hard deadlines

RIDOT's Managing Engineer of Bridge Engineering, David Fish, responded to my request for information about ambient chloride levels at the Sakonnet River Bridge site, and based on the numbers provided, it appears that the environment is within appropriate limits. Fish acknowledged that salt corrosion was a concern, but stressed that RIDOT had done their due diligence.

"We initially had similar concerns," Fish wrote in an e-mail, "and hired a national expert, Dr. Gallagher, to study and report on the suitability of using weathering steel at the Sakonnet River Bridge site. Dr. Gallagher's analysis of the Sakonnet River Bridge site uses a conservative ocean front chloride deposition rate of 33.9 kg/ha. While using this conservative chloride deposition rate, the rate of corrosion still yielded results that were 1/6th of the upper bound range for satisfactory performance of weathering steel." (You can click the image to enlarge a section of Gallagher's report.)

Indeed, if I did the math right, that deposition rate works out to .034 mg/100cm2, which is below the level of concern even in Japan, which has much stricter requirements than the U.S.

And they aren't kidding about their expert having a national reputation — W. Patrick Gallagher literally co-wrote the book on this stuff: Guidelines for the use of weathering steel in bridges. Gallagher also has, on his site, an illuminating take on weathering steel as a sculptural medium.

I could quibble about the methodology used for coming up with the estimate, but I think we can reasonably suppose that Portsmouth is similar to Cape Cod for these purposes. I may sometimes be cranky, but I'm not a crank. RIDOT appears to have done their homework, and weathering steel would seem to be suitable for the Sakonnet River Bridge.