This post is the second in a three-part series looking back to the Northridge earthquake of Jan 17, 1994.
The Magnitude 6.7 Northridge earthquake changed the practice of structural engineering for the better. The big surprise was that welds cracked* on thick steel columns of brand-new buildings, and this led to five important lessons:
- Spread-out the lateral resistance
- For the prior decade or so, there had been an increasing trend to beef-up only a few of the building columns to resist earthquake forces, designing the remaining majority only strong enough to bear weight. This meant earthquake forces were concentrated in only a handful of ‘working’ columns while the rest of the building ‘went along for the ride.’ This put “all the eggs” in too few “baskets”, so to speak, and under all that pressure, some of the columns cracked.
- LESSON: Design with more redundancy so more columns to resist earthquake forces.
- Do testing at full-scale
- The justification for using fewer, bigger seismic columns came from tests in research labs where a welded steel assembly was literally pushed side-to-side with jacks until the steel deformed. These tests showed beautifully-ductile response, or ‘bending without breaking,’ which means the structure is absorbing earthquake forces without collapse. The catch? These tests were done with smaller-scale beams and columns, because of the limits of the jacks, and results were extrapolated upward. Turns out, the material properties of 2-inch-thick steel are quite different from those of half-inch-thick steel. Thick steel cools unevenly, resulting in more-brittle material.
- LESSON: Do research tests at full-scale and don’t extrapolate.
- More quality assurance for welds
- The material itself wasn’t the only culprit. Most of the cracks initiated at weld locations. Welds introduced discontinuities and even more brittleness in the steel material, including microcracks at locations of ‘backing bars,’ a construction technique that prevented molten weld material from dripping down the side of the column. The outcome? an enormous increase in specified welding procedures and inspection, including pre-heating, as well as removal and repair of backing bars.
- LESSON: Take extra quality assurance precautions on the structural elements doing the most work.
- Check buildings that went through prior earthquakes
- I don’t know how cracked columns were first discovered after Northridge. But later, engineers started wondering whether they would find similar damage in similarly-designed steel buildings that went through the Loma Prieta earthquake in Northern California 4 years earlier. No one had known to look. Good thing they checked, because sure enough, similar cracking was found, which means the earthquake resistance of those buildings had been severely compromised. Luckily there hadn’t been an earthquake since! (They have subsequently been retrofitted.)
- LESSON: Check for hidden damage.
- Build-in fuses
- One of the more clever inventions for steel buildings designed after Northridge was “reduced beam sections” or beams that had cut-outs near – but not at – the columns they were connected to. The idea is to concentrate damage away from connections that are critical for holding up floors (and thus ensuring safety). In other words, build-in a ductile “fuse,” kind of like the “crumple zone” of a car.
- LESSON: Protect safety-critical elements by intentionally designing other elements to absorb damage.
*There are literally hundreds of publications on the issue of cracked welds, most notably from the SAC Steel Project.