A theoretical technique is presented for computing the stress distribution, the torque-rotation relation, and the torsional capacity for prismatic beams with arbitrary cross-sections subjected to pure torsional loading. The technique may be applied to any beam with isotropic, homogeneous, linear or non-linear elastic properties and a known shearing stress–shearing strain relation. The theoretical analysis is substantiated with experimental data obtained from pure torsion tests on five unreinforced plaster model beams: three circular, one rectangular, and one T-shaped in cross-section. Results are also presented on a second series of twenty-nine qualitative tests made on plaster model T-beams reinforced with small gage wire to determine the crack pattern and failure mechanisms. / Ph. D.