Crumbled buildings renew calls for stronger designs

SAN FRANCISCO - Seismic safety experts long have warned that brittle concrete frame buildings pose a particularly deadly risk during a major earthquake.

But a horrifying video taken during last week's magnitude 7.1 Mexico quake may do more to highlight the risk than years of reports and studies.

In it, sirens blare, utility poles sway. Then in the background, a building wobbles. Concrete starts falling out of a ground-floor column. Then the columns flex, and the upper floors come crashing down, sinking into a cloud of dust.

"Dios mio! Dios mio!" a woman is heard saying. "My God! My God!"

The crumbled Enrique Rebsamen school in Mexico City - a three-story structure where at least 25 died, including 21 students - was made of concrete, as were many other structures that fell to the ground.

While they may be stout and muscular in appearance, concrete buildings without a robust level of steel reinforcement can see their columns peel off in chunks and then explode when exposed to violent side-to-side shaking.

Collapses of concrete buildings have been documented worldwide for