Direct design of partially prestressed concrete hollow beams

Tests were conducted on three hollow partially prestressed concrete beams subjected to combined loading in bending, shear and torsion. The beams were designed using the Direct Design Method which is based on the Lower Bound Theorem of the Theory of Plasticity. All beams were of 300 × 300 mm cross-section, 3.8 m in length, and possessed a 200 × 200 mm hollow core and 50 mm wall thickness. The two main variables studied were the ratio in the web of the maximum shear stress due to the twisting moment, to the shear stress arising from the shear force, which varied between 1.09 and 6.85, and the ratio of the maximum twisting moment to the maximum bending moment which varied between 0.35 and 2.62. Good agreement was found between design and experimental failure loads. All beams failed near the design loads and underwent ductile behaviour up to failure. The results indicate that the Direct Design Method can be successfully used to design partially prestressed concrete hollow beams which will cater for the combined effect of bending, shear and torsion loads.