Northridge earthquake

Brief reference. Used to illustrate that codes designed only to force-of-fracture (not energy-of-fracture) failed when static structures were vibrated. Five billion dollars of damage led to code revision in California.

So we've been talking about fracture toughness and strength of materials. There's the force of fracture, which is tensile strength and yield strength — or in composites and polymers it may be just the ultimate tensile strength, because they don't really have a yield point. But we also have energy of fracture. Griffith pointed it out in 1925. It didn't become important until World War Two and the Liberty ships kind of emphasized it. Even today we are still running into problems. The Northridge earthquake in the early '90s in California — all the codes and standards had designed to force of fracture, and they had no real criteria for energy of fracture. You come along and have these static structures that get vibrated in an earthquake, and it caused five billion dollars worth of damage. Since then we've rewritten the codes and standards, and those structures now in California have to be able to absorb a little bit of energy, a little vibration that comes around every now and then. But there's still other industries where we haven't really gotten around to helping people figure out what the energy of fracture should be.

Used to illustrate how buildings absorbed Irwin's energy-of-fracture lesson 40 years late. "Building snapped in the earthquake and caused billions of dollars worth of damage." Triggered impact / energy-of-fracture requirements being added to the steel-in-buildings codes in the 1990s.

Let's go with a building. I'll go to the structural welding code, or to the American Institute of Steel Construction, and — based on yield stress rather than ultimate tensile stress — it's typically about two-thirds of yield. That's for the base material. The weld metal will be one-third of yield. We have twice as much safety factor for welds because we know they're going to have defects in them. So it depends on whether it's a straight steel plate or a welded plate. This is all in the Structural Welding Code of the American Welding Society and in the American Institute of Steel Construction for buildings. The safety factor is 1.67 on force. Then the question is, do you need an energy-of-fracture criterion for a building? We never really worried about that a whole lot after the 1950s, when Irwin explained how important the energy of fracture was. We didn't change the building codes. So when the Northridge earthquake hit Southern California in the 1990s, buildings snapped in the earthquake and caused billions of dollars worth of damage. In the 1990s, forty years after Irwin, we started incorporating impact, or energy-of-fracture requirements, into the steel in buildings.

Referenced as the empirical source for the modern LRFD (load-and-resistance factor design) approach in the AISC code. Tom credits Northridge building failures as a major input to the new methodology.

There actually is a new thing in the last ten years called load — what is it, Small, do you remember? Load-based moment design or something. It kind of came out of the Northridge earthquake in southern California in the early 90s, and it's so complex I've tried to look at it a few times. I need to take a course in it, okay.

$5–10 billion in building damage from steels lacking a fracture-toughness specification. Building toughness specs only entered civil engineering codes by 2000, decades after the Coast Guard and Navy required them.

We didn't have specifications for high toughness in buildings until the 1990 Northridge earthquake in southern California, which did five to ten billion dollars worth of damage because the steels didn't have a fracture toughness specification. 1990. Some of these buildings were built in the 1960s — we knew about this, and the Coast Guard was requiring it for some ships in the 1950s and 1960s. But no one was forcing the civil engineers to put those specifications into their requirements for buildings, in the '60s, '70s, or '80s. Then in the early '90s we have the Northridge earthquake, ten billion dollars worth of damage to buildings. By 2000 the civil engineers have a requirement for toughness in their buildings. But they don't on railroads. Until somebody decides they have a big enough problem, no one wants to write it into the spec, because it's going to cost money. So they keep living dangerously until something happens. There are lots of industries like that — we still haven't written it into a lot of other industries.

So the code — this is the good part of the code — it gives us a history of failures, and we fix things after the failure. After the Northridge earthquake in Los Angeles, they revised all kinds of parts of the building code for seismic loading. There was no problem with buildings being able to support their weight and everything in a static situation. But when the ground starts shaking like Jell-O and the buildings start wobbling from side to side, some of those moment connections, where you have a T intersection between the floor beams and the columns — when they're wobbling like this in the earthquake, they just popped, and buildings collapsed and bridges collapsed. So in the 1990s the civil engineers were doing a huge amount of redesign of the code for seismic applications. That's one of the reasons the codes get thicker and thicker with time.

$20B damage event that finally brought fracture mechanics into civil engineering codes. Welded steel connections snapped during seismic vibration.

The civil engineers never learned about it until 1994, when we had an earthquake in southern California called the Northridge earthquake. Twenty billion dollars worth of damage. They had never put real seismic requirements into their codes and standards for buildings vibrating. Well, the buildings vibrated in the earthquake and a lot of the steel connections snapped. That's one of the reasons it was a twenty-billion-dollar loss. The civil engineers have redone all those codes and standards in the last ten or fifteen years, and our design has gotten to be much more sophisticated.

Cited alongside Hyatt Regency as a code-revision-triggering event.

One of the things I want to talk about in codes and standards is, you were taught as a student that you need to learn how to design things, but no one ever told you that you have limitations on your design which are written into law that say you cannot do certain things. So there's the good of codes and standards: they are a historical repository of things that went bad. The Hyatt Regency walkway collapse, we'll probably talk about — where I think over a hundred people, maybe two hundred people, got killed. The Northridge earthquake in Southern California. When there's a major problem, they often revise the code or standard to make things safer. After 9/11 they revised a lot of the building egress code so people could get out of buildings faster, so the fire wouldn't bring the building down as quickly. Their interpretations of the code — codes are not always the easiest things to read.

Used to make the "codes lag the science" point. Buildings built in the 1960s, when fracture energy was understood but not coded, snapped at connections.

That concludes the strength part of comparing materials. Strength has to be measured by two parameters. One parameter is force and the other is energy. Nobody knew that until fifty years ago, when George Irwin came along — well, actually, the Liberty ships. And they still don't have energy in most of the codes. Civil engineers got it in the 1990s because of the Northridge earthquake. They found that buildings built in the 60s, when we knew about the need for fracture energy but it wasn't part of the code — no one's going to pay for it back then — when an earthquake came along, those buildings' connections just snapped. It cost billions of dollars, and the civil engineers decided, hmm, maybe energy of fracture is important.

Tom (mis)dates as "early '80s" — actually 1994. Code response was not to increase strength safety factor but toughness safety factor. Buildings designed as statically loaded became dynamically loaded under earthquake; impact loading produced brittle fractures at connections. Triggered transition from beam-formula design to Load Factor Resistance Design (LRFD). Buildings condemned not from collapse but from broken connections too expensive to repair.

Another example: the Northridge earthquake, which I think was early '80s in Los Angeles. They had a fairly major earthquake out there. They didn't increase the strength safety factor. What they increased was the toughness safety factor. Buildings that were supposed to be statically loaded structures, all of a sudden became dynamically loaded structures — the building swaying in the breeze. I don't know if any of you have been in a tall building and felt the building swaying in the breeze.

Cited as the canonical example of how engineering codes get rewritten after major failures — $15 billion in damage, building codes modified to accommodate earthquake vibrations.

Historically, some of our codes and standards have been written to correct for major mistakes. For example, the Northridge earthquake in Southern California 20 years ago — a lot of buildings and bridges came tumbling down, or were damaged to the tune of like $15 billion worth of damage. A lot of the building codes were modified to allow for earthquake vibrations. The Japanese have been doing that for 20 years because they have a lot more earthquakes than we do, but back in the 50s and 60s we didn't design bridges and buildings in Southern California to accommodate those things. You still read in the news about an earthquake in Chile or Iran, and 10,000 people died because the concrete buildings just collapsed on people. We actually can design things, because of different codes and standards.