United States Department of Transportation
i
Field verification for the effectiveness of continuity diaphragms for skewed continuous P/C P/S concrete girder bridges.
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2009-10-01
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Edition:Final report; April 2006-March 2008.
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Abstract:The research presented herein describes the field verification for the effectiveness of continuity diaphragms for
skewed continuous precast, prestressed, concrete girder bridges. The objectives of this research are (1) to perform
field load testing on the Burlington Northern Santa Fe (BNSF) overpass and compare measured strains with those
determined through the theoretical analyses and (2) to study the effects of continuity diaphragms on stresses and
deflections from truck loading on bridge deck slab and bridge girders.
The current design concept of continuity diaphragms was examined to determine the effectiveness of the
diaphragms in skewed bridges. The bridge parameters that were considered include skew angle, length of the
span, beam spacing, the ratio of beam spacing to span (aspect ratio), and the ratio of girder stiffness to that of the
slab. A prestressed concrete bridge with continuity diaphragms and a skewed angle of 48° was selected by a team
of engineers from the Louisiana Department of Transportation and Development (LADOTD), the Louisiana
Transportation Research Center (LTRC), the Federal Highway Agency (FHWA).
The BNSF Overpass Bridge is located on US-90 in Jennings, Louisiana. The field verification was performed
using a comprehensive instrumentation plan and live load tests as described in this report. The field and
theoretical results from this study provided a fundamental understanding of the load transfer mechanism through
these diaphragms of skewed, continuous span bridges. The findings in this study on stresses, strains, and
deflections in the bridge deck and girders indicated that the effects of the continuity diaphragms on skewed
continuous span precast prestressed concrete girder bridges were negligible. The results presented in this report
also confirmed the theoretical findings published in LTRC Report 383 titled “Continuity Diaphragm for Skewed
Continuous Span Precast Prestressed Concrete Girder Bridges.” Continuity diaphragms used in prestressed
concrete girder bridges on skewed bents provided additional redundancy in the bridge but caused difficulties in
detailing and construction. As the skew angle increases or the girder spacing decreases, the construction becomes
more difficult and the effectiveness of the diaphragms becomes questionable. It is also recommended that the use
of continuity diaphragms be evaluated based on the need for the enhanced structural redundancy, the reduced
expansion joint installation and maintenance costs, and the associated construction difficulties and costs. The
outcome of this research will reduce the construction and maintenance costs of bridges throughout Louisiana and
the United States.
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