Starting with the calculation method of ultimate bearing capacity of rectangular section beam with double layer steel-bar in concrete structure design joints and general design principle,construction method of upper longitudinal steel-bars of primary beam is analyzed in theory.

Analysis of the affecting factor of single-steel rectangle section beam bearing capacity;

Considering the damage of concrete and steel as well as the condition that materials are deteriorated with time at the same time,the time-dependent computation model of existing bending structure with double reinforced rectangular section is studied according to the design and computing formulas in the code.