Based on structural deformation analysis, we developed calculation models of externally prestressing tendon stresses increments for simply supported beams on service loads. The Windows program SICPEM for calculating stress increments, based on energy principle, was programmed using the objectoriented method. Six beams results, tested by Tan, and the free length of externally straight tendons were recomputed by the program. The ratio of computed results to test results is 0.895, and the standard deviation is 0.013. Furthermore, the free length of externally straight tendons, for which the secondorder effect can be ignored safely, is ten times beam depth. This coincides with the reference literature.