NUMERICAL STUDIES ON STABILITY PROBLEMS IN THIN-WALLED BOX-GIRDERS

Abstract

Composite laminated structures are increasingly being used in the design of load-carrying members for the aerospace, civil and modern engineering applications. Laminated composite box beam is the basic building block. It can be used both as a compression member and flexural member. Over the past decade FRP laminated composites are gaining popularity in marine, aerospace and civil industry for numerous structural applications, Being Lightweight it is easy handle and transport leading to short installation times and speedy construction which is a major criteria during disasters. Also the quality of work is improved in terms of corrosion resistance, endurance limit, damage tolerance, aesthetic appearance, longer life, low maintenance, life retardancy etc. For an instance laminated composite FRP deck weighs approximately 80 percent less than a concrete deck. Reducing the dead load increases allowable live load capacity of bridge without significant damage to the existing superstructure, thus lengthening its service life. Further the flexibility to tailor different properties of the structural elements by varying fiber orientation, number of layers etc. to achieve the strength and stiffness requirements in the required direction has enhanced their usage. These composites are often very susceptible to buckle in various modes. Moreover, the classical theories have been shown to under predict the deflections and to over predict the buckling loads of these structures. Since much emphasis is given in achieving an optimized ratio of the structure's weight to strength ratio in modern structures, the accurate prediction of their stability limit state is of fundamental importance in the design of thin-walled laminated composite structures. Further, for the case of Box panels, the buckling of top flange and their buckling modes usually occur very close to the critical buckling loads. So there is need of appropriate structural form with FRP is required to utilize its advantages. Thin-walled box beam is chosen for study as it offers excellent flexural and torsional stiffness with minimum weight. Use of FRP in box beam form combines the advantages of both the material and form and results in an efficient generic structural element. FRP box girders comprising of FRP stiffened panels are predominantly under membrane compression (top flange), in plane shear (web), and membrane tension (bottom flange). Hence the use of laminated composite materials for these structural elements is aptly suited as they provide flexibility to designer to specify different material properties for different elements of the beam cross-section. iii FRP being a high strength material, thickness required to satisfy the strength constraint is very less resulting in thin walled structure where the design is governed mainly by stability requirements. Stability of plate increases with increase in plate thickness but a more economical solution is obtained by keeping the thickness of plate as small as possible and increasing the stability by introducing stiffeners. In the present work studies are carried out on un-stiffened, and stiffened single cell box beams using finite element analysis and an effort is made to the stability characteristics of these thin walled structure. The FEA generated is used to develop the computationally efficient analysis approaches in the form of some constant or relation between Conventional theory and FEM analysis. The FEM based analysis tool ABAQUS is used to predict the local and global buckling strength of FRP box girder. Also for stiffened box sections few important parameters affecting the buckling behavior are identified and guidelines for better stiffener proportioning are developed which will be useful for the designer. This work presents the buckling analysis of laminated composite thin walled structures by the finite element method of analysis using ABAQUS. In nut shell present work is carried out to understand the buckling behavior of laminated composite box sections with the help of vast number of numerical studies. Additionally, formulated relation between simple theory and FEM based analysis theory can be used in predicting the buckling strength of top flange of box section. Also some guidelines for better stiffener proportioning are suggested which can be used by the designer and for further analysis and research on the subject. iv