A reader sends a letter regarding my recent article on the 520 bridge on this piece of paper:

letter_big.jpg

To Dominic Holden

Thank you for your 520 bridge article. I wondered if you know what may be the real reason for cracks on the bridge. My guess is that the cracks are caused by water reaching re-bar – which then rusts which causes swelling which breaks concrete encasement which causes cracks. That is why rebar on new ferry docks is green-colored. It is expoxy coated to bar moisture. I wonder if new 520 bridge will have expoxy-coated re bar.

Thanks,
Cal McCune

That’s all the letter says. It says nothing about THE TERRIFYING FACE.

13 replies on “We Get Mail”

  3. McCune, Calmar (1911-1996), unofficial Mayor of Seattle’s University District

    Calmar M. “Cal” McCune was a leading attorney and civic activist in Seattle’s University District in the 1960s and 1970s. Born in Polk, Nebraska, in 1911, he moved to Seattle in 1932 to take law at the University of Washington and married Margaret Ann “Peg” Griffiths in 1933. He began practicing in 1940 and built a successful firm, McCune Godfrey & Emerick, in the University District. McCune served as president of the University District Chamber of Commerce and Rotary Club and on the City of Seattle Housing Advisory Board and the City Planning Commission. He led efforts to preserve the Meany Hotel, to expand neighborhood housing and public parking, to ease tensions between U District merchants and young “street people” in the late 1960s, and to create a pedestrian mall on “the Ave” (University Way NE) in the early 1970s. Cal McCune published a memoir, From Romance to Riot, shortly before his death in 1996. He is survived by his wife and two children, Leslie Grace (b. 1937) and Cal Jr. (b. 1939).

    source

  6. Epoxy is less for weather proofing, and more for bonding with the concrete. Ideally water/the elements should never reach the steel reinforcement, thus for concrete exposed to the elements the steel should be no closer to its surface than 2 inches. For a floating structure such as the 520 bridge cover is more probably around 3 inches.

    However, after so long on the water, the structure has received far more repetitive loading from both traffic and waves than most concrete structures will ever see. After a while the steel, which carries all of the tension forces in concrete structures, can fatigue and slowly creep along the strain-axis at relatively low stresses (on a stress-strain plot). Basically, even steel, which is a relatively elastic material, slowly stretches over time when subjected to repetitive loading. After all that fatigue, cracks (very small at first) are bound to open up admitting water into the pontoons, and rusting rebar.

    I don’t remember the design life for the 520 bridge, but i think it must be approaching it, if not past it already.

  7. Correction: some epoxies are used to aid in weather proofing. However they only started using epoxies in the last couple of decades or so, and the bridge was built in the 60’s, so epoxies were not applied to the steel reinforcement.

  8. Sorry, one more note:
    Talked to my friend at WSDOT; any new bridge would likely contain all new measures for weather proofing and fatigue-proofing. Individual pontoons would likely be several pre-fab parts post-tensioned together, and PT cables are run through conduits which further protect them from the elements. Also, cracks are far less likely to open on PT structures anyway. Jargon jargon jargon…end of a long day at work and my brain needed to do a memory dump from college. Hope someone finds it interesting/useful.

  9. @6, The 520 bridge was built in 1963 and is more-or-less at the end of its practical life now. It is nearly 50 years old. It also carries about twice as much traffic as it was originally designed for, which means twice as much of the stresses and flexing you are talking about, than it was designed for.

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