The influence coefficient method of analysis has been used to derive closed-form expressions for the mid-span deflection and end rotations of shear deformable uniform section beams with semi-rigid end connections. The formulae have been recast into performance indices which define the reduction in mid-span deflection, the increase in load carrying capacity and the increase in span relative to an otherwise identical simply supported beam for two practical load distributions: (1) a point load at mid-span and (2) a uniform load over the entire span. Expressions are also presented for the required rotation capacity of the semi-rigid end connections. Initial rotational stiffness data, derived from full-scale tests on web and web and flange cleat connections between two sizes of pultruded glass reinforced plastic (GRP) WF-section are used in the formulae to evaluate the performance indices for the practical range of span-to-depth ratios for load case (1). The values obtained quantify the benefits to be derived from exploiting semi-rigid end connection stiffness in the design of pultruded GRP beams with the current, very limited, range of section sizes.