THE SHELL STRUCTURE IS TYPICALLY FOUND IN NATURE AS WELL AS IN CLASSICAL ARCHITECTURE. ITS EFFICIENCY IS BASED ON ITS CURVATURE (SINGLE OR DOUBLE), WHICH ALLOWS A MULTIPLICITY OF ALTERNATIVE STRESS PATHS AND GIVES THE OPTIMUM FORM FOR TRANSMISSION OF MANY DIFFERENT LOAD TYPES. VARIOUS DIFFERENT TYPES OF STEEL SHELL STRUCTURES HAVE BEEN USED FOR INDUSTRIAL PURPOSES; SINGLY CURVED SHELLS, FOR EXAMPLE, CAN BE FOUND IN OIL STORAGE TANKS, THE CENTRAL PART OF SOME PRESSURE VESSELS, IN STORAGE STRUCTURES SUCH AS SILOS, IN INDUSTRIAL CHIMNEYS AND EVEN IN SMALL STRUCTURES LIKE LIGHTING COLUMNS. THE SINGLE CURVATURE ALLOWS A VERY SIMPLE CONSTRUCTION PROCESS AND IS VERY EFFICIENT IN RESISTING CERTAIN TYPES OF LOADS. IN SOME CASES, IT IS BETTER TO TAKE ADVANTAGE OF DOUBLE CURVATURE. DOUBLE CURVED SHELLS ARE USED TO BUILD SPHERICAL GAS RESERVOIRS, ROOFS.

DISTRIBUTED LOADS DUE TO INTERNAL PRESSURE IN STORAGE TANKS, PRESSURE VESSELS OR SILOS, OR TO EXTERNAL PRESSURE FROM WIND, MARINE CURRENTS AND HYDROSTATIC PRESSURES ARE VERY WELL RESISTED BY THE IN-PLANE BEHAVIOUR OF SHELLS. SHELL STRUCTURES OFTEN NEED TO BE STRENGTHENED IN CERTAIN PROBLEM AREAS BY LOCAL REINFORCEMENT. LOCAL REINFORCEMENT IS ALSO OFTEN REQUIRED AT CONNECTIONS BETWEEN SHELL STRUCTURES, SUCH AS COMMONLY OCCUR IN GENERAL PIPING WORK AND IN THE OFFSHORE INDUSTRY. IN CONTRAST TO LOCAL REINFORCEMENT, GLOBAL REINFORCEMENT IS GENERALLY USED TO IMPROVE THE OVERALL SHELL BEHAVIOUR. IN AXISYMMETRIC SHELLS, THE OBVIOUS LOCATION FOR THE STIFFENERS IS ALONG SELECTED MERIDIANS AND PARALLEL LINES, CREATING IN THIS WAY A TRUE MESH WHICH REINFORCES THE PURE SHELL STRUCTURE. 

THERE ARE TWO MAIN MECHANISMS BY WHICH A SHELL CAN SUPPORT LOADS. ON THE ONE HAND, THE STRUCTURE CAN REACT WITH ONLY IN-PLANE FORCES, IN WHICH CASE IT IS SAID TO ACT AS A MEMBRANE. IN PRACTICE, HOWEVER, REAL STRUCTURES HAVE LOCAL AREAS WHERE EQUILIBRIUM OR COMPATIBILITY OF DISPLACEMENTS AND DEFORMATIONS IS NOT