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Robust, or Risky? What Makes an Earthquake-Resistant Building

Building codes were not always so rigorous—especially in earthquake-prone areas. Here's how to eyeball the risk posed by various structures.

BY KALEE THOMPSON

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A skyscraper's credentials aren't visible from the sidewalk, but you can rest assured that steel is more pliable than brick or concrete, and that modern skyscrapers are subject to stringent seismic standards. "You don't just want to build something that's stronger, you want to be able to control where it gives," engineer Aaron Reynolds says. To do that, architects add "dog bones" to the frame—areas of reduced width on the building's steel beams. These weak zones absorb shaking, diverting strain from its welded joints. If you're inside a high-rise during a quake, stay put; you run a greater risk of being hit by debris outside.

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One of the safest places to be during an earthquake is inside a well-made wood-framed home. Metal connectors join the floors and walls in newer houses; older ones have often been professionally retrofitted, or bolted. This creates what's called a continuous load path, in which the frame of the house moves as a single unit during tremors. Hardware should also connect the foundation and mudsill to wall studs, work that can be easily seen from the basement.

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Intense shaking can cause old masonry walls to crack, crumble, and collapse. Brick buildings can be strengthened with concrete and have steel "moment frames" installed around storefronts. Often, steel wall-to-floor reinforcements are visible from the street, where sandwich-size metal plates delineate the line between stories. While such retrofitting work is often mandated by building codes, there are still old brick buildings with little or no quake reinforcement.

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Without sufficient rebar, required by most building codes after the 1970s, concrete buildings are prone to pancaking. The risk is often higher in a building with a "soft" first story—one with high ceilings and multiple openings. Though rebar isn't visible, other reinforcement is: Contractors adhere carbon fiber to concrete walls and posts. "It looks like somebody glued on a burlap bag," Reynolds says. You may also see shear walls bracing the soft first story.

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Building with base isolation rests atop a series of flexible rubber cylinders; these act as a suspension system, turning violent jolts into subtle vibrations. The approach is the gold standard for hospitals, schools, and buildings that contain computer servers, refrigerated medical samples, and other valuable equipment. Such buildings are usually surrounded by a concrete moat at street level—look for a tiny gap in the sidewalk a few feet from the building's exterior walls.