Introduction:
In January 1988, ISG/Bethlehem issued Technical Bulletin TB-301, "Suggested Guidelines for Improving Weathering Steel Bridges." These guidelines have recently been updated by the FHWA and incorporated into their Technical Advisory entitled, "Uncoated Weathering Steel Structures." FHWA,s new Advisory provides the latest and most complete information available concerning the application of weathering steel for highway and bridge structures. ISG/Bethlehem supports this new Advisory. The following is a reprint in its complete form.

Purpose:
To provide engineers with suggested guidelines for proper application of uncoated (unpainted) weathering grade steels in highway structures and recommendations for maintenance to ensure continued successful performance of the steel.

Background:
a. Uncoated weathering grade steels have been available to the bridge engineering profession for many years. The cost-effectiveness of use of this material has been demonstrated in both short and long-term savings. The additional cost of this grade of steel is offset by the elimination of the need for initial painting of the structure. These steels are currently supplied under American Association of State Highway and Transportation Officials (AASHTO) Specification M270 (ASTM A709) with grades 50, 70 and 100 available. Where enhanced atmospheric corrosion resistance is desired, the letter "W" follows the grade.

b. Environmental benefits also result from the use of this material. The reduction in initial painting reduces emissions of volatile organic compounds (VOC) when oil based coatings are used. The elimination of removal of the coating and disposal of contaminated blast cleaning debris over the life span of the structure is another significant environmental benefit. There are documented cases where the estimated cost of the collection and disposal of materials from a structure repainting project were so great that the structure was either abandoned or replaced with a new bridge.

1. At the same time, there are documented cases where application of this material in improper locations or under improper conditions has resulted in less than desirable performance of the structure.
2. In most cases, this poor performance was the result of a lack of understanding of the limitations of weathering grade steels, or from poor detailing which caused exposure conditions which would cause distress in any structure, coated or uncoated, concrete or steel.

c. To better define the performance record of this material, the FHWA sponsored a Weathering Steel Forum in July of 1988 where knowledgeable speakers from across the nation were invited to present case histories and research data on the performance of this product in highway structures. The outgrowth of this forum was the suggested guidelines included herein. If these guidelines are followed, the potential for satisfactory performance and long term durability of weathering grade steels in highway structures is greatly enhanced. Proceedings from this forum are available from the Federal Highway Administration Office of Implementation, HRT-10.

Guidelines:
If the proposed structure is to be located at a site with any of the characteristics noted in paragraph a or b below, the use of uncoated (AASHTO M270 Weathering Grade) steels should be considered with caution and a study of both the macro-environment and micro-environment by a corrosion consultant may be required. In all environments, the designer must pay careful attention to detailing, specifically noted in paragraph c, and the owner should implement, as a minimum, the maintenance actions as noted in paragraph.

a. Environment

1. Marine Coastal Areas.
2. Frequent High Rainfall, High Humidity or Persistent Fog (Condensing Conditions).
3. Industrial Areas where concentrated chemical fumes may drift directly onto the structure.

b. Location

1. Grade Separations in "Tunnel-like" Conditions.
2. Low Level Water Crossings.
   (a) Ten feet or less over stagnant, sheltered water.
   (b) Eight feet or less over moving water.

c. Design Details
For uncoated steel in bridges and other highway structures, the following items should receive careful consideration:

1. Eliminate bridge joints where possible.
2. Expansion joints must be able to control water that is on the deck. Consider the use of a trough under the deck joint to divert water away from vulnerable elements.
3. Paint all superstructure steel within a distance of 1 1/2 times the depth of girder from bridge joints.
4. Do not use welded drip bars where fatigue stresses may be critical.
5. Minimize the number of bridge deck scuppers.
6. Eliminate details that serve as water and debris "traps".
7. "Hermetically seal" box members when possible, or provide weep holes to allow proper drainage and circulation of air.
8. Cover or screen all openings in boxes that are not sealed.
9. Consider protecting pier caps and abutment walls to minimize staining.
10. Seal overlapping surfaces exposed to water (to prevent capillary penetration action).

d. Maintenance Actions

1. Implement maintenance and inspection procedures designed to detect and minimize corrosion.
2. Control roadway drainage:
   1. Divert roadway drainage away from the bridge structure.
   2. Clean troughs or reseal deck joints.
   3. Maintain deck drainage systems.
   4. Periodically clean and, when needed, repaint all steel within a minimum distance of 1 1/2 times the depth of the girder from bridge joints.
3. Regularly remove all dirt, debris and other deposits that trap moisture.
4. Regularly remove all vegetation which can prevent the natural drying of wet steel surfaces.
5. Maintain covers and screens over access holes.

Discussion:

a. General. Controlling the corrosion of steel highway bridges and other steel appurtenances and mitigating the corrosion related damage is a major problem facing bridge owners. A special aspect of the problem is ensuring that highway structures utilizing uncoated (AASHTO M270 Weathering Grade) steels are located in an environment, and incorporate details, that will ensure cost-effective performance over the expected service life of the structure. For existing weathering steel structures, where proper guidelines have not been followed, another part of the problem is controlling the corrosion damage of uncoated steel. In a number of cases, bridges, light poles and guardrail have experienced excessive corrosion damage, and some have ultimately experienced loss of section and/or localized structural failure because of improper applications of this material.

Further work is needed to quantify and understand the performance of uncoated weathering steel in a variety of circumstances and conditions. These guidelines are intended to aid the engineer in making a prudent decision to use coated or uncoated steel in highway environments and applications. A more precise technical evaluation of the suitability of uncoated weathering steel for a particular site may be obtained from a corrosion consultant, from conducting standardized environmental tests, or from both. If serious doubt remains after applying the guidelines in the selection process, then engineering judgement should lean towards coated steel.