United States Department of Transportation
i
Load rating and retrofit testing of bridge timber piles subjected to eccentric loading.
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2012-11-01
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Abstract:This report first evaluated the load rating procedure currently in use by the Illinois Department of Transportation (IDOT) for rating timber
piles supporting multiple-span, simply supported bridges. For simplicity, these piles are often rated under concentric loads, and the effect
of bending in the piles is neglected. Recent studies have shown, however, that under highly eccentric live loads, the effect of bending
moments in the piles is of great importance and could have an impact on the piles’ load rating. The report proposed an alternative
structural load rating method for timber piles based on the National Design Specification for Wood Construction (NDS), which took into
consideration the effect of combined compression-flexure behavior of piles. This method was used to conduct a parametric study to
investigate the effect of several geometric and structural parameters on the load rating of bridge timber piles using 3-D finite element
models of concrete deck bridges supported on groups of timber piles. The results showed that the proposed load rating method produced
significantly lower ratings for piles with moderate to high levels of deterioration, as compared to the ratings obtained using the
conventional approach. Among the studied parameters, the length of piles was found to have the most significant impact on the load
rating of the piles. The report also presents a study on examining a fiber reinforced polymer (FRP)-based retrofitting method for timber
piles subjected to combined axial and bending loading. A total of twenty pile specimens were tested in the study, four under compressiononly load and sixteen under compression-flexure load. Each specimen was tested twice, before and after retrofitting with glass FRP
(GFRP) or carbon FRP (CFRP) sheets. To assess the impact of realistic field conditions, different details of the FRP retrofit technique
were investigated, including using mortar shell, introducing a mortar-filled wedge in the tested specimen to mimic the effect of decayed
wood, and “posting” the piles with nails instead of steel drift pins. The test results showed that the strength of the tested specimens using
the proposed GFRP retrofit technique was fully recovered or even enhanced compared to that of the unretrofitted specimens, regardless
of the retrofit details adopted in the tests. On average, specimens retrofitted with GFRPs showed strength 10% greater than that of
unretrofitted specimens. The behavior of CFRP sheets, however, was less satisfactory due to the small thickness of the CFRP shell used
as a result of the high strength of CFRP compared to GFRP. It was also found from the study that using mortar shell along with FRP
sheets helped enhance the stiffness of the retrofitted pile. Finally, linear regression analysis was conducted on the test data to develop a
formula that could be used for the design of FRP retrofit for bridge timber piles subjected to axial-bending loading.
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