عملکرد پل آهنی در حال سرویس با تاندون های بیرونی از پیش تقویت شده

In this study the effect of strengthening on a bridge by means of external prestressing tendons was investigated. The target bridge was a plate girder bridge which had been in service for 23 years before the strengthening. The loss of prestressing force with time was investigated while the existing prestressing force was released/re-prestressed after four years of service since the strengthening. Also a field load test was performed using a live-truck load to evaluate the behavior of the bridge before and after the strengthening. An additional numerical model using the finite element method was developed and used in the analysis. From the experiments, it was found that the long term loss of the prestressing force on the external tendons was negligible and that the strengthening did not significantly affect the dynamic responses of the bridge. However, the strengthening reduced the mid-span deflection by 10–24%. The strengthening also reduced the difference in deflection between girders while the truck was riding on the outer girder and this difference arising from the strengthening became more apparent as the truck speed increased. It was also found from the dynamic load test that the strengthening did not significantly influence the natural frequency and impact factor.

Bridge structures sustain repeated live load, and, accordingly, their structural capacity diminishes continuously as the service period increases. In addition, an increase in traffic load can accelerate the deterioration and can make the load-carrying capacity of the bridge lower. Therefore, a rational strategy for maintenance, rehabilitation and strengthening should be sought and applied in the field as required. In the upgrading process, strategies including construction of a new bridge should be considered in order to minimize cost. Generally the construction of a new bridge will be the most expensive solution not only because of construction cost but also because of other invisible extraneous costs resulting from traffic jams, increase of fuel consumption, etc. For these reasons, various alternative ways have been suggested to upgrade existing bridges. For steel bridges, strengthening with external prestressing tendons has been known to have great advantages among others: (1) its structural analysis for design is relatively easy and fairly straightforward; (2) its effectiveness has been well recognized through many researches; and (3) this method can economically minimize the disruption of the traffic flow [1] and [2]. This study used an in-service bridge that had been, after 23 years of service, strengthened with external prestressing tendons for upgrading to a higher rate. After four years of service the prestressing force was released and re-applied for the testing purposes. While the existing prestressing force was released and re-stressed, the behavior of the bridge and the loss of prestressing force were investigated. A field load test utilized a live truck-load for dynamic behavior evaluations. In addition, a numerical analysis was performed for a further in-depth analysis of the static and dynamic behavior.

The study herein investigated the behavior of a plate girder bridge strengthened with external prestressing tendons. The field loading test was performed using a live-truck load to evaluate the behavior of the bridge before and after strengthening. In addition, analyses using the numerical FEM model and analytical equations were performed for further discussions. Based on the research studies presented herein, the following conclusions are given: (1) The long term loss of prestressing force on the external tendons was found to be very small. The compressive strain exerted by the prestressing force in the field was in a good agreement with the results from both the FEM modeling analysis and the suggested design equation for strengthening. (2) The strengthening with external prestressing tendons did not significantly affect the shape of the dynamic responses of the bridge. However, the strengthening reduced the mid-span deflection by 10–24%. It is also anticipated that the strengthening increases the load-carrying capacity for live load. (3) It was found that the strengthening reduced the difference in the deflection between girder 1 and girder 2 while the truck was riding on girder 1, and also that the reduction of the difference became more apparent as the truck speed increased. (4) The effects of strengthening on both the natural frequency and the impact factor were found to be negligible from the dynamic load test. The measured natural frequencies before strengthening did not deviate much from the values obtained after strengthening. A similar tendency was observed for the natural frequencies obtained from the analytical equation and the FEM model analysis.