The failure analysis report was recently released for the pipe break that disrupted the city of Boston's water supply last May. It is a good piece of forensic engineering by the authors, who had little to go by.
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Progressive bolt failure likely cause of pipe break in Boston
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JedClampett
Structural
I fought through the report, although it seemed a bit academic to me. Maybe I missed something when I slipped into a temporary coma, but I never saw a root cause for the progressive bolt failure. The stresses were manageable.
I’m no metallurgist, but the fact that the bolts were “zinc plated” caught my eye. I know that other high strength bolting materials are susceptible to embrittlement when hot dip galvanized Could this have something to do with it? I did some very cursory research on A193 material embrittlement due to HDG, but the answers were contradictory.
I’m no metallurgist, but the fact that the bolts were “zinc plated” caught my eye. I know that other high strength bolting materials are susceptible to embrittlement when hot dip galvanized Could this have something to do with it? I did some very cursory research on A193 material embrittlement due to HDG, but the answers were contradictory.
I only scanned the report, but didn't see anything about the origin of the bolts. Were they domestic or from some foreign source? "Zinc plated" struck me as well, but for a different reason than Jed's concern. Zinc plating to me doesn't mean HDG, and is not much good for corrosion protection. They did say there was some sort of cathodic protection.
JedClampett
Structural
The coupling was made by a reputable manufacturer, Victaulic, but that doesn't mean the bolts weren't supplied by others or counterfeit.
I understand that zinc plated isn't the same as HDG, but it's a common mistake to confuse the two, so I thought maybe the authors did.
I understand that zinc plated isn't the same as HDG, but it's a common mistake to confuse the two, so I thought maybe the authors did.
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I think one thing to take from the report is that the bolts were highly overstressed if you take into account the theoretical thread stress concentration factor of 3~4. The pre-existing cracks from the manufacturing process (indicated by zinc present in some of the cracks) likely did cause stress concentrations in the thread roots. Over time, with the constant tension on the bolts, the cracks elongated and the bolts fractured.
What I take away fom this is that the devil is in the details with tension connections. Over time, the tension will exploit any flaws and lead to a failure. Reinforces the principle for providing redundancy in tension connections to insure against these manufacturing, installation and mother nature factors that engineers cannot usually control.
Another example of this would be the Big Dig tunnel roof panel collapse.
What I take away fom this is that the devil is in the details with tension connections. Over time, the tension will exploit any flaws and lead to a failure. Reinforces the principle for providing redundancy in tension connections to insure against these manufacturing, installation and mother nature factors that engineers cannot usually control.
Another example of this would be the Big Dig tunnel roof panel collapse.
Lot's of repetition and padding in the report. Could of summed-up
their conclusions in one page and presented back-up evidence in an Appendix.
Anyway, one would assume that Victaulic would have also supplied the bolts and inspected and approved the installation as the integrity of their product is highly dependent on this.
What surprised me was not making an "empty pipe" one of the design conditions.....
their conclusions in one page and presented back-up evidence in an Appendix.
Anyway, one would assume that Victaulic would have also supplied the bolts and inspected and approved the installation as the integrity of their product is highly dependent on this.
What surprised me was not making an "empty pipe" one of the design conditions.....
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