Slender and thin-walled forms of construction are a natural choice for any large-scale structure employing reinforced concrete. Reinforced concrete flanged beams provide structural strength and stability, and their applications range from buildings, offshore structures, flotation units for housing in marshy land to floating hotels. Tests were carried out on a total of 21 reinforced concrete flanged beams, with overall depth web thickness ratios between 40 and 80. The test specimens were mostly large structural elements, some of which measured 2 m x 4 m overall. The web thickness ranged from 20 mm to 33 mm. The test beams represent some of the most slender forms of reinforced concrete construction attempted in a laboratory. Details of beams, method of construction, test procedure, measurements taken and the observed structural behaviour are described. The main modes of failure observed in the test were diagonal splitting, flexural shear and web crushing. The characteristics of the modes of failure and the mechanisms are discussed. The web crushing mode of failure happens suddenly and can be violent.