Two Very Different Signs Of Age

It will come as no surprise to any engineer looking at that photo to hear that it was taken in a multi-story garage. What we’re seeing is the logical conclusion of water seeping through concrete (concrete is not waterproof, really) carrying road salt and causing accelerated rusting of the rebar in the concrete beams. Those spalls are a sign of aging of the structure. The damage to both steel and concrete is repairable at this point, but it will get worse of not addressed.

The first thing I noticed, after I got past the reaction of “they really need to fix that,” was how few ties there are in the beam closer to me. That’s exceedingly rare today but was fairly common when I first started work, and when I spent much of 1987 and 88 designing and detailing concrete for 712 Fifth Avenue. Back then, in New York, columns had “ties” and beams had “stirrups” because the beam ties were usually not closed across the top, but instead hooked down over the outer pair of top bars. I spent more time than I care to think about looking at the shear diagrams for the beams I was designing (by hand, of course) to figure out where the spacing of stirrups could be increased because the shear had dropped below a threshold value. (You can see the more-closely spaced stirrups on the beam further away from me, at a spall near the end of its span.) That was done, I had been taught, to not waste valuable steel, but it had the effect of making the shop drawings and placement of rebar more complicated. I suspect that the economic analysis that had material cost as more important than labor cost was already out of date when I learned it, but that spacing idea died not long after.

What changed this was the adoption of seismic design in the parts of the country that did not already have it – the relatively low seismicity areas like New York – and the changes that forced in terms of thinking about loads. Shear was no longer so boring in amount or direction, so determining cut-off thresholds was more difficult at best and would often lead to the answer that there were no cut-offs. And the combination of different forms of lateral loading and resilience requirements meant that stirrups were replaced by closed ties. If I knew nothing else about this garage, I would know it was built before New York adopted seismic design in 1996.

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