Traditionally, joints are either assumed to be pinned or fully rigid, but in reality many can possess a significant level of stiffness giving them a semi-rigid classification. For low-rise steel frames, the stiffness in such semi-rigid joints may be sufficient for serviceability limit state (SLS) design to control the sway deflection. This paper presents a design procedure on how the semi-rigid design approach may be applied to steel frames using Hollow Structural Sections (HSS) as columns and blindbolted endplate connections. It will assess the accuracy of a simplified hand calculation method for lateral deflections with semi-rigid connections and compare this with that obtained from using the wind moment design method in the Steel Construction Institute (SCI) Publication P2631. The proposed method can be used to quickly estimate the required semi-rigid connection stiffness to control sway deflections. This paper then presents a method to determine endplate connection detailing to HSS columns to achieve the required connection stiffness using the stiffness equations recently derived by the authors for the HSS component and the existing equations in Eurocode 3: Part 1-82 for the endplates and bolts. A. Y. Park, MEng (Hons) School of Mechanical, Aerospace and Civil Engineering, University of Manchester, UK Y. C. Wang, BEng, PhD, CEng, FIStructE School of Mechanical, Aerospace and Civil Engineering, University of Manchester, UK