What Are The Type Of Steels Used In Constructing Bridges?

Steels for application in bridges are covered by A709, which includes steel in several of the categories mentioned above. Under this specification, grades 36, 50, 70, and 100 are steels with yield strengths of 36, 50, 70, and 100 ksi, respectively.

The grade designation is followed by the letter W, indicating whether ordinary or high atmospheric corrosion resistance is required. An additional letter, T or F, indicates that Charpy V-notch impact tests must be conducted on the steel.

The T designation indicates that the material is to be used in a non-fracture-critical application as defined by AASHTO; the F indicates use in a fracture-critical application.

A trailing numeral, 1, 2, or 3, indicates the testing zone, which relates to the lowest ambient temperature expected at the bridge site. (see Table Below)

As indicated by the first footnote in the table, the service temperature for each zone is considerably less than the Charpy V-notch impact-test temperature.

This accounts for the fact that the dynamic loading rate in the impact test is more severe than that to which the structure is subjected.

The toughness requirements depend on fracture criticality, grade, thickness, and method of connection. A709-HPS70W, designated as a High Performance Steel (HPS), is also now available for highway bridge construction.

This is a weathering plate steel, designated HPS because it possesses superior weldability and toughness as compared to conventional steels of similar strength.

For example, for welded construction with plates over 21⁄2 in thick, A709-70W must have a minimum average Charpy V-notch toughness of 35 ft lb at 10 F in Zone III, the most severe climate.

Toughness values reported for some heats of A709-HPS70W have been much higher, in the range of 120 to 240 ft lb at 10 F. Such extra toughness provides a very high resistance to brittle fracture.

(R. L. Brockenbrough, Sec. 9 in Standard Handbook for Civil Engineers, 4th ed., F. S.
Merritt, ed., McGraw-Hill, Inc., New York.)

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