Following up on yesterday’s following up on Pat’s discussion of distress versus failure, I want to give set of examples of similar-type signs of distress. As is probably obvious from the picture above, they’re all variations on sagging.
Let’s start with the picture above, in which the federal Farm Security Administration was warning that all the work put into those jars of preserves would be lost if the shelves fail. As I’ve talked about a number of times, strength and stiffness are two different things. Each shelf is a beam spanning between the wall to the left and the upright to the right. They appear to be strong enough – they’re each carrying the load without splitting or otherwise physically degrading – but they are not stiff enough to remain even remotely straight. So there’s distress in the form of the visible sag, but not necessarily evidence of potential structural failure.
Time to define some terms that all sounds very much the same to non-engineers. “Deflection” is the downward curve of a beam element (usually, something spanning horizontally to carry vertical loads) when loaded. This is expected and normal behavior. No material is infinitely stiff, so all beams defect under load, although the amount may be too small to see without careful measurement. I misused “sag” above. Sagging is usually defined as a downward curve that isn’t necessarily connected to load. There are different reasons a beam could sag, including “creep,” which is deflection (originally caused by load) that gradually becomes permanent due to the inelastic nature of the material. Wood creeps, as does concrete. Metal does not. If that sounds esoteric, ask yourself if you’re surprised when you see a sag in an unloaded wood-just floor in an old house or if you expect that. Finally, “settlement” is usually reserved for foundation and earth-related issues. So when someone calls up and says their floor is settling, it always takes me a second to realize they likely mean deflection or sag.
Deflection, even in an extreme case like the picture above, is not necessarily a structural problem. Those shelves appear to be strong enough, and their strength can be easily checked. It’s a serviceability problem, in that the geometric change to the structure from deflection may, as above, interfere with its use or even make it unusable. But it’s not in itself evidence of imminent failure. You have to look at the ends of the beam, though: a curved line connecting the two end points is longer than a straight line between those two points, and the shelf isn’t getting longer as it defects, so its ends move in slightly. If they move in too much, they will come off the cleats that support them and fall.
Creep is even less likely to be evidence of possible failure. It too causes serviceability problems, and it too can cause bearing problems at the ends.
Sag is a different issue. Sagging can possibly be caused by creep, by past overload on a steel beam that exceeded the elastic limit on the beam and caused permanent deformation, or by material degradation that has weakened the beam. I’ve probably missed a few possible causes, but the important thing to note is that material degradation can mean imminent failure. So the same sign of distress in a wood beam – sagging – can be not-harmful creep or can be harmful rot.
Finally, settlement can range from structurally meaningless but annoying to incredibly destructive. An example of the first would be the buildings in Mexico City that had settled straight down a couple of meters. Their structure is undamaged, but they have a serviceability problem in that their ground floors are now basements. An example of the second is when there are changes to the subgrade (for example, caused by human intervention that permanently changes the ground water elevation) that lead to differential settlement. If one edge of a building drops and the remainder of the building does not, it can rip a wall off or worse. On the other hand, a gradual and overall tilt is differential settlement that may cause little or no structural damage. This can be seen near a lot of the old subway lines, where one side of building was underpinned during the tunnel construction and the whole building is now tilted away from that side. This is common along Fourth Avenue in Brooklyn. So, again, the same sign of distress can mean anything from no structural damage to severe damage.
This was a long blog post to repeat yesterday’s point: distress is not directly correlated to failure.
