Based on the experimental resultof4 three-span externally pre-stressed concrete continuous
beamswith different pre-stress degrees, reinforcement ratios and sections, the stress increment of
externally pre-stressed tendonswas investigated. The result shows that the stress increment is directly
proportional to the deflection atmid-section in the middle span, and the relation among the stress
incremen,t the deflection and the load may be described using a model of tri-broken line. In this
mode,l crackings of themid-section and themid-support section are taken as its turning points. As a
resul,t a simplified method of calculating the stress increment based on the elastic theory was
proposed, the calculated resultbeing accordedwith thatof a non-linear analysis.