Home > Uncategorized > Stitch in time saves nine – 2nd in a series on cliches that made our country strong

Stitch in time saves nine – 2nd in a series on cliches that made our country strong

September 5, 2011

Things we used to say: “A stitch in time saves nine” (Labor Day edition)

Rusty bridge support link

This bridge support link isn't as bad as it looks - but because of the rust it will need more frequent inspections (click to embiggen)

A few weeks ago I was walking across a multi-level bridge on a tour of Chicago. Bridges – in fact structures of all kinds – fascinate me and I like learning how they stand, and how they fall. So naturally I was looking at the structure, figuring out how the loads were distributed, how they were sent to ground, and if there was any redundant load-path to ground. And following the load from the span to where it connects to ground support I looked up and saw this:

You are looking at a link, between the bridge support (left side) and the bridge span (right side). The link consists of a pair of vertical steel bars with 2 holes, bolted onto the support and the span I-beam respectively. The span literally hangs from the support; load applied to the span is conveyed to ground via the lower bolt, the link, then the upper bolt. I’m going to describe the best of my understanding of this structure, and if any inspectors or engineers happen by, feel free to expand or correct in the comments.

Since the bridge span consists of many I-beams in parallel, each with one link at each end, we would not expect all the links to fail at once unless they were in such bad condition that a cascading failure became possible. The challenge is that these links necessarily exist near where the span connects to the support, so road salt can fall right on them from the road above. The links and bolts themselves seldom break. A common failure mode of this kind of link is that expanding rust on the beam forces the washer outward, shearing off the cotter pin that holds it in place, allowing the bolt to migrate horizontally and leave the I-beam unsupported. Mitigations include regular inspection to insure that the pins are intact (as these are), holding the bolt in place. Application of water-repellent grease isn’t a bad idea either.

OK, fine, that’s cool. This bridge support is actually in fair condition structurally even though it should be watched carefully on account of the rust. The problem is there are hundreds of these links on this one complex bridge alone, and Chicago has more bridges than any other city anywhere. Some of them are of different designs and could have different failure modes. Bridge inspecting is, to say the very least, a full-time job, and it is not without political pressures. Add sewers, water mains, buried power lines, buildings, and roads and you need a lot of people who just go look at things and make predictions about them. And the looking sometimes involves advanced imaging and analysis techniques, plus knowledge of chemistry, engineering, physics and even technology history. People who can do that don’t work for minimum wage.  (They don’t exist in isolation from a working education system either, but I’ll cover that later in this series.)

Bridge traffic in Chicago

Another problem of bridge repair is handling traffic load while it takes place (click to embiggen)

Roads, bridges, electrical grid, water mains, sewers, schools, dams and river engineering projects of days past contributed in two ways to our economy. First directly, in the wages of the people who designed them and built them. These people spent their wages, creating a cascade of economic activity from service to manufacturing. And second, the infrastructure itself reliably enabled economic growth – something politicians were pleased to attribute instead to their charm and ideologies. Today that same infrastructure is falling into disrepair, as thoroughly documented by the excellent Slactivist series you will find in the Notes section.

We are in effect resting on past accomplishments – infrastructure built from the 1930′s through the 1970′s. No politician wants to hear from the bridge inspectors that they must sell a multi-million dollar “expense” to the taxpaying, voting public. We’d rather be afraid of swarthy foreigners than rusting bridges – even though in the long run the rusty bridges are a much greater hazard to our economy. Infrastructure is one of several necessary conditions for a sustainable economy.

Is there anyone alive who really thinks it is better to wait for disasters than to prevent them? The only explanation I can conjure for our national behavior is that we’ve become accustomed to not thinking it through. Strong infrastructure makes for strong economies – and the opposite is true when infrastructure is allowed to fall apart. By the time disaster strikes, the economy is immune-compromised, vulnerable. We have stopped telling ourselves; “A stitch in time saves nine”, dismissing the old aphorism as a cliche.  There is another cliche that now applies: “better late than never” – but late unpredictably turns into “too late” so we’d better start right away.


Categories: Uncategorized
  1. Dana Hunter
    September 5, 2011 at 12:32 | #1

    Loving this series. I hope it creates a cadre of sensible people who can elect sensible politicians and pressure them to do sensible things, because without that, this country is going to fall apart.

  2. Chas, PE SE
    September 5, 2011 at 21:00 | #2

    Hi, George:

    You pretty much got it right. The hanger assembly is under the expansion joint, and they haven’t invented the good one of those yet–though they keep trying.

    As for “stitch in time”–it isn’t quigte as bad as you make out. Every bridge in the US is inspected at least every two years, “Arm’s Reach”, ie from right close up. Some are inspected more often, if they’re in poorer shape. It is standard practice to replace those hangers with splice plates, and to eliminate the joint openings in the decks so salty water doesn’t attack the steel. This is done as funds are available, and as bridge conditions require, and it is a judgement call by people holding purse strings–but the work does get done.

    And the bit about how there are several “I beams” (actually Wide Flange Sections, or W shapes): That’s called Redundancy, and is an important factor in bridge design. This was first realized in the Silver Bridge collapse of 1966. This bridge had been designed in such a way that a single crack in an eyebar caused the catastreophic collapse of a 1000 foor suspension bridge. Now we design so that, if one member fails, the bridge might be unusable, but it won’t fall.

    If you have any more bridge-type questions, send me an E-mail.

    Chas, PE SE, NHI qualified bridge inspector, and currently unemployed.

  3. dof
    September 5, 2011 at 21:38 | #3

    Thanks Chas! I looked for splice plates but couldn’t find anything that really helped me visualize them. Would appreciate a link if you have one.

    I’m not so much worried all the bridges are going to collapse; I’m worried a bunch of them will get so bad that they’ll have to be shut down, and an anemic economy (which would have been improved by, say, bridge-maintenence and building) won’t be able to provide the funding to replace them, further impeding transportation and hurting the economy even more. Same goes for water mains, sewer systems, the grid, etc. The purse-string people only have as big a purse as we give them.

  4. dof
    September 5, 2011 at 21:41 | #4

    Oh, and you’re right: a “good” expansion joint is an entertaining problem. I’ve sketched several over the years only to find 1) they’ve been tried and found wanting for XYZ reasons, or 2) see number 1. It’s one of those things that seems like it should be easy, but…

  5. September 16, 2011 at 04:56 | #5

    Hi George, Thanks for this post and this series. Lucas turned me onto your blog.

    For the past few months I’ve been staying in Seoul and marveling every day at the investments here: in transportation, healthcare, business, education, parks, housing, etc. etc. etc. Seeing all these things done right here makes me really worry about the future of our country, above and beyond the obvious political and macroeconomic concerns. I’ve recently started trying to get my thoughts down at http://log.shilman.net.

    I’m really inspired by this principles-based look at things and will use it as yet another lens through which to understand what’s going on here in Seoul. Please keep these posts coming!

  6. dof
    September 18, 2011 at 09:15 | #6

    Welcome Shilman!

    Almost on cue, an example of exactly what I was concerned about: Sherman-Minton bridge in Louisville shut down. I’d love to hear a credible estimate of the weekly economic cost of having that route shut down.

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