An experimental investigation was conducted into thin-walled square hollow section (SHS) steel beams subjected to a uniform transverse blast load using PE4 high explosive. The aim of the tests was to determine the failure modes and deformations of clamped SHS steel beams. The uniform load was achieved by using equally spaced explosive strips. The mass of the explosive ranged from 10 g to 75 g. A ballistic pendulum was used to measure the impulse. In total six beam sections, divided equally into compact, non-compact and slender categories were tested over a span of 600 mm. The SHS cross-section dimensions tested were 20 x 20 x 1.6 mm, 35 x 35 x 1.6 mm, 40 x 40 x 1.6 mm, 50 x 50 x 1.6 mm, 75 x 75 x 2 mm and 100 x 100 x 2 mm. The failure mode of the beams was generally mode I (large inelastic deformation). The beams exhibited varying degrees of localised flange and global bending inelastic deformation. The compact beams showed a slight localised flange deformation accompanied by a large global bending deformation, whereas the slender beams showed a large flange deformation and slight global deformation. This behaviour conforms to the generally accepted behaviour of cold formed hollow sections under static loading conditions. Existing theoretical rigid-plastic solutions for solid beams were found to give reasonable engineering estimates of the final midspan displacement of compact thin-walled hollow beams. However, these solutions become non- conservative as the slenderness ratio of the section is increased.