Experimental Research On Damage Identification Of Concrete Bridge Structure

Structural damage identification is the important components of the bridgestructural health monitoring system. The structural damage identification based on thedynamic response is the current research focus. The paper is supported by the nationalnatural science foundation of china (51078134), it studied the model updating ofconcrete bridge structure and structural damage identification based on stochasticsubspace method. The main contents include:I. Modal test and model updating of cable-stayed bridge(1) Modal testing was carried out on a single tower concrete cable-stayed modelbridge with a12m main span. A serial of modal tests on the model bridge were doneby hammering test, the test results including the vertical and reverse mode of the mainbeam, the cross-bridge mode of the main beam, the bridge tower in-plane mode andthe tower out-plane mode, the measured modal results were integrated to provide abasis for model updating.(2) Model updating of the cable-stayed bridge was done based on dynamic testand static test results, the updated model was verified too. According to thecharacteristics of Π section beam, a beam plate model was built by ANSYS software.Optimized parameters were chosen through frequency sensitivity analysis, andboundary conditions were simulated by spring elements. Mode shape was classifiedand modal matching between the same type of mode shape, practice shows that themethod can solve the modal matching problem of complex structure. The objectivefunction of model updating was constructed by dynamic parameters and staticdisplacement. After the model updating, not only the static displacement and modalresults which involved in optimization are more consistent with the measured values,but also the high level frequencies which not involved in optimization are closer tothe measured values. Finally, the updated model was verified by the modal test resultof another state.II. Damage identification method based on stochastic subspaceStochastic subspace-based damage indicators were analyzed and evaluated, anew damage indicator (I A) is proposed. The results of simulation show that the modelresidual damage indicator, the covariance of Hankel matrix damage indicator and thediscrete state space matrix damage indicator can identify the damage degree effectively; indicator value versus damage degree is almost linear. The proposeddamage indicatorI Ais the most sensitive to damage, with the same damage degree thevalue of it is the maximum. When use the model residual damage indicator, the effectis better when the order is slightly larger than the actual order. When use thecovariance of Hankel matrix damage index, the effect is better when the order issmaller than the actual order.III. Damage experimental study on concrete cable-stayed bridge(1) The impact of prestress to frequency of model bridge was tested. Tower andbeam frequency increased with the increase of prestressing. After the tower tensionedprestress, the first and the second out plane frequencies were increased1.98%and2.81%respectively. After the beam tensioned prestress, the maximum of the first fourfrequencies was increased1.69%. The reason may be parts of the concrete micro-cracks are closed after tensioning, which improved cross-section stiffness. In addition,after tensioning the tendons and the concrete structure become an entity, the two partswork together, so the stiffness of the structure increased.(2) The cable force change of four beam damage cases were tested, cable force ofthe side span all has a small increase under each case. The cable force change of case1and2is small and the variation rule is not obvious. The cable force decreasesobviously in damage region of case3and4, which can be able to judge the location ofdamage, when the beam is serious damaged, the damage location can be find by thechange of cable force.(3) The damage tests were carried out on beam and downstream side tower. Ineach damage case, the stochastic subspace-based damage indictors were used toidentify the damage. The results show that: the observation standard damage indicatorcan not identify the damage correctly; the model residual damage indicator isdifficulty to identify the damage; the covariance of Hankel matrix damage indicator(I U、I UV) and the discrete state space matrix damage indicator (I A) can effectivelyidentify whether the structure damaged or not and the damage degree,I Aindicator isslightly better thanI U(orI UV) indicator on the sensitivity and correlation of damagedegree.(4) The damage degrees of tower and beam were analyzed by model updatingmethod quantitatively. By assuming the elastic modulus decreases in damage regionelement to simulate the damage. In the maximum damage degree, elastic modulus indamage region element of tower and beam were reduced to61%and88%of theundamaged state. IV. Full scale model test of prestressed concrete box girder(1) A nonlinear analysis program of prestressed concrete beam was compiledused the finite strip method, and compared the analytical results with the experimentresults, it can be seen that the program is accurate and reliable. The static stiffness ofeach damage state can be calculated by the program. The unloading curve wascalculated by a simplified method which corresponds well to the measured curve.(2) The crack distribution of box girder and hollow slab was analyzed using boxcounting method. The results show that: the surface crack distributions of the boxgirder and hollow slab possess definite fractal character. With the gradual increase inthe damage severity the box-counting dimension increases gradually. Approximately abilinear relationship exist between the fractal quantities derived from the surfacecrack distributions and the natural frequency, the turning point is the same as thefeature point at which the prestressed reinforcement yields of load deflection curve.(3) The dynamic and static stiffness of each damage state was analyzed, theresults show that: at the undamaged state, static stiffness and dynamic stiffnessbasically the same, the decease of dynamic stiffness at each damage state issignificantly less than the decrease of static stiffness. The damage form of dynamicstiffness and static stiffness is consistent; it can be use the static stiffness mode toidentify the dynamic stiffness.