The Study On Effective Prestress Detection Of Simply Supported Unbonded Prestressed Concrete Beam By Dynamical Method

During their service span of several decades or even about a century, maybe prestressed concrete structures would inevitably lead to damaged accumulation and resistance reduction, cause accidents to happen because of many unfavorable conditions and factors, for example, overload, overuse, corrosion, aging and environmental problems. Therefore, it is urgent and necessary to apply the damage detection and health evaluation to engineering structures including prestressed structures. This task has been one of the most vital research topics in civil engineering fields, which is of great theoretical and practical significance.This project aims to do the research upon the vibrating feature of unbonded prestressed concrete beams, in order to create an optimized evaluating method of effective prestress in prestressed concrete works. The current modeling on which the testing is based seems too simplified, not covering enough factors related with vibrating frequency of prestressed concrete works. In order to make it clear the relationship among effective prestress, its vibration frequency and related factors, the project focuses on the following topics: the effect of stretching eccentricity of prestressed steel bars on structural vibrating frequency; the effect of stretching force of prestressed steel bars on structural vibrating frequency; the effect of stretching eccentricity of prestressed steel bars and stretching force themselves on the damping ratio of structural vibrating frequency. The research findings are as follows: (1) With the condition of the unchanged eccentric distance, the first order vibrating frequency of unbonded prestressed concrete beams increases linearly as the prestress value multiplies, which shows that there exists the effect of prestress values on the first order vibrating frequency of unbonded prestressed concrete beams. (2) With the condition of the unchanged prestress values, the first order vibrating frequency of the experimental beams increases with the eccentric distance increasing, which shows that there also exists the effect of eccentric distance on the first order vibrating frequency of unbonded prestressed concrete beams. (3) When eccentric distance is 0, the slope will be 0.0142; when eccentric distance is 11.21mm, the slope will be 0.001; when eccentric distance is 111.21mm, the slope will be 0.0008; when eccentric distance is -88.79mm, the slope will be 0.0003. These results indicate that the prestress value will have much greater effect on the first order vibrating frequency of experimental unbonded prestressed concrete beams, when the prestressed bars are put close to the core of the project rather than outside of the core. (4) To what extent does the eccentric distance have the effect on the first order vibrating frequency of unbonded prestressed concrete beams just depends on the value of prestress. When the pretress value is little, with the expansion of the eccentric distance, the effect on the first order vibrating frequency of unbonded prestressed concrete beams becomes greater. When the pretress value multiplies, with the expansion of the eccentric distance, the effect on the first order vibrating frequency of unbonded prestressed concrete beams is declining.At last, based on the regressive formula, a new method has been carried out about evaluating effective prestress of prestressed steel bars.