In order to investigate the optimum section of beam, the criterion for rupture of beam with unsymmetrical section is proposed, and it is shown that the rupture of graphite beam with unsymmetrical I-section can be accounted for by the criterion. Let n be the ratio of the distance between the outermost layer on tension side and the neutral axis to the height of the beam, the criterion is as follows. (1) For the beam with n > 0.5, [numerical formula] where σ^
- is the modified mean stress taken over the section, σ
s is the stress at outermost layer on tension side, σ
b is the tensile strength of the material. (2) For the beam with n < 0.5, [numerical formula] where σ^
-t is the modified mean stress on the section of tension side.
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