April 16, 2019
Steel is utilized in the fabrication of bridge superstructures for different advantages: 100% recyclable, high resistance, light weight, etc. There are several steel grades on the market today. At the design stage, when it comes to choosing the steel grade for different members of a bridge, the designer must consider several parameters. This article aims to discuss a few.
In steel bridges, the weldability of the steel used is one of the most important parameters. The weldability can be defined as the ease to which quality welds can be performed. The most current index used to qualify the weldability is the carbon index equivalent (CE). An index inferior to 0.45 generally indicates a good steel weldability. In Canada, as per CAN/CSA G40.21, the weldable steel is identified with the letter « W ». For example, the grade of steel 350W has a yield strength (Fy) of 350 MPa and the suffix « W » is used to indicate that it is weldable.
Steel used in bridges can be protected from corrosion to insure the product durability. In order to do so, a protective coating like painting, metalizing and galvanizing can be used. However, weathering steel, which means that the steel is more resistant to atmospheric corrosion, can also be used and combined with a protective coating. This steel can be up to 4 times more resistant to corrosion than non-weathering steel. The standard CAN/CSA G40.21 denominates the weldable and corrosion resistant steel with the suffix « A ». The letter « A » means «Atmospheric corrosion-resistant». In the US, the standard ASTM A709, weldable steel, denominates the weldable and weathering steel by the letter « W ». We must not confuse the « W » (Weldable) of the standard G40.21 with the “W” (Weathering) of the standard A709. The steel from the American standard ASTM A588 is also corrosion resistant steel. This steel is generally used for bridge secondary members as well as plates thicker than 100 mm.
Toughness at low temperature
The toughness, which is the material capacity of energy absorption when it deforms under shock, is measured by Charpy tests. It is known that low temperatures reduce the toughness of steel. An impact to the material at low temperature could increase the risk of a brittle fracture of a piece. The imposition of a minimum toughness level in the contract documents ensures the steel ductility behavior under impact loads at low temperatures. A classification by levels of Charpy tests is used. The standard CAN/CSA G40.21 prescribes 5 categories of Charpy tests based on the temperature (Figure 1) of the test going from a category 1 with testing at a low temperature of 0°C to a category 4 with testing at a colder temperature of -45°C. Category 5 was created for special demands for which the testing temperature must be specified when the material is ordered. The standard CAN/CSA G40.21 denominates the steel, where notch toughness at low temperature is a design requirement, by the suffix « T » (Toughness) and adding the achieved category. So, the steel, weldable and having the required toughness at low temperature will carry the letters « WT », while the weldable weathering steel that has the required toughness will carry the letters « AT». In the US, the classification is different using Zones 1, 2 & 3 instead of categories. Also, the standard ASTM A709 denominates the steel with required toughness at low temperature with the suffix « F » (Fracture-Critical) or « T » (Non-Fracture-Critical). The testing temperature is the same for these two types of steel, however, the minimum dissipated energy is superior for fracture-critical steel (Figures 2 et 3).
Figure 1: Minimum dissipated temperature and energy for the Charpy tests according to the intended category and standard CAN/CSA G40.21-13.
Figure 2 : Minimum temperature and energy dissipated for the steel that is not at a critical resistance to rupture according to the standard ASTM A709-16.
Figure 3 : Minimum temperature and energy dissipated for the steel that is at a critical resistance to rupture according to the standard ASTM A709-16.