Analysis on stress-strain complete process of unbonded prestressing tendons in concrete subjected to fire

In fire, thermal expansion and thermal creep appear obviously in prestressing tendons and affect the mechanical behavior of the tendons. Based on analysis stress and strain caused by stress, thermal expansion, thermal creep of each micro-segment in the unbonded tendons and total elongation of the tendons at each time step in fire, the analysis method for stress-strain complete process of unbonded tendons in concrete subjected to fire was proposed. And computer program was compiled for the calculation. The calculated values of stress variation of unbonded tendons subjected to fire agreed well with the related experimental results. Analysis results showed that the stress variation trend of tensile non-prestressing steel at mid-span in fire was similar to that of unbonded tendons. That is the stress increases in the early stages in fire, and decreases at some time later. All these may be helpful for understanding fire resistance mechanism of unbonded prestressed concrete structures.