One of the most important decisions designers have to make is selection of the proper grade of steel and corrosion-protection system. These should not only meet structural needs but also provide an economical structure capable of long-term, low-maintenance performance.
Specifications of the American Association of State Highway and Transportation Officials recognize structural steels designated M270 with a specified grade. These are equivalent to ASTM A709 steels of the same grades except for AASHTO-specified mandatory notch toughness. Material properties of M270 steels and other equivalent ASTM steels are listed in Table 11.28. (See also Table 1.1 and Art. 1.1.5). Steels that meet AASHTO M270 requirements are prequalified for use in welded bridges.
Designers should have available AASHTO Standard Specifications for Transportation Materials and Methods of Sampling and Testing, Part 1, Specifications, and Part 2, Tests, to ensure that appropriate material properties are specified for their designs.
High Performance Steels (HPS), a new addition to the family of bridge steels, provide an opportunity to significantly increase reliability while reducing cost. Only Grade HPS70W, with a minimum yield point of 70,000 psi, has been fully developed and is presently available for bridge design. To qualify as HPS, the material has to provide improved weathering characteristics and significantly higher impact toughness. HPS has a corrosion index, I, of 6.5 and higher, thus providing increased resistance to weathering over earlier grades of steels designated as weathering (W). Weathering grades are defined as having a corrosion index, I, of 6.0 and higher as calculated using ASTM Standard G101. In addition, Charpy V-notch (CVN) impact properties for this steel usually exceed 100 ft-lbs at -10F.
Minimum Steel Thickness
Because structural steel in bridges is exposed to the weather, minimum-thickness requirements are imposed on components to obtain a long life despite corrosion. Where steel will be exposed to unusual corrosive influences, the component should be increased in thickness beyond required thickness or specially protected against corrosion.
In highway bridges, structural steel components, except railings, fillers, and webs of certain rolled shapes, should be at least 5â„16 in thick. Web thickness of rolled beams or channels should be at least 0.23 in (0.25 in for LRFD). Closed ribs in orthotropic-plate decks should be at least 3â„16 in thick (0.25 in for LRFD). Fillers less than 1â„4 in thick should not be extended beyond splicing material. In addition, minimum thickness may be governed by slenderness ratios (Table 11.24) or maximum width-thickness or depth-thickness ratios (Table 11.25).
A preferred way to achieve economy for bridges is to use steel of a weathering grade when conditions permit. This is a type of steel that has enhanced atmospheric corrosion resistance when properly used and does not require painting under most conditions. Although it costs slightly more per pound than other steels of equivalent grade its initial cost and lifecycle cost is usually less than that of painted structural steel. The weathering grades are available only with yield points of 50 ksi and higher. Before selecting a weathering steel, designers should determine the corrosivity of the environment in which the bridge will be located as a first step. This will determine whether the use of an unpainted steel of grade 50W, 70W, HPS70W, or 100W (Table 11.28 and Arts. 1.1.4 and 1.1.5) is appropriate. These steels provide the most cost-effective grade that can be used in most situations and have proven to be capable of excellent performance even in areas where deicing salts are used.
But use of good detailing practices, such as jointless bridges, is imperative to assure adequate performance (Art. 11.26).
The Federal Highway Administration Guidelines for the Use of Unpainted Weathering Steel, to ensure a long-term and adequate performance of unpainted steels, recommends the following:
If the proposed structure is to be located at a site with any of the environmental or location characteristics noted below, use of uncoated weathering-grade steels should be considered with caution. A study of both the macroenvironment and microenvironment by a corrosion consultant may be required. In all environments, designers must pay careful attention to detailing, specifically as noted in the following recommendations for design details. Also, owners should implement, as a minimum, the maintenance actions as noted in the following. Environments to be treated with caution include marine coastal areas; regions with frequent high rainfall, high humidity, or persistent fog; and industrial areas where concentrated chemical fumes may drift directly onto structures.
Locations to be treated with caution include grade separations in tunnel-like conditions, where concentration of vehicle exhausts may be highly corrosive; also, low-level water cross ings, with clearance of 10 ft or less over stagnant, sheltered water or 8 ft or less over moving water.
Design details for uncoated steel in bridges and other highway structures require careful consideration of the following:
1. Elimination of bridge joints where possible.
2. If expansion joints are used, they must be able to control water that comes on the deck.
A trough under the deck joint may serve to divert water away from vulnerable elements.
3. Painting all superstructure steel within a distance of 11â„2 times the depth of girder from bridge joints.
4. Avoiding use of welded drip bars where fatigue stresses may be critical.
5. Minimizing the number of bridge-deck scuppers.
6. Eliminating details that serve as water and debris traps.
7. If box girders are used, they should be hermetically sealed, when possible, or provided with weep holes to allow proper drainage and circulation of air. All openings in boxes that are not sealed should be covered or screened.
8. Protecting pier caps and abutment walls to minimize staining.
9. Sealing overlapping surfaces exposed to water, to prevent capillary penetration of moisture.
Maintenance actions advisable include the following:
1. Implementing procedures designed to detect and minimize corrosion.
2. Controlling roadway drainage by diverting roadway drainage away from the bridge structure, cleaning troughs or resealing deck joints, maintaining deck drainage systems, and periodically cleaning and, when needed, repainting all steel within a minimum distance of 11â„2 times the depth of the girder from bridge joints.
3. Regularly removing all dirt, debris, and other deposits that trap moisture.
4. Regularly removing all vegetation and other matter that can prevent the natural drying of wet steel surfaces.
5. Maintaining covers and screens over access holes.
The preceding recommendations are applicable to all structures, painted or unpainted, to ensure satisfactory performance. Unpainted structures that have been in existence for 30 or more years in environments consistent with these recommendations have provided excellent service, testifying to the adequacy of the weathering grades of steel. (Performance ofWeathering Steel in Highway Bridges A Third Phase Report, American Iron and Steel Institute, Washington, DC, 1995.)
Where weathering grades of steel are not appropriate, only high-performance paint systems should be specified for corrosion protection. Designers should be aware, however, that recommendations for paint systems change periodically, primarily due to the need for consideration of environmental impacts. Lead-based paints, for example, are no longer acceptable due to their health hazard. Also, concern for the effect of volatile organic compounds on the ozone in the atmosphere has caused a change from mineral-based to water-based paints.
Consequently, designers should ensure that only current technology is specified in contract documents.
The AASHTO Guide for Painting Steel Structures provides state-of-the-art information for the painting of new bridge steels, as well as paint removal and repainting of existing steel bridges.