Research On The Long-term Loss And The Method Of Construction & Detection Of Vertical Prestress

Prestressed concrete continuous(rigid frame) box-girder bridge structural systems are widely used and develop rapidly.With the increasing of the span,in order to reduce and control the principal tensile stress at the webs,it becomes an important design aspect to set vertical prestressed steel bars in box-girder webs.But after the vertical prestressed steel bar is set, cracking in webs is still widespread.Excessive loss of vertical prestress is the main reason which causes the web crack of the box-girder bridge.The calculation of vertical prestress loss lacks of the support of theories and practice,especially disturbed by man-made factors and it is difficult to establish the effective computing model.In addition,there is no strong detection method to monitor the construction of vertical prestress,and eventually it leads to excessive loss of vertical prestress accompanying anchorage,even failure of prestress.This paper focuses on several aspects of the above-mentioned,the specific study contents as follows:(1) Use the wireless data acquisition system,make a n-day data collection of vertical prestress,use GM(1,1) gray prediction model,and get the long-term vertical prestress loss prediction formula and the vertical prestress loss prediction of 1200 days.At the same time,simplify the web model of vertical prestress,choose different wet-week parameters and formulas of the old and new Bridge Regulation,compare the calculated values with the predicted value,and finally determine the calculation model which is fit for the long-term loss of vertical prestress.(2) Through theoretical analysis and field test,for excessive losses of the vertical prestress and the non-uniform stress distribution of webs, study the nature of tightening nuts,refer to the construction method of vertical prestress based on stretch-torque methods,combine vertical prestress tension lag with the tension technology of the second process,and finally refer to the Comprehensive Construction Guide for the vertical prestress construction method.(3) By comparing a variety of stress measurement method,first of all, combine the characteristics of vertical prestress bars to determine the frequency method to measure pretension force,and then derive the vibration equation of the cantilever beam with the boundary condition of an elastic rotational stiffness.By the analysis of sensitivity of solid cantilever vibration parameters,the influence degree of the length,diameter of vertical prestress bars,measuring sensors,and processing errors on vibration frequency is got.Finally by laboratory tests detect of the frequency of the exposed bar of different length and different tension,the modified calculating length is obtained to substitude the affect of nut,so to inversdeduce the vibration stiffness of the exposed bar segement,then come to the relationship conclusion of exposed lengths,tonnages with the vibration frequencies.