For the economy of material in composite building constructions of ceiling and coverage prestressing of a lower subdiagonal is used. To simplify the process of optimal cross-section elements selection in the combined subdiagonal systems and their geometrical descriptions equation of a strain unbreak has been used. The example of the 6m span combined steel construction calculation that works in composition of the monolithic reinforce-concrete ceiling has been executed with the prestressing of a lower subdiagonal and the analysis of the results. As the research has shown, prestressing in such a construction allows decreasing a cut of an upper steel beam by 20–24%.

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