Research On The Crack Identification Method And Strain-deflection Transferring Technology In Beam Bridge Monitoring

Bridge is the key part of road traffic, whose traffic capacity directly determines that ofthe whole road. The construction quality, material ageing and the increase in traffic load allmay lead to the degradation of bridge service state, or even the failure in meeting the needsof traffic load increase. In this case, the effective monitoring of bridge to accurately grasp ofthe bridge service condition is essential to avoid bridge collapse, which does great harm tothe people’s life and property. Thus, bridge monitoring is a meaningful task..The so called bridge montoring is to set some monitoring devices in the bridge structureby embedment or other method, so as to timely collect the different responses of the bridge,by which the bridge service state is judged. Of all the monitoring parameters, such asdeflection, strain, displacement, tempreture and dynamic response, deflection is an importantparameter easy to be measured accurately. Because the relationship between the deflectionand the bridge stiffness can be easily found, deflection monitoring of the bridge structure isspecially favored by scholrs. When the bridge steps over the rivers,mountains or urban roads,the traditional contacting displacement measurement devices are difficult to be installed andrisk the artificial damage. What’s more, the static displacement measurement devices (suchas hydrostatic level) can not collect the bridge dynamic deflection information. To handle allthe trouble, it’s a meaningful subject to find a method of obtaining the bridge displacementinformation timely.For beam bridges, there are some inner relationships between the bridge strain anddeflection. So it’s viable to to obtain the deflection information from the timely monitoringstrain data. For the concrete beam bridge, the measured strain data cover the strain arousedby the creep and shrink of the concrete, tempreture, dead load and live load. The obtain ofthe strain caused by live load needs actual measurement. While in the concrete beammontoring, the actual measurement includes the strain caused by creep, load and so on, soit’s necessary to separate the strain caused by different factors.Crack is one of the most common damage in concrete beam bridges, which has effecton the bridge durability, strength and stability. Therefore, it’s meaningful to grasp the correctcrack information for taking some proper miantenaince and reinforcement measures toensure the bridge safety. Many factors may lead to the concrete cracks, such as the load, tempreture chang, shrink, foundation deformation, steel corrosion, frost heaving, quality ofthe construction material and the construction process. The cracks in the concrete beambridge have the following two features：first is the concealment, including the elusive crackcause and the crack location; second is the great amount, to a bridge in service for a longtime, cracks are in a large number. The traditional nondestructive detection method costs alarge amount of labour power and material resources, and is hard to find out the hiddencracks. Study shows that the bridge dynamic parameters have certain sensitivity to cracks.So how to detect the cracks accurately by dynamic parameters is an important researchdirection in bridge engineering field.This paper relies on National High Technology Research and Development Program(“863Program”) of China (Project No.2009AA11Z104) and does some work on the thecrack identification method and strain-deflection transferring technology in beam bridgemonitoring, the researches conducted are as follows:1. Based on the basic definition of the deflection and strain, the relationship isestablished to form the displacement dynamic measurement method for beam bridges bymeans of strain gauge. No displacement measurement devices are needed in this method,and only by the existing strain data can the bridge displacement be analyzed timely to formthe displacement response. Numerical simulation analysis for both the simply-supportedbeam and continuous beam bridge are conducted, and the results show that the proposedmethod in this paper provides a high level precision for displacement static response and thedynamic response of the simply-supported beam and the continuous beam bridge. The errorsof the displacement static and dynamic responses are less than5%with1%random errors instrain.2. With the deep researches of creep mechanism, creep theory and the creep predictionmodel and the combination of the finite element method and the recursive method, the creeptwo-separative-stage recursive method is formed which can separate the strain caused bycreep and load. The Changchun Dong Feng Street Overpass, as a practical project, verifiesthe propsed separative method of this paper. The results show that, the method is practicableand effective.3. Analysis is conducted on the Euler-Bernoulli beam model and the finite elementmodel of simply-supported beam with cracks. Based on the theoretical analysis models, thebeam modal parameters under several crack damage working conditions are calculated andthe influence laws of the crack with and the crack hight on the simply-supported modal parameters are analyzed. Then it discusses the features of the two theoretical models.4. Taking the multi-span simply-supported beam as the study object, the detection ofdamage location and the damage extent are achieved by means of PSO-SVM. This methodfirst uses the change in curvature of flexibility to detect the damage location, then usespaticle optimization-based support vector machine to detect the damage extent. Thenumerical simulation results of a simply-supported beam bridge with five girders show thatthe proposed method is precise and effective. The results also show that PSO-SVM canaccurately identify the damage location of both the single and multiple location damage ofthe multi-girder simply-supported beam with a high level in damage extent identificationwhose maximum relative errors are4.13%...