well as in-plane actions caused by structural responses at the system level.To estimate the ultimate strength of a stiffened panel by simplified methods, it is often necessary to accurately evaluate the post-buckling effective width of a local plate panel.For an intact vessel, the bending moment about the horizontal neutral axis is the predominant sectional force. Because of the very high stress levels, this moment may create in the deck and bottom of the structure, these regions may suffer failure due to buckling or plastic collapse.For the practical part, it is divided into three mains parts:-ultimate strength of the simple plate in which it's studying buckling (uni- compressions, bi-compressions, combined loads) and shear with/without combined loads.-Ultimate strength of the stiffened panel; and here buckling was evaluating with/without combined loads.In those two parts considers outer bottom stiffened-plate structures of a 100,000 ton class double-hull oil tanker, the magnitude (p) of lateral pressure actions is taken asp=0.16MPa in extreme (design) condition.-The last part studies the ultimate bending moment of Nishihara box with applying moment loads in one master node.For the all paper welding residual stress is ignored, only take the initial deformation following the classical theory of structural mechanics.In the end of this paper a tutorial is presented as an appendix how to evaluate the ultimate strength of simple plate using symmetry boundary condition and pressure load on line.