Overview of Recent Bay Bridge Bolt Failures
Hello everyone, I hope y’all have done well since I’ve been away. School got very busy for me in the couple weeks before finals but I have final finished up the semester and I am now just waiting for grades. No more course work to do which I’m glad about – just need to take the comprehensive exam and I will have finished my ME program. Now I’m going to look for work over the summer while I work part time and study for my comprehensive exam.
Today I want to talk about the Bay Bridge in California which is currently scheduled to open at the end of summer. I shared a link earlier this week of an article describing it in Popular Mechanics (http://tinyurl.com/cqxdmtp). The bridge structure is interesting due to a couple issues they had to deal with. One issue they had to deal with is the soft soil on the shore – due to the soft soil they couldn’t use it stabilize the bridge. To deal with this, they designed the bridge to have on long cable that wrapped around the deck structure to stabilize the tower structure. Along with that, to dissipate energy absorbed in earthquakes they used four drill shaft posts that are connected by a shear key. The shear key would resist the earthquake forces – above the critical limit it would deform and then fracture absorbing a lot of energy from the earthquake load as well.
While that structural system seems to be adequate and well planned there have been some recent flaws found upon inspection. It has been discovered that 32 of the threaded rods that connect the bridge deck, shear key/bearing, and pier have cracked. These threaded rods are 17 to 24 inches length and have a diameter of 3 in – these threaded rods are referred to as A354 BD bolts according to ASTM. This failure directly makes 96 more threaded rods unsafe and calls into question 2000 more threaded rods. It is unclear whether the bolts failed in the production or installation/on site stage of construction. According to the Caltrans director, the threaded rods were overly hard and susceptible to crack and that the decision to use threaded rods of this type was based on proper treating. This is backed up by studies that show that this type of bolt is susceptible to Hydrogen contamination which gets into the steel and weakens the material properties related to strength. In this case, it is believed that the galvanizing process further exacerbated the issue by trapping the hydrogen into the steel. Testing proved that this could be an issue – 2 bolts were above the hydrogen limit and 5 bolts were in the critical zone out of a test of 12 sample bolts. This was to be accounted for by using a dry grit cleaning process before the galvanizing of the bolts. One contractor refused to bid on the project due to the risks involved and the Caltrans Bridge Specification also doesn’t allow for this. As it turns out, the design of the bolts called for the process to not include something called “pickling” which may have been done anyways. Pickling is a step in the middle of the bolt treating process in which the bolt is dipped in either hydrochloric or sulfuric acid to remove oxidation on the bolt. Details about the process are explained on this website – http://tinyurl.com/cp4yuwn. Currently, a steel saddle to replace the threaded rod support structure is being built at an additional cost of $10 million. The Caltrans director believes that this future usage of these threaded rods in this capacity should more restrictions based on this failure. A revised schedule for construction will be determined by May 29th.
In my opinion, there a several things we can learn from the threaded rod failures in the Bay Bridge. The first and most obvious one is that hydrogen contamination can be a critical issue – especially when the bolt or threaded rod is galvanized. Secondly, when a treatment procedure is critical in maintaining the strength of a connection, design oversight is critical and proper testing needs to be done after the manufacturing process is complete. And finally, if testing of any part of structure falls into a critical zone, the design should be re-examined with the idea of setting a more stringent restriction or strength limit in mind. What are your thoughts on the recent Bay Bridge issues? What do you think the ramifications will be for this mistake in the design? Hope you enjoyed the article – have a good week!
Popular Mechanics Link: http://tinyurl.com/cqxdmtp
San Francisco Chronicle Link: http://tinyurl.com/c72jrp3
Contra Costa Times: http://tinyurl.com/c3w6v38
CBS Link: http://tinyurl.com/cwq62kt
American Galvanization Association Link: http://tinyurl.com/cp4yuwn