Effective breadth for top-hat stiffened composite structures

Structural design often considers single beams instead of larger structures. To allow the single stiffener to be considered instead of the plate requires an assumption that the stress in the structure is carried through the stiffeners and not the attached plate, and therefore the stresses in each member do not interact. However, this assumption is not completely realistic so effective breadth has been developed to calculate an area of plate, carrying a uniform stress ensuring that the stresses are close to those in the larger structure. It is commonly used to design uniformly loaded structures such as ships, bridges and aircraft; allowing the replacement of complex and computationally expensive structural units with smaller monodimensional elements. Despite the effective breadth having been widely investigated for metallic structures specific derivations for composites are still limited, as they are still based on these original definitions. Almost every study that has been performed leads to the creation of a new formula but these studies tend not to compare back to the original larger structural units. This paper investigates the use of effective breadth for composite top-hat stiffened structures by comparing a number of definitions of effective breadth. It is shown that there is a wide variation in the different definitions and that comparison to realistic structural units is important, to ensure that the individual beams are replicating the behaviour of the full structure. The position of the stiffener is important, with intersection stresses calculated accurately but edge stresses giving poorer results, and a new formula is proposed to account for this