A MARTINEZ, HOST:
It's been six days since an explosion damaged the bridge linking Russia to the Crimean Peninsula. And since then, it still hasn't been publicly determined just how it was pulled off. Russia's security service arrested eight people who it alleges planted a truck bomb. The Ukrainian government has not claimed responsibility. NPR's Julian Hayda looked for clues about who did it based on how it happened.
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UNIDENTIFIED PERSON: (Non-English language spoken).
JULIAN HAYDA, BYLINE: Ukrainian television and social media abound with theories about what happened to the Crimea bridge.
OLEH ZHDANOV: (Non-English language spoken).
HAYDA: "It couldn't have been a rocket," says Ukrainian military expert Oleh Zhdanov, "because the Ukrainian military doesn't have the kind of long-range rockets it would have required." Before ballistics experts can figure out how a bridge collapsed, they need to know what kind of bridge it is.
ANDREW BARR: The Crimean bridge has some quite large arch structures, but the majority of the bridge is a much simpler beam bridge design.
HAYDA: That's Andrew Barr. He researches blast and impact dynamics at the University of Sheffield. He says it's like one of those highway overpasses that we've all seen, with big vertical columns holding up the road above. That's the kind of section where the blast in the Crimea bridge happened. Three big chunks of the bridge, still barely held together, slipped to the bottom of the sea. But if it was one truck bomb, like Russian authorities say, why are there three damaged sections? The answer, says Barr, has to do with the sound you hear when you drive over a long bridge.
BARR: That regular clunk, clunk noise as you drive is the sound of your tires passing over one of these joints.
HAYDA: Instead of a bridge like this being one long road, it's a bunch of short roads that just butt together. So if one section falls off the cement column, it can pull others down with it. Now, Ukrainian authorities have said that Russia may have orchestrated the blast itself. Here's Andrew Barr.
BARR: The sparks in the air immediately after the explosion suggests the addition of some kind of reactive material.
HAYDA: Such as thermite, which human rights groups have accused Russia of using in Syria and Ukraine. No matter who's responsible, Barr says the attack took a lot of expert planning to break a bridge with rare explosives. After all, bridges are engineered to withstand a lot.
BARR: Pressures involved are so high that none of our standard building materials are strong enough to withstand them. So there's no realistic way of completely avoiding damage.
HAYDA: For now, Russia has vowed to step up security of the bridge to prevent further damage.
Julian Hayda, NPR News, Kyiv.
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