Pat Sparks – a ridiculously good engineer and a friend of OSE – wrote a short twitter thread on Saturday that is worth reading. Pat is generally a man of fewer words than I am, which is a fancy way of saying he gets to the point faster. In any case, I want to expand a bit on one of his points, specifically to relate it to a problem I see on a regular basis. Here’s Pat’s definition of distress versus damage and failure:
(Note that he managed to make several other useful observations in the space of four tweets, which is what I meant about getting to the point fast.)
You can find distress in any structure anywhere at any time. No building (or bridge, or dam, etc.) is without minor signs of distress. In some cases we count on it: reinforced-concrete design is predicated on the concrete cracking when loaded in tension. Once the concrete cracks, all of the tension load is thrown into the reinforcing, which is an assumption we use when we analyze a reinforced concrete beam. Those cracks are often visible. For example, concrete floor slabs in steel-frame buildings are typically designed as continuous over each support provided by a beam. That means that we have “negative moment” in the slabs at the beams, with tension on top. So, ordinary and expected structural action will cause cracks to develop in the top surface of the slab directly over the beams. On average I get asked about such cracks maybe every six months or so; maybe every two years or so my response that those cracks are to be expected and do not signify any structural defect is met with disbelief. This conversation sometimes takes place after interior demolition, when removal of carpet or vinyl tile shows the cracks; it sometimes takes place after completion of a project when a newly exposed and polished concrete floor shows cracks.
(I should probably mention here that I semi-regularly boast that, if allowed to wander around any building that I have never seen before, I can find something wrong with it in less than ten minutes.)
I’d like to think that engineers performing investigations are reasonably good at distinguishing distress from damage. The problem that Pat’s tweets highlighted for me is that non-engineers often react to distress and so part of an engineer’s job – as Hamish Harvey said in a follow-up to Pat’s thread – is to perform triage, separating trivial defects from dangerous damage. I have recently seen several reports where distress that I consider to be ordinary and not dangerous was used as justification for extensive structural intervention. Pat has effectively summarized my response: just because there’s some distress doesn’t mean there’s structural damage.
The picture at the top is the collapse of a railroad bridge near Cochrane, Alabama in 1973. That’s a good example of what I have in mind when I write “catastrophic failure” in a report.
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