Continuity of bridges composed of simple-span precast prestressed concrete girders made continuous
Department of Civil and Environmental Engineering
Doctor of Philosophy
Saadeghvaziri, M. Ala
Spillers, William R.
Schuring, John R.
Bridges composed of simple-span, precast, prestressed concrete girders made continuous via cast-in-place decks and diaphragms are continuous only for live loads and superimposed dead loads. The continuity diaphragms often crack due to time dependent effects in the girders. These cracks not only impair bridge ascetics and durability, but also reduce "degree of continuity". A related issue is that joint construction is time consuming and expensive due to reinforcement congestion. This dissertation presents a series of field tests, analytical studies, and laboratory experiments concerning the design and performance of this type of bridge.
Based on a survey of the state departments of transportation in the U.S. and a literature review, the current practice is evaluated. Three bridges in New Jersey were instrumented and tested. Results show that the degree of continuity ranges from 0% to 90%. A comparison of the support detail suggests that anchor bolts be sheathed to allow free rotation of the girders.
A computer program called "CONTINUITY" is developed to analyze the restraint moments and the degree of continuity of bridges up to four continuous spans. The program takes into account concrete creep and shrinkage and strand relaxation. For concrete creep and shrinkage, users can choose from three different models: ACI-209 (American Concrete Institute), CEB-FIP (European) and HPC (High Performance Concrete). Support details and cracking of the composite girder and diaphragm sections are also considered in the program.
Three-dimensional finite element analyses have been carried out to further study factors affecting restraint moments. The study confirms that the girder age at continuity plays a vital role in developing the restraint moments and that the amount of positive moment reinforcement at the support has a negligible effect on the resultant mid-span moment.
As part of this research a new continuity connection is developed using Carbon Fiber Reinforced Polymer (CFRP) composites. By making the girders continuous for slab self-weight as well, the additional negative moment over the continuity support will counteract the positive restraint moment and limit it below the cracking moment. Thus, cracks will not form and positive moment reinforcement is not needed in the diaphragm. Total 20 laboratory tests were carried out to validate the new connection. Results show that CFRP is effective for improving the continuity and performance of bridges of this type. Recommendations for the use of CFRP reinforcement and a design example are also presented.
njit-etd2004-128 (174 pages ~ 15,193 KB pdf)
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Created January 10, 2005