ABSTRACT
Within the structural rehabilitation intervention carried out in a steel railway bridge, which included the addition of pre-stress bars, several structural tests were performed to experimentally assess the effects in the bridge structural behavior of the tensioning of the referred pre-stress bars.
The railway bridge, which is the subject of the work presented in this paper, is a steel structure originally built in 1887. It has a total length of 120 m, subdivided in three equal spans of 40 m each.
The bridge structure is composed by two main truss beams with a height of 5.8 m and a distance between them of 5.0 m. These main truss beams have vertical elements with a distance of 2.86 m between them and a grid of cross diagonals at 45° with a double L cross section.
During it’s more than 100 years of lifetime, the railway bridge was subjected to several rehabilitation interventions for the improvement of its structural condition. The last of those interventions was concluded in 2007 although it had been planned and designed about a decade before.
In the referred last rehabilitation works performed in the bridge, one of the interventions that was carried out was the addition and stressing of pre-stress bars connected to anchor blocks placed outside the existing steel structure. It was within this last intervention that the structural tests, which are presented in this paper, were performed.
The structural tests that were performed included static load tests, moving load tests and dynamic ambient vibration measurements for modal identification purposes. A precise topographic levelling of the bridge deck was also carried out to evaluate its vertical displacements. All these tests and measurements were performed before and after the stressing of the added strengthening pre-stress bars.
Besides the experimental work, the developed studies included also the preparation of numerical models of the structure based on the information of the bridge rehabilitation design documents. These models were important for the preparation of the tests and also for the interpretation of the test results.
This paper presents some of the results obtained in the several tests that were performed, comparing the measurements made before and after the stressing of the pre-stress bars. In some cases, the tests results are also compared with the ones computed with the finite element models of the bridge.
The results of the tests that were performed were useful to evaluate the structural effects of the adopted intervention, which are reflected in the following aspects:
– In the introduction of deformations and stresses which are contrary to the ones due to the dead load or to a uniformly distributed live load; this is clearly reflected in the vertical displacements evaluated with the precise topographic levelling and in the measured rotations and strains induced in the structure by the pre-stress bars stressing;
– In a change in the bridge structural behavior, that, after the stressing of the pre-stress bars, exhibits a slight increase in its vertical stiffness, which is reflected in the lower vertical displacements at midspan of the lateral spans, the lower longitudinal rotations of the two main truss beams in the sections over the abutments, and the higher frequencies of the first two vertical vibration modes – with this change in the structural behavior, the tests that were performed showed also that in most of the elements instrumented with strain gauges there was a decrease in the maximum recorded strains for the same applied loads.
The results of the tests performed in the railway bridge are also important for the characterization of its current status and behavior and should be considered in the future monitoring of its condition.
