Inadvertent Structural Action

The title of this post is recycled from a talk I gave 16 years ago, “Inadvertent Structural Action in Traditional Buildings; or Why Hasn’t That Fallen Down Yet?” That talk was about how buildings that were constructed without structural design are often stronger than we think they are. The picture above is not that, but rather a modernish structure that could not have been functioning as intended by its designers.

What you’re looking at: the partially-demolished ground floor of a commercial building, where a restaurant kitchen used to be. The four visible steel beams are spanning left to right between the foundation wall and a girder at the first interior column line. The girder is still encased in its concrete fireproofing, so it’s not visible. The fireproofing was cast integrally with the floor slabs, which have been removed in this bay but are still present elsewhere. The first two beams from the top of the photo are rust-damaged but mostly intact. The third and fourth beams are no longer viable: their top flanges and webs can’t work as intended where they are physically missing from rust. The severe and localized damage is the result of water on the floor in the kitchen.

So, why didn’t this floor collapse? I saw the floor before our design began, and there were some small cracks and spalls in the slab but no noticeable sagging or any kind of severe visible damage that would suggest the floor was failing. But three entire panels of slab (between the two worst beams and on either side of them) no longer had the support that their design was based on. I’ve mentioned my dislike of the joke that a structure “forgot to fall down” – if we don’t know why something hasn’t fallen down, then we don’t know how it works. This simply brings me to where I always start an investigation, which is trying to understand the mechanisms by which the structure carries load. Here are a few scenarios:

  1. The beams, even in their damaged state, have enough capacity to span as intended. That’s ridiculous. Removing the top flange and web of an I-shaped beam removes 90 percent or more of its strength and stiffness, and these beams were not ten times the size they needed to be. A simple reverse-design check shows them to be (in their undamaged state) just about the size I’d expect them to be for a 1930 NYC steel design.
  2. The beams were strengthened by the concrete encasement, and so worked until we demolished the encasement. Just because the designers intended the encasement to be non-structural doesn’t mean it didn’t carry load. The problem from scenario 1 still exists, however. Even if the encasement doubled the strength of the beams bay accident, it’s not enough.
  3. The encasement acts as a group of concrete beams, reinforced by the bottom flanges of the steel beams.This is not exactly a recommended design for reinforced concrete, but it is feasible, particularly if the slab acted as a compression flange in a T-beam design. It also is different from scenario 2 only by how we assume stress is distributed between the materials.
  4. The slabs were strong enough to span “the wrong way.” Maybe the secondary reinforcement in the slabs, running perpendicular to the assumed (beam to beam) span of the slab was strong enough to span between the foundation wall and the girder.
  5. Various combinations of these scenarios are also possible, such as the slab spanning the wrong way far enough to reduce the moment on the damaged steel beams to the point where they work.

Here’s the punchline: it doesn’t matter that we don’t know which scenario is true. Empirical fact – the floor was intact and carrying a significant live load in addition to its own weight – says that is was working somehow, and we have at least two scenarios (3 and 4) that can be shown to work. That doesn’t mean that repair was not required, but rather that there is always an explanation for a lack of collapse.

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