Evaluation of the Luling bridge retrofit details under service loads : final report.
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1991-08-01
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Alternative Title:Evaluation of the Luling bridge retrofit details under service loads.
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Abstract:Extensive strain measurements were carried out on three cross girder boxes used to connect the cable stays to the orthotropic deck-trapezoidal box steel structure. The measurements were obtained at CG3, CG4 and CG5 adjacent to the tower at pier 2.
The measurements wre focused on details which had known cracks that had been retrofitted by installing holes in the trapezoidal box girder webs at the crack tip. the hole placement resulted in several residual uncracked web segments. These uncracked segments could be modeled for crack growth based on measured stress histories and compared with field observations of crack extension.
The strain gages were installed in October 1986. The gages were protected and connected to junction boxes accessible from the bridge deck. Two types of stress measurements were acquired. On November 4, 1986 two test trucks with known weights of 80,140 pounds and 82,180 pounds were used to obtain the structural response as they traveled across the bridge either side-by-side in two lanes or in tandem in the traveling lane. Test runs were made in the southbound and northbound directions at a crawl speed of about 5 mph and at 60 mph.
In addition, measurements were made under regular trunk traffic for the four day period from November 3 to November 6, 1986 using a magnetic tape recorder for continuous analog records. The frequency of trucks was low as only light traffic used the bridge. The measured maximum stress range at the details with cracks varied from 0.1 ksi to 0.74 ksi. These levels of stress range did not appear to exceed the crack growth threshold. Hence no crack extension would develop from the truck traffic using the structure.
During the measurements over 15-1/3 hours, a single large stress range excursion occurred at CG5. The maximum stress range was 16.3 ksi at one of the retrofit holes. It was hypothesized that this resulted from a wind gust and/or an unknown structural release.
Because of the low live load response and single high stress event, no additional measurements were deemed desirable until the connecting roadways are completed and much higher traffic frequency develops.
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