This chapter presents a broad perspective of the recent research done on laminated composite hypar shells used as roofing units. Different types of analysis including bending, vibration, buckling, impact, and failure are included. The chapter is collated and categorized based on various aspect of research. The first aspect concentrates on typical analyses including problems in which various boundary conditions and laminations are considered. Then it focuses on the structural complexities which include stiffened shells, shells with cutouts, etc. The basic results of theoretical and experimental investigations of stress-strain state, vibration, buckling, and failure are summarized in this chapter incorporating the review of materials published in scientific and technical journals and proceedings in recent times.
TopIntroduction
One prime achievement of structural engineering profession is the design, construction and analysis of thin walled constructions and buildings in the shape of different shell surfaces. Laminated composite hyper shells (Hyperbolic paraboloids bounded by straight edges) are landmarks and testimonials to this achievement. They are true three dimensional representations of our equilibrium equations and affirmations of our analytical techniques, design standards and construction practices and choice of smart materials. These shells are useful as roofing units in civil engineering industry to cover large column free areas such as in stadiums, airports, shopping malls and parking lots. Being a doubly curved and doubly ruled surface, it satisfies aesthetic and ease of casting requirements of the industry. Moreover, hyper shell allows entry of daylight and natural air which is preferred in food processing and medicine units. Cutout is sometimes necessary in roof structure for the passage of light, to provide accessibility of other parts of the structure, for venting and also sometimes for alteration of resonant frequency. Shell structures that are normally thin walled, when provided with cutout, exhibits improved performances with stiffeners. Interest in composite shell behavior has grown day by day due to increased usage of laminated composite shells in many engineering applications. Laminated composites offer higher strength-to-weight and stiffness-to weight ratios, improved chemical and environmental resistance and ability to tailor properties compared to traditional materials. Additionally, the advances in composite manufacturing methods have also contributed to the increased usage of laminated composite materials in many modern applications.
Shell is an arbitrarily curved structural surface capable of resisting any externally superimposed load by combined in-plane thrusts and bending of the surface. Coupling of in-plane forces and bending moments provides high strength to the shell forms. Essentially a shell form gets its strength through its form rather than mass. The doubly curved, non-developable shell surfaces have added efficiency both in terms of strength and aesthetics. Some of the earliest applications of shell structures in the history of civilization include the first prototype submarine in 1620, the first ever pressure cooker in 1688, the use of balloon as a thin walled pressure vessel, the pneumatic tyre in 1845 etc. Shell roofs of civil engineering interests include the Pantheon of Rome built in AD 125, the Mosque of Santa Sophia, Istanbul built in AD 538 etc. Subsequently, shell forms were used in civil engineering practices as roofs, grain-storage structures, chimneys, water tanks, etc. From the early years of human civilization up to the present time, the course of shell construction, application and research has been developed quite a lot with the focus being shifted from one aspect to another with time. Structures made of thin plates/shells and strengthened by integrating them with a row of ribs appeared as a revolutionary concept.
Wide use of stiffened structural elements in engineering came into existence in the nineteenth century, mainly with the application of steel plates for hulls of ships and with the development of steel bridges and aircraft structures. As the use of stiffened structures gained popularity, the engineers started thinking of using different materials in different applications depending on the case specific requirements. The search in the field of advanced materials resulted in the development of lightweight laminated composites with high stiffness-to-weight and strength-to-weight ratios. The potentially high resistance of these materials to environmental degradation results in lower life-cycle costs and they may be tailored to any desired shape. Due to the wide spectrum of advantages the laminated composites can offer, these materials started being applied in different branches of engineering. The use of composites in engineering structures evolved as a result of a necessity during the Second World War.