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Research and Development (R&D) Project Sites

Project Information
Project ID:   FHWA-PROJ-08-0024
Project Name:   Improved Analytical Techniques for Historic Covered Bridges
Project Status:   Active
Start Date:  September 8, 2008
End Date:  September 30, 2013
Contact Information
Last Name:  Duwadi
First Name:  Sheila R
Telephone:  202-493-3106
E-mail:  sheila.duwadi@dot.gov
Office:   Office of Infrastructure Research and Development
Team:   Hazard Mitigation Team [HRDI-50]
Program:   National Historic Covered Bridge Preservation Program
Project detail
Roadmap/Focus area(s):   Historic Preservation
Project Description:   Covered bridges are complex structures to analyze as they are constructed with timber, which is highly variable and with designs that are redundant with material properties influencing the behavior. When combined, it is easy to understand why some analyses conclude that some bridges should not even be standing. As noted in the Federal Highway Administration (FHWA) publication FHWA-HRT-04-098, Covered Bridge Manual, there are inconsistencies with the assumptions of the traditional, simple, static analysis of trusses. Generally, in most covered bridge analyses, it is assumed that the primary truss elements behave in a manner similar to steel trusses. However, the fact that timber trusses are more significant means that covered bridge members tend to behave more like frame members (i.e., have both axial and bending forces) than as truss members (i.e., axial forces only). Some of the complexities involve eccentric connections at the joints, various load paths from the stringers and floor beams to the truss, uncertainty of the connections between the trusses and the arches, and interaction between the trusses and their housing. Most engineers are relatively comfortable with completing a standard truss analysis but are unprepared to complete a sophisticated two-dimensional (2-D) or three-dimensional (3-D) frame analysis. As such, much simplified and inaccurate analyses are often performed with overly conservative safety factors applied to account for known inaccuracies. In addition, many covered bridge trusses consist of multiple kingpost trusses superimposed with an arch that has complex connectively between the various elements. An accurate analysis of such systems clearly requires sophisticated analysis techniques with which most engineers are not familiar. Because of the issues discussed above, it is clear that engineers need assistance with the analysis and modeling of covered bridges. This is especially true as they conduct repair and load rating calculations.
Goals:   This study will develop guidelines for improving the analysis of covered bridges.
Product Type:   Research report
Test Methodology:   It is envisioned this will be done through collection of specific behavior data such that modeling geometries and configurations can be optimized and validated. Specific areas of interest include intersection/interconnection of lattice members, impact of classic arch behavior, behavior and interaction of bolster beams, floor systems, distribution beams, influence of bracing, and others. To collect the needed behavioral information, a number of covered bridges need to be instrumented and response data (e.g., strain, deflection, etc.) collected while controlled loads cross the bridge. This data is necessary to develop and calibrate modeling methodologies.
Other Information:   Funding is $130,000 from the Federal Highway Administration) and $39,000 from the Forest Service—Forest Products Laboratory.
Expected Benefits:   This was phase I of the study and has resulted in better understanding of how to analyze wood trusses that make up covered bridges.
Deliverables: Name: Analytical Techniques for Historic Covered Bridges
Product Type(s): Research report, Techbrief
Description: The report will provide guidance on analyzing timber trusses for use by designers.
FHWA Topics:   Roads and Bridges--Design
TRT Terms:   Covered Bridges
Wooden Bridges
FHWA Disciplines:   Design
Subject Areas:   Bridges and other structures


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