# Plastic Properties of Prestressed High-Strength Steel Reinforced Concrete Continuous Beams
## 4 Test Results and Analysis
### Steel Strain Analysis and Plastic Hinge Zone Determination  
The tensile strain of the steel flange can be monitored using the steel strain gauges arranged on both sides of the middle support. When the test beam reached the failure state, the strain distribution curve of the steel tensile flange in the plastic hinge section was observed. This is shown in Figure 8; the horizontal red line in all the strain distribution curves is the yield strain value of the steel tensile flange. When the compressive strain of the concrete edge of the compression zone of the control section at the middle support of the continuous beam reached 3300 \(\upmu\varepsilon\), the section curvature at this time was taken as the ultimate curvature. When the strain of the tensile flange of the test  
Figure 8: Strain distribution curve of the steel tensile flange in the plastic hinge zone. (**a**) A\({}_{1}\)Q\({}_{3}\)S\({}_{4}\)P\({}_{2}\)R\({}_{5}\); (**b**) A\({}_{2}\)Q\({}_{4}\)S\({}_{4}\)P\({}_{1}\)R\({}_{5}\); (**c**) A\({}_{3}\)Q\({}_{4}\)S\({}_{4}\)P\({}_{2}\)R\({}_{5}\); (**d**) A\({}_{4}\)Q\({}_{4}\)S\({}_{4}\)P\({}_{3}\)R\({}_{5}\); (**e**) A\({}_{5}\)Q\({}_{6}\)S\({}_{2}\)P\({}_{2}\)R\({}_{5}\); (**f**) A\({}_{6}\)Q\({}_{6}\)S\({}_{4}\)P\({}_{2}\)R\({}_{5}\); (**g**) A\({}_{7}\)Q\({}_{6}\)S\({}_{5}\)P\({}_{2}\)R\({}_{5}\); (**h**) A\({}_{8}\)Q\({}_{6}\)S\({}_{4}\)P\({}_{2}\)R\({}_{6}\); (**i**) A\({}_{9}\)Q\({}_{6}\)S\({}_{4}\)P\({}_{2}\)R\({}_{8}\).  
The curvature ductility of the section is an important index of the plastic performance of the continuous beam. The curvature ductility coefficient of the section is defined as \(\mu_{\Phi}=\Phi_{u}/\Phi_{y}\)[46], where \(\Phi_{u}\) is the ultimate curvature of the section, and \(\Phi_{y}\) is the curvature of the section steel when it yields. According to the plane section assumption, the ultimate curvature and yield curvature of the beam section are obtained by the strain of the upper and lower flanges of the steel and the strain of the concrete. The curvature ductility coefficient of the test beam is shown in Table 6.  
The curvature ductility of the specimens with high-strength steel was reduced due to the large increase in the yield strain. When the steel strength grade of the composite beam is changed from Q355 to Q690, the yield strain is increased by 76.8%, and the curvature ductility of the section is decreased by 44.4%. Reducing the steel ratio of high-strength steel specimens can enhance the curvature ductility of the section, which is conducive to giving full play to the plastic deformation capacity of prestressed high-strength steel reinforced concrete continuous beams. After the steel ratio of the Q690 steel specimens decreased from 5.65% to 4.55% and 2.88%, the section curvature ductility increased by 8.69% and 26.95%, respectively. After the steel ratio of the Q690 specimen decreased from 4.55% to 2.88%, the curvature ductility of the section increased by 16.8%.  
With increasing steel strength grade and steel ratio, the height of the relative compression zone of the middle bearing control section increased. The actual plastic hinge zone shortened with increasing steel strength grade and steel ratio. After the steel strength grade changed from Q355 to Q690, the actual plastic hinge zone length decreased from 385 mm to 300 mm. Increasing the comprehensive reinforcement ratio and the stirrup ratio can enhance the plastic development ability of the control section of the middle support, increase the ratio of the ultimate bending moment to the yield bending moment of the control section of the middle support, and extend the actual plastic hinge zone. After the comprehensive reinforcement ratio was changed from 0.36% to 0.52%, the actual plastic hinge zone length was expanded from 290 mm to 362 mm.