In Situ Load Testing of Bridge A6358 and Bridge A6101 and Bridge A6102

The scope of this project is the assessment of three High Performance Steel (HPS) using innovative measuring techniques. For each of these bridges a report is presented. The following is a brief description of the bridges, and of the technologies and analysis used to assess them.

Bridge A6101 and Bridge A6102

Both bridges were just opened to traffic at the time of the load test. They were designed using a standard MS18 truck with military 106 kN tandem axle. The bridges were built with two continuous 42 m long spans with central support consisting of a reinforced concrete (RC) bent supported by three RC circular piers. Bridge A6101 is 12.82 m wide, while Bridge A6102 is 14.02 m wide. Both bridges carrie two lanes of traffic. The superstructures consist of two continuous spans having a skew angle of about 12 degrees.

The bridges deflection under static load was measured using a robotic tacheometry ("total station") system (RTS) that offers a non-contact deflection measurement technique. In this project, both RTS and conventional extensometers (LVDTs) were used to measure the bridge deflections. The RTS was used to monitor the deflection of 19 points located on the bridge girders. The LVDTs were primarily used to measure the dynamic deflection of the bridge; they were mounted in correspondence to three prisms monitored by the total station in order to validate the accuracy of the latter during the static test. The comparison between theoretical results according to AASTHO Standard Specifications and experimental data and between static and dynamic loads allowed establishing the safety of the structure. A Finite Element Method (FEM) analysis was undertaken. The numerical model was able to represent the behavior of the bridges and therefore could be used to determine their load rating.

Bridge A6358

Bridge A6358 was recently open to traffic. The bridge is built with five continuous symmetrical spans: the two external are 45 m and 56 m long, respectively, while the central one has a length of 61 m, resulting in a total bridge length of 263 m. Each internal support consists of RC bents supported by two RC circular piers having a 2 m diameter. The cross section comprises five composite, equally spaced, HPS I-girders acting compositely with a 216 mm thick RC deck, with an out-to-out deck and roadway width of 12.4 m and 11.6 m, respectively.

Two different innovative techniques for Structural Health Monitoring (SHM) were used:

1. An optical non-contact measurement technique, based on an automated target recognition total station system, used to measure the vertical deflections of the girders during the pouring of the concrete deck and the diagnostic load test, conducted after the completion of the bridge;
2. A distributed Brillouin sensing technique, utilizing a smart GFRP tape with embedded sensing fibers and bare optical fibers, used to determine the strain distribution at different depths of the web for some of the girders. Strains were measured during the diagnostic load test only, since at the time when the deck was poured the optical sensors had not been installed yet.

Experimental deflection and strain data were verified to be consistent with each other and allowed to evaluate the actual girder distribution factors. The comparison between experimental and theoretical results based on the AASHTO LRFD Bridge Design Specifications (AASHTO, 1998) allowed establishing the safety of the structure, although pointing out a significant drop in the expected behavior of one of the external girders, which may call for further assessment.

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