Bridge Load Testing
- Objectives: Measure bridge’s loadbearing capacity
- Solution: Jewell Instruments Series 700 Tiltmeter
- Benefits: High-precision and exceptional repeatability
- Results: Achieved a more accurate assessment of capacity
Tiltmeters installed on a scenic, but sagging, concrete bridge in California’s historic 49er Gold Country measured structural response to a series of load tests. Project engineers rated the tiltmeter results “more accurate” than theodolite measurements that were also performed.
Parrotts Ferry Bridge, part of the New Melones Dam Project completed by the Sacramento District of the U.S. Army Corps of Engineers, spans the Stanislaus River near Vallecito, California. Howard, Needles, Tammen & Bergendoff (HNTB) designed the bridge as a three-span, cast-in-place, post-tensioned single cell box girder structure with a center span of 640 ft and two end spans of 325 ft. T.Y. Lin performed the design review. In 1979 S.J. Groves and Sons complet- ed the structure after a construction period lasting two years.
The deck elevation is 350 ft above the former streambed (inundated by New Melones Lake as a result of the dam project). The two bridge piers are about 225 ft high. The superstructure, of lightweight concrete, was erected using the segmental balanced cantilever construction method. It was post-tensioned longitudinally, vertically and transversely by high- strength bar and strand tendons.
Creep deflection during the five months after con- struction resulted in midspan sag of about 11 inches. Over the next 10 years, inspections and surveys dis- closed an additional 11 inch deflection of the middle span. Concerned about the nearly 2 ft of displace- ment that had occurred, and the fact that the mid- span deflection was growing larger, the Corps decided to investigate these phenomena.
T.Y. Lin led the study team. A critical aspect of the 1989 field investigation was load testing. The objec- tives of the load testing program were 1) to deter- mine the stiffness of the concrete and 2) to evaluate whether the overall stiffness of the bridge has deteri- orated beyond safe limits.
Two gravel trucks, each with a double trailer, pro- vided a static test load of 140 kips (about 70 kips or 70,000 pounds each). Care was taken to ensure that the loads were symmetrical with respect to the longitudinal centerline of the bridge. All three spans were load tested.
Structural response to the imposed loads was mea- sured in two ways: 1) deflections were measured with a theodolite and an electronic distance measurement device, and 2) rotations were measured using 700 Series Ultra Precision tiltmeters supplied by Jewell Instruments. One of the problems with the deflection data, according to T.Y. Lin, was their degradation by temperature effects.
“Though the deflection measurements were correct- ed for temperature effects as far as possible, it was felt that the rotation (tiltmeter) measurements were more accurate,” said T.Y. Lin.
The study disclosed another important advantage of the rotation measurement approach: tiltmeters can record continuously, revealing the complete history of flexure during the loading/unloading cycle.
The overall investigation, which also involved lab- oratory testing and computer modeling, established that the bridge superstructure’s unusual profile is mainly due to creep of the lightweight concrete. T.Y. Lin developed recommendations for controlling sag over the life of the bridge including annual surveys and inspection.
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