Safety Evaluation On The Cable Stayed Bridges For Single Tower Based On Test

With the expansion of the national traffic and speeding up of bridge construction, The long span cable beam complex system is wide public concerned by researchers and designers, single-pagoda cable stayed bridge as a composite system is one of the main structural forms which mechanical behavior is very complex, so we need to experimental test this type of bridge, definite the transmission rules and working conditions between the cable support tower and beam. This paper based on the experimental analysis of the single-tower cable-stayed bridge analyzed the appearance of the bridge situation,the geometric condition of the bridge structure, the cable force parameter, the static force parameter, the dynamic parameter and safety evaluation. The main research contents as follows:(1) Appearance and geometric state of the bridge is tested.â‘ The tests of the anchorage zone of cable and tower as well as the anchorage zone of cable and beam, dampers between tower and beam, dampers of stayed-cable on the bridge deck, bearings and extension joints, cable bent tower and piers as well as the surface state of their concrete show that the presentation quality of southern bridge is good, obvious damage and defects have not been discovered.â‘¡After seting up GPS measuring halftone dots of main tower ,cushion cap and deck control points, 5 sets of Ashtech UZ-12 dual-frequency GPS receivers are used for synchronously and statically relative with e fix observation of established halftone dots of GPS control network according to B-class network standards,observational data can be used as technical information of late maintenance management and health monitoring.â‘¢It can be seen from the measurement results of deck height that the measured value of deck height is slightly higher than the design value of deck height, the value of span centre is slightly higher,the depth displacement is gradually decreasing near to the pivot point , the vertical curve of deck is continuous and smooth,the line shape of deck is good.(2) Ambient excitation frequency method is used to test the dead load cable power of the 80 kinds of stayed cables of the whole bridges.The results of the test show that the variation of dead load cable power of two rows of southen waterway bridge longitudinally inosculate the designed andcalculated value. The upsteam and downsteam cable power is closed to its corresponding cable power value, which shows symmetrical, the distribution of observed cable power accords with the general regularity of cable of stayed bridges.(3) Static test is prefomed on the bridge.â‘ The actual measuring strains in testing sections of steel box girder have a relatively good regularity under different working conditions, which indicates that actual measuring parameters and structural analysis can reflect the mechanical characteristics of real bridge structure, the mechanical behaviors of main girder meet the design requirements; it can be deduced from the absolute value of main girder actual measuring strains that main girder has a biggish safety margin; the shear lag effect of main girder has little effect on the structure; main tower works in the elastic stage.â‘¡Under test loads, the strain increment of main tower steel anchor box is relatively small, measuring data are discrete, it can be seen from the distribution of strain value that steel anchor box is whole section compressed and its bending effect is not obvious; the absolute values of measuring and theoretical principal stress are both small, therefore the structure is safe; From comparison between measuring and theoretical deflection value of control sections it can be seen that stiffening girder works in the elastic stage.â‘¢The distribution of deflection measuring value under main working conditions along the longitudinal bridge span is in accordance with that of the theoretical deflection curve, deflection measuring value in control points is smaller than that of theoretical ones, under central loaded condition, stiffening girder deflections of observation points in the same location of the upstream and downstream are almost the same; under eccentric loaded condition, eccentric load effect is more obvious near the acting position of testing cars, as the distance from testing cars increases, eccentric load effect decreases gradually, the transverse distribution of deflection is more even when it passes over adjacent fulcrum, which indicates that lateral stiffness of the main girder is large, and its torsional behavior is good.â‘£The test result of the cable force incremental of the typical work condition shows that: The measured cable force increment is less than the result of theory, The measured results are all uniformity distribution than theoretical value, This show that the main beam of the actual stiffness is larger than the theoretical calculation of stiffness; After the main operating condition test, It notes that the overall stiffness of the main tower is larger, The main work condition of the horizontal displacement of the stiffen girder beam-ends is all less than the theoretical values, The horizontal displacement indicator of the stiffen girder meets the design requirements.(4) Dynamic testing is performed on the bridgeâ‘ The measured Vibration mode shape of the first five bands of the Main Bridge is Consistent in the theoretical analysis value. Also,the emerging order of mode shape of the measured results is as the same the theoretical calculation. That is,The first two mode is mainly the vibration of the cross-bridge,and the next three mode is mainly the vertical vibration of the Main girder.â‘¡The main bridge's the first five frequency bands measured is slightly larger than the theoretical analysis' results. It shows that the overall dynamic stiffness of the main bridge is good.