Bolts, again.

And so the story of the Bay Bridge rods continues.

Today’s article in the SF Chronicle sheds some light on the testing that was done on the rods.

According to the article, tests were done to verify the rods’ ability to stretch without breaking and their hardness, but no tests were done for hydrogen embrittlement, which is what is currently believed to be the cause of the rod breakage. (For those who haven’t read the linked articles, the short form of the issue is that exposure to hydrogen can make steel prone to cracking; hydrogen can be introduced to the steel at several points during the manufacturing process.)

I gather from the article that no tests were done for hydrogen embrittlement because the specific processes used in manufacturing the rods were at a lower risk to introduce hydrogen. It’s not stated, but I suspect that the length of time necessary to do the tests was a significant factor. Since failures are triggered when the steel is placed under stress for extended periods of time, a useful test must also run for an extended period.

Putting on my QA hat for a moment, this kind of cost/benefit analysis is standard. It’s impossible to test everything at every point in every project, so you look at what testing will give you the biggest benefit for the time and money spent, and focus on the highest risks. I haven’t seen anything that would suggest that there were problems with the risk analysis or cost/benefit analysis done by Caltrans that led to the omission of hydrogen embrittlement testing, so it seems that they just got bitten by a lower-risk possibility.

None of this answers my original question, though: how did a design that makes replacing the rods difficult or impossible make it through the review process?

Today’s article talks about the planned fix being the addition of metal collars “to allow room for new rods to be inserted”. The language is a little ambiguous; it’s not clear to me if this means removing some of the concrete to allow the broken rods to be replaced, or if it is a way to add new rods without removing the current broken ones. If it’s the first case, why wasn’t the structure designed this way to begin with? I’d hate to think it was an aesthetic decision. In the second case, I have to go along with the materials engineer quoted in the story, and ask what Caltrans plans to do if more of the rods fail. Will there be another round of collar and rod addition?

One final thought, at least for now: Just how long are these bolts? I think I’ve seen a different maximum length given in every story the Chronicle has run so far. I realize it’s a minor point, but it does contribute to the overall mystery of the piece. Shouldn’t the size of the bridge’s components be clearly documented?

One thought on “Bolts, again.

  1. Pingback: That grinding noise… | Koi Scribblings

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