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Federal Highway Administration Research and Technology
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|Publication Number: Date: Winter 1997|
Issue No: Vol. 60 No. 3
Date: Winter 1997
The National Bridge Inventory (NBI) includes more than 475,000 bridges (excluding culverts) made of various materials, including concrete, wood, and steel. More than 40 percent of the bridges listed in NBI have superstructures made of steel. These steel bridges range in age from new bridges, built to present day material and design standards, to bridges built before the turn of the century. The age distribution for these bridges is shown in figure 1, which indicates that more than 63,000 steel bridges were constructed before 1940, when the concept of closely controlled chemistry, manufacturing, and material properties first began to emerge.
These older steel bridges must be periodically evaluated to determine if their load-carrying capacity is sufficient to support modern traffic requirements. This type of analysis is often complicated by limited information concerning the material properties of the steel. Material properties that are critical to this type of evaluation include the yield strength, ultimate strength, and toughness of the steel. Lost plans, incomplete records, and minimal testing during fabrication make determining these properties difficult without removing samples for testing, and removing samples is generally not a viable option.
Although much research has been done on old bridge steel, this type of information is not easily accessible. Studies have been performed by many different state highway agencies and research organizations. Individuals from one agency may not be aware of the studies done by others, so the information gained cannot be transferred and used. The goal of the Historic Bridge Steel Database is to consolidate information from various studies so that it can be widely used. Test results included in the database were obtained from a recent Federal Highway Administration (FHWA) study of historic bridge steel, along with results of previous studies performed by both FHWA and other agencies. Steels included in the database were primarily those manufactured before 1940 although some steels produced later in this century have been included.
This searchable database includes a collection of material properties such as hardness, yield strength, ultimate strength, percent area reduction, and percent elongation. Hardness values give the user an indication of a steel's ability to resist deformation and can be correlated with tensile strength. The yield and ultimate strength values are used to analyze the load-carrying capacity of the structure. The values of percent area reduction and percent elongation are key indicators of a material's ductility at controlled loading rates.
Graphs of the stress-strain curves and of the Charpy V-notch toughness are also provided in the database. In addition to showing the yield and ultimate strength, the stress-strain curve displays the material's modulus of elasticity. Charpy V-notch tests are used to determine fracture toughness -- a key parameter in determining the risk of brittle fracture and structural collapse.
The database also includes the chemical analysis of the various steels, and this factor can be used as a correlational value to relate the material properties of similar steel. This analysis may also be used to find the weldability of the steel.
Presently, there is data from hundreds of tests that were run on more than 130 specimens of steel from approximately 30 bridges. The data taken from these tests can be used to identify general trends of steel from different eras. The database can be searched to find test results for steel based on the year of manufacture or any of the material properties listed in the database.
For example, to analyze a bridge with unknown material properties, the database may be queried to identify test results from steel manufactured in the same period. The search can be refined by including other known data, such as the chemical content of the steel or the hardness. Properties such as these can be found by testing a small core sample about 25 millimeters in diameter. Testing the small sample is more practical than removing full-sized test specimens for tensile tests.
Figure 2 illustrates the typical query form with all the parameters that may be used to define a search. A query may include as few or as many parameters as desired, and the parameters can be changed at any time during a search.
The Historic Steel Database includes a large array of data on the performance and properties of old bridge steel. This data has not been previously available in one location. This type of data is critical to maintaining the high performance, reliability, and efficiency of the aging infrastructure, and ensuring that safety is maintained as bridges age and traffic loads increase. The database can be a powerful tool for evaluating the safety of older bridges and for deciding to rehabilitate or replace those structures.
The database is available on the Internet and can be accessed through the home page of the Turner-Fairbank Highway Research Center at http://www.tfhrc.gov. For more information on the Historic Steel Database, please contact Glenn Washer at (202 493-3082.
Glenn Washer is a research engineer in the Special Projects and Engineering Division of the Office of Engineering Research and Development at the Turner-Fairbank Highway Research Center in McLean, Va. He has a bachelor's degree in civil engineering from Worcester Polytechnic Institute and a master's degree in civil engineering from the University of Maryland. He is a licensed professional engineer in Virginia.
Greg Nelson is a graduate student in the Civil and Environmental Engineering Department of the University of Wisconsin at Madison. He spent a year as an Eisenhower graduate research fellow in the nondestructive evaluation group at the Turner-Fairbank Highway Research Center.