Now to the buildings that do not burn. Masonry, steel and concrete are non-combustible, and people reason very naturally from non-combustible to safe. Those are different claims. A material that does not burn can still conduct heat into the next compartment, expand until it pushes a wall over, lose half its strength well below the temperature of a room fire, or throw pieces of itself across a space explosively. And the contents of a non-combustible building burn exactly as well as the contents of a wooden one, since the fuel in a modern fire is mostly furniture, finishes and stock rather than structure. So this lesson is about three material behaviours that are each counterintuitive, and about a classification system that gets misread constantly. The framing question throughout is not will it burn. It is how does it fail, and what does it do to the building around it on the way.

Three objectives for this lesson. First, you should be able to place a building in the five construction types, and, just as importantly, say what the classification does not tell you. Second, you should be able to describe how ordinary masonry-and-joist construction is designed to behave when its floors fail, because it was designed deliberately and the design is surprising. Third, you should be able to state what steel and concrete each do under fire, in terms specific enough to change a decision: not that they weaken, but how, at roughly what point, and with what effect on the rest of the structure. These three between them cover most of the commercial and multi-family building stock you will meet.

The five construction types are the shared vocabulary, so it is worth having them straight. Type one is fire-resistive: structural elements are non-combustible and protected, which is high-rise construction. Type two is non-combustible but unprotected or less protected, the steel-and-metal-deck commercial box. Type three is ordinary: masonry exterior walls with a combustible interior structure, which is most older main streets. Type four is heavy timber, with large-section members and the char behaviour we discussed. Type five is wood frame, everything in lesson two. Now the caveat that matters more than the list. The type describes the structure and says nothing at all about the contents. A Type one high-rise full of upholstered furniture and plastics has the same fuel load as a Type five house with the same furniture in it. The classification tells you how the building will behave. It tells you nothing about how the fire will behave inside it.

Ordinary construction deserves particular attention because its failure mode was chosen on purpose. Masonry outside, wood inside, and the wooden floor joists are seated in pockets in the masonry wall. Now consider the problem the builders faced. If a joist burns through and drops, and its end is square in that pocket, the falling joist levers against the masonry and pushes the wall outward, and the wall comes down into the street on whoever is standing there. So the joists are cut on an angle where they enter the wall, a fire cut, sized so that a failing joist rotates cleanly out of its pocket and drops without prying the wall over. That is elegant, and it is a deliberate decision that the floors are expendable and the walls are not. It also means the building is designed to have its interior collapse while standing, so the intact masonry facade of an ordinary building during a fire is not evidence that anything behind it is still there.

Steel does not burn, and that is most of what people know about it. Two other behaviours matter more. First, steel expands as it heats, on the order of an inch for every ten feet of length per hundred degrees Fahrenheit of rise, which sounds small until you apply it to a fifty-foot beam in a compartment fire and get several inches of elongation with nowhere to go. The beam pushes. It can shove a bearing wall out of plumb or off its seat, and this happens well before the steel is close to failing. Second, steel loses strength as it heats, and by around eleven hundred degrees Fahrenheit it has shed roughly half of its load capacity, a temperature routinely exceeded in a fully involved compartment. Then, as it fails, it does not break cleanly. It sags, and a sagging beam pulls inward on whatever its ends are attached to. So steel first pushes the building apart and then pulls it in, and neither behaviour is intuitive from the outside.

Concrete has a different problem, and it comes from water. Concrete holds moisture within its matrix long after it has cured. Heat it quickly and that moisture turns to steam inside the material, faster than it can escape through the surrounding mass, and the pressure has nowhere to go. So it breaks pieces off the face, sometimes with real violence, which is spalling. Two consequences follow. The immediate one is that fragments of concrete are being thrown off a surface, which is a hazard to anyone working near it. The more serious one is that concrete's fire resistance depends on the depth of cover protecting the reinforcing steel inside it, and spalling removes that cover. Once the rebar is exposed to fire it heats rapidly, loses strength on the same curve as any other steel, and the element begins to fail. Fast heating and high moisture content make it worse, which is why newer, denser concrete and heavy fire loads are a bad combination.

Take a couple of minutes on the non-combustible buildings in your own area. Where is the ordinary construction, meaning the masonry-fronted commercial rows, the older main street, the converted warehouse? Then ask what has been done to them since, because the answer is usually a great deal. Ordinary buildings are the ones most often renovated, subdivided, given a new storefront, converted from retail to residential above, or joined internally to the building next door. Every one of those changes touches the interior structure while leaving the masonry facade looking exactly as it did. And ask about the cocklofts, the common attic spaces running above the ceilings of an entire row, because in ordinary construction those are frequently continuous across party walls that stop short of the roof deck. That single feature has cost more firefighters their lives than any other characteristic of these buildings.

Three things to remember. The construction type describes the frame and not the contents, so a fire-resistive building can hold a thoroughly modern fuel load and usually does. Ordinary construction is deliberately designed for its floors to fail while its walls stand, by way of the fire-cut joist, which means an intact facade tells you nothing about the interior. And the two non-combustible materials each fail in a way that acts on the rest of the building: steel expands and pushes, then sags and pulls, losing about half its strength by eleven hundred degrees Fahrenheit; concrete spalls, throwing fragments and stripping the cover from the reinforcement it depends on.